tcp_input.c

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/*
 * INET     An implementation of the TCP/IP protocol suite for the LINUX
 *      operating system.  INET is implemented using the  BSD Socket
 *      interface as the means of communication with the user level.
 *
 *      Implementation of the Transmission Control Protocol(TCP).
 *
 * Authors: Ross Biro
 *      Fred N. van Kempen, <[email protected]>
 *      Mark Evans, <[email protected]>
 *      Corey Minyard <[email protected]>
 *      Florian La Roche, <[email protected]>
 *      Charles Hedrick, <[email protected]>
 *      Linus Torvalds, <[email protected]>
 *      Alan Cox, <[email protected]>
 *      Matthew Dillon, <[email protected]>
 *      Arnt Gulbrandsen, <[email protected]>
 *      Jorge Cwik, <[email protected]>
 */

/*
 * Changes:
 *      Pedro Roque :   Fast Retransmit/Recovery.
 *                  Two receive queues.
 *                  Retransmit queue handled by TCP.
 *                  Better retransmit timer handling.
 *                  New congestion avoidance.
 *                  Header prediction.
 *                  Variable renaming.
 *
 *      Eric        :   Fast Retransmit.
 *      Randy Scott :   MSS option defines.
 *      Eric Schenk :   Fixes to slow start algorithm.
 *      Eric Schenk :   Yet another double ACK bug.
 *      Eric Schenk :   Delayed ACK bug fixes.
 *      Eric Schenk :   Floyd style fast retrans war avoidance.
 *      David S. Miller :   Don't allow zero congestion window.
 *      Eric Schenk :   Fix retransmitter so that it sends
 *                  next packet on ack of previous packet.
 *      Andi Kleen  :   Moved open_request checking here
 *                  and process RSTs for open_requests.
 *      Andi Kleen  :   Better prune_queue, and other fixes.
 *      Andrey Savochkin:   Fix RTT measurements in the presence of
 *                  timestamps.
 *      Andrey Savochkin:   Check sequence numbers correctly when
 *                  removing SACKs due to in sequence incoming
 *                  data segments.
 *      Andi Kleen:     Make sure we never ack data there is not
 *                  enough room for. Also make this condition
 *                  a fatal error if it might still happen.
 *      Andi Kleen:     Add tcp_measure_rcv_mss to make
 *                  connections with MSS<min(MTU,ann. MSS)
 *                  work without delayed acks.
 *      Andi Kleen:     Process packets with PSH set in the
 *                  fast path.
 *      J Hadi Salim:       ECN support
 *      Andrei Gurtov,
 *      Pasi Sarolahti,
 *      Panu Kuhlberg:      Experimental audit of TCP (re)transmission
 *                  engine. Lots of bugs are found.
 *      Pasi Sarolahti:     F-RTO for dealing with spurious RTOs
 */

#define pr_fmt(fmt) "TCP: " fmt

#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/module.h>
#include <linux/sysctl.h>
#include <linux/kernel.h>
#include <net/dst.h>
#include <net/tcp.h>
#include <net/inet_common.h>
#include <linux/ipsec.h>
#include <asm/unaligned.h>
#include <net/netdma.h>

int sysctl_tcp_timestamps __read_mostly = 1;
int sysctl_tcp_window_scaling __read_mostly = 1;
int sysctl_tcp_sack __read_mostly = 1;
int sysctl_tcp_fack __read_mostly = 1;
int sysctl_tcp_reordering __read_mostly = TCP_FASTRETRANS_THRESH;
EXPORT_SYMBOL(sysctl_tcp_reordering);
int sysctl_tcp_ecn __read_mostly = 2;
EXPORT_SYMBOL(sysctl_tcp_ecn);
int sysctl_tcp_dsack __read_mostly = 1;
int sysctl_tcp_app_win __read_mostly = 31;
int sysctl_tcp_adv_win_scale __read_mostly = 1;
EXPORT_SYMBOL(sysctl_tcp_adv_win_scale);

/* rfc5961 challenge ack rate limiting */
int sysctl_tcp_challenge_ack_limit = 100;

int sysctl_tcp_stdurg __read_mostly;
int sysctl_tcp_rfc1337 __read_mostly;
int sysctl_tcp_max_orphans __read_mostly = NR_FILE;
int sysctl_tcp_frto __read_mostly = 2;
int sysctl_tcp_frto_response __read_mostly;

int sysctl_tcp_thin_dupack __read_mostly;

int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
int sysctl_tcp_abc __read_mostly;
int sysctl_tcp_early_retrans __read_mostly = 2;

#define FLAG_DATA       0x01 /* Incoming frame contained data.      */
#define FLAG_WIN_UPDATE     0x02 /* Incoming ACK was a window update.   */
#define FLAG_DATA_ACKED     0x04 /* This ACK acknowledged new data.     */
#define FLAG_RETRANS_DATA_ACKED 0x08 /* "" "" some of which was retransmitted.  */
#define FLAG_SYN_ACKED      0x10 /* This ACK acknowledged SYN.      */
#define FLAG_DATA_SACKED    0x20 /* New SACK.               */
#define FLAG_ECE        0x40 /* ECE in this ACK             */
#define FLAG_SLOWPATH       0x100 /* Do not skip RFC checks for window update.*/
#define FLAG_ONLY_ORIG_SACKED   0x200 /* SACKs only non-rexmit sent before RTO */
#define FLAG_SND_UNA_ADVANCED   0x400 /* Snd_una was changed (!= FLAG_DATA_ACKED) */
#define FLAG_DSACKING_ACK   0x800 /* SACK blocks contained D-SACK info */
#define FLAG_NONHEAD_RETRANS_ACKED  0x1000 /* Non-head rexmitted data was ACKed */
#define FLAG_SACK_RENEGING  0x2000 /* snd_una advanced to a sacked seq */

#define FLAG_ACKED      (FLAG_DATA_ACKED|FLAG_SYN_ACKED)
#define FLAG_NOT_DUP        (FLAG_DATA|FLAG_WIN_UPDATE|FLAG_ACKED)
#define FLAG_CA_ALERT       (FLAG_DATA_SACKED|FLAG_ECE)
#define FLAG_FORWARD_PROGRESS   (FLAG_ACKED|FLAG_DATA_SACKED)
#define FLAG_ANY_PROGRESS   (FLAG_FORWARD_PROGRESS|FLAG_SND_UNA_ADVANCED)

#define TCP_REMNANT (TCP_FLAG_FIN|TCP_FLAG_URG|TCP_FLAG_SYN|TCP_FLAG_PSH)
#define TCP_HP_BITS (~(TCP_RESERVED_BITS|TCP_FLAG_PSH))

/* Adapt the MSS value used to make delayed ack decision to the
 * real world.
 */
static void tcp_measure_rcv_mss(struct sock *sk, const struct sk_buff *skb)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    const unsigned int lss = icsk->icsk_ack.last_seg_size;
    unsigned int len;

    icsk->icsk_ack.last_seg_size = 0;

    /* skb->len may jitter because of SACKs, even if peer
     * sends good full-sized frames.
     */
    len = skb_shinfo(skb)->gso_size ? : skb->len;
    if (len >= icsk->icsk_ack.rcv_mss) {
        icsk->icsk_ack.rcv_mss = len;
    } else {
        /* Otherwise, we make more careful check taking into account,
         * that SACKs block is variable.
         *
         * "len" is invariant segment length, including TCP header.
         */
        len += skb->data - skb_transport_header(skb);
        if (len >= TCP_MSS_DEFAULT + sizeof(struct tcphdr) ||
            /* If PSH is not set, packet should be
             * full sized, provided peer TCP is not badly broken.
             * This observation (if it is correct 8)) allows
             * to handle super-low mtu links fairly.
             */
            (len >= TCP_MIN_MSS + sizeof(struct tcphdr) &&
             !(tcp_flag_word(tcp_hdr(skb)) & TCP_REMNANT))) {
            /* Subtract also invariant (if peer is RFC compliant),
             * tcp header plus fixed timestamp option length.
             * Resulting "len" is MSS free of SACK jitter.
             */
            len -= tcp_sk(sk)->tcp_header_len;
            icsk->icsk_ack.last_seg_size = len;
            if (len == lss) {
                icsk->icsk_ack.rcv_mss = len;
                return;
            }
        }
        if (icsk->icsk_ack.pending & ICSK_ACK_PUSHED)
            icsk->icsk_ack.pending |= ICSK_ACK_PUSHED2;
        icsk->icsk_ack.pending |= ICSK_ACK_PUSHED;
    }
}

static void tcp_incr_quickack(struct sock *sk)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    unsigned int quickacks = tcp_sk(sk)->rcv_wnd / (2 * icsk->icsk_ack.rcv_mss);

    if (quickacks == 0)
        quickacks = 2;
    if (quickacks > icsk->icsk_ack.quick)
        icsk->icsk_ack.quick = min(quickacks, TCP_MAX_QUICKACKS);
}

static void tcp_enter_quickack_mode(struct sock *sk)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    tcp_incr_quickack(sk);
    icsk->icsk_ack.pingpong = 0;
    icsk->icsk_ack.ato = TCP_ATO_MIN;
}

/* Send ACKs quickly, if "quick" count is not exhausted
 * and the session is not interactive.
 */

static inline bool tcp_in_quickack_mode(const struct sock *sk)
{
    const struct inet_connection_sock *icsk = inet_csk(sk);

    return icsk->icsk_ack.quick && !icsk->icsk_ack.pingpong;
}

static inline void TCP_ECN_queue_cwr(struct tcp_sock *tp)
{
    if (tp->ecn_flags & TCP_ECN_OK)
        tp->ecn_flags |= TCP_ECN_QUEUE_CWR;
}

static inline void TCP_ECN_accept_cwr(struct tcp_sock *tp, const struct sk_buff *skb)
{
    if (tcp_hdr(skb)->cwr)
        tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
}

static inline void TCP_ECN_withdraw_cwr(struct tcp_sock *tp)
{
    tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
}

static inline void TCP_ECN_check_ce(struct tcp_sock *tp, const struct sk_buff *skb)
{
    if (!(tp->ecn_flags & TCP_ECN_OK))
        return;

    switch (TCP_SKB_CB(skb)->ip_dsfield & INET_ECN_MASK) {
    case INET_ECN_NOT_ECT:
        /* Funny extension: if ECT is not set on a segment,
         * and we already seen ECT on a previous segment,
         * it is probably a retransmit.
         */
        if (tp->ecn_flags & TCP_ECN_SEEN)
            tcp_enter_quickack_mode((struct sock *)tp);
        break;
    case INET_ECN_CE:
        if (!(tp->ecn_flags & TCP_ECN_DEMAND_CWR)) {
            /* Better not delay acks, sender can have a very low cwnd */
            tcp_enter_quickack_mode((struct sock *)tp);
            tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
        }
        /* fallinto */
    default:
        tp->ecn_flags |= TCP_ECN_SEEN;
    }
}

static inline void TCP_ECN_rcv_synack(struct tcp_sock *tp, const struct tcphdr *th)
{
    if ((tp->ecn_flags & TCP_ECN_OK) && (!th->ece || th->cwr))
        tp->ecn_flags &= ~TCP_ECN_OK;
}

static inline void TCP_ECN_rcv_syn(struct tcp_sock *tp, const struct tcphdr *th)
{
    if ((tp->ecn_flags & TCP_ECN_OK) && (!th->ece || !th->cwr))
        tp->ecn_flags &= ~TCP_ECN_OK;
}

static bool TCP_ECN_rcv_ecn_echo(const struct tcp_sock *tp, const struct tcphdr *th)
{
    if (th->ece && !th->syn && (tp->ecn_flags & TCP_ECN_OK))
        return true;
    return false;
}

/* Buffer size and advertised window tuning.
 *
 * 1. Tuning sk->sk_sndbuf, when connection enters established state.
 */

static void tcp_fixup_sndbuf(struct sock *sk)
{
    int sndmem = SKB_TRUESIZE(tcp_sk(sk)->rx_opt.mss_clamp + MAX_TCP_HEADER);

    sndmem *= TCP_INIT_CWND;
    if (sk->sk_sndbuf < sndmem)
        sk->sk_sndbuf = min(sndmem, sysctl_tcp_wmem[2]);
}

/* 2. Tuning advertised window (window_clamp, rcv_ssthresh)
 *
 * All tcp_full_space() is split to two parts: "network" buffer, allocated
 * forward and advertised in receiver window (tp->rcv_wnd) and
 * "application buffer", required to isolate scheduling/application
 * latencies from network.
 * window_clamp is maximal advertised window. It can be less than
 * tcp_full_space(), in this case tcp_full_space() - window_clamp
 * is reserved for "application" buffer. The less window_clamp is
 * the smoother our behaviour from viewpoint of network, but the lower
 * throughput and the higher sensitivity of the connection to losses. 8)
 *
 * rcv_ssthresh is more strict window_clamp used at "slow start"
 * phase to predict further behaviour of this connection.
 * It is used for two goals:
 * - to enforce header prediction at sender, even when application
 *   requires some significant "application buffer". It is check #1.
 * - to prevent pruning of receive queue because of misprediction
 *   of receiver window. Check #2.
 *
 * The scheme does not work when sender sends good segments opening
 * window and then starts to feed us spaghetti. But it should work
 * in common situations. Otherwise, we have to rely on queue collapsing.
 */

/* Slow part of check#2. */
static int __tcp_grow_window(const struct sock *sk, const struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);
    /* Optimize this! */
    int truesize = tcp_win_from_space(skb->truesize) >> 1;
    int window = tcp_win_from_space(sysctl_tcp_rmem[2]) >> 1;

    while (tp->rcv_ssthresh <= window) {
        if (truesize <= skb->len)
            return 2 * inet_csk(sk)->icsk_ack.rcv_mss;

        truesize >>= 1;
        window >>= 1;
    }
    return 0;
}

static void tcp_grow_window(struct sock *sk, const struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);

    /* Check #1 */
    if (tp->rcv_ssthresh < tp->window_clamp &&
        (int)tp->rcv_ssthresh < tcp_space(sk) &&
        !sk_under_memory_pressure(sk)) {
        int incr;

        /* Check #2. Increase window, if skb with such overhead
         * will fit to rcvbuf in future.
         */
        if (tcp_win_from_space(skb->truesize) <= skb->len)
            incr = 2 * tp->advmss;
        else
            incr = __tcp_grow_window(sk, skb);

        if (incr) {
            incr = max_t(int, incr, 2 * skb->len);
            tp->rcv_ssthresh = min(tp->rcv_ssthresh + incr,
                           tp->window_clamp);
            inet_csk(sk)->icsk_ack.quick |= 1;
        }
    }
}

/* 3. Tuning rcvbuf, when connection enters established state. */

static void tcp_fixup_rcvbuf(struct sock *sk)
{
    u32 mss = tcp_sk(sk)->advmss;
    u32 icwnd = TCP_DEFAULT_INIT_RCVWND;
    int rcvmem;

    /* Limit to 10 segments if mss <= 1460,
     * or 14600/mss segments, with a minimum of two segments.
     */
    if (mss > 1460)
        icwnd = max_t(u32, (1460 * TCP_DEFAULT_INIT_RCVWND) / mss, 2);

    rcvmem = SKB_TRUESIZE(mss + MAX_TCP_HEADER);
    while (tcp_win_from_space(rcvmem) < mss)
        rcvmem += 128;

    rcvmem *= icwnd;

    if (sk->sk_rcvbuf < rcvmem)
        sk->sk_rcvbuf = min(rcvmem, sysctl_tcp_rmem[2]);
}

/* 4. Try to fixup all. It is made immediately after connection enters
 *    established state.
 */
void tcp_init_buffer_space(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    int maxwin;

    if (!(sk->sk_userlocks & SOCK_RCVBUF_LOCK))
        tcp_fixup_rcvbuf(sk);
    if (!(sk->sk_userlocks & SOCK_SNDBUF_LOCK))
        tcp_fixup_sndbuf(sk);

    tp->rcvq_space.space = tp->rcv_wnd;

    maxwin = tcp_full_space(sk);

    if (tp->window_clamp >= maxwin) {
        tp->window_clamp = maxwin;

        if (sysctl_tcp_app_win && maxwin > 4 * tp->advmss)
            tp->window_clamp = max(maxwin -
                           (maxwin >> sysctl_tcp_app_win),
                           4 * tp->advmss);
    }

    /* Force reservation of one segment. */
    if (sysctl_tcp_app_win &&
        tp->window_clamp > 2 * tp->advmss &&
        tp->window_clamp + tp->advmss > maxwin)
        tp->window_clamp = max(2 * tp->advmss, maxwin - tp->advmss);

    tp->rcv_ssthresh = min(tp->rcv_ssthresh, tp->window_clamp);
    tp->snd_cwnd_stamp = tcp_time_stamp;
}

/* 5. Recalculate window clamp after socket hit its memory bounds. */
static void tcp_clamp_window(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct inet_connection_sock *icsk = inet_csk(sk);

    icsk->icsk_ack.quick = 0;

    if (sk->sk_rcvbuf < sysctl_tcp_rmem[2] &&
        !(sk->sk_userlocks & SOCK_RCVBUF_LOCK) &&
        !sk_under_memory_pressure(sk) &&
        sk_memory_allocated(sk) < sk_prot_mem_limits(sk, 0)) {
        sk->sk_rcvbuf = min(atomic_read(&sk->sk_rmem_alloc),
                    sysctl_tcp_rmem[2]);
    }
    if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf)
        tp->rcv_ssthresh = min(tp->window_clamp, 2U * tp->advmss);
}

/* Initialize RCV_MSS value.
 * RCV_MSS is an our guess about MSS used by the peer.
 * We haven't any direct information about the MSS.
 * It's better to underestimate the RCV_MSS rather than overestimate.
 * Overestimations make us ACKing less frequently than needed.
 * Underestimations are more easy to detect and fix by tcp_measure_rcv_mss().
 */
void tcp_initialize_rcv_mss(struct sock *sk)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    unsigned int hint = min_t(unsigned int, tp->advmss, tp->mss_cache);

    hint = min(hint, tp->rcv_wnd / 2);
    hint = min(hint, TCP_MSS_DEFAULT);
    hint = max(hint, TCP_MIN_MSS);

    inet_csk(sk)->icsk_ack.rcv_mss = hint;
}
EXPORT_SYMBOL(tcp_initialize_rcv_mss);

/* Receiver "autotuning" code.
 *
 * The algorithm for RTT estimation w/o timestamps is based on
 * Dynamic Right-Sizing (DRS) by Wu Feng and Mike Fisk of LANL.
 * <http://public.lanl.gov/radiant/pubs.html#DRS>
 *
 * More detail on this code can be found at
 * <http://staff.psc.edu/jheffner/>,
 * though this reference is out of date.  A new paper
 * is pending.
 */
static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)
{
    u32 new_sample = tp->rcv_rtt_est.rtt;
    long m = sample;

    if (m == 0)
        m = 1;

    if (new_sample != 0) {
        /* If we sample in larger samples in the non-timestamp
         * case, we could grossly overestimate the RTT especially
         * with chatty applications or bulk transfer apps which
         * are stalled on filesystem I/O.
         *
         * Also, since we are only going for a minimum in the
         * non-timestamp case, we do not smooth things out
         * else with timestamps disabled convergence takes too
         * long.
         */
        if (!win_dep) {
            m -= (new_sample >> 3);
            new_sample += m;
        } else {
            m <<= 3;
            if (m < new_sample)
                new_sample = m;
        }
    } else {
        /* No previous measure. */
        new_sample = m << 3;
    }

    if (tp->rcv_rtt_est.rtt != new_sample)
        tp->rcv_rtt_est.rtt = new_sample;
}

static inline void tcp_rcv_rtt_measure(struct tcp_sock *tp)
{
    if (tp->rcv_rtt_est.time == 0)
        goto new_measure;
    if (before(tp->rcv_nxt, tp->rcv_rtt_est.seq))
        return;
    tcp_rcv_rtt_update(tp, tcp_time_stamp - tp->rcv_rtt_est.time, 1);

new_measure:
    tp->rcv_rtt_est.seq = tp->rcv_nxt + tp->rcv_wnd;
    tp->rcv_rtt_est.time = tcp_time_stamp;
}

static inline void tcp_rcv_rtt_measure_ts(struct sock *sk,
                      const struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);
    if (tp->rx_opt.rcv_tsecr &&
        (TCP_SKB_CB(skb)->end_seq -
         TCP_SKB_CB(skb)->seq >= inet_csk(sk)->icsk_ack.rcv_mss))
        tcp_rcv_rtt_update(tp, tcp_time_stamp - tp->rx_opt.rcv_tsecr, 0);
}

/*
 * This function should be called every time data is copied to user space.
 * It calculates the appropriate TCP receive buffer space.
 */
void tcp_rcv_space_adjust(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    int time;
    int space;

    if (tp->rcvq_space.time == 0)
        goto new_measure;

    time = tcp_time_stamp - tp->rcvq_space.time;
    if (time < (tp->rcv_rtt_est.rtt >> 3) || tp->rcv_rtt_est.rtt == 0)
        return;

    space = 2 * (tp->copied_seq - tp->rcvq_space.seq);

    space = max(tp->rcvq_space.space, space);

    if (tp->rcvq_space.space != space) {
        int rcvmem;

        tp->rcvq_space.space = space;

        if (sysctl_tcp_moderate_rcvbuf &&
            !(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
            int new_clamp = space;

            /* Receive space grows, normalize in order to
             * take into account packet headers and sk_buff
             * structure overhead.
             */
            space /= tp->advmss;
            if (!space)
                space = 1;
            rcvmem = SKB_TRUESIZE(tp->advmss + MAX_TCP_HEADER);
            while (tcp_win_from_space(rcvmem) < tp->advmss)
                rcvmem += 128;
            space *= rcvmem;
            space = min(space, sysctl_tcp_rmem[2]);
            if (space > sk->sk_rcvbuf) {
                sk->sk_rcvbuf = space;

                /* Make the window clamp follow along.  */
                tp->window_clamp = new_clamp;
            }
        }
    }

new_measure:
    tp->rcvq_space.seq = tp->copied_seq;
    tp->rcvq_space.time = tcp_time_stamp;
}

/* There is something which you must keep in mind when you analyze the
 * behavior of the tp->ato delayed ack timeout interval.  When a
 * connection starts up, we want to ack as quickly as possible.  The
 * problem is that "good" TCP's do slow start at the beginning of data
 * transmission.  The means that until we send the first few ACK's the
 * sender will sit on his end and only queue most of his data, because
 * he can only send snd_cwnd unacked packets at any given time.  For
 * each ACK we send, he increments snd_cwnd and transmits more of his
 * queue.  -DaveM
 */
static void tcp_event_data_recv(struct sock *sk, struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct inet_connection_sock *icsk = inet_csk(sk);
    u32 now;

    inet_csk_schedule_ack(sk);

    tcp_measure_rcv_mss(sk, skb);

    tcp_rcv_rtt_measure(tp);

    now = tcp_time_stamp;

    if (!icsk->icsk_ack.ato) {
        /* The _first_ data packet received, initialize
         * delayed ACK engine.
         */
        tcp_incr_quickack(sk);
        icsk->icsk_ack.ato = TCP_ATO_MIN;
    } else {
        int m = now - icsk->icsk_ack.lrcvtime;

        if (m <= TCP_ATO_MIN / 2) {
            /* The fastest case is the first. */
            icsk->icsk_ack.ato = (icsk->icsk_ack.ato >> 1) + TCP_ATO_MIN / 2;
        } else if (m < icsk->icsk_ack.ato) {
            icsk->icsk_ack.ato = (icsk->icsk_ack.ato >> 1) + m;
            if (icsk->icsk_ack.ato > icsk->icsk_rto)
                icsk->icsk_ack.ato = icsk->icsk_rto;
        } else if (m > icsk->icsk_rto) {
            /* Too long gap. Apparently sender failed to
             * restart window, so that we send ACKs quickly.
             */
            tcp_incr_quickack(sk);
            sk_mem_reclaim(sk);
        }
    }
    icsk->icsk_ack.lrcvtime = now;

    TCP_ECN_check_ce(tp, skb);

    if (skb->len >= 128)
        tcp_grow_window(sk, skb);
}

/* Called to compute a smoothed rtt estimate. The data fed to this
 * routine either comes from timestamps, or from segments that were
 * known _not_ to have been retransmitted [see Karn/Partridge
 * Proceedings SIGCOMM 87]. The algorithm is from the SIGCOMM 88
 * piece by Van Jacobson.
 * NOTE: the next three routines used to be one big routine.
 * To save cycles in the RFC 1323 implementation it was better to break
 * it up into three procedures. -- erics
 */
static void tcp_rtt_estimator(struct sock *sk, const __u32 mrtt)
{
    struct tcp_sock *tp = tcp_sk(sk);
    long m = mrtt; /* RTT */

    /*  The following amusing code comes from Jacobson's
     *  article in SIGCOMM '88.  Note that rtt and mdev
     *  are scaled versions of rtt and mean deviation.
     *  This is designed to be as fast as possible
     *  m stands for "measurement".
     *
     *  On a 1990 paper the rto value is changed to:
     *  RTO = rtt + 4 * mdev
     *
     * Funny. This algorithm seems to be very broken.
     * These formulae increase RTO, when it should be decreased, increase
     * too slowly, when it should be increased quickly, decrease too quickly
     * etc. I guess in BSD RTO takes ONE value, so that it is absolutely
     * does not matter how to _calculate_ it. Seems, it was trap
     * that VJ failed to avoid. 8)
     */
    if (m == 0)
        m = 1;
    if (tp->srtt != 0) {
        m -= (tp->srtt >> 3);   /* m is now error in rtt est */
        tp->srtt += m;      /* rtt = 7/8 rtt + 1/8 new */
        if (m < 0) {
            m = -m;     /* m is now abs(error) */
            m -= (tp->mdev >> 2);   /* similar update on mdev */
            /* This is similar to one of Eifel findings.
             * Eifel blocks mdev updates when rtt decreases.
             * This solution is a bit different: we use finer gain
             * for mdev in this case (alpha*beta).
             * Like Eifel it also prevents growth of rto,
             * but also it limits too fast rto decreases,
             * happening in pure Eifel.
             */
            if (m > 0)
                m >>= 3;
        } else {
            m -= (tp->mdev >> 2);   /* similar update on mdev */
        }
        tp->mdev += m;          /* mdev = 3/4 mdev + 1/4 new */
        if (tp->mdev > tp->mdev_max) {
            tp->mdev_max = tp->mdev;
            if (tp->mdev_max > tp->rttvar)
                tp->rttvar = tp->mdev_max;
        }
        if (after(tp->snd_una, tp->rtt_seq)) {
            if (tp->mdev_max < tp->rttvar)
                tp->rttvar -= (tp->rttvar - tp->mdev_max) >> 2;
            tp->rtt_seq = tp->snd_nxt;
            tp->mdev_max = tcp_rto_min(sk);
        }
    } else {
        /* no previous measure. */
        tp->srtt = m << 3;  /* take the measured time to be rtt */
        tp->mdev = m << 1;  /* make sure rto = 3*rtt */
        tp->mdev_max = tp->rttvar = max(tp->mdev, tcp_rto_min(sk));
        tp->rtt_seq = tp->snd_nxt;
    }
}

/* Calculate rto without backoff.  This is the second half of Van Jacobson's
 * routine referred to above.
 */
void tcp_set_rto(struct sock *sk)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    /* Old crap is replaced with new one. 8)
     *
     * More seriously:
     * 1. If rtt variance happened to be less 50msec, it is hallucination.
     *    It cannot be less due to utterly erratic ACK generation made
     *    at least by solaris and freebsd. "Erratic ACKs" has _nothing_
     *    to do with delayed acks, because at cwnd>2 true delack timeout
     *    is invisible. Actually, Linux-2.4 also generates erratic
     *    ACKs in some circumstances.
     */
    inet_csk(sk)->icsk_rto = __tcp_set_rto(tp);

    /* 2. Fixups made earlier cannot be right.
     *    If we do not estimate RTO correctly without them,
     *    all the algo is pure shit and should be replaced
     *    with correct one. It is exactly, which we pretend to do.
     */

    /* NOTE: clamping at TCP_RTO_MIN is not required, current algo
     * guarantees that rto is higher.
     */
    tcp_bound_rto(sk);
}

__u32 tcp_init_cwnd(const struct tcp_sock *tp, const struct dst_entry *dst)
{
    __u32 cwnd = (dst ? dst_metric(dst, RTAX_INITCWND) : 0);

    if (!cwnd)
        cwnd = TCP_INIT_CWND;
    return min_t(__u32, cwnd, tp->snd_cwnd_clamp);
}

/*
 * Packet counting of FACK is based on in-order assumptions, therefore TCP
 * disables it when reordering is detected
 */
void tcp_disable_fack(struct tcp_sock *tp)
{
    /* RFC3517 uses different metric in lost marker => reset on change */
    if (tcp_is_fack(tp))
        tp->lost_skb_hint = NULL;
    tp->rx_opt.sack_ok &= ~TCP_FACK_ENABLED;
}

/* Take a notice that peer is sending D-SACKs */
static void tcp_dsack_seen(struct tcp_sock *tp)
{
    tp->rx_opt.sack_ok |= TCP_DSACK_SEEN;
}

static void tcp_update_reordering(struct sock *sk, const int metric,
                  const int ts)
{
    struct tcp_sock *tp = tcp_sk(sk);
    if (metric > tp->reordering) {
        int mib_idx;

        tp->reordering = min(TCP_MAX_REORDERING, metric);

        /* This exciting event is worth to be remembered. 8) */
        if (ts)
            mib_idx = LINUX_MIB_TCPTSREORDER;
        else if (tcp_is_reno(tp))
            mib_idx = LINUX_MIB_TCPRENOREORDER;
        else if (tcp_is_fack(tp))
            mib_idx = LINUX_MIB_TCPFACKREORDER;
        else
            mib_idx = LINUX_MIB_TCPSACKREORDER;

        NET_INC_STATS_BH(sock_net(sk), mib_idx);
#if FASTRETRANS_DEBUG > 1
        pr_debug("Disorder%d %d %u f%u s%u rr%d\n",
             tp->rx_opt.sack_ok, inet_csk(sk)->icsk_ca_state,
             tp->reordering,
             tp->fackets_out,
             tp->sacked_out,
             tp->undo_marker ? tp->undo_retrans : 0);
#endif
        tcp_disable_fack(tp);
    }

    if (metric > 0)
        tcp_disable_early_retrans(tp);
}

/* This must be called before lost_out is incremented */
static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb)
{
    if ((tp->retransmit_skb_hint == NULL) ||
        before(TCP_SKB_CB(skb)->seq,
           TCP_SKB_CB(tp->retransmit_skb_hint)->seq))
        tp->retransmit_skb_hint = skb;

    if (!tp->lost_out ||
        after(TCP_SKB_CB(skb)->end_seq, tp->retransmit_high))
        tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
}

static void tcp_skb_mark_lost(struct tcp_sock *tp, struct sk_buff *skb)
{
    if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_ACKED))) {
        tcp_verify_retransmit_hint(tp, skb);

        tp->lost_out += tcp_skb_pcount(skb);
        TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
    }
}

static void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp,
                        struct sk_buff *skb)
{
    tcp_verify_retransmit_hint(tp, skb);

    if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_ACKED))) {
        tp->lost_out += tcp_skb_pcount(skb);
        TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
    }
}

/* This procedure tags the retransmission queue when SACKs arrive.
 *
 * We have three tag bits: SACKED(S), RETRANS(R) and LOST(L).
 * Packets in queue with these bits set are counted in variables
 * sacked_out, retrans_out and lost_out, correspondingly.
 *
 * Valid combinations are:
 * Tag  InFlight    Description
 * 0    1       - orig segment is in flight.
 * S    0       - nothing flies, orig reached receiver.
 * L    0       - nothing flies, orig lost by net.
 * R    2       - both orig and retransmit are in flight.
 * L|R  1       - orig is lost, retransmit is in flight.
 * S|R  1       - orig reached receiver, retrans is still in flight.
 * (L|S|R is logically valid, it could occur when L|R is sacked,
 *  but it is equivalent to plain S and code short-curcuits it to S.
 *  L|S is logically invalid, it would mean -1 packet in flight 8))
 *
 * These 6 states form finite state machine, controlled by the following events:
 * 1. New ACK (+SACK) arrives. (tcp_sacktag_write_queue())
 * 2. Retransmission. (tcp_retransmit_skb(), tcp_xmit_retransmit_queue())
 * 3. Loss detection event of two flavors:
 *  A. Scoreboard estimator decided the packet is lost.
 *     A'. Reno "three dupacks" marks head of queue lost.
 *     A''. Its FACK modification, head until snd.fack is lost.
 *  B. SACK arrives sacking SND.NXT at the moment, when the
 *     segment was retransmitted.
 * 4. D-SACK added new rule: D-SACK changes any tag to S.
 *
 * It is pleasant to note, that state diagram turns out to be commutative,
 * so that we are allowed not to be bothered by order of our actions,
 * when multiple events arrive simultaneously. (see the function below).
 *
 * Reordering detection.
 * --------------------
 * Reordering metric is maximal distance, which a packet can be displaced
 * in packet stream. With SACKs we can estimate it:
 *
 * 1. SACK fills old hole and the corresponding segment was not
 *    ever retransmitted -> reordering. Alas, we cannot use it
 *    when segment was retransmitted.
 * 2. The last flaw is solved with D-SACK. D-SACK arrives
 *    for retransmitted and already SACKed segment -> reordering..
 * Both of these heuristics are not used in Loss state, when we cannot
 * account for retransmits accurately.
 *
 * SACK block validation.
 * ----------------------
 *
 * SACK block range validation checks that the received SACK block fits to
 * the expected sequence limits, i.e., it is between SND.UNA and SND.NXT.
 * Note that SND.UNA is not included to the range though being valid because
 * it means that the receiver is rather inconsistent with itself reporting
 * SACK reneging when it should advance SND.UNA. Such SACK block this is
 * perfectly valid, however, in light of RFC2018 which explicitly states
 * that "SACK block MUST reflect the newest segment.  Even if the newest
 * segment is going to be discarded ...", not that it looks very clever
 * in case of head skb. Due to potentional receiver driven attacks, we
 * choose to avoid immediate execution of a walk in write queue due to
 * reneging and defer head skb's loss recovery to standard loss recovery
 * procedure that will eventually trigger (nothing forbids us doing this).
 *
 * Implements also blockage to start_seq wrap-around. Problem lies in the
 * fact that though start_seq (s) is before end_seq (i.e., not reversed),
 * there's no guarantee that it will be before snd_nxt (n). The problem
 * happens when start_seq resides between end_seq wrap (e_w) and snd_nxt
 * wrap (s_w):
 *
 *         <- outs wnd ->                          <- wrapzone ->
 *         u     e      n                         u_w   e_w  s n_w
 *         |     |      |                          |     |   |  |
 * |<------------+------+----- TCP seqno space --------------+---------->|
 * ...-- <2^31 ->|                                           |<--------...
 * ...---- >2^31 ------>|                                    |<--------...
 *
 * Current code wouldn't be vulnerable but it's better still to discard such
 * crazy SACK blocks. Doing this check for start_seq alone closes somewhat
 * similar case (end_seq after snd_nxt wrap) as earlier reversed check in
 * snd_nxt wrap -> snd_una region will then become "well defined", i.e.,
 * equal to the ideal case (infinite seqno space without wrap caused issues).
 *
 * With D-SACK the lower bound is extended to cover sequence space below
 * SND.UNA down to undo_marker, which is the last point of interest. Yet
 * again, D-SACK block must not to go across snd_una (for the same reason as
 * for the normal SACK blocks, explained above). But there all simplicity
 * ends, TCP might receive valid D-SACKs below that. As long as they reside
 * fully below undo_marker they do not affect behavior in anyway and can
 * therefore be safely ignored. In rare cases (which are more or less
 * theoretical ones), the D-SACK will nicely cross that boundary due to skb
 * fragmentation and packet reordering past skb's retransmission. To consider
 * them correctly, the acceptable range must be extended even more though
 * the exact amount is rather hard to quantify. However, tp->max_window can
 * be used as an exaggerated estimate.
 */
static bool tcp_is_sackblock_valid(struct tcp_sock *tp, bool is_dsack,
                   u32 start_seq, u32 end_seq)
{
    /* Too far in future, or reversed (interpretation is ambiguous) */
    if (after(end_seq, tp->snd_nxt) || !before(start_seq, end_seq))
        return false;

    /* Nasty start_seq wrap-around check (see comments above) */
    if (!before(start_seq, tp->snd_nxt))
        return false;

    /* In outstanding window? ...This is valid exit for D-SACKs too.
     * start_seq == snd_una is non-sensical (see comments above)
     */
    if (after(start_seq, tp->snd_una))
        return true;

    if (!is_dsack || !tp->undo_marker)
        return false;

    /* ...Then it's D-SACK, and must reside below snd_una completely */
    if (after(end_seq, tp->snd_una))
        return false;

    if (!before(start_seq, tp->undo_marker))
        return true;

    /* Too old */
    if (!after(end_seq, tp->undo_marker))
        return false;

    /* Undo_marker boundary crossing (overestimates a lot). Known already:
     *   start_seq < undo_marker and end_seq >= undo_marker.
     */
    return !before(start_seq, end_seq - tp->max_window);
}

/* Check for lost retransmit. This superb idea is borrowed from "ratehalving".
 * Event "B". Later note: FACK people cheated me again 8), we have to account
 * for reordering! Ugly, but should help.
 *
 * Search retransmitted skbs from write_queue that were sent when snd_nxt was
 * less than what is now known to be received by the other end (derived from
 * highest SACK block). Also calculate the lowest snd_nxt among the remaining
 * retransmitted skbs to avoid some costly processing per ACKs.
 */
static void tcp_mark_lost_retrans(struct sock *sk)
{
    const struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb;
    int cnt = 0;
    u32 new_low_seq = tp->snd_nxt;
    u32 received_upto = tcp_highest_sack_seq(tp);

    if (!tcp_is_fack(tp) || !tp->retrans_out ||
        !after(received_upto, tp->lost_retrans_low) ||
        icsk->icsk_ca_state != TCP_CA_Recovery)
        return;

    tcp_for_write_queue(skb, sk) {
        u32 ack_seq = TCP_SKB_CB(skb)->ack_seq;

        if (skb == tcp_send_head(sk))
            break;
        if (cnt == tp->retrans_out)
            break;
        if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
            continue;

        if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS))
            continue;

        /* TODO: We would like to get rid of tcp_is_fack(tp) only
         * constraint here (see above) but figuring out that at
         * least tp->reordering SACK blocks reside between ack_seq
         * and received_upto is not easy task to do cheaply with
         * the available datastructures.
         *
         * Whether FACK should check here for tp->reordering segs
         * in-between one could argue for either way (it would be
         * rather simple to implement as we could count fack_count
         * during the walk and do tp->fackets_out - fack_count).
         */
        if (after(received_upto, ack_seq)) {
            TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
            tp->retrans_out -= tcp_skb_pcount(skb);

            tcp_skb_mark_lost_uncond_verify(tp, skb);
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT);
        } else {
            if (before(ack_seq, new_low_seq))
                new_low_seq = ack_seq;
            cnt += tcp_skb_pcount(skb);
        }
    }

    if (tp->retrans_out)
        tp->lost_retrans_low = new_low_seq;
}

static bool tcp_check_dsack(struct sock *sk, const struct sk_buff *ack_skb,
                struct tcp_sack_block_wire *sp, int num_sacks,
                u32 prior_snd_una)
{
    struct tcp_sock *tp = tcp_sk(sk);
    u32 start_seq_0 = get_unaligned_be32(&sp[0].start_seq);
    u32 end_seq_0 = get_unaligned_be32(&sp[0].end_seq);
    bool dup_sack = false;

    if (before(start_seq_0, TCP_SKB_CB(ack_skb)->ack_seq)) {
        dup_sack = true;
        tcp_dsack_seen(tp);
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDSACKRECV);
    } else if (num_sacks > 1) {
        u32 end_seq_1 = get_unaligned_be32(&sp[1].end_seq);
        u32 start_seq_1 = get_unaligned_be32(&sp[1].start_seq);

        if (!after(end_seq_0, end_seq_1) &&
            !before(start_seq_0, start_seq_1)) {
            dup_sack = true;
            tcp_dsack_seen(tp);
            NET_INC_STATS_BH(sock_net(sk),
                    LINUX_MIB_TCPDSACKOFORECV);
        }
    }

    /* D-SACK for already forgotten data... Do dumb counting. */
    if (dup_sack && tp->undo_marker && tp->undo_retrans &&
        !after(end_seq_0, prior_snd_una) &&
        after(end_seq_0, tp->undo_marker))
        tp->undo_retrans--;

    return dup_sack;
}

struct tcp_sacktag_state {
    int reord;
    int fack_count;
    int flag;
};

/* Check if skb is fully within the SACK block. In presence of GSO skbs,
 * the incoming SACK may not exactly match but we can find smaller MSS
 * aligned portion of it that matches. Therefore we might need to fragment
 * which may fail and creates some hassle (caller must handle error case
 * returns).
 *
 * FIXME: this could be merged to shift decision code
 */
static int tcp_match_skb_to_sack(struct sock *sk, struct sk_buff *skb,
                  u32 start_seq, u32 end_seq)
{
    int err;
    bool in_sack;
    unsigned int pkt_len;
    unsigned int mss;

    in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq) &&
          !before(end_seq, TCP_SKB_CB(skb)->end_seq);

    if (tcp_skb_pcount(skb) > 1 && !in_sack &&
        after(TCP_SKB_CB(skb)->end_seq, start_seq)) {
        mss = tcp_skb_mss(skb);
        in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq);

        if (!in_sack) {
            pkt_len = start_seq - TCP_SKB_CB(skb)->seq;
            if (pkt_len < mss)
                pkt_len = mss;
        } else {
            pkt_len = end_seq - TCP_SKB_CB(skb)->seq;
            if (pkt_len < mss)
                return -EINVAL;
        }

        /* Round if necessary so that SACKs cover only full MSSes
         * and/or the remaining small portion (if present)
         */
        if (pkt_len > mss) {
            unsigned int new_len = (pkt_len / mss) * mss;
            if (!in_sack && new_len < pkt_len) {
                new_len += mss;
                if (new_len > skb->len)
                    return 0;
            }
            pkt_len = new_len;
        }
        err = tcp_fragment(sk, skb, pkt_len, mss);
        if (err < 0)
            return err;
    }

    return in_sack;
}

/* Mark the given newly-SACKed range as such, adjusting counters and hints. */
static u8 tcp_sacktag_one(struct sock *sk,
              struct tcp_sacktag_state *state, u8 sacked,
              u32 start_seq, u32 end_seq,
              bool dup_sack, int pcount)
{
    struct tcp_sock *tp = tcp_sk(sk);
    int fack_count = state->fack_count;

    /* Account D-SACK for retransmitted packet. */
    if (dup_sack && (sacked & TCPCB_RETRANS)) {
        if (tp->undo_marker && tp->undo_retrans &&
            after(end_seq, tp->undo_marker))
            tp->undo_retrans--;
        if (sacked & TCPCB_SACKED_ACKED)
            state->reord = min(fack_count, state->reord);
    }

    /* Nothing to do; acked frame is about to be dropped (was ACKed). */
    if (!after(end_seq, tp->snd_una))
        return sacked;

    if (!(sacked & TCPCB_SACKED_ACKED)) {
        if (sacked & TCPCB_SACKED_RETRANS) {
            /* If the segment is not tagged as lost,
             * we do not clear RETRANS, believing
             * that retransmission is still in flight.
             */
            if (sacked & TCPCB_LOST) {
                sacked &= ~(TCPCB_LOST|TCPCB_SACKED_RETRANS);
                tp->lost_out -= pcount;
                tp->retrans_out -= pcount;
            }
        } else {
            if (!(sacked & TCPCB_RETRANS)) {
                /* New sack for not retransmitted frame,
                 * which was in hole. It is reordering.
                 */
                if (before(start_seq,
                       tcp_highest_sack_seq(tp)))
                    state->reord = min(fack_count,
                               state->reord);

                /* SACK enhanced F-RTO (RFC4138; Appendix B) */
                if (!after(end_seq, tp->frto_highmark))
                    state->flag |= FLAG_ONLY_ORIG_SACKED;
            }

            if (sacked & TCPCB_LOST) {
                sacked &= ~TCPCB_LOST;
                tp->lost_out -= pcount;
            }
        }

        sacked |= TCPCB_SACKED_ACKED;
        state->flag |= FLAG_DATA_SACKED;
        tp->sacked_out += pcount;

        fack_count += pcount;

        /* Lost marker hint past SACKed? Tweak RFC3517 cnt */
        if (!tcp_is_fack(tp) && (tp->lost_skb_hint != NULL) &&
            before(start_seq, TCP_SKB_CB(tp->lost_skb_hint)->seq))
            tp->lost_cnt_hint += pcount;

        if (fack_count > tp->fackets_out)
            tp->fackets_out = fack_count;
    }

    /* D-SACK. We can detect redundant retransmission in S|R and plain R
     * frames and clear it. undo_retrans is decreased above, L|R frames
     * are accounted above as well.
     */
    if (dup_sack && (sacked & TCPCB_SACKED_RETRANS)) {
        sacked &= ~TCPCB_SACKED_RETRANS;
        tp->retrans_out -= pcount;
    }

    return sacked;
}

/* Shift newly-SACKed bytes from this skb to the immediately previous
 * already-SACKed sk_buff. Mark the newly-SACKed bytes as such.
 */
static bool tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
                struct tcp_sacktag_state *state,
                unsigned int pcount, int shifted, int mss,
                bool dup_sack)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *prev = tcp_write_queue_prev(sk, skb);
    u32 start_seq = TCP_SKB_CB(skb)->seq;   /* start of newly-SACKed */
    u32 end_seq = start_seq + shifted;  /* end of newly-SACKed */

    BUG_ON(!pcount);

    /* Adjust counters and hints for the newly sacked sequence
     * range but discard the return value since prev is already
     * marked. We must tag the range first because the seq
     * advancement below implicitly advances
     * tcp_highest_sack_seq() when skb is highest_sack.
     */
    tcp_sacktag_one(sk, state, TCP_SKB_CB(skb)->sacked,
            start_seq, end_seq, dup_sack, pcount);

    if (skb == tp->lost_skb_hint)
        tp->lost_cnt_hint += pcount;

    TCP_SKB_CB(prev)->end_seq += shifted;
    TCP_SKB_CB(skb)->seq += shifted;

    skb_shinfo(prev)->gso_segs += pcount;
    BUG_ON(skb_shinfo(skb)->gso_segs < pcount);
    skb_shinfo(skb)->gso_segs -= pcount;

    /* When we're adding to gso_segs == 1, gso_size will be zero,
     * in theory this shouldn't be necessary but as long as DSACK
     * code can come after this skb later on it's better to keep
     * setting gso_size to something.
     */
    if (!skb_shinfo(prev)->gso_size) {
        skb_shinfo(prev)->gso_size = mss;
        skb_shinfo(prev)->gso_type = sk->sk_gso_type;
    }

    /* CHECKME: To clear or not to clear? Mimics normal skb currently */
    if (skb_shinfo(skb)->gso_segs <= 1) {
        skb_shinfo(skb)->gso_size = 0;
        skb_shinfo(skb)->gso_type = 0;
    }

    /* Difference in this won't matter, both ACKed by the same cumul. ACK */
    TCP_SKB_CB(prev)->sacked |= (TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS);

    if (skb->len > 0) {
        BUG_ON(!tcp_skb_pcount(skb));
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SACKSHIFTED);
        return false;
    }

    /* Whole SKB was eaten :-) */

    if (skb == tp->retransmit_skb_hint)
        tp->retransmit_skb_hint = prev;
    if (skb == tp->scoreboard_skb_hint)
        tp->scoreboard_skb_hint = prev;
    if (skb == tp->lost_skb_hint) {
        tp->lost_skb_hint = prev;
        tp->lost_cnt_hint -= tcp_skb_pcount(prev);
    }

    TCP_SKB_CB(skb)->tcp_flags |= TCP_SKB_CB(prev)->tcp_flags;
    if (skb == tcp_highest_sack(sk))
        tcp_advance_highest_sack(sk, skb);

    tcp_unlink_write_queue(skb, sk);
    sk_wmem_free_skb(sk, skb);

    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SACKMERGED);

    return true;
}

/* I wish gso_size would have a bit more sane initialization than
 * something-or-zero which complicates things
 */
static int tcp_skb_seglen(const struct sk_buff *skb)
{
    return tcp_skb_pcount(skb) == 1 ? skb->len : tcp_skb_mss(skb);
}

/* Shifting pages past head area doesn't work */
static int skb_can_shift(const struct sk_buff *skb)
{
    return !skb_headlen(skb) && skb_is_nonlinear(skb);
}

/* Try collapsing SACK blocks spanning across multiple skbs to a single
 * skb.
 */
static struct sk_buff *tcp_shift_skb_data(struct sock *sk, struct sk_buff *skb,
                      struct tcp_sacktag_state *state,
                      u32 start_seq, u32 end_seq,
                      bool dup_sack)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *prev;
    int mss;
    int pcount = 0;
    int len;
    int in_sack;

    if (!sk_can_gso(sk))
        goto fallback;

    /* Normally R but no L won't result in plain S */
    if (!dup_sack &&
        (TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_RETRANS)) == TCPCB_SACKED_RETRANS)
        goto fallback;
    if (!skb_can_shift(skb))
        goto fallback;
    /* This frame is about to be dropped (was ACKed). */
    if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
        goto fallback;

    /* Can only happen with delayed DSACK + discard craziness */
    if (unlikely(skb == tcp_write_queue_head(sk)))
        goto fallback;
    prev = tcp_write_queue_prev(sk, skb);

    if ((TCP_SKB_CB(prev)->sacked & TCPCB_TAGBITS) != TCPCB_SACKED_ACKED)
        goto fallback;

    in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq) &&
          !before(end_seq, TCP_SKB_CB(skb)->end_seq);

    if (in_sack) {
        len = skb->len;
        pcount = tcp_skb_pcount(skb);
        mss = tcp_skb_seglen(skb);

        /* TODO: Fix DSACKs to not fragment already SACKed and we can
         * drop this restriction as unnecessary
         */
        if (mss != tcp_skb_seglen(prev))
            goto fallback;
    } else {
        if (!after(TCP_SKB_CB(skb)->end_seq, start_seq))
            goto noop;
        /* CHECKME: This is non-MSS split case only?, this will
         * cause skipped skbs due to advancing loop btw, original
         * has that feature too
         */
        if (tcp_skb_pcount(skb) <= 1)
            goto noop;

        in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq);
        if (!in_sack) {
            /* TODO: head merge to next could be attempted here
             * if (!after(TCP_SKB_CB(skb)->end_seq, end_seq)),
             * though it might not be worth of the additional hassle
             *
             * ...we can probably just fallback to what was done
             * previously. We could try merging non-SACKed ones
             * as well but it probably isn't going to buy off
             * because later SACKs might again split them, and
             * it would make skb timestamp tracking considerably
             * harder problem.
             */
            goto fallback;
        }

        len = end_seq - TCP_SKB_CB(skb)->seq;
        BUG_ON(len < 0);
        BUG_ON(len > skb->len);

        /* MSS boundaries should be honoured or else pcount will
         * severely break even though it makes things bit trickier.
         * Optimize common case to avoid most of the divides
         */
        mss = tcp_skb_mss(skb);

        /* TODO: Fix DSACKs to not fragment already SACKed and we can
         * drop this restriction as unnecessary
         */
        if (mss != tcp_skb_seglen(prev))
            goto fallback;

        if (len == mss) {
            pcount = 1;
        } else if (len < mss) {
            goto noop;
        } else {
            pcount = len / mss;
            len = pcount * mss;
        }
    }

    /* tcp_sacktag_one() won't SACK-tag ranges below snd_una */
    if (!after(TCP_SKB_CB(skb)->seq + len, tp->snd_una))
        goto fallback;

    if (!skb_shift(prev, skb, len))
        goto fallback;
    if (!tcp_shifted_skb(sk, skb, state, pcount, len, mss, dup_sack))
        goto out;

    /* Hole filled allows collapsing with the next as well, this is very
     * useful when hole on every nth skb pattern happens
     */
    if (prev == tcp_write_queue_tail(sk))
        goto out;
    skb = tcp_write_queue_next(sk, prev);

    if (!skb_can_shift(skb) ||
        (skb == tcp_send_head(sk)) ||
        ((TCP_SKB_CB(skb)->sacked & TCPCB_TAGBITS) != TCPCB_SACKED_ACKED) ||
        (mss != tcp_skb_seglen(skb)))
        goto out;

    len = skb->len;
    if (skb_shift(prev, skb, len)) {
        pcount += tcp_skb_pcount(skb);
        tcp_shifted_skb(sk, skb, state, tcp_skb_pcount(skb), len, mss, 0);
    }

out:
    state->fack_count += pcount;
    return prev;

noop:
    return skb;

fallback:
    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SACKSHIFTFALLBACK);
    return NULL;
}

static struct sk_buff *tcp_sacktag_walk(struct sk_buff *skb, struct sock *sk,
                    struct tcp_sack_block *next_dup,
                    struct tcp_sacktag_state *state,
                    u32 start_seq, u32 end_seq,
                    bool dup_sack_in)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *tmp;

    tcp_for_write_queue_from(skb, sk) {
        int in_sack = 0;
        bool dup_sack = dup_sack_in;

        if (skb == tcp_send_head(sk))
            break;

        /* queue is in-order => we can short-circuit the walk early */
        if (!before(TCP_SKB_CB(skb)->seq, end_seq))
            break;

        if ((next_dup != NULL) &&
            before(TCP_SKB_CB(skb)->seq, next_dup->end_seq)) {
            in_sack = tcp_match_skb_to_sack(sk, skb,
                            next_dup->start_seq,
                            next_dup->end_seq);
            if (in_sack > 0)
                dup_sack = true;
        }

        /* skb reference here is a bit tricky to get right, since
         * shifting can eat and free both this skb and the next,
         * so not even _safe variant of the loop is enough.
         */
        if (in_sack <= 0) {
            tmp = tcp_shift_skb_data(sk, skb, state,
                         start_seq, end_seq, dup_sack);
            if (tmp != NULL) {
                if (tmp != skb) {
                    skb = tmp;
                    continue;
                }

                in_sack = 0;
            } else {
                in_sack = tcp_match_skb_to_sack(sk, skb,
                                start_seq,
                                end_seq);
            }
        }

        if (unlikely(in_sack < 0))
            break;

        if (in_sack) {
            TCP_SKB_CB(skb)->sacked =
                tcp_sacktag_one(sk,
                        state,
                        TCP_SKB_CB(skb)->sacked,
                        TCP_SKB_CB(skb)->seq,
                        TCP_SKB_CB(skb)->end_seq,
                        dup_sack,
                        tcp_skb_pcount(skb));

            if (!before(TCP_SKB_CB(skb)->seq,
                    tcp_highest_sack_seq(tp)))
                tcp_advance_highest_sack(sk, skb);
        }

        state->fack_count += tcp_skb_pcount(skb);
    }
    return skb;
}

/* Avoid all extra work that is being done by sacktag while walking in
 * a normal way
 */
static struct sk_buff *tcp_sacktag_skip(struct sk_buff *skb, struct sock *sk,
                    struct tcp_sacktag_state *state,
                    u32 skip_to_seq)
{
    tcp_for_write_queue_from(skb, sk) {
        if (skb == tcp_send_head(sk))
            break;

        if (after(TCP_SKB_CB(skb)->end_seq, skip_to_seq))
            break;

        state->fack_count += tcp_skb_pcount(skb);
    }
    return skb;
}

static struct sk_buff *tcp_maybe_skipping_dsack(struct sk_buff *skb,
                        struct sock *sk,
                        struct tcp_sack_block *next_dup,
                        struct tcp_sacktag_state *state,
                        u32 skip_to_seq)
{
    if (next_dup == NULL)
        return skb;

    if (before(next_dup->start_seq, skip_to_seq)) {
        skb = tcp_sacktag_skip(skb, sk, state, next_dup->start_seq);
        skb = tcp_sacktag_walk(skb, sk, NULL, state,
                       next_dup->start_seq, next_dup->end_seq,
                       1);
    }

    return skb;
}

static int tcp_sack_cache_ok(const struct tcp_sock *tp, const struct tcp_sack_block *cache)
{
    return cache < tp->recv_sack_cache + ARRAY_SIZE(tp->recv_sack_cache);
}

static int
tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
            u32 prior_snd_una)
{
    const struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    const unsigned char *ptr = (skb_transport_header(ack_skb) +
                    TCP_SKB_CB(ack_skb)->sacked);
    struct tcp_sack_block_wire *sp_wire = (struct tcp_sack_block_wire *)(ptr+2);
    struct tcp_sack_block sp[TCP_NUM_SACKS];
    struct tcp_sack_block *cache;
    struct tcp_sacktag_state state;
    struct sk_buff *skb;
    int num_sacks = min(TCP_NUM_SACKS, (ptr[1] - TCPOLEN_SACK_BASE) >> 3);
    int used_sacks;
    bool found_dup_sack = false;
    int i, j;
    int first_sack_index;

    state.flag = 0;
    state.reord = tp->packets_out;

    if (!tp->sacked_out) {
        if (WARN_ON(tp->fackets_out))
            tp->fackets_out = 0;
        tcp_highest_sack_reset(sk);
    }

    found_dup_sack = tcp_check_dsack(sk, ack_skb, sp_wire,
                     num_sacks, prior_snd_una);
    if (found_dup_sack)
        state.flag |= FLAG_DSACKING_ACK;

    /* Eliminate too old ACKs, but take into
     * account more or less fresh ones, they can
     * contain valid SACK info.
     */
    if (before(TCP_SKB_CB(ack_skb)->ack_seq, prior_snd_una - tp->max_window))
        return 0;

    if (!tp->packets_out)
        goto out;

    used_sacks = 0;
    first_sack_index = 0;
    for (i = 0; i < num_sacks; i++) {
        bool dup_sack = !i && found_dup_sack;

        sp[used_sacks].start_seq = get_unaligned_be32(&sp_wire[i].start_seq);
        sp[used_sacks].end_seq = get_unaligned_be32(&sp_wire[i].end_seq);

        if (!tcp_is_sackblock_valid(tp, dup_sack,
                        sp[used_sacks].start_seq,
                        sp[used_sacks].end_seq)) {
            int mib_idx;

            if (dup_sack) {
                if (!tp->undo_marker)
                    mib_idx = LINUX_MIB_TCPDSACKIGNOREDNOUNDO;
                else
                    mib_idx = LINUX_MIB_TCPDSACKIGNOREDOLD;
            } else {
                /* Don't count olds caused by ACK reordering */
                if ((TCP_SKB_CB(ack_skb)->ack_seq != tp->snd_una) &&
                    !after(sp[used_sacks].end_seq, tp->snd_una))
                    continue;
                mib_idx = LINUX_MIB_TCPSACKDISCARD;
            }

            NET_INC_STATS_BH(sock_net(sk), mib_idx);
            if (i == 0)
                first_sack_index = -1;
            continue;
        }

        /* Ignore very old stuff early */
        if (!after(sp[used_sacks].end_seq, prior_snd_una))
            continue;

        used_sacks++;
    }

    /* order SACK blocks to allow in order walk of the retrans queue */
    for (i = used_sacks - 1; i > 0; i--) {
        for (j = 0; j < i; j++) {
            if (after(sp[j].start_seq, sp[j + 1].start_seq)) {
                swap(sp[j], sp[j + 1]);

                /* Track where the first SACK block goes to */
                if (j == first_sack_index)
                    first_sack_index = j + 1;
            }
        }
    }

    skb = tcp_write_queue_head(sk);
    state.fack_count = 0;
    i = 0;

    if (!tp->sacked_out) {
        /* It's already past, so skip checking against it */
        cache = tp->recv_sack_cache + ARRAY_SIZE(tp->recv_sack_cache);
    } else {
        cache = tp->recv_sack_cache;
        /* Skip empty blocks in at head of the cache */
        while (tcp_sack_cache_ok(tp, cache) && !cache->start_seq &&
               !cache->end_seq)
            cache++;
    }

    while (i < used_sacks) {
        u32 start_seq = sp[i].start_seq;
        u32 end_seq = sp[i].end_seq;
        bool dup_sack = (found_dup_sack && (i == first_sack_index));
        struct tcp_sack_block *next_dup = NULL;

        if (found_dup_sack && ((i + 1) == first_sack_index))
            next_dup = &sp[i + 1];

        /* Skip too early cached blocks */
        while (tcp_sack_cache_ok(tp, cache) &&
               !before(start_seq, cache->end_seq))
            cache++;

        /* Can skip some work by looking recv_sack_cache? */
        if (tcp_sack_cache_ok(tp, cache) && !dup_sack &&
            after(end_seq, cache->start_seq)) {

            /* Head todo? */
            if (before(start_seq, cache->start_seq)) {
                skb = tcp_sacktag_skip(skb, sk, &state,
                               start_seq);
                skb = tcp_sacktag_walk(skb, sk, next_dup,
                               &state,
                               start_seq,
                               cache->start_seq,
                               dup_sack);
            }

            /* Rest of the block already fully processed? */
            if (!after(end_seq, cache->end_seq))
                goto advance_sp;

            skb = tcp_maybe_skipping_dsack(skb, sk, next_dup,
                               &state,
                               cache->end_seq);

            /* ...tail remains todo... */
            if (tcp_highest_sack_seq(tp) == cache->end_seq) {
                /* ...but better entrypoint exists! */
                skb = tcp_highest_sack(sk);
                if (skb == NULL)
                    break;
                state.fack_count = tp->fackets_out;
                cache++;
                goto walk;
            }

            skb = tcp_sacktag_skip(skb, sk, &state, cache->end_seq);
            /* Check overlap against next cached too (past this one already) */
            cache++;
            continue;
        }

        if (!before(start_seq, tcp_highest_sack_seq(tp))) {
            skb = tcp_highest_sack(sk);
            if (skb == NULL)
                break;
            state.fack_count = tp->fackets_out;
        }
        skb = tcp_sacktag_skip(skb, sk, &state, start_seq);

walk:
        skb = tcp_sacktag_walk(skb, sk, next_dup, &state,
                       start_seq, end_seq, dup_sack);

advance_sp:
        /* SACK enhanced FRTO (RFC4138, Appendix B): Clearing correct
         * due to in-order walk
         */
        if (after(end_seq, tp->frto_highmark))
            state.flag &= ~FLAG_ONLY_ORIG_SACKED;

        i++;
    }

    /* Clear the head of the cache sack blocks so we can skip it next time */
    for (i = 0; i < ARRAY_SIZE(tp->recv_sack_cache) - used_sacks; i++) {
        tp->recv_sack_cache[i].start_seq = 0;
        tp->recv_sack_cache[i].end_seq = 0;
    }
    for (j = 0; j < used_sacks; j++)
        tp->recv_sack_cache[i++] = sp[j];

    tcp_mark_lost_retrans(sk);

    tcp_verify_left_out(tp);

    if ((state.reord < tp->fackets_out) &&
        ((icsk->icsk_ca_state != TCP_CA_Loss) || tp->undo_marker) &&
        (!tp->frto_highmark || after(tp->snd_una, tp->frto_highmark)))
        tcp_update_reordering(sk, tp->fackets_out - state.reord, 0);

out:

#if FASTRETRANS_DEBUG > 0
    WARN_ON((int)tp->sacked_out < 0);
    WARN_ON((int)tp->lost_out < 0);
    WARN_ON((int)tp->retrans_out < 0);
    WARN_ON((int)tcp_packets_in_flight(tp) < 0);
#endif
    return state.flag;
}

/* Limits sacked_out so that sum with lost_out isn't ever larger than
 * packets_out. Returns false if sacked_out adjustement wasn't necessary.
 */
static bool tcp_limit_reno_sacked(struct tcp_sock *tp)
{
    u32 holes;

    holes = max(tp->lost_out, 1U);
    holes = min(holes, tp->packets_out);

    if ((tp->sacked_out + holes) > tp->packets_out) {
        tp->sacked_out = tp->packets_out - holes;
        return true;
    }
    return false;
}

/* If we receive more dupacks than we expected counting segments
 * in assumption of absent reordering, interpret this as reordering.
 * The only another reason could be bug in receiver TCP.
 */
static void tcp_check_reno_reordering(struct sock *sk, const int addend)
{
    struct tcp_sock *tp = tcp_sk(sk);
    if (tcp_limit_reno_sacked(tp))
        tcp_update_reordering(sk, tp->packets_out + addend, 0);
}

/* Emulate SACKs for SACKless connection: account for a new dupack. */

static void tcp_add_reno_sack(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    tp->sacked_out++;
    tcp_check_reno_reordering(sk, 0);
    tcp_verify_left_out(tp);
}

/* Account for ACK, ACKing some data in Reno Recovery phase. */

static void tcp_remove_reno_sacks(struct sock *sk, int acked)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (acked > 0) {
        /* One ACK acked hole. The rest eat duplicate ACKs. */
        if (acked - 1 >= tp->sacked_out)
            tp->sacked_out = 0;
        else
            tp->sacked_out -= acked - 1;
    }
    tcp_check_reno_reordering(sk, acked);
    tcp_verify_left_out(tp);
}

static inline void tcp_reset_reno_sack(struct tcp_sock *tp)
{
    tp->sacked_out = 0;
}

static int tcp_is_sackfrto(const struct tcp_sock *tp)
{
    return (sysctl_tcp_frto == 0x2) && !tcp_is_reno(tp);
}

/* F-RTO can only be used if TCP has never retransmitted anything other than
 * head (SACK enhanced variant from Appendix B of RFC4138 is more robust here)
 */
bool tcp_use_frto(struct sock *sk)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    const struct inet_connection_sock *icsk = inet_csk(sk);
    struct sk_buff *skb;

    if (!sysctl_tcp_frto)
        return false;

    /* MTU probe and F-RTO won't really play nicely along currently */
    if (icsk->icsk_mtup.probe_size)
        return false;

    if (tcp_is_sackfrto(tp))
        return true;

    /* Avoid expensive walking of rexmit queue if possible */
    if (tp->retrans_out > 1)
        return false;

    skb = tcp_write_queue_head(sk);
    if (tcp_skb_is_last(sk, skb))
        return true;
    skb = tcp_write_queue_next(sk, skb);    /* Skips head */
    tcp_for_write_queue_from(skb, sk) {
        if (skb == tcp_send_head(sk))
            break;
        if (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS)
            return false;
        /* Short-circuit when first non-SACKed skb has been checked */
        if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
            break;
    }
    return true;
}

/* RTO occurred, but do not yet enter Loss state. Instead, defer RTO
 * recovery a bit and use heuristics in tcp_process_frto() to detect if
 * the RTO was spurious. Only clear SACKED_RETRANS of the head here to
 * keep retrans_out counting accurate (with SACK F-RTO, other than head
 * may still have that bit set); TCPCB_LOST and remaining SACKED_RETRANS
 * bits are handled if the Loss state is really to be entered (in
 * tcp_enter_frto_loss).
 *
 * Do like tcp_enter_loss() would; when RTO expires the second time it
 * does:
 *  "Reduce ssthresh if it has not yet been made inside this window."
 */
void tcp_enter_frto(struct sock *sk)
{
    const struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb;

    if ((!tp->frto_counter && icsk->icsk_ca_state <= TCP_CA_Disorder) ||
        tp->snd_una == tp->high_seq ||
        ((icsk->icsk_ca_state == TCP_CA_Loss || tp->frto_counter) &&
         !icsk->icsk_retransmits)) {
        tp->prior_ssthresh = tcp_current_ssthresh(sk);
        /* Our state is too optimistic in ssthresh() call because cwnd
         * is not reduced until tcp_enter_frto_loss() when previous F-RTO
         * recovery has not yet completed. Pattern would be this: RTO,
         * Cumulative ACK, RTO (2xRTO for the same segment does not end
         * up here twice).
         * RFC4138 should be more specific on what to do, even though
         * RTO is quite unlikely to occur after the first Cumulative ACK
         * due to back-off and complexity of triggering events ...
         */
        if (tp->frto_counter) {
            u32 stored_cwnd;
            stored_cwnd = tp->snd_cwnd;
            tp->snd_cwnd = 2;
            tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
            tp->snd_cwnd = stored_cwnd;
        } else {
            tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
        }
        /* ... in theory, cong.control module could do "any tricks" in
         * ssthresh(), which means that ca_state, lost bits and lost_out
         * counter would have to be faked before the call occurs. We
         * consider that too expensive, unlikely and hacky, so modules
         * using these in ssthresh() must deal these incompatibility
         * issues if they receives CA_EVENT_FRTO and frto_counter != 0
         */
        tcp_ca_event(sk, CA_EVENT_FRTO);
    }

    tp->undo_marker = tp->snd_una;
    tp->undo_retrans = 0;

    skb = tcp_write_queue_head(sk);
    if (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS)
        tp->undo_marker = 0;
    if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
        TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
        tp->retrans_out -= tcp_skb_pcount(skb);
    }
    tcp_verify_left_out(tp);

    /* Too bad if TCP was application limited */
    tp->snd_cwnd = min(tp->snd_cwnd, tcp_packets_in_flight(tp) + 1);

    /* Earlier loss recovery underway (see RFC4138; Appendix B).
     * The last condition is necessary at least in tp->frto_counter case.
     */
    if (tcp_is_sackfrto(tp) && (tp->frto_counter ||
        ((1 << icsk->icsk_ca_state) & (TCPF_CA_Recovery|TCPF_CA_Loss))) &&
        after(tp->high_seq, tp->snd_una)) {
        tp->frto_highmark = tp->high_seq;
    } else {
        tp->frto_highmark = tp->snd_nxt;
    }
    tcp_set_ca_state(sk, TCP_CA_Disorder);
    tp->high_seq = tp->snd_nxt;
    tp->frto_counter = 1;
}

/* Enter Loss state after F-RTO was applied. Dupack arrived after RTO,
 * which indicates that we should follow the traditional RTO recovery,
 * i.e. mark everything lost and do go-back-N retransmission.
 */
static void tcp_enter_frto_loss(struct sock *sk, int allowed_segments, int flag)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb;

    tp->lost_out = 0;
    tp->retrans_out = 0;
    if (tcp_is_reno(tp))
        tcp_reset_reno_sack(tp);

    tcp_for_write_queue(skb, sk) {
        if (skb == tcp_send_head(sk))
            break;

        TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
        /*
         * Count the retransmission made on RTO correctly (only when
         * waiting for the first ACK and did not get it)...
         */
        if ((tp->frto_counter == 1) && !(flag & FLAG_DATA_ACKED)) {
            /* For some reason this R-bit might get cleared? */
            if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)
                tp->retrans_out += tcp_skb_pcount(skb);
            /* ...enter this if branch just for the first segment */
            flag |= FLAG_DATA_ACKED;
        } else {
            if (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS)
                tp->undo_marker = 0;
            TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
        }

        /* Marking forward transmissions that were made after RTO lost
         * can cause unnecessary retransmissions in some scenarios,
         * SACK blocks will mitigate that in some but not in all cases.
         * We used to not mark them but it was causing break-ups with
         * receivers that do only in-order receival.
         *
         * TODO: we could detect presence of such receiver and select
         * different behavior per flow.
         */
        if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)) {
            TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
            tp->lost_out += tcp_skb_pcount(skb);
            tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
        }
    }
    tcp_verify_left_out(tp);

    tp->snd_cwnd = tcp_packets_in_flight(tp) + allowed_segments;
    tp->snd_cwnd_cnt = 0;
    tp->snd_cwnd_stamp = tcp_time_stamp;
    tp->frto_counter = 0;
    tp->bytes_acked = 0;

    tp->reordering = min_t(unsigned int, tp->reordering,
                   sysctl_tcp_reordering);
    tcp_set_ca_state(sk, TCP_CA_Loss);
    tp->high_seq = tp->snd_nxt;
    TCP_ECN_queue_cwr(tp);

    tcp_clear_all_retrans_hints(tp);
}

static void tcp_clear_retrans_partial(struct tcp_sock *tp)
{
    tp->retrans_out = 0;
    tp->lost_out = 0;

    tp->undo_marker = 0;
    tp->undo_retrans = 0;
}

void tcp_clear_retrans(struct tcp_sock *tp)
{
    tcp_clear_retrans_partial(tp);

    tp->fackets_out = 0;
    tp->sacked_out = 0;
}

/* Enter Loss state. If "how" is not zero, forget all SACK information
 * and reset tags completely, otherwise preserve SACKs. If receiver
 * dropped its ofo queue, we will know this due to reneging detection.
 */
void tcp_enter_loss(struct sock *sk, int how)
{
    const struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb;

    /* Reduce ssthresh if it has not yet been made inside this window. */
    if (icsk->icsk_ca_state <= TCP_CA_Disorder || tp->snd_una == tp->high_seq ||
        (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)) {
        tp->prior_ssthresh = tcp_current_ssthresh(sk);
        tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
        tcp_ca_event(sk, CA_EVENT_LOSS);
    }
    tp->snd_cwnd       = 1;
    tp->snd_cwnd_cnt   = 0;
    tp->snd_cwnd_stamp = tcp_time_stamp;

    tp->bytes_acked = 0;
    tcp_clear_retrans_partial(tp);

    if (tcp_is_reno(tp))
        tcp_reset_reno_sack(tp);

    if (!how) {
        /* Push undo marker, if it was plain RTO and nothing
         * was retransmitted. */
        tp->undo_marker = tp->snd_una;
    } else {
        tp->sacked_out = 0;
        tp->fackets_out = 0;
    }
    tcp_clear_all_retrans_hints(tp);

    tcp_for_write_queue(skb, sk) {
        if (skb == tcp_send_head(sk))
            break;

        if (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS)
            tp->undo_marker = 0;
        TCP_SKB_CB(skb)->sacked &= (~TCPCB_TAGBITS)|TCPCB_SACKED_ACKED;
        if (!(TCP_SKB_CB(skb)->sacked&TCPCB_SACKED_ACKED) || how) {
            TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_ACKED;
            TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
            tp->lost_out += tcp_skb_pcount(skb);
            tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
        }
    }
    tcp_verify_left_out(tp);

    tp->reordering = min_t(unsigned int, tp->reordering,
                   sysctl_tcp_reordering);
    tcp_set_ca_state(sk, TCP_CA_Loss);
    tp->high_seq = tp->snd_nxt;
    TCP_ECN_queue_cwr(tp);
    /* Abort F-RTO algorithm if one is in progress */
    tp->frto_counter = 0;
}

/* If ACK arrived pointing to a remembered SACK, it means that our
 * remembered SACKs do not reflect real state of receiver i.e.
 * receiver _host_ is heavily congested (or buggy).
 *
 * Do processing similar to RTO timeout.
 */
static bool tcp_check_sack_reneging(struct sock *sk, int flag)
{
    if (flag & FLAG_SACK_RENEGING) {
        struct inet_connection_sock *icsk = inet_csk(sk);
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSACKRENEGING);

        tcp_enter_loss(sk, 1);
        icsk->icsk_retransmits++;
        tcp_retransmit_skb(sk, tcp_write_queue_head(sk));
        inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                      icsk->icsk_rto, TCP_RTO_MAX);
        return true;
    }
    return false;
}

static inline int tcp_fackets_out(const struct tcp_sock *tp)
{
    return tcp_is_reno(tp) ? tp->sacked_out + 1 : tp->fackets_out;
}

/* Heurestics to calculate number of duplicate ACKs. There's no dupACKs
 * counter when SACK is enabled (without SACK, sacked_out is used for
 * that purpose).
 *
 * Instead, with FACK TCP uses fackets_out that includes both SACKed
 * segments up to the highest received SACK block so far and holes in
 * between them.
 *
 * With reordering, holes may still be in flight, so RFC3517 recovery
 * uses pure sacked_out (total number of SACKed segments) even though
 * it violates the RFC that uses duplicate ACKs, often these are equal
 * but when e.g. out-of-window ACKs or packet duplication occurs,
 * they differ. Since neither occurs due to loss, TCP should really
 * ignore them.
 */
static inline int tcp_dupack_heuristics(const struct tcp_sock *tp)
{
    return tcp_is_fack(tp) ? tp->fackets_out : tp->sacked_out + 1;
}

static bool tcp_pause_early_retransmit(struct sock *sk, int flag)
{
    struct tcp_sock *tp = tcp_sk(sk);
    unsigned long delay;

    /* Delay early retransmit and entering fast recovery for
     * max(RTT/4, 2msec) unless ack has ECE mark, no RTT samples
     * available, or RTO is scheduled to fire first.
     */
    if (sysctl_tcp_early_retrans < 2 || (flag & FLAG_ECE) || !tp->srtt)
        return false;

    delay = max_t(unsigned long, (tp->srtt >> 5), msecs_to_jiffies(2));
    if (!time_after(inet_csk(sk)->icsk_timeout, (jiffies + delay)))
        return false;

    inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, delay, TCP_RTO_MAX);
    tp->early_retrans_delayed = 1;
    return true;
}

static inline int tcp_skb_timedout(const struct sock *sk,
                   const struct sk_buff *skb)
{
    return tcp_time_stamp - TCP_SKB_CB(skb)->when > inet_csk(sk)->icsk_rto;
}

static inline int tcp_head_timedout(const struct sock *sk)
{
    const struct tcp_sock *tp = tcp_sk(sk);

    return tp->packets_out &&
           tcp_skb_timedout(sk, tcp_write_queue_head(sk));
}

/* Linux NewReno/SACK/FACK/ECN state machine.
 * --------------------------------------
 *
 * "Open"   Normal state, no dubious events, fast path.
 * "Disorder"   In all the respects it is "Open",
 *      but requires a bit more attention. It is entered when
 *      we see some SACKs or dupacks. It is split of "Open"
 *      mainly to move some processing from fast path to slow one.
 * "CWR"    CWND was reduced due to some Congestion Notification event.
 *      It can be ECN, ICMP source quench, local device congestion.
 * "Recovery"   CWND was reduced, we are fast-retransmitting.
 * "Loss"   CWND was reduced due to RTO timeout or SACK reneging.
 *
 * tcp_fastretrans_alert() is entered:
 * - each incoming ACK, if state is not "Open"
 * - when arrived ACK is unusual, namely:
 *  * SACK
 *  * Duplicate ACK.
 *  * ECN ECE.
 *
 * Counting packets in flight is pretty simple.
 *
 *  in_flight = packets_out - left_out + retrans_out
 *
 *  packets_out is SND.NXT-SND.UNA counted in packets.
 *
 *  retrans_out is number of retransmitted segments.
 *
 *  left_out is number of segments left network, but not ACKed yet.
 *
 *      left_out = sacked_out + lost_out
 *
 *     sacked_out: Packets, which arrived to receiver out of order
 *         and hence not ACKed. With SACKs this number is simply
 *         amount of SACKed data. Even without SACKs
 *         it is easy to give pretty reliable estimate of this number,
 *         counting duplicate ACKs.
 *
 *       lost_out: Packets lost by network. TCP has no explicit
 *         "loss notification" feedback from network (for now).
 *         It means that this number can be only _guessed_.
 *         Actually, it is the heuristics to predict lossage that
 *         distinguishes different algorithms.
 *
 *  F.e. after RTO, when all the queue is considered as lost,
 *  lost_out = packets_out and in_flight = retrans_out.
 *
 *      Essentially, we have now two algorithms counting
 *      lost packets.
 *
 *      FACK: It is the simplest heuristics. As soon as we decided
 *      that something is lost, we decide that _all_ not SACKed
 *      packets until the most forward SACK are lost. I.e.
 *      lost_out = fackets_out - sacked_out and left_out = fackets_out.
 *      It is absolutely correct estimate, if network does not reorder
 *      packets. And it loses any connection to reality when reordering
 *      takes place. We use FACK by default until reordering
 *      is suspected on the path to this destination.
 *
 *      NewReno: when Recovery is entered, we assume that one segment
 *      is lost (classic Reno). While we are in Recovery and
 *      a partial ACK arrives, we assume that one more packet
 *      is lost (NewReno). This heuristics are the same in NewReno
 *      and SACK.
 *
 *  Imagine, that's all! Forget about all this shamanism about CWND inflation
 *  deflation etc. CWND is real congestion window, never inflated, changes
 *  only according to classic VJ rules.
 *
 * Really tricky (and requiring careful tuning) part of algorithm
 * is hidden in functions tcp_time_to_recover() and tcp_xmit_retransmit_queue().
 * The first determines the moment _when_ we should reduce CWND and,
 * hence, slow down forward transmission. In fact, it determines the moment
 * when we decide that hole is caused by loss, rather than by a reorder.
 *
 * tcp_xmit_retransmit_queue() decides, _what_ we should retransmit to fill
 * holes, caused by lost packets.
 *
 * And the most logically complicated part of algorithm is undo
 * heuristics. We detect false retransmits due to both too early
 * fast retransmit (reordering) and underestimated RTO, analyzing
 * timestamps and D-SACKs. When we detect that some segments were
 * retransmitted by mistake and CWND reduction was wrong, we undo
 * window reduction and abort recovery phase. This logic is hidden
 * inside several functions named tcp_try_undo_<something>.
 */

/* This function decides, when we should leave Disordered state
 * and enter Recovery phase, reducing congestion window.
 *
 * Main question: may we further continue forward transmission
 * with the same cwnd?
 */
static bool tcp_time_to_recover(struct sock *sk, int flag)
{
    struct tcp_sock *tp = tcp_sk(sk);
    __u32 packets_out;

    /* Do not perform any recovery during F-RTO algorithm */
    if (tp->frto_counter)
        return false;

    /* Trick#1: The loss is proven. */
    if (tp->lost_out)
        return true;

    /* Not-A-Trick#2 : Classic rule... */
    if (tcp_dupack_heuristics(tp) > tp->reordering)
        return true;

    /* Trick#3 : when we use RFC2988 timer restart, fast
     * retransmit can be triggered by timeout of queue head.
     */
    if (tcp_is_fack(tp) && tcp_head_timedout(sk))
        return true;

    /* Trick#4: It is still not OK... But will it be useful to delay
     * recovery more?
     */
    packets_out = tp->packets_out;
    if (packets_out <= tp->reordering &&
        tp->sacked_out >= max_t(__u32, packets_out/2, sysctl_tcp_reordering) &&
        !tcp_may_send_now(sk)) {
        /* We have nothing to send. This connection is limited
         * either by receiver window or by application.
         */
        return true;
    }

    /* If a thin stream is detected, retransmit after first
     * received dupack. Employ only if SACK is supported in order
     * to avoid possible corner-case series of spurious retransmissions
     * Use only if there are no unsent data.
     */
    if ((tp->thin_dupack || sysctl_tcp_thin_dupack) &&
        tcp_stream_is_thin(tp) && tcp_dupack_heuristics(tp) > 1 &&
        tcp_is_sack(tp) && !tcp_send_head(sk))
        return true;

    /* Trick#6: TCP early retransmit, per RFC5827.  To avoid spurious
     * retransmissions due to small network reorderings, we implement
     * Mitigation A.3 in the RFC and delay the retransmission for a short
     * interval if appropriate.
     */
    if (tp->do_early_retrans && !tp->retrans_out && tp->sacked_out &&
        (tp->packets_out == (tp->sacked_out + 1) && tp->packets_out < 4) &&
        !tcp_may_send_now(sk))
        return !tcp_pause_early_retransmit(sk, flag);

    return false;
}

/* New heuristics: it is possible only after we switched to restart timer
 * each time when something is ACKed. Hence, we can detect timed out packets
 * during fast retransmit without falling to slow start.
 *
 * Usefulness of this as is very questionable, since we should know which of
 * the segments is the next to timeout which is relatively expensive to find
 * in general case unless we add some data structure just for that. The
 * current approach certainly won't find the right one too often and when it
 * finally does find _something_ it usually marks large part of the window
 * right away (because a retransmission with a larger timestamp blocks the
 * loop from advancing). -ij
 */
static void tcp_timeout_skbs(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb;

    if (!tcp_is_fack(tp) || !tcp_head_timedout(sk))
        return;

    skb = tp->scoreboard_skb_hint;
    if (tp->scoreboard_skb_hint == NULL)
        skb = tcp_write_queue_head(sk);

    tcp_for_write_queue_from(skb, sk) {
        if (skb == tcp_send_head(sk))
            break;
        if (!tcp_skb_timedout(sk, skb))
            break;

        tcp_skb_mark_lost(tp, skb);
    }

    tp->scoreboard_skb_hint = skb;

    tcp_verify_left_out(tp);
}

/* Detect loss in event "A" above by marking head of queue up as lost.
 * For FACK or non-SACK(Reno) senders, the first "packets" number of segments
 * are considered lost. For RFC3517 SACK, a segment is considered lost if it
 * has at least tp->reordering SACKed seqments above it; "packets" refers to
 * the maximum SACKed segments to pass before reaching this limit.
 */
static void tcp_mark_head_lost(struct sock *sk, int packets, int mark_head)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb;
    int cnt, oldcnt;
    int err;
    unsigned int mss;
    /* Use SACK to deduce losses of new sequences sent during recovery */
    const u32 loss_high = tcp_is_sack(tp) ?  tp->snd_nxt : tp->high_seq;

    WARN_ON(packets > tp->packets_out);
    if (tp->lost_skb_hint) {
        skb = tp->lost_skb_hint;
        cnt = tp->lost_cnt_hint;
        /* Head already handled? */
        if (mark_head && skb != tcp_write_queue_head(sk))
            return;
    } else {
        skb = tcp_write_queue_head(sk);
        cnt = 0;
    }

    tcp_for_write_queue_from(skb, sk) {
        if (skb == tcp_send_head(sk))
            break;
        /* TODO: do this better */
        /* this is not the most efficient way to do this... */
        tp->lost_skb_hint = skb;
        tp->lost_cnt_hint = cnt;

        if (after(TCP_SKB_CB(skb)->end_seq, loss_high))
            break;

        oldcnt = cnt;
        if (tcp_is_fack(tp) || tcp_is_reno(tp) ||
            (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
            cnt += tcp_skb_pcount(skb);

        if (cnt > packets) {
            if ((tcp_is_sack(tp) && !tcp_is_fack(tp)) ||
                (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) ||
                (oldcnt >= packets))
                break;

            mss = skb_shinfo(skb)->gso_size;
            err = tcp_fragment(sk, skb, (packets - oldcnt) * mss, mss);
            if (err < 0)
                break;
            cnt = packets;
        }

        tcp_skb_mark_lost(tp, skb);

        if (mark_head)
            break;
    }
    tcp_verify_left_out(tp);
}

/* Account newly detected lost packet(s) */

static void tcp_update_scoreboard(struct sock *sk, int fast_rexmit)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (tcp_is_reno(tp)) {
        tcp_mark_head_lost(sk, 1, 1);
    } else if (tcp_is_fack(tp)) {
        int lost = tp->fackets_out - tp->reordering;
        if (lost <= 0)
            lost = 1;
        tcp_mark_head_lost(sk, lost, 0);
    } else {
        int sacked_upto = tp->sacked_out - tp->reordering;
        if (sacked_upto >= 0)
            tcp_mark_head_lost(sk, sacked_upto, 0);
        else if (fast_rexmit)
            tcp_mark_head_lost(sk, 1, 1);
    }

    tcp_timeout_skbs(sk);
}

/* CWND moderation, preventing bursts due to too big ACKs
 * in dubious situations.
 */
static inline void tcp_moderate_cwnd(struct tcp_sock *tp)
{
    tp->snd_cwnd = min(tp->snd_cwnd,
               tcp_packets_in_flight(tp) + tcp_max_burst(tp));
    tp->snd_cwnd_stamp = tcp_time_stamp;
}

/* Nothing was retransmitted or returned timestamp is less
 * than timestamp of the first retransmission.
 */
static inline bool tcp_packet_delayed(const struct tcp_sock *tp)
{
    return !tp->retrans_stamp ||
        (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
         before(tp->rx_opt.rcv_tsecr, tp->retrans_stamp));
}

/* Undo procedures. */

#if FASTRETRANS_DEBUG > 1
static void DBGUNDO(struct sock *sk, const char *msg)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct inet_sock *inet = inet_sk(sk);

    if (sk->sk_family == AF_INET) {
        pr_debug("Undo %s %pI4/%u c%u l%u ss%u/%u p%u\n",
             msg,
             &inet->inet_daddr, ntohs(inet->inet_dport),
             tp->snd_cwnd, tcp_left_out(tp),
             tp->snd_ssthresh, tp->prior_ssthresh,
             tp->packets_out);
    }
#if IS_ENABLED(CONFIG_IPV6)
    else if (sk->sk_family == AF_INET6) {
        struct ipv6_pinfo *np = inet6_sk(sk);
        pr_debug("Undo %s %pI6/%u c%u l%u ss%u/%u p%u\n",
             msg,
             &np->daddr, ntohs(inet->inet_dport),
             tp->snd_cwnd, tcp_left_out(tp),
             tp->snd_ssthresh, tp->prior_ssthresh,
             tp->packets_out);
    }
#endif
}
#else
#define DBGUNDO(x...) do { } while (0)
#endif

static void tcp_undo_cwr(struct sock *sk, const bool undo_ssthresh)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (tp->prior_ssthresh) {
        const struct inet_connection_sock *icsk = inet_csk(sk);

        if (icsk->icsk_ca_ops->undo_cwnd)
            tp->snd_cwnd = icsk->icsk_ca_ops->undo_cwnd(sk);
        else
            tp->snd_cwnd = max(tp->snd_cwnd, tp->snd_ssthresh << 1);

        if (undo_ssthresh && tp->prior_ssthresh > tp->snd_ssthresh) {
            tp->snd_ssthresh = tp->prior_ssthresh;
            TCP_ECN_withdraw_cwr(tp);
        }
    } else {
        tp->snd_cwnd = max(tp->snd_cwnd, tp->snd_ssthresh);
    }
    tp->snd_cwnd_stamp = tcp_time_stamp;
}

static inline bool tcp_may_undo(const struct tcp_sock *tp)
{
    return tp->undo_marker && (!tp->undo_retrans || tcp_packet_delayed(tp));
}

/* People celebrate: "We love our President!" */
static bool tcp_try_undo_recovery(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (tcp_may_undo(tp)) {
        int mib_idx;

        /* Happy end! We did not retransmit anything
         * or our original transmission succeeded.
         */
        DBGUNDO(sk, inet_csk(sk)->icsk_ca_state == TCP_CA_Loss ? "loss" : "retrans");
        tcp_undo_cwr(sk, true);
        if (inet_csk(sk)->icsk_ca_state == TCP_CA_Loss)
            mib_idx = LINUX_MIB_TCPLOSSUNDO;
        else
            mib_idx = LINUX_MIB_TCPFULLUNDO;

        NET_INC_STATS_BH(sock_net(sk), mib_idx);
        tp->undo_marker = 0;
    }
    if (tp->snd_una == tp->high_seq && tcp_is_reno(tp)) {
        /* Hold old state until something *above* high_seq
         * is ACKed. For Reno it is MUST to prevent false
         * fast retransmits (RFC2582). SACK TCP is safe. */
        tcp_moderate_cwnd(tp);
        return true;
    }
    tcp_set_ca_state(sk, TCP_CA_Open);
    return false;
}

/* Try to undo cwnd reduction, because D-SACKs acked all retransmitted data */
static void tcp_try_undo_dsack(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (tp->undo_marker && !tp->undo_retrans) {
        DBGUNDO(sk, "D-SACK");
        tcp_undo_cwr(sk, true);
        tp->undo_marker = 0;
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDSACKUNDO);
    }
}

/* We can clear retrans_stamp when there are no retransmissions in the
 * window. It would seem that it is trivially available for us in
 * tp->retrans_out, however, that kind of assumptions doesn't consider
 * what will happen if errors occur when sending retransmission for the
 * second time. ...It could the that such segment has only
 * TCPCB_EVER_RETRANS set at the present time. It seems that checking
 * the head skb is enough except for some reneging corner cases that
 * are not worth the effort.
 *
 * Main reason for all this complexity is the fact that connection dying
 * time now depends on the validity of the retrans_stamp, in particular,
 * that successive retransmissions of a segment must not advance
 * retrans_stamp under any conditions.
 */
static bool tcp_any_retrans_done(const struct sock *sk)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb;

    if (tp->retrans_out)
        return true;

    skb = tcp_write_queue_head(sk);
    if (unlikely(skb && TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS))
        return true;

    return false;
}

/* Undo during fast recovery after partial ACK. */

static int tcp_try_undo_partial(struct sock *sk, int acked)
{
    struct tcp_sock *tp = tcp_sk(sk);
    /* Partial ACK arrived. Force Hoe's retransmit. */
    int failed = tcp_is_reno(tp) || (tcp_fackets_out(tp) > tp->reordering);

    if (tcp_may_undo(tp)) {
        /* Plain luck! Hole if filled with delayed
         * packet, rather than with a retransmit.
         */
        if (!tcp_any_retrans_done(sk))
            tp->retrans_stamp = 0;

        tcp_update_reordering(sk, tcp_fackets_out(tp) + acked, 1);

        DBGUNDO(sk, "Hoe");
        tcp_undo_cwr(sk, false);
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPPARTIALUNDO);

        /* So... Do not make Hoe's retransmit yet.
         * If the first packet was delayed, the rest
         * ones are most probably delayed as well.
         */
        failed = 0;
    }
    return failed;
}

/* Undo during loss recovery after partial ACK. */
static bool tcp_try_undo_loss(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (tcp_may_undo(tp)) {
        struct sk_buff *skb;
        tcp_for_write_queue(skb, sk) {
            if (skb == tcp_send_head(sk))
                break;
            TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
        }

        tcp_clear_all_retrans_hints(tp);

        DBGUNDO(sk, "partial loss");
        tp->lost_out = 0;
        tcp_undo_cwr(sk, true);
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPLOSSUNDO);
        inet_csk(sk)->icsk_retransmits = 0;
        tp->undo_marker = 0;
        if (tcp_is_sack(tp))
            tcp_set_ca_state(sk, TCP_CA_Open);
        return true;
    }
    return false;
}

/* The cwnd reduction in CWR and Recovery use the PRR algorithm
 * https://datatracker.ietf.org/doc/draft-ietf-tcpm-proportional-rate-reduction/
 * It computes the number of packets to send (sndcnt) based on packets newly
 * delivered:
 *   1) If the packets in flight is larger than ssthresh, PRR spreads the
 *  cwnd reductions across a full RTT.
 *   2) If packets in flight is lower than ssthresh (such as due to excess
 *  losses and/or application stalls), do not perform any further cwnd
 *  reductions, but instead slow start up to ssthresh.
 */
static void tcp_init_cwnd_reduction(struct sock *sk, const bool set_ssthresh)
{
    struct tcp_sock *tp = tcp_sk(sk);

    tp->high_seq = tp->snd_nxt;
    tp->bytes_acked = 0;
    tp->snd_cwnd_cnt = 0;
    tp->prior_cwnd = tp->snd_cwnd;
    tp->prr_delivered = 0;
    tp->prr_out = 0;
    if (set_ssthresh)
        tp->snd_ssthresh = inet_csk(sk)->icsk_ca_ops->ssthresh(sk);
    TCP_ECN_queue_cwr(tp);
}

static void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked,
                   int fast_rexmit)
{
    struct tcp_sock *tp = tcp_sk(sk);
    int sndcnt = 0;
    int delta = tp->snd_ssthresh - tcp_packets_in_flight(tp);

    tp->prr_delivered += newly_acked_sacked;
    if (tcp_packets_in_flight(tp) > tp->snd_ssthresh) {
        u64 dividend = (u64)tp->snd_ssthresh * tp->prr_delivered +
                   tp->prior_cwnd - 1;
        sndcnt = div_u64(dividend, tp->prior_cwnd) - tp->prr_out;
    } else {
        sndcnt = min_t(int, delta,
                   max_t(int, tp->prr_delivered - tp->prr_out,
                     newly_acked_sacked) + 1);
    }

    sndcnt = max(sndcnt, (fast_rexmit ? 1 : 0));
    tp->snd_cwnd = tcp_packets_in_flight(tp) + sndcnt;
}

static inline void tcp_end_cwnd_reduction(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);

    /* Reset cwnd to ssthresh in CWR or Recovery (unless it's undone) */
    if (inet_csk(sk)->icsk_ca_state == TCP_CA_CWR ||
        (tp->undo_marker && tp->snd_ssthresh < TCP_INFINITE_SSTHRESH)) {
        tp->snd_cwnd = tp->snd_ssthresh;
        tp->snd_cwnd_stamp = tcp_time_stamp;
    }
    tcp_ca_event(sk, CA_EVENT_COMPLETE_CWR);
}

/* Enter CWR state. Disable cwnd undo since congestion is proven with ECN */
void tcp_enter_cwr(struct sock *sk, const int set_ssthresh)
{
    struct tcp_sock *tp = tcp_sk(sk);

    tp->prior_ssthresh = 0;
    tp->bytes_acked = 0;
    if (inet_csk(sk)->icsk_ca_state < TCP_CA_CWR) {
        tp->undo_marker = 0;
        tcp_init_cwnd_reduction(sk, set_ssthresh);
        tcp_set_ca_state(sk, TCP_CA_CWR);
    }
}

static void tcp_try_keep_open(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    int state = TCP_CA_Open;

    if (tcp_left_out(tp) || tcp_any_retrans_done(sk))
        state = TCP_CA_Disorder;

    if (inet_csk(sk)->icsk_ca_state != state) {
        tcp_set_ca_state(sk, state);
        tp->high_seq = tp->snd_nxt;
    }
}

static void tcp_try_to_open(struct sock *sk, int flag, int newly_acked_sacked)
{
    struct tcp_sock *tp = tcp_sk(sk);

    tcp_verify_left_out(tp);

    if (!tp->frto_counter && !tcp_any_retrans_done(sk))
        tp->retrans_stamp = 0;

    if (flag & FLAG_ECE)
        tcp_enter_cwr(sk, 1);

    if (inet_csk(sk)->icsk_ca_state != TCP_CA_CWR) {
        tcp_try_keep_open(sk);
        if (inet_csk(sk)->icsk_ca_state != TCP_CA_Open)
            tcp_moderate_cwnd(tp);
    } else {
        tcp_cwnd_reduction(sk, newly_acked_sacked, 0);
    }
}

static void tcp_mtup_probe_failed(struct sock *sk)
{
    struct inet_connection_sock *icsk = inet_csk(sk);

    icsk->icsk_mtup.search_high = icsk->icsk_mtup.probe_size - 1;
    icsk->icsk_mtup.probe_size = 0;
}

static void tcp_mtup_probe_success(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct inet_connection_sock *icsk = inet_csk(sk);

    /* FIXME: breaks with very large cwnd */
    tp->prior_ssthresh = tcp_current_ssthresh(sk);
    tp->snd_cwnd = tp->snd_cwnd *
               tcp_mss_to_mtu(sk, tp->mss_cache) /
               icsk->icsk_mtup.probe_size;
    tp->snd_cwnd_cnt = 0;
    tp->snd_cwnd_stamp = tcp_time_stamp;
    tp->snd_ssthresh = tcp_current_ssthresh(sk);

    icsk->icsk_mtup.search_low = icsk->icsk_mtup.probe_size;
    icsk->icsk_mtup.probe_size = 0;
    tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
}

/* Do a simple retransmit without using the backoff mechanisms in
 * tcp_timer. This is used for path mtu discovery.
 * The socket is already locked here.
 */
void tcp_simple_retransmit(struct sock *sk)
{
    const struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb;
    unsigned int mss = tcp_current_mss(sk);
    u32 prior_lost = tp->lost_out;

    tcp_for_write_queue(skb, sk) {
        if (skb == tcp_send_head(sk))
            break;
        if (tcp_skb_seglen(skb) > mss &&
            !(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)) {
            if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
                TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
                tp->retrans_out -= tcp_skb_pcount(skb);
            }
            tcp_skb_mark_lost_uncond_verify(tp, skb);
        }
    }

    tcp_clear_retrans_hints_partial(tp);

    if (prior_lost == tp->lost_out)
        return;

    if (tcp_is_reno(tp))
        tcp_limit_reno_sacked(tp);

    tcp_verify_left_out(tp);

    /* Don't muck with the congestion window here.
     * Reason is that we do not increase amount of _data_
     * in network, but units changed and effective
     * cwnd/ssthresh really reduced now.
     */
    if (icsk->icsk_ca_state != TCP_CA_Loss) {
        tp->high_seq = tp->snd_nxt;
        tp->snd_ssthresh = tcp_current_ssthresh(sk);
        tp->prior_ssthresh = 0;
        tp->undo_marker = 0;
        tcp_set_ca_state(sk, TCP_CA_Loss);
    }
    tcp_xmit_retransmit_queue(sk);
}
EXPORT_SYMBOL(tcp_simple_retransmit);

static void tcp_enter_recovery(struct sock *sk, bool ece_ack)
{
    struct tcp_sock *tp = tcp_sk(sk);
    int mib_idx;

    if (tcp_is_reno(tp))
        mib_idx = LINUX_MIB_TCPRENORECOVERY;
    else
        mib_idx = LINUX_MIB_TCPSACKRECOVERY;

    NET_INC_STATS_BH(sock_net(sk), mib_idx);

    tp->prior_ssthresh = 0;
    tp->undo_marker = tp->snd_una;
    tp->undo_retrans = tp->retrans_out;

    if (inet_csk(sk)->icsk_ca_state < TCP_CA_CWR) {
        if (!ece_ack)
            tp->prior_ssthresh = tcp_current_ssthresh(sk);
        tcp_init_cwnd_reduction(sk, true);
    }
    tcp_set_ca_state(sk, TCP_CA_Recovery);
}

/* Process an event, which can update packets-in-flight not trivially.
 * Main goal of this function is to calculate new estimate for left_out,
 * taking into account both packets sitting in receiver's buffer and
 * packets lost by network.
 *
 * Besides that it does CWND reduction, when packet loss is detected
 * and changes state of machine.
 *
 * It does _not_ decide what to send, it is made in function
 * tcp_xmit_retransmit_queue().
 */
static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
                  int prior_sacked, bool is_dupack,
                  int flag)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    int do_lost = is_dupack || ((flag & FLAG_DATA_SACKED) &&
                    (tcp_fackets_out(tp) > tp->reordering));
    int newly_acked_sacked = 0;
    int fast_rexmit = 0;

    if (WARN_ON(!tp->packets_out && tp->sacked_out))
        tp->sacked_out = 0;
    if (WARN_ON(!tp->sacked_out && tp->fackets_out))
        tp->fackets_out = 0;

    /* Now state machine starts.
     * A. ECE, hence prohibit cwnd undoing, the reduction is required. */
    if (flag & FLAG_ECE)
        tp->prior_ssthresh = 0;

    /* B. In all the states check for reneging SACKs. */
    if (tcp_check_sack_reneging(sk, flag))
        return;

    /* C. Check consistency of the current state. */
    tcp_verify_left_out(tp);

    /* D. Check state exit conditions. State can be terminated
     *    when high_seq is ACKed. */
    if (icsk->icsk_ca_state == TCP_CA_Open) {
        WARN_ON(tp->retrans_out != 0);
        tp->retrans_stamp = 0;
    } else if (!before(tp->snd_una, tp->high_seq)) {
        switch (icsk->icsk_ca_state) {
        case TCP_CA_Loss:
            icsk->icsk_retransmits = 0;
            if (tcp_try_undo_recovery(sk))
                return;
            break;

        case TCP_CA_CWR:
            /* CWR is to be held something *above* high_seq
             * is ACKed for CWR bit to reach receiver. */
            if (tp->snd_una != tp->high_seq) {
                tcp_end_cwnd_reduction(sk);
                tcp_set_ca_state(sk, TCP_CA_Open);
            }
            break;

        case TCP_CA_Recovery:
            if (tcp_is_reno(tp))
                tcp_reset_reno_sack(tp);
            if (tcp_try_undo_recovery(sk))
                return;
            tcp_end_cwnd_reduction(sk);
            break;
        }
    }

    /* E. Process state. */
    switch (icsk->icsk_ca_state) {
    case TCP_CA_Recovery:
        if (!(flag & FLAG_SND_UNA_ADVANCED)) {
            if (tcp_is_reno(tp) && is_dupack)
                tcp_add_reno_sack(sk);
        } else
            do_lost = tcp_try_undo_partial(sk, pkts_acked);
        newly_acked_sacked = pkts_acked + tp->sacked_out - prior_sacked;
        break;
    case TCP_CA_Loss:
        if (flag & FLAG_DATA_ACKED)
            icsk->icsk_retransmits = 0;
        if (tcp_is_reno(tp) && flag & FLAG_SND_UNA_ADVANCED)
            tcp_reset_reno_sack(tp);
        if (!tcp_try_undo_loss(sk)) {
            tcp_moderate_cwnd(tp);
            tcp_xmit_retransmit_queue(sk);
            return;
        }
        if (icsk->icsk_ca_state != TCP_CA_Open)
            return;
        /* Loss is undone; fall through to processing in Open state. */
    default:
        if (tcp_is_reno(tp)) {
            if (flag & FLAG_SND_UNA_ADVANCED)
                tcp_reset_reno_sack(tp);
            if (is_dupack)
                tcp_add_reno_sack(sk);
        }
        newly_acked_sacked = pkts_acked + tp->sacked_out - prior_sacked;

        if (icsk->icsk_ca_state <= TCP_CA_Disorder)
            tcp_try_undo_dsack(sk);

        if (!tcp_time_to_recover(sk, flag)) {
            tcp_try_to_open(sk, flag, newly_acked_sacked);
            return;
        }

        /* MTU probe failure: don't reduce cwnd */
        if (icsk->icsk_ca_state < TCP_CA_CWR &&
            icsk->icsk_mtup.probe_size &&
            tp->snd_una == tp->mtu_probe.probe_seq_start) {
            tcp_mtup_probe_failed(sk);
            /* Restores the reduction we did in tcp_mtup_probe() */
            tp->snd_cwnd++;
            tcp_simple_retransmit(sk);
            return;
        }

        /* Otherwise enter Recovery state */
        tcp_enter_recovery(sk, (flag & FLAG_ECE));
        fast_rexmit = 1;
    }

    if (do_lost || (tcp_is_fack(tp) && tcp_head_timedout(sk)))
        tcp_update_scoreboard(sk, fast_rexmit);
    tcp_cwnd_reduction(sk, newly_acked_sacked, fast_rexmit);
    tcp_xmit_retransmit_queue(sk);
}

void tcp_valid_rtt_meas(struct sock *sk, u32 seq_rtt)
{
    tcp_rtt_estimator(sk, seq_rtt);
    tcp_set_rto(sk);
    inet_csk(sk)->icsk_backoff = 0;
}
EXPORT_SYMBOL(tcp_valid_rtt_meas);

/* Read draft-ietf-tcplw-high-performance before mucking
 * with this code. (Supersedes RFC1323)
 */
static void tcp_ack_saw_tstamp(struct sock *sk, int flag)
{
    /* RTTM Rule: A TSecr value received in a segment is used to
     * update the averaged RTT measurement only if the segment
     * acknowledges some new data, i.e., only if it advances the
     * left edge of the send window.
     *
     * See draft-ietf-tcplw-high-performance-00, section 3.3.
     * 1998/04/10 Andrey V. Savochkin <[email protected]>
     *
     * Changed: reset backoff as soon as we see the first valid sample.
     * If we do not, we get strongly overestimated rto. With timestamps
     * samples are accepted even from very old segments: f.e., when rtt=1
     * increases to 8, we retransmit 5 times and after 8 seconds delayed
     * answer arrives rto becomes 120 seconds! If at least one of segments
     * in window is lost... Voila.              --ANK (010210)
     */
    struct tcp_sock *tp = tcp_sk(sk);

    tcp_valid_rtt_meas(sk, tcp_time_stamp - tp->rx_opt.rcv_tsecr);
}

static void tcp_ack_no_tstamp(struct sock *sk, u32 seq_rtt, int flag)
{
    /* We don't have a timestamp. Can only use
     * packets that are not retransmitted to determine
     * rtt estimates. Also, we must not reset the
     * backoff for rto until we get a non-retransmitted
     * packet. This allows us to deal with a situation
     * where the network delay has increased suddenly.
     * I.e. Karn's algorithm. (SIGCOMM '87, p5.)
     */

    if (flag & FLAG_RETRANS_DATA_ACKED)
        return;

    tcp_valid_rtt_meas(sk, seq_rtt);
}

static inline void tcp_ack_update_rtt(struct sock *sk, const int flag,
                      const s32 seq_rtt)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    /* Note that peer MAY send zero echo. In this case it is ignored. (rfc1323) */
    if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
        tcp_ack_saw_tstamp(sk, flag);
    else if (seq_rtt >= 0)
        tcp_ack_no_tstamp(sk, seq_rtt, flag);
}

static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
{
    const struct inet_connection_sock *icsk = inet_csk(sk);
    icsk->icsk_ca_ops->cong_avoid(sk, ack, in_flight);
    tcp_sk(sk)->snd_cwnd_stamp = tcp_time_stamp;
}

/* Restart timer after forward progress on connection.
 * RFC2988 recommends to restart timer to now+rto.
 */
void tcp_rearm_rto(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);

    /* If the retrans timer is currently being used by Fast Open
     * for SYN-ACK retrans purpose, stay put.
     */
    if (tp->fastopen_rsk)
        return;

    if (!tp->packets_out) {
        inet_csk_clear_xmit_timer(sk, ICSK_TIME_RETRANS);
    } else {
        u32 rto = inet_csk(sk)->icsk_rto;
        /* Offset the time elapsed after installing regular RTO */
        if (tp->early_retrans_delayed) {
            struct sk_buff *skb = tcp_write_queue_head(sk);
            const u32 rto_time_stamp = TCP_SKB_CB(skb)->when + rto;
            s32 delta = (s32)(rto_time_stamp - tcp_time_stamp);
            /* delta may not be positive if the socket is locked
             * when the delayed ER timer fires and is rescheduled.
             */
            if (delta > 0)
                rto = delta;
        }
        inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, rto,
                      TCP_RTO_MAX);
    }
    tp->early_retrans_delayed = 0;
}

/* This function is called when the delayed ER timer fires. TCP enters
 * fast recovery and performs fast-retransmit.
 */
void tcp_resume_early_retransmit(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);

    tcp_rearm_rto(sk);

    /* Stop if ER is disabled after the delayed ER timer is scheduled */
    if (!tp->do_early_retrans)
        return;

    tcp_enter_recovery(sk, false);
    tcp_update_scoreboard(sk, 1);
    tcp_xmit_retransmit_queue(sk);
}

/* If we get here, the whole TSO packet has not been acked. */
static u32 tcp_tso_acked(struct sock *sk, struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);
    u32 packets_acked;

    BUG_ON(!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una));

    packets_acked = tcp_skb_pcount(skb);
    if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
        return 0;
    packets_acked -= tcp_skb_pcount(skb);

    if (packets_acked) {
        BUG_ON(tcp_skb_pcount(skb) == 0);
        BUG_ON(!before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq));
    }

    return packets_acked;
}

/* Remove acknowledged frames from the retransmission queue. If our packet
 * is before the ack sequence we can discard it as it's confirmed to have
 * arrived at the other end.
 */
static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
                   u32 prior_snd_una)
{
    struct tcp_sock *tp = tcp_sk(sk);
    const struct inet_connection_sock *icsk = inet_csk(sk);
    struct sk_buff *skb;
    u32 now = tcp_time_stamp;
    int fully_acked = true;
    int flag = 0;
    u32 pkts_acked = 0;
    u32 reord = tp->packets_out;
    u32 prior_sacked = tp->sacked_out;
    s32 seq_rtt = -1;
    s32 ca_seq_rtt = -1;
    ktime_t last_ackt = net_invalid_timestamp();

    while ((skb = tcp_write_queue_head(sk)) && skb != tcp_send_head(sk)) {
        struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
        u32 acked_pcount;
        u8 sacked = scb->sacked;

        /* Determine how many packets and what bytes were acked, tso and else */
        if (after(scb->end_seq, tp->snd_una)) {
            if (tcp_skb_pcount(skb) == 1 ||
                !after(tp->snd_una, scb->seq))
                break;

            acked_pcount = tcp_tso_acked(sk, skb);
            if (!acked_pcount)
                break;

            fully_acked = false;
        } else {
            acked_pcount = tcp_skb_pcount(skb);
        }

        if (sacked & TCPCB_RETRANS) {
            if (sacked & TCPCB_SACKED_RETRANS)
                tp->retrans_out -= acked_pcount;
            flag |= FLAG_RETRANS_DATA_ACKED;
            ca_seq_rtt = -1;
            seq_rtt = -1;
            if ((flag & FLAG_DATA_ACKED) || (acked_pcount > 1))
                flag |= FLAG_NONHEAD_RETRANS_ACKED;
        } else {
            ca_seq_rtt = now - scb->when;
            last_ackt = skb->tstamp;
            if (seq_rtt < 0) {
                seq_rtt = ca_seq_rtt;
            }
            if (!(sacked & TCPCB_SACKED_ACKED))
                reord = min(pkts_acked, reord);
        }

        if (sacked & TCPCB_SACKED_ACKED)
            tp->sacked_out -= acked_pcount;
        if (sacked & TCPCB_LOST)
            tp->lost_out -= acked_pcount;

        tp->packets_out -= acked_pcount;
        pkts_acked += acked_pcount;

        /* Initial outgoing SYN's get put onto the write_queue
         * just like anything else we transmit.  It is not
         * true data, and if we misinform our callers that
         * this ACK acks real data, we will erroneously exit
         * connection startup slow start one packet too
         * quickly.  This is severely frowned upon behavior.
         */
        if (!(scb->tcp_flags & TCPHDR_SYN)) {
            flag |= FLAG_DATA_ACKED;
        } else {
            flag |= FLAG_SYN_ACKED;
            tp->retrans_stamp = 0;
        }

        if (!fully_acked)
            break;

        tcp_unlink_write_queue(skb, sk);
        sk_wmem_free_skb(sk, skb);
        tp->scoreboard_skb_hint = NULL;
        if (skb == tp->retransmit_skb_hint)
            tp->retransmit_skb_hint = NULL;
        if (skb == tp->lost_skb_hint)
            tp->lost_skb_hint = NULL;
    }

    if (likely(between(tp->snd_up, prior_snd_una, tp->snd_una)))
        tp->snd_up = tp->snd_una;

    if (skb && (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
        flag |= FLAG_SACK_RENEGING;

    if (flag & FLAG_ACKED) {
        const struct tcp_congestion_ops *ca_ops
            = inet_csk(sk)->icsk_ca_ops;

        if (unlikely(icsk->icsk_mtup.probe_size &&
                 !after(tp->mtu_probe.probe_seq_end, tp->snd_una))) {
            tcp_mtup_probe_success(sk);
        }

        tcp_ack_update_rtt(sk, flag, seq_rtt);
        tcp_rearm_rto(sk);

        if (tcp_is_reno(tp)) {
            tcp_remove_reno_sacks(sk, pkts_acked);
        } else {
            int delta;

            /* Non-retransmitted hole got filled? That's reordering */
            if (reord < prior_fackets)
                tcp_update_reordering(sk, tp->fackets_out - reord, 0);

            delta = tcp_is_fack(tp) ? pkts_acked :
                          prior_sacked - tp->sacked_out;
            tp->lost_cnt_hint -= min(tp->lost_cnt_hint, delta);
        }

        tp->fackets_out -= min(pkts_acked, tp->fackets_out);

        if (ca_ops->pkts_acked) {
            s32 rtt_us = -1;

            /* Is the ACK triggering packet unambiguous? */
            if (!(flag & FLAG_RETRANS_DATA_ACKED)) {
                /* High resolution needed and available? */
                if (ca_ops->flags & TCP_CONG_RTT_STAMP &&
                    !ktime_equal(last_ackt,
                         net_invalid_timestamp()))
                    rtt_us = ktime_us_delta(ktime_get_real(),
                                last_ackt);
                else if (ca_seq_rtt >= 0)
                    rtt_us = jiffies_to_usecs(ca_seq_rtt);
            }

            ca_ops->pkts_acked(sk, pkts_acked, rtt_us);
        }
    }

#if FASTRETRANS_DEBUG > 0
    WARN_ON((int)tp->sacked_out < 0);
    WARN_ON((int)tp->lost_out < 0);
    WARN_ON((int)tp->retrans_out < 0);
    if (!tp->packets_out && tcp_is_sack(tp)) {
        icsk = inet_csk(sk);
        if (tp->lost_out) {
            pr_debug("Leak l=%u %d\n",
                 tp->lost_out, icsk->icsk_ca_state);
            tp->lost_out = 0;
        }
        if (tp->sacked_out) {
            pr_debug("Leak s=%u %d\n",
                 tp->sacked_out, icsk->icsk_ca_state);
            tp->sacked_out = 0;
        }
        if (tp->retrans_out) {
            pr_debug("Leak r=%u %d\n",
                 tp->retrans_out, icsk->icsk_ca_state);
            tp->retrans_out = 0;
        }
    }
#endif
    return flag;
}

static void tcp_ack_probe(struct sock *sk)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    struct inet_connection_sock *icsk = inet_csk(sk);

    /* Was it a usable window open? */

    if (!after(TCP_SKB_CB(tcp_send_head(sk))->end_seq, tcp_wnd_end(tp))) {
        icsk->icsk_backoff = 0;
        inet_csk_clear_xmit_timer(sk, ICSK_TIME_PROBE0);
        /* Socket must be waked up by subsequent tcp_data_snd_check().
         * This function is not for random using!
         */
    } else {
        inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
                      min(icsk->icsk_rto << icsk->icsk_backoff, TCP_RTO_MAX),
                      TCP_RTO_MAX);
    }
}

static inline bool tcp_ack_is_dubious(const struct sock *sk, const int flag)
{
    return !(flag & FLAG_NOT_DUP) || (flag & FLAG_CA_ALERT) ||
        inet_csk(sk)->icsk_ca_state != TCP_CA_Open;
}

static inline bool tcp_may_raise_cwnd(const struct sock *sk, const int flag)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    return (!(flag & FLAG_ECE) || tp->snd_cwnd < tp->snd_ssthresh) &&
        !tcp_in_cwnd_reduction(sk);
}

/* Check that window update is acceptable.
 * The function assumes that snd_una<=ack<=snd_next.
 */
static inline bool tcp_may_update_window(const struct tcp_sock *tp,
                    const u32 ack, const u32 ack_seq,
                    const u32 nwin)
{
    return  after(ack, tp->snd_una) ||
        after(ack_seq, tp->snd_wl1) ||
        (ack_seq == tp->snd_wl1 && nwin > tp->snd_wnd);
}

/* Update our send window.
 *
 * Window update algorithm, described in RFC793/RFC1122 (used in linux-2.2
 * and in FreeBSD. NetBSD's one is even worse.) is wrong.
 */
static int tcp_ack_update_window(struct sock *sk, const struct sk_buff *skb, u32 ack,
                 u32 ack_seq)
{
    struct tcp_sock *tp = tcp_sk(sk);
    int flag = 0;
    u32 nwin = ntohs(tcp_hdr(skb)->window);

    if (likely(!tcp_hdr(skb)->syn))
        nwin <<= tp->rx_opt.snd_wscale;

    if (tcp_may_update_window(tp, ack, ack_seq, nwin)) {
        flag |= FLAG_WIN_UPDATE;
        tcp_update_wl(tp, ack_seq);

        if (tp->snd_wnd != nwin) {
            tp->snd_wnd = nwin;

            /* Note, it is the only place, where
             * fast path is recovered for sending TCP.
             */
            tp->pred_flags = 0;
            tcp_fast_path_check(sk);

            if (nwin > tp->max_window) {
                tp->max_window = nwin;
                tcp_sync_mss(sk, inet_csk(sk)->icsk_pmtu_cookie);
            }
        }
    }

    tp->snd_una = ack;

    return flag;
}

/* A very conservative spurious RTO response algorithm: reduce cwnd and
 * continue in congestion avoidance.
 */
static void tcp_conservative_spur_to_response(struct tcp_sock *tp)
{
    tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);
    tp->snd_cwnd_cnt = 0;
    tp->bytes_acked = 0;
    TCP_ECN_queue_cwr(tp);
    tcp_moderate_cwnd(tp);
}

/* A conservative spurious RTO response algorithm: reduce cwnd using
 * PRR and continue in congestion avoidance.
 */
static void tcp_cwr_spur_to_response(struct sock *sk)
{
    tcp_enter_cwr(sk, 0);
}

static void tcp_undo_spur_to_response(struct sock *sk, int flag)
{
    if (flag & FLAG_ECE)
        tcp_cwr_spur_to_response(sk);
    else
        tcp_undo_cwr(sk, true);
}

/* F-RTO spurious RTO detection algorithm (RFC4138)
 *
 * F-RTO affects during two new ACKs following RTO (well, almost, see inline
 * comments). State (ACK number) is kept in frto_counter. When ACK advances
 * window (but not to or beyond highest sequence sent before RTO):
 *   On First ACK,  send two new segments out.
 *   On Second ACK, RTO was likely spurious. Do spurious response (response
 *                  algorithm is not part of the F-RTO detection algorithm
 *                  given in RFC4138 but can be selected separately).
 * Otherwise (basically on duplicate ACK), RTO was (likely) caused by a loss
 * and TCP falls back to conventional RTO recovery. F-RTO allows overriding
 * of Nagle, this is done using frto_counter states 2 and 3, when a new data
 * segment of any size sent during F-RTO, state 2 is upgraded to 3.
 *
 * Rationale: if the RTO was spurious, new ACKs should arrive from the
 * original window even after we transmit two new data segments.
 *
 * SACK version:
 *   on first step, wait until first cumulative ACK arrives, then move to
 *   the second step. In second step, the next ACK decides.
 *
 * F-RTO is implemented (mainly) in four functions:
 *   - tcp_use_frto() is used to determine if TCP is can use F-RTO
 *   - tcp_enter_frto() prepares TCP state on RTO if F-RTO is used, it is
 *     called when tcp_use_frto() showed green light
 *   - tcp_process_frto() handles incoming ACKs during F-RTO algorithm
 *   - tcp_enter_frto_loss() is called if there is not enough evidence
 *     to prove that the RTO is indeed spurious. It transfers the control
 *     from F-RTO to the conventional RTO recovery
 */
static bool tcp_process_frto(struct sock *sk, int flag)
{
    struct tcp_sock *tp = tcp_sk(sk);

    tcp_verify_left_out(tp);

    /* Duplicate the behavior from Loss state (fastretrans_alert) */
    if (flag & FLAG_DATA_ACKED)
        inet_csk(sk)->icsk_retransmits = 0;

    if ((flag & FLAG_NONHEAD_RETRANS_ACKED) ||
        ((tp->frto_counter >= 2) && (flag & FLAG_RETRANS_DATA_ACKED)))
        tp->undo_marker = 0;

    if (!before(tp->snd_una, tp->frto_highmark)) {
        tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 2 : 3), flag);
        return true;
    }

    if (!tcp_is_sackfrto(tp)) {
        /* RFC4138 shortcoming in step 2; should also have case c):
         * ACK isn't duplicate nor advances window, e.g., opposite dir
         * data, winupdate
         */
        if (!(flag & FLAG_ANY_PROGRESS) && (flag & FLAG_NOT_DUP))
            return true;

        if (!(flag & FLAG_DATA_ACKED)) {
            tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 0 : 3),
                        flag);
            return true;
        }
    } else {
        if (!(flag & FLAG_DATA_ACKED) && (tp->frto_counter == 1)) {
            /* Prevent sending of new data. */
            tp->snd_cwnd = min(tp->snd_cwnd,
                       tcp_packets_in_flight(tp));
            return true;
        }

        if ((tp->frto_counter >= 2) &&
            (!(flag & FLAG_FORWARD_PROGRESS) ||
             ((flag & FLAG_DATA_SACKED) &&
              !(flag & FLAG_ONLY_ORIG_SACKED)))) {
            /* RFC4138 shortcoming (see comment above) */
            if (!(flag & FLAG_FORWARD_PROGRESS) &&
                (flag & FLAG_NOT_DUP))
                return true;

            tcp_enter_frto_loss(sk, 3, flag);
            return true;
        }
    }

    if (tp->frto_counter == 1) {
        /* tcp_may_send_now needs to see updated state */
        tp->snd_cwnd = tcp_packets_in_flight(tp) + 2;
        tp->frto_counter = 2;

        if (!tcp_may_send_now(sk))
            tcp_enter_frto_loss(sk, 2, flag);

        return true;
    } else {
        switch (sysctl_tcp_frto_response) {
        case 2:
            tcp_undo_spur_to_response(sk, flag);
            break;
        case 1:
            tcp_conservative_spur_to_response(tp);
            break;
        default:
            tcp_cwr_spur_to_response(sk);
            break;
        }
        tp->frto_counter = 0;
        tp->undo_marker = 0;
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSPURIOUSRTOS);
    }
    return false;
}

/* This routine deals with incoming acks, but not outgoing ones. */
static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    u32 prior_snd_una = tp->snd_una;
    u32 ack_seq = TCP_SKB_CB(skb)->seq;
    u32 ack = TCP_SKB_CB(skb)->ack_seq;
    bool is_dupack = false;
    u32 prior_in_flight;
    u32 prior_fackets;
    int prior_packets;
    int prior_sacked = tp->sacked_out;
    int pkts_acked = 0;
    bool frto_cwnd = false;

    /* If the ack is older than previous acks
     * then we can probably ignore it.
     */
    if (before(ack, prior_snd_una))
        goto old_ack;

    /* If the ack includes data we haven't sent yet, discard
     * this segment (RFC793 Section 3.9).
     */
    if (after(ack, tp->snd_nxt))
        goto invalid_ack;

    if (tp->early_retrans_delayed)
        tcp_rearm_rto(sk);

    if (after(ack, prior_snd_una))
        flag |= FLAG_SND_UNA_ADVANCED;

    if (sysctl_tcp_abc) {
        if (icsk->icsk_ca_state < TCP_CA_CWR)
            tp->bytes_acked += ack - prior_snd_una;
        else if (icsk->icsk_ca_state == TCP_CA_Loss)
            /* we assume just one segment left network */
            tp->bytes_acked += min(ack - prior_snd_una,
                           tp->mss_cache);
    }

    prior_fackets = tp->fackets_out;
    prior_in_flight = tcp_packets_in_flight(tp);

    if (!(flag & FLAG_SLOWPATH) && after(ack, prior_snd_una)) {
        /* Window is constant, pure forward advance.
         * No more checks are required.
         * Note, we use the fact that SND.UNA>=SND.WL2.
         */
        tcp_update_wl(tp, ack_seq);
        tp->snd_una = ack;
        flag |= FLAG_WIN_UPDATE;

        tcp_ca_event(sk, CA_EVENT_FAST_ACK);

        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPHPACKS);
    } else {
        if (ack_seq != TCP_SKB_CB(skb)->end_seq)
            flag |= FLAG_DATA;
        else
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPPUREACKS);

        flag |= tcp_ack_update_window(sk, skb, ack, ack_seq);

        if (TCP_SKB_CB(skb)->sacked)
            flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una);

        if (TCP_ECN_rcv_ecn_echo(tp, tcp_hdr(skb)))
            flag |= FLAG_ECE;

        tcp_ca_event(sk, CA_EVENT_SLOW_ACK);
    }

    /* We passed data and got it acked, remove any soft error
     * log. Something worked...
     */
    sk->sk_err_soft = 0;
    icsk->icsk_probes_out = 0;
    tp->rcv_tstamp = tcp_time_stamp;
    prior_packets = tp->packets_out;
    if (!prior_packets)
        goto no_queue;

    /* See if we can take anything off of the retransmit queue. */
    flag |= tcp_clean_rtx_queue(sk, prior_fackets, prior_snd_una);

    pkts_acked = prior_packets - tp->packets_out;

    if (tp->frto_counter)
        frto_cwnd = tcp_process_frto(sk, flag);
    /* Guarantee sacktag reordering detection against wrap-arounds */
    if (before(tp->frto_highmark, tp->snd_una))
        tp->frto_highmark = 0;

    if (tcp_ack_is_dubious(sk, flag)) {
        /* Advance CWND, if state allows this. */
        if ((flag & FLAG_DATA_ACKED) && !frto_cwnd &&
            tcp_may_raise_cwnd(sk, flag))
            tcp_cong_avoid(sk, ack, prior_in_flight);
        is_dupack = !(flag & (FLAG_SND_UNA_ADVANCED | FLAG_NOT_DUP));
        tcp_fastretrans_alert(sk, pkts_acked, prior_sacked,
                      is_dupack, flag);
    } else {
        if ((flag & FLAG_DATA_ACKED) && !frto_cwnd)
            tcp_cong_avoid(sk, ack, prior_in_flight);
    }

    if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP)) {
        struct dst_entry *dst = __sk_dst_get(sk);
        if (dst)
            dst_confirm(dst);
    }
    return 1;

no_queue:
    /* If data was DSACKed, see if we can undo a cwnd reduction. */
    if (flag & FLAG_DSACKING_ACK)
        tcp_fastretrans_alert(sk, pkts_acked, prior_sacked,
                      is_dupack, flag);
    /* If this ack opens up a zero window, clear backoff.  It was
     * being used to time the probes, and is probably far higher than
     * it needs to be for normal retransmission.
     */
    if (tcp_send_head(sk))
        tcp_ack_probe(sk);
    return 1;

invalid_ack:
    SOCK_DEBUG(sk, "Ack %u after %u:%u\n", ack, tp->snd_una, tp->snd_nxt);
    return -1;

old_ack:
    /* If data was SACKed, tag it and see if we should send more data.
     * If data was DSACKed, see if we can undo a cwnd reduction.
     */
    if (TCP_SKB_CB(skb)->sacked) {
        flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una);
        tcp_fastretrans_alert(sk, pkts_acked, prior_sacked,
                      is_dupack, flag);
    }

    SOCK_DEBUG(sk, "Ack %u before %u:%u\n", ack, tp->snd_una, tp->snd_nxt);
    return 0;
}

/* Look for tcp options. Normally only called on SYN and SYNACK packets.
 * But, this can also be called on packets in the established flow when
 * the fast version below fails.
 */
void tcp_parse_options(const struct sk_buff *skb, struct tcp_options_received *opt_rx,
               const u8 **hvpp, int estab,
               struct tcp_fastopen_cookie *foc)
{
    const unsigned char *ptr;
    const struct tcphdr *th = tcp_hdr(skb);
    int length = (th->doff * 4) - sizeof(struct tcphdr);

    ptr = (const unsigned char *)(th + 1);
    opt_rx->saw_tstamp = 0;

    while (length > 0) {
        int opcode = *ptr++;
        int opsize;

        switch (opcode) {
        case TCPOPT_EOL:
            return;
        case TCPOPT_NOP:    /* Ref: RFC 793 section 3.1 */
            length--;
            continue;
        default:
            opsize = *ptr++;
            if (opsize < 2) /* "silly options" */
                return;
            if (opsize > length)
                return; /* don't parse partial options */
            switch (opcode) {
            case TCPOPT_MSS:
                if (opsize == TCPOLEN_MSS && th->syn && !estab) {
                    u16 in_mss = get_unaligned_be16(ptr);
                    if (in_mss) {
                        if (opt_rx->user_mss &&
                            opt_rx->user_mss < in_mss)
                            in_mss = opt_rx->user_mss;
                        opt_rx->mss_clamp = in_mss;
                    }
                }
                break;
            case TCPOPT_WINDOW:
                if (opsize == TCPOLEN_WINDOW && th->syn &&
                    !estab && sysctl_tcp_window_scaling) {
                    __u8 snd_wscale = *(__u8 *)ptr;
                    opt_rx->wscale_ok = 1;
                    if (snd_wscale > 14) {
                        net_info_ratelimited("%s: Illegal window scaling value %d >14 received\n",
                                     __func__,
                                     snd_wscale);
                        snd_wscale = 14;
                    }
                    opt_rx->snd_wscale = snd_wscale;
                }
                break;
            case TCPOPT_TIMESTAMP:
                if ((opsize == TCPOLEN_TIMESTAMP) &&
                    ((estab && opt_rx->tstamp_ok) ||
                     (!estab && sysctl_tcp_timestamps))) {
                    opt_rx->saw_tstamp = 1;
                    opt_rx->rcv_tsval = get_unaligned_be32(ptr);
                    opt_rx->rcv_tsecr = get_unaligned_be32(ptr + 4);
                }
                break;
            case TCPOPT_SACK_PERM:
                if (opsize == TCPOLEN_SACK_PERM && th->syn &&
                    !estab && sysctl_tcp_sack) {
                    opt_rx->sack_ok = TCP_SACK_SEEN;
                    tcp_sack_reset(opt_rx);
                }
                break;

            case TCPOPT_SACK:
                if ((opsize >= (TCPOLEN_SACK_BASE + TCPOLEN_SACK_PERBLOCK)) &&
                   !((opsize - TCPOLEN_SACK_BASE) % TCPOLEN_SACK_PERBLOCK) &&
                   opt_rx->sack_ok) {
                    TCP_SKB_CB(skb)->sacked = (ptr - 2) - (unsigned char *)th;
                }
                break;
#ifdef CONFIG_TCP_MD5SIG
            case TCPOPT_MD5SIG:
                /*
                 * The MD5 Hash has already been
                 * checked (see tcp_v{4,6}_do_rcv()).
                 */
                break;
#endif
            case TCPOPT_COOKIE:
                /* This option is variable length.
                 */
                switch (opsize) {
                case TCPOLEN_COOKIE_BASE:
                    /* not yet implemented */
                    break;
                case TCPOLEN_COOKIE_PAIR:
                    /* not yet implemented */
                    break;
                case TCPOLEN_COOKIE_MIN+0:
                case TCPOLEN_COOKIE_MIN+2:
                case TCPOLEN_COOKIE_MIN+4:
                case TCPOLEN_COOKIE_MIN+6:
                case TCPOLEN_COOKIE_MAX:
                    /* 16-bit multiple */
                    opt_rx->cookie_plus = opsize;
                    *hvpp = ptr;
                    break;
                default:
                    /* ignore option */
                    break;
                }
                break;

            case TCPOPT_EXP:
                /* Fast Open option shares code 254 using a
                 * 16 bits magic number. It's valid only in
                 * SYN or SYN-ACK with an even size.
                 */
                if (opsize < TCPOLEN_EXP_FASTOPEN_BASE ||
                    get_unaligned_be16(ptr) != TCPOPT_FASTOPEN_MAGIC ||
                    foc == NULL || !th->syn || (opsize & 1))
                    break;
                foc->len = opsize - TCPOLEN_EXP_FASTOPEN_BASE;
                if (foc->len >= TCP_FASTOPEN_COOKIE_MIN &&
                    foc->len <= TCP_FASTOPEN_COOKIE_MAX)
                    memcpy(foc->val, ptr + 2, foc->len);
                else if (foc->len != 0)
                    foc->len = -1;
                break;

            }
            ptr += opsize-2;
            length -= opsize;
        }
    }
}
EXPORT_SYMBOL(tcp_parse_options);

static bool tcp_parse_aligned_timestamp(struct tcp_sock *tp, const struct tcphdr *th)
{
    const __be32 *ptr = (const __be32 *)(th + 1);

    if (*ptr == htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16)
              | (TCPOPT_TIMESTAMP << 8) | TCPOLEN_TIMESTAMP)) {
        tp->rx_opt.saw_tstamp = 1;
        ++ptr;
        tp->rx_opt.rcv_tsval = ntohl(*ptr);
        ++ptr;
        tp->rx_opt.rcv_tsecr = ntohl(*ptr);
        return true;
    }
    return false;
}

/* Fast parse options. This hopes to only see timestamps.
 * If it is wrong it falls back on tcp_parse_options().
 */
static bool tcp_fast_parse_options(const struct sk_buff *skb,
                   const struct tcphdr *th,
                   struct tcp_sock *tp, const u8 **hvpp)
{
    /* In the spirit of fast parsing, compare doff directly to constant
     * values.  Because equality is used, short doff can be ignored here.
     */
    if (th->doff == (sizeof(*th) / 4)) {
        tp->rx_opt.saw_tstamp = 0;
        return false;
    } else if (tp->rx_opt.tstamp_ok &&
           th->doff == ((sizeof(*th) + TCPOLEN_TSTAMP_ALIGNED) / 4)) {
        if (tcp_parse_aligned_timestamp(tp, th))
            return true;
    }
    tcp_parse_options(skb, &tp->rx_opt, hvpp, 1, NULL);
    return true;
}

#ifdef CONFIG_TCP_MD5SIG
/*
 * Parse MD5 Signature option
 */
const u8 *tcp_parse_md5sig_option(const struct tcphdr *th)
{
    int length = (th->doff << 2) - sizeof(*th);
    const u8 *ptr = (const u8 *)(th + 1);

    /* If the TCP option is too short, we can short cut */
    if (length < TCPOLEN_MD5SIG)
        return NULL;

    while (length > 0) {
        int opcode = *ptr++;
        int opsize;

        switch(opcode) {
        case TCPOPT_EOL:
            return NULL;
        case TCPOPT_NOP:
            length--;
            continue;
        default:
            opsize = *ptr++;
            if (opsize < 2 || opsize > length)
                return NULL;
            if (opcode == TCPOPT_MD5SIG)
                return opsize == TCPOLEN_MD5SIG ? ptr : NULL;
        }
        ptr += opsize - 2;
        length -= opsize;
    }
    return NULL;
}
EXPORT_SYMBOL(tcp_parse_md5sig_option);
#endif

static inline void tcp_store_ts_recent(struct tcp_sock *tp)
{
    tp->rx_opt.ts_recent = tp->rx_opt.rcv_tsval;
    tp->rx_opt.ts_recent_stamp = get_seconds();
}

static inline void tcp_replace_ts_recent(struct tcp_sock *tp, u32 seq)
{
    if (tp->rx_opt.saw_tstamp && !after(seq, tp->rcv_wup)) {
        /* PAWS bug workaround wrt. ACK frames, the PAWS discard
         * extra check below makes sure this can only happen
         * for pure ACK frames.  -DaveM
         *
         * Not only, also it occurs for expired timestamps.
         */

        if (tcp_paws_check(&tp->rx_opt, 0))
            tcp_store_ts_recent(tp);
    }
}

/* Sorry, PAWS as specified is broken wrt. pure-ACKs -DaveM
 *
 * It is not fatal. If this ACK does _not_ change critical state (seqs, window)
 * it can pass through stack. So, the following predicate verifies that
 * this segment is not used for anything but congestion avoidance or
 * fast retransmit. Moreover, we even are able to eliminate most of such
 * second order effects, if we apply some small "replay" window (~RTO)
 * to timestamp space.
 *
 * All these measures still do not guarantee that we reject wrapped ACKs
 * on networks with high bandwidth, when sequence space is recycled fastly,
 * but it guarantees that such events will be very rare and do not affect
 * connection seriously. This doesn't look nice, but alas, PAWS is really
 * buggy extension.
 *
 * [ Later note. Even worse! It is buggy for segments _with_ data. RFC
 * states that events when retransmit arrives after original data are rare.
 * It is a blatant lie. VJ forgot about fast retransmit! 8)8) It is
 * the biggest problem on large power networks even with minor reordering.
 * OK, let's give it small replay window. If peer clock is even 1hz, it is safe
 * up to bandwidth of 18Gigabit/sec. 8) ]
 */

static int tcp_disordered_ack(const struct sock *sk, const struct sk_buff *skb)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    const struct tcphdr *th = tcp_hdr(skb);
    u32 seq = TCP_SKB_CB(skb)->seq;
    u32 ack = TCP_SKB_CB(skb)->ack_seq;

    return (/* 1. Pure ACK with correct sequence number. */
        (th->ack && seq == TCP_SKB_CB(skb)->end_seq && seq == tp->rcv_nxt) &&

        /* 2. ... and duplicate ACK. */
        ack == tp->snd_una &&

        /* 3. ... and does not update window. */
        !tcp_may_update_window(tp, ack, seq, ntohs(th->window) << tp->rx_opt.snd_wscale) &&

        /* 4. ... and sits in replay window. */
        (s32)(tp->rx_opt.ts_recent - tp->rx_opt.rcv_tsval) <= (inet_csk(sk)->icsk_rto * 1024) / HZ);
}

static inline bool tcp_paws_discard(const struct sock *sk,
                   const struct sk_buff *skb)
{
    const struct tcp_sock *tp = tcp_sk(sk);

    return !tcp_paws_check(&tp->rx_opt, TCP_PAWS_WINDOW) &&
           !tcp_disordered_ack(sk, skb);
}

/* Check segment sequence number for validity.
 *
 * Segment controls are considered valid, if the segment
 * fits to the window after truncation to the window. Acceptability
 * of data (and SYN, FIN, of course) is checked separately.
 * See tcp_data_queue(), for example.
 *
 * Also, controls (RST is main one) are accepted using RCV.WUP instead
 * of RCV.NXT. Peer still did not advance his SND.UNA when we
 * delayed ACK, so that hisSND.UNA<=ourRCV.WUP.
 * (borrowed from freebsd)
 */

static inline bool tcp_sequence(const struct tcp_sock *tp, u32 seq, u32 end_seq)
{
    return  !before(end_seq, tp->rcv_wup) &&
        !after(seq, tp->rcv_nxt + tcp_receive_window(tp));
}

/* When we get a reset we do this. */
void tcp_reset(struct sock *sk)
{
    /* We want the right error as BSD sees it (and indeed as we do). */
    switch (sk->sk_state) {
    case TCP_SYN_SENT:
        sk->sk_err = ECONNREFUSED;
        break;
    case TCP_CLOSE_WAIT:
        sk->sk_err = EPIPE;
        break;
    case TCP_CLOSE:
        return;
    default:
        sk->sk_err = ECONNRESET;
    }
    /* This barrier is coupled with smp_rmb() in tcp_poll() */
    smp_wmb();

    if (!sock_flag(sk, SOCK_DEAD))
        sk->sk_error_report(sk);

    tcp_done(sk);
}

/*
 *  Process the FIN bit. This now behaves as it is supposed to work
 *  and the FIN takes effect when it is validly part of sequence
 *  space. Not before when we get holes.
 *
 *  If we are ESTABLISHED, a received fin moves us to CLOSE-WAIT
 *  (and thence onto LAST-ACK and finally, CLOSE, we never enter
 *  TIME-WAIT)
 *
 *  If we are in FINWAIT-1, a received FIN indicates simultaneous
 *  close and we go into CLOSING (and later onto TIME-WAIT)
 *
 *  If we are in FINWAIT-2, a received FIN moves us to TIME-WAIT.
 */
static void tcp_fin(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);

    inet_csk_schedule_ack(sk);

    sk->sk_shutdown |= RCV_SHUTDOWN;
    sock_set_flag(sk, SOCK_DONE);

    switch (sk->sk_state) {
    case TCP_SYN_RECV:
    case TCP_ESTABLISHED:
        /* Move to CLOSE_WAIT */
        tcp_set_state(sk, TCP_CLOSE_WAIT);
        inet_csk(sk)->icsk_ack.pingpong = 1;
        break;

    case TCP_CLOSE_WAIT:
    case TCP_CLOSING:
        /* Received a retransmission of the FIN, do
         * nothing.
         */
        break;
    case TCP_LAST_ACK:
        /* RFC793: Remain in the LAST-ACK state. */
        break;

    case TCP_FIN_WAIT1:
        /* This case occurs when a simultaneous close
         * happens, we must ack the received FIN and
         * enter the CLOSING state.
         */
        tcp_send_ack(sk);
        tcp_set_state(sk, TCP_CLOSING);
        break;
    case TCP_FIN_WAIT2:
        /* Received a FIN -- send ACK and enter TIME_WAIT. */
        tcp_send_ack(sk);
        tcp_time_wait(sk, TCP_TIME_WAIT, 0);
        break;
    default:
        /* Only TCP_LISTEN and TCP_CLOSE are left, in these
         * cases we should never reach this piece of code.
         */
        pr_err("%s: Impossible, sk->sk_state=%d\n",
               __func__, sk->sk_state);
        break;
    }

    /* It _is_ possible, that we have something out-of-order _after_ FIN.
     * Probably, we should reset in this case. For now drop them.
     */
    __skb_queue_purge(&tp->out_of_order_queue);
    if (tcp_is_sack(tp))
        tcp_sack_reset(&tp->rx_opt);
    sk_mem_reclaim(sk);

    if (!sock_flag(sk, SOCK_DEAD)) {
        sk->sk_state_change(sk);

        /* Do not send POLL_HUP for half duplex close. */
        if (sk->sk_shutdown == SHUTDOWN_MASK ||
            sk->sk_state == TCP_CLOSE)
            sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_HUP);
        else
            sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_IN);
    }
}

static inline bool tcp_sack_extend(struct tcp_sack_block *sp, u32 seq,
                  u32 end_seq)
{
    if (!after(seq, sp->end_seq) && !after(sp->start_seq, end_seq)) {
        if (before(seq, sp->start_seq))
            sp->start_seq = seq;
        if (after(end_seq, sp->end_seq))
            sp->end_seq = end_seq;
        return true;
    }
    return false;
}

static void tcp_dsack_set(struct sock *sk, u32 seq, u32 end_seq)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (tcp_is_sack(tp) && sysctl_tcp_dsack) {
        int mib_idx;

        if (before(seq, tp->rcv_nxt))
            mib_idx = LINUX_MIB_TCPDSACKOLDSENT;
        else
            mib_idx = LINUX_MIB_TCPDSACKOFOSENT;

        NET_INC_STATS_BH(sock_net(sk), mib_idx);

        tp->rx_opt.dsack = 1;
        tp->duplicate_sack[0].start_seq = seq;
        tp->duplicate_sack[0].end_seq = end_seq;
    }
}

static void tcp_dsack_extend(struct sock *sk, u32 seq, u32 end_seq)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (!tp->rx_opt.dsack)
        tcp_dsack_set(sk, seq, end_seq);
    else
        tcp_sack_extend(tp->duplicate_sack, seq, end_seq);
}

static void tcp_send_dupack(struct sock *sk, const struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
        before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
        tcp_enter_quickack_mode(sk);

        if (tcp_is_sack(tp) && sysctl_tcp_dsack) {
            u32 end_seq = TCP_SKB_CB(skb)->end_seq;

            if (after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt))
                end_seq = tp->rcv_nxt;
            tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, end_seq);
        }
    }

    tcp_send_ack(sk);
}

/* These routines update the SACK block as out-of-order packets arrive or
 * in-order packets close up the sequence space.
 */
static void tcp_sack_maybe_coalesce(struct tcp_sock *tp)
{
    int this_sack;
    struct tcp_sack_block *sp = &tp->selective_acks[0];
    struct tcp_sack_block *swalk = sp + 1;

    /* See if the recent change to the first SACK eats into
     * or hits the sequence space of other SACK blocks, if so coalesce.
     */
    for (this_sack = 1; this_sack < tp->rx_opt.num_sacks;) {
        if (tcp_sack_extend(sp, swalk->start_seq, swalk->end_seq)) {
            int i;

            /* Zap SWALK, by moving every further SACK up by one slot.
             * Decrease num_sacks.
             */
            tp->rx_opt.num_sacks--;
            for (i = this_sack; i < tp->rx_opt.num_sacks; i++)
                sp[i] = sp[i + 1];
            continue;
        }
        this_sack++, swalk++;
    }
}

static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct tcp_sack_block *sp = &tp->selective_acks[0];
    int cur_sacks = tp->rx_opt.num_sacks;
    int this_sack;

    if (!cur_sacks)
        goto new_sack;

    for (this_sack = 0; this_sack < cur_sacks; this_sack++, sp++) {
        if (tcp_sack_extend(sp, seq, end_seq)) {
            /* Rotate this_sack to the first one. */
            for (; this_sack > 0; this_sack--, sp--)
                swap(*sp, *(sp - 1));
            if (cur_sacks > 1)
                tcp_sack_maybe_coalesce(tp);
            return;
        }
    }

    /* Could not find an adjacent existing SACK, build a new one,
     * put it at the front, and shift everyone else down.  We
     * always know there is at least one SACK present already here.
     *
     * If the sack array is full, forget about the last one.
     */
    if (this_sack >= TCP_NUM_SACKS) {
        this_sack--;
        tp->rx_opt.num_sacks--;
        sp--;
    }
    for (; this_sack > 0; this_sack--, sp--)
        *sp = *(sp - 1);

new_sack:
    /* Build the new head SACK, and we're done. */
    sp->start_seq = seq;
    sp->end_seq = end_seq;
    tp->rx_opt.num_sacks++;
}

/* RCV.NXT advances, some SACKs should be eaten. */

static void tcp_sack_remove(struct tcp_sock *tp)
{
    struct tcp_sack_block *sp = &tp->selective_acks[0];
    int num_sacks = tp->rx_opt.num_sacks;
    int this_sack;

    /* Empty ofo queue, hence, all the SACKs are eaten. Clear. */
    if (skb_queue_empty(&tp->out_of_order_queue)) {
        tp->rx_opt.num_sacks = 0;
        return;
    }

    for (this_sack = 0; this_sack < num_sacks;) {
        /* Check if the start of the sack is covered by RCV.NXT. */
        if (!before(tp->rcv_nxt, sp->start_seq)) {
            int i;

            /* RCV.NXT must cover all the block! */
            WARN_ON(before(tp->rcv_nxt, sp->end_seq));

            /* Zap this SACK, by moving forward any other SACKS. */
            for (i=this_sack+1; i < num_sacks; i++)
                tp->selective_acks[i-1] = tp->selective_acks[i];
            num_sacks--;
            continue;
        }
        this_sack++;
        sp++;
    }
    tp->rx_opt.num_sacks = num_sacks;
}

/* This one checks to see if we can put data from the
 * out_of_order queue into the receive_queue.
 */
static void tcp_ofo_queue(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    __u32 dsack_high = tp->rcv_nxt;
    struct sk_buff *skb;

    while ((skb = skb_peek(&tp->out_of_order_queue)) != NULL) {
        if (after(TCP_SKB_CB(skb)->seq, tp->rcv_nxt))
            break;

        if (before(TCP_SKB_CB(skb)->seq, dsack_high)) {
            __u32 dsack = dsack_high;
            if (before(TCP_SKB_CB(skb)->end_seq, dsack_high))
                dsack_high = TCP_SKB_CB(skb)->end_seq;
            tcp_dsack_extend(sk, TCP_SKB_CB(skb)->seq, dsack);
        }

        if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
            SOCK_DEBUG(sk, "ofo packet was already received\n");
            __skb_unlink(skb, &tp->out_of_order_queue);
            __kfree_skb(skb);
            continue;
        }
        SOCK_DEBUG(sk, "ofo requeuing : rcv_next %X seq %X - %X\n",
               tp->rcv_nxt, TCP_SKB_CB(skb)->seq,
               TCP_SKB_CB(skb)->end_seq);

        __skb_unlink(skb, &tp->out_of_order_queue);
        __skb_queue_tail(&sk->sk_receive_queue, skb);
        tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
        if (tcp_hdr(skb)->fin)
            tcp_fin(sk);
    }
}

static bool tcp_prune_ofo_queue(struct sock *sk);
static int tcp_prune_queue(struct sock *sk);

static int tcp_try_rmem_schedule(struct sock *sk, struct sk_buff *skb,
                 unsigned int size)
{
    if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf ||
        !sk_rmem_schedule(sk, skb, size)) {

        if (tcp_prune_queue(sk) < 0)
            return -1;

        if (!sk_rmem_schedule(sk, skb, size)) {
            if (!tcp_prune_ofo_queue(sk))
                return -1;

            if (!sk_rmem_schedule(sk, skb, size))
                return -1;
        }
    }
    return 0;
}

static bool tcp_try_coalesce(struct sock *sk,
                 struct sk_buff *to,
                 struct sk_buff *from,
                 bool *fragstolen)
{
    int delta;

    *fragstolen = false;

    if (tcp_hdr(from)->fin)
        return false;

    /* Its possible this segment overlaps with prior segment in queue */
    if (TCP_SKB_CB(from)->seq != TCP_SKB_CB(to)->end_seq)
        return false;

    if (!skb_try_coalesce(to, from, fragstolen, &delta))
        return false;

    atomic_add(delta, &sk->sk_rmem_alloc);
    sk_mem_charge(sk, delta);
    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRCVCOALESCE);
    TCP_SKB_CB(to)->end_seq = TCP_SKB_CB(from)->end_seq;
    TCP_SKB_CB(to)->ack_seq = TCP_SKB_CB(from)->ack_seq;
    return true;
}

static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb1;
    u32 seq, end_seq;

    TCP_ECN_check_ce(tp, skb);

    if (unlikely(tcp_try_rmem_schedule(sk, skb, skb->truesize))) {
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPOFODROP);
        __kfree_skb(skb);
        return;
    }

    /* Disable header prediction. */
    tp->pred_flags = 0;
    inet_csk_schedule_ack(sk);

    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPOFOQUEUE);
    SOCK_DEBUG(sk, "out of order segment: rcv_next %X seq %X - %X\n",
           tp->rcv_nxt, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);

    skb1 = skb_peek_tail(&tp->out_of_order_queue);
    if (!skb1) {
        /* Initial out of order segment, build 1 SACK. */
        if (tcp_is_sack(tp)) {
            tp->rx_opt.num_sacks = 1;
            tp->selective_acks[0].start_seq = TCP_SKB_CB(skb)->seq;
            tp->selective_acks[0].end_seq =
                        TCP_SKB_CB(skb)->end_seq;
        }
        __skb_queue_head(&tp->out_of_order_queue, skb);
        goto end;
    }

    seq = TCP_SKB_CB(skb)->seq;
    end_seq = TCP_SKB_CB(skb)->end_seq;

    if (seq == TCP_SKB_CB(skb1)->end_seq) {
        bool fragstolen;

        if (!tcp_try_coalesce(sk, skb1, skb, &fragstolen)) {
            __skb_queue_after(&tp->out_of_order_queue, skb1, skb);
        } else {
            kfree_skb_partial(skb, fragstolen);
            skb = NULL;
        }

        if (!tp->rx_opt.num_sacks ||
            tp->selective_acks[0].end_seq != seq)
            goto add_sack;

        /* Common case: data arrive in order after hole. */
        tp->selective_acks[0].end_seq = end_seq;
        goto end;
    }

    /* Find place to insert this segment. */
    while (1) {
        if (!after(TCP_SKB_CB(skb1)->seq, seq))
            break;
        if (skb_queue_is_first(&tp->out_of_order_queue, skb1)) {
            skb1 = NULL;
            break;
        }
        skb1 = skb_queue_prev(&tp->out_of_order_queue, skb1);
    }

    /* Do skb overlap to previous one? */
    if (skb1 && before(seq, TCP_SKB_CB(skb1)->end_seq)) {
        if (!after(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
            /* All the bits are present. Drop. */
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPOFOMERGE);
            __kfree_skb(skb);
            skb = NULL;
            tcp_dsack_set(sk, seq, end_seq);
            goto add_sack;
        }
        if (after(seq, TCP_SKB_CB(skb1)->seq)) {
            /* Partial overlap. */
            tcp_dsack_set(sk, seq,
                      TCP_SKB_CB(skb1)->end_seq);
        } else {
            if (skb_queue_is_first(&tp->out_of_order_queue,
                           skb1))
                skb1 = NULL;
            else
                skb1 = skb_queue_prev(
                    &tp->out_of_order_queue,
                    skb1);
        }
    }
    if (!skb1)
        __skb_queue_head(&tp->out_of_order_queue, skb);
    else
        __skb_queue_after(&tp->out_of_order_queue, skb1, skb);

    /* And clean segments covered by new one as whole. */
    while (!skb_queue_is_last(&tp->out_of_order_queue, skb)) {
        skb1 = skb_queue_next(&tp->out_of_order_queue, skb);

        if (!after(end_seq, TCP_SKB_CB(skb1)->seq))
            break;
        if (before(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
            tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
                     end_seq);
            break;
        }
        __skb_unlink(skb1, &tp->out_of_order_queue);
        tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
                 TCP_SKB_CB(skb1)->end_seq);
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPOFOMERGE);
        __kfree_skb(skb1);
    }

add_sack:
    if (tcp_is_sack(tp))
        tcp_sack_new_ofo_skb(sk, seq, end_seq);
end:
    if (skb)
        skb_set_owner_r(skb, sk);
}

static int __must_check tcp_queue_rcv(struct sock *sk, struct sk_buff *skb, int hdrlen,
          bool *fragstolen)
{
    int eaten;
    struct sk_buff *tail = skb_peek_tail(&sk->sk_receive_queue);

    __skb_pull(skb, hdrlen);
    eaten = (tail &&
         tcp_try_coalesce(sk, tail, skb, fragstolen)) ? 1 : 0;
    tcp_sk(sk)->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
    if (!eaten) {
        __skb_queue_tail(&sk->sk_receive_queue, skb);
        skb_set_owner_r(skb, sk);
    }
    return eaten;
}

int tcp_send_rcvq(struct sock *sk, struct msghdr *msg, size_t size)
{
    struct sk_buff *skb = NULL;
    struct tcphdr *th;
    bool fragstolen;

    if (size == 0)
        return 0;

    skb = alloc_skb(size + sizeof(*th), sk->sk_allocation);
    if (!skb)
        goto err;

    if (tcp_try_rmem_schedule(sk, skb, size + sizeof(*th)))
        goto err_free;

    th = (struct tcphdr *)skb_put(skb, sizeof(*th));
    skb_reset_transport_header(skb);
    memset(th, 0, sizeof(*th));

    if (memcpy_fromiovec(skb_put(skb, size), msg->msg_iov, size))
        goto err_free;

    TCP_SKB_CB(skb)->seq = tcp_sk(sk)->rcv_nxt;
    TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq + size;
    TCP_SKB_CB(skb)->ack_seq = tcp_sk(sk)->snd_una - 1;

    if (tcp_queue_rcv(sk, skb, sizeof(*th), &fragstolen)) {
        WARN_ON_ONCE(fragstolen); /* should not happen */
        __kfree_skb(skb);
    }
    return size;

err_free:
    kfree_skb(skb);
err:
    return -ENOMEM;
}

static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
{
    const struct tcphdr *th = tcp_hdr(skb);
    struct tcp_sock *tp = tcp_sk(sk);
    int eaten = -1;
    bool fragstolen = false;

    if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq)
        goto drop;

    skb_dst_drop(skb);
    __skb_pull(skb, th->doff * 4);

    TCP_ECN_accept_cwr(tp, skb);

    tp->rx_opt.dsack = 0;

    /*  Queue data for delivery to the user.
     *  Packets in sequence go to the receive queue.
     *  Out of sequence packets to the out_of_order_queue.
     */
    if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt) {
        if (tcp_receive_window(tp) == 0)
            goto out_of_window;

        /* Ok. In sequence. In window. */
        if (tp->ucopy.task == current &&
            tp->copied_seq == tp->rcv_nxt && tp->ucopy.len &&
            sock_owned_by_user(sk) && !tp->urg_data) {
            int chunk = min_t(unsigned int, skb->len,
                      tp->ucopy.len);

            __set_current_state(TASK_RUNNING);

            local_bh_enable();
            if (!skb_copy_datagram_iovec(skb, 0, tp->ucopy.iov, chunk)) {
                tp->ucopy.len -= chunk;
                tp->copied_seq += chunk;
                eaten = (chunk == skb->len);
                tcp_rcv_space_adjust(sk);
            }
            local_bh_disable();
        }

        if (eaten <= 0) {
queue_and_out:
            if (eaten < 0 &&
                tcp_try_rmem_schedule(sk, skb, skb->truesize))
                goto drop;

            eaten = tcp_queue_rcv(sk, skb, 0, &fragstolen);
        }
        tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
        if (skb->len)
            tcp_event_data_recv(sk, skb);
        if (th->fin)
            tcp_fin(sk);

        if (!skb_queue_empty(&tp->out_of_order_queue)) {
            tcp_ofo_queue(sk);

            /* RFC2581. 4.2. SHOULD send immediate ACK, when
             * gap in queue is filled.
             */
            if (skb_queue_empty(&tp->out_of_order_queue))
                inet_csk(sk)->icsk_ack.pingpong = 0;
        }

        if (tp->rx_opt.num_sacks)
            tcp_sack_remove(tp);

        tcp_fast_path_check(sk);

        if (eaten > 0)
            kfree_skb_partial(skb, fragstolen);
        if (!sock_flag(sk, SOCK_DEAD))
            sk->sk_data_ready(sk, 0);
        return;
    }

    if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
        /* A retransmit, 2nd most common case.  Force an immediate ack. */
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
        tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);

out_of_window:
        tcp_enter_quickack_mode(sk);
        inet_csk_schedule_ack(sk);
drop:
        __kfree_skb(skb);
        return;
    }

    /* Out of window. F.e. zero window probe. */
    if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt + tcp_receive_window(tp)))
        goto out_of_window;

    tcp_enter_quickack_mode(sk);

    if (before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
        /* Partial packet, seq < rcv_next < end_seq */
        SOCK_DEBUG(sk, "partial packet: rcv_next %X seq %X - %X\n",
               tp->rcv_nxt, TCP_SKB_CB(skb)->seq,
               TCP_SKB_CB(skb)->end_seq);

        tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, tp->rcv_nxt);

        /* If window is closed, drop tail of packet. But after
         * remembering D-SACK for its head made in previous line.
         */
        if (!tcp_receive_window(tp))
            goto out_of_window;
        goto queue_and_out;
    }

    tcp_data_queue_ofo(sk, skb);
}

static struct sk_buff *tcp_collapse_one(struct sock *sk, struct sk_buff *skb,
                    struct sk_buff_head *list)
{
    struct sk_buff *next = NULL;

    if (!skb_queue_is_last(list, skb))
        next = skb_queue_next(list, skb);

    __skb_unlink(skb, list);
    __kfree_skb(skb);
    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRCVCOLLAPSED);

    return next;
}

/* Collapse contiguous sequence of skbs head..tail with
 * sequence numbers start..end.
 *
 * If tail is NULL, this means until the end of the list.
 *
 * Segments with FIN/SYN are not collapsed (only because this
 * simplifies code)
 */
static void
tcp_collapse(struct sock *sk, struct sk_buff_head *list,
         struct sk_buff *head, struct sk_buff *tail,
         u32 start, u32 end)
{
    struct sk_buff *skb, *n;
    bool end_of_skbs;

    /* First, check that queue is collapsible and find
     * the point where collapsing can be useful. */
    skb = head;
restart:
    end_of_skbs = true;
    skb_queue_walk_from_safe(list, skb, n) {
        if (skb == tail)
            break;
        /* No new bits? It is possible on ofo queue. */
        if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
            skb = tcp_collapse_one(sk, skb, list);
            if (!skb)
                break;
            goto restart;
        }

        /* The first skb to collapse is:
         * - not SYN/FIN and
         * - bloated or contains data before "start" or
         *   overlaps to the next one.
         */
        if (!tcp_hdr(skb)->syn && !tcp_hdr(skb)->fin &&
            (tcp_win_from_space(skb->truesize) > skb->len ||
             before(TCP_SKB_CB(skb)->seq, start))) {
            end_of_skbs = false;
            break;
        }

        if (!skb_queue_is_last(list, skb)) {
            struct sk_buff *next = skb_queue_next(list, skb);
            if (next != tail &&
                TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(next)->seq) {
                end_of_skbs = false;
                break;
            }
        }

        /* Decided to skip this, advance start seq. */
        start = TCP_SKB_CB(skb)->end_seq;
    }
    if (end_of_skbs || tcp_hdr(skb)->syn || tcp_hdr(skb)->fin)
        return;

    while (before(start, end)) {
        struct sk_buff *nskb;
        unsigned int header = skb_headroom(skb);
        int copy = SKB_MAX_ORDER(header, 0);

        /* Too big header? This can happen with IPv6. */
        if (copy < 0)
            return;
        if (end - start < copy)
            copy = end - start;
        nskb = alloc_skb(copy + header, GFP_ATOMIC);
        if (!nskb)
            return;

        skb_set_mac_header(nskb, skb_mac_header(skb) - skb->head);
        skb_set_network_header(nskb, (skb_network_header(skb) -
                          skb->head));
        skb_set_transport_header(nskb, (skb_transport_header(skb) -
                        skb->head));
        skb_reserve(nskb, header);
        memcpy(nskb->head, skb->head, header);
        memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
        TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(nskb)->end_seq = start;
        __skb_queue_before(list, skb, nskb);
        skb_set_owner_r(nskb, sk);

        /* Copy data, releasing collapsed skbs. */
        while (copy > 0) {
            int offset = start - TCP_SKB_CB(skb)->seq;
            int size = TCP_SKB_CB(skb)->end_seq - start;

            BUG_ON(offset < 0);
            if (size > 0) {
                size = min(copy, size);
                if (skb_copy_bits(skb, offset, skb_put(nskb, size), size))
                    BUG();
                TCP_SKB_CB(nskb)->end_seq += size;
                copy -= size;
                start += size;
            }
            if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
                skb = tcp_collapse_one(sk, skb, list);
                if (!skb ||
                    skb == tail ||
                    tcp_hdr(skb)->syn ||
                    tcp_hdr(skb)->fin)
                    return;
            }
        }
    }
}

/* Collapse ofo queue. Algorithm: select contiguous sequence of skbs
 * and tcp_collapse() them until all the queue is collapsed.
 */
static void tcp_collapse_ofo_queue(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb = skb_peek(&tp->out_of_order_queue);
    struct sk_buff *head;
    u32 start, end;

    if (skb == NULL)
        return;

    start = TCP_SKB_CB(skb)->seq;
    end = TCP_SKB_CB(skb)->end_seq;
    head = skb;

    for (;;) {
        struct sk_buff *next = NULL;

        if (!skb_queue_is_last(&tp->out_of_order_queue, skb))
            next = skb_queue_next(&tp->out_of_order_queue, skb);
        skb = next;

        /* Segment is terminated when we see gap or when
         * we are at the end of all the queue. */
        if (!skb ||
            after(TCP_SKB_CB(skb)->seq, end) ||
            before(TCP_SKB_CB(skb)->end_seq, start)) {
            tcp_collapse(sk, &tp->out_of_order_queue,
                     head, skb, start, end);
            head = skb;
            if (!skb)
                break;
            /* Start new segment */
            start = TCP_SKB_CB(skb)->seq;
            end = TCP_SKB_CB(skb)->end_seq;
        } else {
            if (before(TCP_SKB_CB(skb)->seq, start))
                start = TCP_SKB_CB(skb)->seq;
            if (after(TCP_SKB_CB(skb)->end_seq, end))
                end = TCP_SKB_CB(skb)->end_seq;
        }
    }
}

/*
 * Purge the out-of-order queue.
 * Return true if queue was pruned.
 */
static bool tcp_prune_ofo_queue(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);
    bool res = false;

    if (!skb_queue_empty(&tp->out_of_order_queue)) {
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_OFOPRUNED);
        __skb_queue_purge(&tp->out_of_order_queue);

        /* Reset SACK state.  A conforming SACK implementation will
         * do the same at a timeout based retransmit.  When a connection
         * is in a sad state like this, we care only about integrity
         * of the connection not performance.
         */
        if (tp->rx_opt.sack_ok)
            tcp_sack_reset(&tp->rx_opt);
        sk_mem_reclaim(sk);
        res = true;
    }
    return res;
}

/* Reduce allocated memory if we can, trying to get
 * the socket within its memory limits again.
 *
 * Return less than zero if we should start dropping frames
 * until the socket owning process reads some of the data
 * to stabilize the situation.
 */
static int tcp_prune_queue(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);

    SOCK_DEBUG(sk, "prune_queue: c=%x\n", tp->copied_seq);

    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PRUNECALLED);

    if (atomic_read(&sk->sk_rmem_alloc) >= sk->sk_rcvbuf)
        tcp_clamp_window(sk);
    else if (sk_under_memory_pressure(sk))
        tp->rcv_ssthresh = min(tp->rcv_ssthresh, 4U * tp->advmss);

    tcp_collapse_ofo_queue(sk);
    if (!skb_queue_empty(&sk->sk_receive_queue))
        tcp_collapse(sk, &sk->sk_receive_queue,
                 skb_peek(&sk->sk_receive_queue),
                 NULL,
                 tp->copied_seq, tp->rcv_nxt);
    sk_mem_reclaim(sk);

    if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
        return 0;

    /* Collapsing did not help, destructive actions follow.
     * This must not ever occur. */

    tcp_prune_ofo_queue(sk);

    if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
        return 0;

    /* If we are really being abused, tell the caller to silently
     * drop receive data on the floor.  It will get retransmitted
     * and hopefully then we'll have sufficient space.
     */
    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_RCVPRUNED);

    /* Massive buffer overcommit. */
    tp->pred_flags = 0;
    return -1;
}

/* RFC2861, slow part. Adjust cwnd, after it was not full during one rto.
 * As additional protections, we do not touch cwnd in retransmission phases,
 * and if application hit its sndbuf limit recently.
 */
void tcp_cwnd_application_limited(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (inet_csk(sk)->icsk_ca_state == TCP_CA_Open &&
        sk->sk_socket && !test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
        /* Limited by application or receiver window. */
        u32 init_win = tcp_init_cwnd(tp, __sk_dst_get(sk));
        u32 win_used = max(tp->snd_cwnd_used, init_win);
        if (win_used < tp->snd_cwnd) {
            tp->snd_ssthresh = tcp_current_ssthresh(sk);
            tp->snd_cwnd = (tp->snd_cwnd + win_used) >> 1;
        }
        tp->snd_cwnd_used = 0;
    }
    tp->snd_cwnd_stamp = tcp_time_stamp;
}

static bool tcp_should_expand_sndbuf(const struct sock *sk)
{
    const struct tcp_sock *tp = tcp_sk(sk);

    /* If the user specified a specific send buffer setting, do
     * not modify it.
     */
    if (sk->sk_userlocks & SOCK_SNDBUF_LOCK)
        return false;

    /* If we are under global TCP memory pressure, do not expand.  */
    if (sk_under_memory_pressure(sk))
        return false;

    /* If we are under soft global TCP memory pressure, do not expand.  */
    if (sk_memory_allocated(sk) >= sk_prot_mem_limits(sk, 0))
        return false;

    /* If we filled the congestion window, do not expand.  */
    if (tp->packets_out >= tp->snd_cwnd)
        return false;

    return true;
}

/* When incoming ACK allowed to free some skb from write_queue,
 * we remember this event in flag SOCK_QUEUE_SHRUNK and wake up socket
 * on the exit from tcp input handler.
 *
 * PROBLEM: sndbuf expansion does not work well with largesend.
 */
static void tcp_new_space(struct sock *sk)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (tcp_should_expand_sndbuf(sk)) {
        int sndmem = SKB_TRUESIZE(max_t(u32,
                        tp->rx_opt.mss_clamp,
                        tp->mss_cache) +
                      MAX_TCP_HEADER);
        int demanded = max_t(unsigned int, tp->snd_cwnd,
                     tp->reordering + 1);
        sndmem *= 2 * demanded;
        if (sndmem > sk->sk_sndbuf)
            sk->sk_sndbuf = min(sndmem, sysctl_tcp_wmem[2]);
        tp->snd_cwnd_stamp = tcp_time_stamp;
    }

    sk->sk_write_space(sk);
}

static void tcp_check_space(struct sock *sk)
{
    if (sock_flag(sk, SOCK_QUEUE_SHRUNK)) {
        sock_reset_flag(sk, SOCK_QUEUE_SHRUNK);
        if (sk->sk_socket &&
            test_bit(SOCK_NOSPACE, &sk->sk_socket->flags))
            tcp_new_space(sk);
    }
}

static inline void tcp_data_snd_check(struct sock *sk)
{
    tcp_push_pending_frames(sk);
    tcp_check_space(sk);
}

/*
 * Check if sending an ack is needed.
 */
static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
{
    struct tcp_sock *tp = tcp_sk(sk);

        /* More than one full frame received... */
    if (((tp->rcv_nxt - tp->rcv_wup) > inet_csk(sk)->icsk_ack.rcv_mss &&
         /* ... and right edge of window advances far enough.
          * (tcp_recvmsg() will send ACK otherwise). Or...
          */
         __tcp_select_window(sk) >= tp->rcv_wnd) ||
        /* We ACK each frame or... */
        tcp_in_quickack_mode(sk) ||
        /* We have out of order data. */
        (ofo_possible && skb_peek(&tp->out_of_order_queue))) {
        /* Then ack it now */
        tcp_send_ack(sk);
    } else {
        /* Else, send delayed ack. */
        tcp_send_delayed_ack(sk);
    }
}

static inline void tcp_ack_snd_check(struct sock *sk)
{
    if (!inet_csk_ack_scheduled(sk)) {
        /* We sent a data segment already. */
        return;
    }
    __tcp_ack_snd_check(sk, 1);
}

/*
 *  This routine is only called when we have urgent data
 *  signaled. Its the 'slow' part of tcp_urg. It could be
 *  moved inline now as tcp_urg is only called from one
 *  place. We handle URGent data wrong. We have to - as
 *  BSD still doesn't use the correction from RFC961.
 *  For 1003.1g we should support a new option TCP_STDURG to permit
 *  either form (or just set the sysctl tcp_stdurg).
 */

static void tcp_check_urg(struct sock *sk, const struct tcphdr *th)
{
    struct tcp_sock *tp = tcp_sk(sk);
    u32 ptr = ntohs(th->urg_ptr);

    if (ptr && !sysctl_tcp_stdurg)
        ptr--;
    ptr += ntohl(th->seq);

    /* Ignore urgent data that we've already seen and read. */
    if (after(tp->copied_seq, ptr))
        return;

    /* Do not replay urg ptr.
     *
     * NOTE: interesting situation not covered by specs.
     * Misbehaving sender may send urg ptr, pointing to segment,
     * which we already have in ofo queue. We are not able to fetch
     * such data and will stay in TCP_URG_NOTYET until will be eaten
     * by recvmsg(). Seems, we are not obliged to handle such wicked
     * situations. But it is worth to think about possibility of some
     * DoSes using some hypothetical application level deadlock.
     */
    if (before(ptr, tp->rcv_nxt))
        return;

    /* Do we already have a newer (or duplicate) urgent pointer? */
    if (tp->urg_data && !after(ptr, tp->urg_seq))
        return;

    /* Tell the world about our new urgent pointer. */
    sk_send_sigurg(sk);

    /* We may be adding urgent data when the last byte read was
     * urgent. To do this requires some care. We cannot just ignore
     * tp->copied_seq since we would read the last urgent byte again
     * as data, nor can we alter copied_seq until this data arrives
     * or we break the semantics of SIOCATMARK (and thus sockatmark())
     *
     * NOTE. Double Dutch. Rendering to plain English: author of comment
     * above did something sort of  send("A", MSG_OOB); send("B", MSG_OOB);
     * and expect that both A and B disappear from stream. This is _wrong_.
     * Though this happens in BSD with high probability, this is occasional.
     * Any application relying on this is buggy. Note also, that fix "works"
     * only in this artificial test. Insert some normal data between A and B and we will
     * decline of BSD again. Verdict: it is better to remove to trap
     * buggy users.
     */
    if (tp->urg_seq == tp->copied_seq && tp->urg_data &&
        !sock_flag(sk, SOCK_URGINLINE) && tp->copied_seq != tp->rcv_nxt) {
        struct sk_buff *skb = skb_peek(&sk->sk_receive_queue);
        tp->copied_seq++;
        if (skb && !before(tp->copied_seq, TCP_SKB_CB(skb)->end_seq)) {
            __skb_unlink(skb, &sk->sk_receive_queue);
            __kfree_skb(skb);
        }
    }

    tp->urg_data = TCP_URG_NOTYET;
    tp->urg_seq = ptr;

    /* Disable header prediction. */
    tp->pred_flags = 0;
}

/* This is the 'fast' part of urgent handling. */
static void tcp_urg(struct sock *sk, struct sk_buff *skb, const struct tcphdr *th)
{
    struct tcp_sock *tp = tcp_sk(sk);

    /* Check if we get a new urgent pointer - normally not. */
    if (th->urg)
        tcp_check_urg(sk, th);

    /* Do we wait for any urgent data? - normally not... */
    if (tp->urg_data == TCP_URG_NOTYET) {
        u32 ptr = tp->urg_seq - ntohl(th->seq) + (th->doff * 4) -
              th->syn;

        /* Is the urgent pointer pointing into this packet? */
        if (ptr < skb->len) {
            u8 tmp;
            if (skb_copy_bits(skb, ptr, &tmp, 1))
                BUG();
            tp->urg_data = TCP_URG_VALID | tmp;
            if (!sock_flag(sk, SOCK_DEAD))
                sk->sk_data_ready(sk, 0);
        }
    }
}

static int tcp_copy_to_iovec(struct sock *sk, struct sk_buff *skb, int hlen)
{
    struct tcp_sock *tp = tcp_sk(sk);
    int chunk = skb->len - hlen;
    int err;

    local_bh_enable();
    if (skb_csum_unnecessary(skb))
        err = skb_copy_datagram_iovec(skb, hlen, tp->ucopy.iov, chunk);
    else
        err = skb_copy_and_csum_datagram_iovec(skb, hlen,
                               tp->ucopy.iov);

    if (!err) {
        tp->ucopy.len -= chunk;
        tp->copied_seq += chunk;
        tcp_rcv_space_adjust(sk);
    }

    local_bh_disable();
    return err;
}

static __sum16 __tcp_checksum_complete_user(struct sock *sk,
                        struct sk_buff *skb)
{
    __sum16 result;

    if (sock_owned_by_user(sk)) {
        local_bh_enable();
        result = __tcp_checksum_complete(skb);
        local_bh_disable();
    } else {
        result = __tcp_checksum_complete(skb);
    }
    return result;
}

static inline bool tcp_checksum_complete_user(struct sock *sk,
                         struct sk_buff *skb)
{
    return !skb_csum_unnecessary(skb) &&
           __tcp_checksum_complete_user(sk, skb);
}

#ifdef CONFIG_NET_DMA
static bool tcp_dma_try_early_copy(struct sock *sk, struct sk_buff *skb,
                  int hlen)
{
    struct tcp_sock *tp = tcp_sk(sk);
    int chunk = skb->len - hlen;
    int dma_cookie;
    bool copied_early = false;

    if (tp->ucopy.wakeup)
        return false;

    if (!tp->ucopy.dma_chan && tp->ucopy.pinned_list)
        tp->ucopy.dma_chan = net_dma_find_channel();

    if (tp->ucopy.dma_chan && skb_csum_unnecessary(skb)) {

        dma_cookie = dma_skb_copy_datagram_iovec(tp->ucopy.dma_chan,
                             skb, hlen,
                             tp->ucopy.iov, chunk,
                             tp->ucopy.pinned_list);

        if (dma_cookie < 0)
            goto out;

        tp->ucopy.dma_cookie = dma_cookie;
        copied_early = true;

        tp->ucopy.len -= chunk;
        tp->copied_seq += chunk;
        tcp_rcv_space_adjust(sk);

        if ((tp->ucopy.len == 0) ||
            (tcp_flag_word(tcp_hdr(skb)) & TCP_FLAG_PSH) ||
            (atomic_read(&sk->sk_rmem_alloc) > (sk->sk_rcvbuf >> 1))) {
            tp->ucopy.wakeup = 1;
            sk->sk_data_ready(sk, 0);
        }
    } else if (chunk > 0) {
        tp->ucopy.wakeup = 1;
        sk->sk_data_ready(sk, 0);
    }
out:
    return copied_early;
}
#endif /* CONFIG_NET_DMA */

static void tcp_send_challenge_ack(struct sock *sk)
{
    /* unprotected vars, we dont care of overwrites */
    static u32 challenge_timestamp;
    static unsigned int challenge_count;
    u32 now = jiffies / HZ;

    if (now != challenge_timestamp) {
        challenge_timestamp = now;
        challenge_count = 0;
    }
    if (++challenge_count <= sysctl_tcp_challenge_ack_limit) {
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPCHALLENGEACK);
        tcp_send_ack(sk);
    }
}

/* Does PAWS and seqno based validation of an incoming segment, flags will
 * play significant role here.
 */
static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
                  const struct tcphdr *th, int syn_inerr)
{
    const u8 *hash_location;
    struct tcp_sock *tp = tcp_sk(sk);

    /* RFC1323: H1. Apply PAWS check first. */
    if (tcp_fast_parse_options(skb, th, tp, &hash_location) &&
        tp->rx_opt.saw_tstamp &&
        tcp_paws_discard(sk, skb)) {
        if (!th->rst) {
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
            tcp_send_dupack(sk, skb);
            goto discard;
        }
        /* Reset is accepted even if it did not pass PAWS. */
    }

    /* Step 1: check sequence number */
    if (!tcp_sequence(tp, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq)) {
        /* RFC793, page 37: "In all states except SYN-SENT, all reset
         * (RST) segments are validated by checking their SEQ-fields."
         * And page 69: "If an incoming segment is not acceptable,
         * an acknowledgment should be sent in reply (unless the RST
         * bit is set, if so drop the segment and return)".
         */
        if (!th->rst) {
            if (th->syn)
                goto syn_challenge;
            tcp_send_dupack(sk, skb);
        }
        goto discard;
    }

    /* Step 2: check RST bit */
    if (th->rst) {
        /* RFC 5961 3.2 :
         * If sequence number exactly matches RCV.NXT, then
         *     RESET the connection
         * else
         *     Send a challenge ACK
         */
        if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt)
            tcp_reset(sk);
        else
            tcp_send_challenge_ack(sk);
        goto discard;
    }

    /* step 3: check security and precedence [ignored] */

    /* step 4: Check for a SYN
     * RFC 5691 4.2 : Send a challenge ack
     */
    if (th->syn) {
syn_challenge:
        if (syn_inerr)
            TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSYNCHALLENGE);
        tcp_send_challenge_ack(sk);
        goto discard;
    }

    return true;

discard:
    __kfree_skb(skb);
    return false;
}

/*
 *  TCP receive function for the ESTABLISHED state.
 *
 *  It is split into a fast path and a slow path. The fast path is
 *  disabled when:
 *  - A zero window was announced from us - zero window probing
 *        is only handled properly in the slow path.
 *  - Out of order segments arrived.
 *  - Urgent data is expected.
 *  - There is no buffer space left
 *  - Unexpected TCP flags/window values/header lengths are received
 *    (detected by checking the TCP header against pred_flags)
 *  - Data is sent in both directions. Fast path only supports pure senders
 *    or pure receivers (this means either the sequence number or the ack
 *    value must stay constant)
 *  - Unexpected TCP option.
 *
 *  When these conditions are not satisfied it drops into a standard
 *  receive procedure patterned after RFC793 to handle all cases.
 *  The first three cases are guaranteed by proper pred_flags setting,
 *  the rest is checked inline. Fast processing is turned on in
 *  tcp_data_queue when everything is OK.
 */
int tcp_rcv_established(struct sock *sk, struct sk_buff *skb,
            const struct tcphdr *th, unsigned int len)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (unlikely(sk->sk_rx_dst == NULL))
        inet_csk(sk)->icsk_af_ops->sk_rx_dst_set(sk, skb);
    /*
     *  Header prediction.
     *  The code loosely follows the one in the famous
     *  "30 instruction TCP receive" Van Jacobson mail.
     *
     *  Van's trick is to deposit buffers into socket queue
     *  on a device interrupt, to call tcp_recv function
     *  on the receive process context and checksum and copy
     *  the buffer to user space. smart...
     *
     *  Our current scheme is not silly either but we take the
     *  extra cost of the net_bh soft interrupt processing...
     *  We do checksum and copy also but from device to kernel.
     */

    tp->rx_opt.saw_tstamp = 0;

    /*  pred_flags is 0xS?10 << 16 + snd_wnd
     *  if header_prediction is to be made
     *  'S' will always be tp->tcp_header_len >> 2
     *  '?' will be 0 for the fast path, otherwise pred_flags is 0 to
     *  turn it off (when there are holes in the receive
     *   space for instance)
     *  PSH flag is ignored.
     */

    if ((tcp_flag_word(th) & TCP_HP_BITS) == tp->pred_flags &&
        TCP_SKB_CB(skb)->seq == tp->rcv_nxt &&
        !after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
        int tcp_header_len = tp->tcp_header_len;

        /* Timestamp header prediction: tcp_header_len
         * is automatically equal to th->doff*4 due to pred_flags
         * match.
         */

        /* Check timestamp */
        if (tcp_header_len == sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) {
            /* No? Slow path! */
            if (!tcp_parse_aligned_timestamp(tp, th))
                goto slow_path;

            /* If PAWS failed, check it more carefully in slow path */
            if ((s32)(tp->rx_opt.rcv_tsval - tp->rx_opt.ts_recent) < 0)
                goto slow_path;

            /* DO NOT update ts_recent here, if checksum fails
             * and timestamp was corrupted part, it will result
             * in a hung connection since we will drop all
             * future packets due to the PAWS test.
             */
        }

        if (len <= tcp_header_len) {
            /* Bulk data transfer: sender */
            if (len == tcp_header_len) {
                /* Predicted packet is in window by definition.
                 * seq == rcv_nxt and rcv_wup <= rcv_nxt.
                 * Hence, check seq<=rcv_wup reduces to:
                 */
                if (tcp_header_len ==
                    (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
                    tp->rcv_nxt == tp->rcv_wup)
                    tcp_store_ts_recent(tp);

                /* We know that such packets are checksummed
                 * on entry.
                 */
                tcp_ack(sk, skb, 0);
                __kfree_skb(skb);
                tcp_data_snd_check(sk);
                return 0;
            } else { /* Header too small */
                TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);
                goto discard;
            }
        } else {
            int eaten = 0;
            int copied_early = 0;
            bool fragstolen = false;

            if (tp->copied_seq == tp->rcv_nxt &&
                len - tcp_header_len <= tp->ucopy.len) {
#ifdef CONFIG_NET_DMA
                if (tp->ucopy.task == current &&
                    sock_owned_by_user(sk) &&
                    tcp_dma_try_early_copy(sk, skb, tcp_header_len)) {
                    copied_early = 1;
                    eaten = 1;
                }
#endif
                if (tp->ucopy.task == current &&
                    sock_owned_by_user(sk) && !copied_early) {
                    __set_current_state(TASK_RUNNING);

                    if (!tcp_copy_to_iovec(sk, skb, tcp_header_len))
                        eaten = 1;
                }
                if (eaten) {
                    /* Predicted packet is in window by definition.
                     * seq == rcv_nxt and rcv_wup <= rcv_nxt.
                     * Hence, check seq<=rcv_wup reduces to:
                     */
                    if (tcp_header_len ==
                        (sizeof(struct tcphdr) +
                         TCPOLEN_TSTAMP_ALIGNED) &&
                        tp->rcv_nxt == tp->rcv_wup)
                        tcp_store_ts_recent(tp);

                    tcp_rcv_rtt_measure_ts(sk, skb);

                    __skb_pull(skb, tcp_header_len);
                    tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
                    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPHPHITSTOUSER);
                }
                if (copied_early)
                    tcp_cleanup_rbuf(sk, skb->len);
            }
            if (!eaten) {
                if (tcp_checksum_complete_user(sk, skb))
                    goto csum_error;

                /* Predicted packet is in window by definition.
                 * seq == rcv_nxt and rcv_wup <= rcv_nxt.
                 * Hence, check seq<=rcv_wup reduces to:
                 */
                if (tcp_header_len ==
                    (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
                    tp->rcv_nxt == tp->rcv_wup)
                    tcp_store_ts_recent(tp);

                tcp_rcv_rtt_measure_ts(sk, skb);

                if ((int)skb->truesize > sk->sk_forward_alloc)
                    goto step5;

                NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPHPHITS);

                /* Bulk data transfer: receiver */
                eaten = tcp_queue_rcv(sk, skb, tcp_header_len,
                              &fragstolen);
            }

            tcp_event_data_recv(sk, skb);

            if (TCP_SKB_CB(skb)->ack_seq != tp->snd_una) {
                /* Well, only one small jumplet in fast path... */
                tcp_ack(sk, skb, FLAG_DATA);
                tcp_data_snd_check(sk);
                if (!inet_csk_ack_scheduled(sk))
                    goto no_ack;
            }

            if (!copied_early || tp->rcv_nxt != tp->rcv_wup)
                __tcp_ack_snd_check(sk, 0);
no_ack:
#ifdef CONFIG_NET_DMA
            if (copied_early)
                __skb_queue_tail(&sk->sk_async_wait_queue, skb);
            else
#endif
            if (eaten)
                kfree_skb_partial(skb, fragstolen);
            sk->sk_data_ready(sk, 0);
            return 0;
        }
    }

slow_path:
    if (len < (th->doff << 2) || tcp_checksum_complete_user(sk, skb))
        goto csum_error;

    /*
     *  Standard slow path.
     */

    if (!tcp_validate_incoming(sk, skb, th, 1))
        return 0;

step5:
    if (th->ack && tcp_ack(sk, skb, FLAG_SLOWPATH) < 0)
        goto discard;

    /* ts_recent update must be made after we are sure that the packet
     * is in window.
     */
    tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq);

    tcp_rcv_rtt_measure_ts(sk, skb);

    /* Process urgent data. */
    tcp_urg(sk, skb, th);

    /* step 7: process the segment text */
    tcp_data_queue(sk, skb);

    tcp_data_snd_check(sk);
    tcp_ack_snd_check(sk);
    return 0;

csum_error:
    TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);

discard:
    __kfree_skb(skb);
    return 0;
}
EXPORT_SYMBOL(tcp_rcv_established);

void tcp_finish_connect(struct sock *sk, struct sk_buff *skb)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct inet_connection_sock *icsk = inet_csk(sk);

    tcp_set_state(sk, TCP_ESTABLISHED);

    if (skb != NULL) {
        icsk->icsk_af_ops->sk_rx_dst_set(sk, skb);
        security_inet_conn_established(sk, skb);
    }

    /* Make sure socket is routed, for correct metrics.  */
    icsk->icsk_af_ops->rebuild_header(sk);

    tcp_init_metrics(sk);

    tcp_init_congestion_control(sk);

    /* Prevent spurious tcp_cwnd_restart() on first data
     * packet.
     */
    tp->lsndtime = tcp_time_stamp;

    tcp_init_buffer_space(sk);

    if (sock_flag(sk, SOCK_KEEPOPEN))
        inet_csk_reset_keepalive_timer(sk, keepalive_time_when(tp));

    if (!tp->rx_opt.snd_wscale)
        __tcp_fast_path_on(tp, tp->snd_wnd);
    else
        tp->pred_flags = 0;

    if (!sock_flag(sk, SOCK_DEAD)) {
        sk->sk_state_change(sk);
        sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
    }
}

static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
                    struct tcp_fastopen_cookie *cookie)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *data = tp->syn_data ? tcp_write_queue_head(sk) : NULL;
    u16 mss = tp->rx_opt.mss_clamp;
    bool syn_drop;

    if (mss == tp->rx_opt.user_mss) {
        struct tcp_options_received opt;
        const u8 *hash_location;

        /* Get original SYNACK MSS value if user MSS sets mss_clamp */
        tcp_clear_options(&opt);
        opt.user_mss = opt.mss_clamp = 0;
        tcp_parse_options(synack, &opt, &hash_location, 0, NULL);
        mss = opt.mss_clamp;
    }

    if (!tp->syn_fastopen)  /* Ignore an unsolicited cookie */
        cookie->len = -1;

    /* The SYN-ACK neither has cookie nor acknowledges the data. Presumably
     * the remote receives only the retransmitted (regular) SYNs: either
     * the original SYN-data or the corresponding SYN-ACK is lost.
     */
    syn_drop = (cookie->len <= 0 && data &&
            inet_csk(sk)->icsk_retransmits);

    tcp_fastopen_cache_set(sk, mss, cookie, syn_drop);

    if (data) { /* Retransmit unacked data in SYN */
        tcp_for_write_queue_from(data, sk) {
            if (data == tcp_send_head(sk) ||
                __tcp_retransmit_skb(sk, data))
                break;
        }
        tcp_rearm_rto(sk);
        return true;
    }
    tp->syn_data_acked = tp->syn_data;
    return false;
}

static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
                     const struct tcphdr *th, unsigned int len)
{
    const u8 *hash_location;
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    struct tcp_cookie_values *cvp = tp->cookie_values;
    struct tcp_fastopen_cookie foc = { .len = -1 };
    int saved_clamp = tp->rx_opt.mss_clamp;

    tcp_parse_options(skb, &tp->rx_opt, &hash_location, 0, &foc);

    if (th->ack) {
        /* rfc793:
         * "If the state is SYN-SENT then
         *    first check the ACK bit
         *      If the ACK bit is set
         *    If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send
         *        a reset (unless the RST bit is set, if so drop
         *        the segment and return)"
         */
        if (!after(TCP_SKB_CB(skb)->ack_seq, tp->snd_una) ||
            after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt))
            goto reset_and_undo;

        if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
            !between(tp->rx_opt.rcv_tsecr, tp->retrans_stamp,
                 tcp_time_stamp)) {
            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSACTIVEREJECTED);
            goto reset_and_undo;
        }

        /* Now ACK is acceptable.
         *
         * "If the RST bit is set
         *    If the ACK was acceptable then signal the user "error:
         *    connection reset", drop the segment, enter CLOSED state,
         *    delete TCB, and return."
         */

        if (th->rst) {
            tcp_reset(sk);
            goto discard;
        }

        /* rfc793:
         *   "fifth, if neither of the SYN or RST bits is set then
         *    drop the segment and return."
         *
         *    See note below!
         *                                        --ANK(990513)
         */
        if (!th->syn)
            goto discard_and_undo;

        /* rfc793:
         *   "If the SYN bit is on ...
         *    are acceptable then ...
         *    (our SYN has been ACKed), change the connection
         *    state to ESTABLISHED..."
         */

        TCP_ECN_rcv_synack(tp, th);

        tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
        tcp_ack(sk, skb, FLAG_SLOWPATH);

        /* Ok.. it's good. Set up sequence numbers and
         * move to established.
         */
        tp->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
        tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1;

        /* RFC1323: The window in SYN & SYN/ACK segments is
         * never scaled.
         */
        tp->snd_wnd = ntohs(th->window);

        if (!tp->rx_opt.wscale_ok) {
            tp->rx_opt.snd_wscale = tp->rx_opt.rcv_wscale = 0;
            tp->window_clamp = min(tp->window_clamp, 65535U);
        }

        if (tp->rx_opt.saw_tstamp) {
            tp->rx_opt.tstamp_ok       = 1;
            tp->tcp_header_len =
                sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
            tp->advmss      -= TCPOLEN_TSTAMP_ALIGNED;
            tcp_store_ts_recent(tp);
        } else {
            tp->tcp_header_len = sizeof(struct tcphdr);
        }

        if (tcp_is_sack(tp) && sysctl_tcp_fack)
            tcp_enable_fack(tp);

        tcp_mtup_init(sk);
        tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
        tcp_initialize_rcv_mss(sk);

        /* Remember, tcp_poll() does not lock socket!
         * Change state from SYN-SENT only after copied_seq
         * is initialized. */
        tp->copied_seq = tp->rcv_nxt;

        if (cvp != NULL &&
            cvp->cookie_pair_size > 0 &&
            tp->rx_opt.cookie_plus > 0) {
            int cookie_size = tp->rx_opt.cookie_plus
                    - TCPOLEN_COOKIE_BASE;
            int cookie_pair_size = cookie_size
                         + cvp->cookie_desired;

            /* A cookie extension option was sent and returned.
             * Note that each incoming SYNACK replaces the
             * Responder cookie.  The initial exchange is most
             * fragile, as protection against spoofing relies
             * entirely upon the sequence and timestamp (above).
             * This replacement strategy allows the correct pair to
             * pass through, while any others will be filtered via
             * Responder verification later.
             */
            if (sizeof(cvp->cookie_pair) >= cookie_pair_size) {
                memcpy(&cvp->cookie_pair[cvp->cookie_desired],
                       hash_location, cookie_size);
                cvp->cookie_pair_size = cookie_pair_size;
            }
        }

        smp_mb();

        tcp_finish_connect(sk, skb);

        if ((tp->syn_fastopen || tp->syn_data) &&
            tcp_rcv_fastopen_synack(sk, skb, &foc))
            return -1;

        if (sk->sk_write_pending ||
            icsk->icsk_accept_queue.rskq_defer_accept ||
            icsk->icsk_ack.pingpong) {
            /* Save one ACK. Data will be ready after
             * several ticks, if write_pending is set.
             *
             * It may be deleted, but with this feature tcpdumps
             * look so _wonderfully_ clever, that I was not able
             * to stand against the temptation 8)     --ANK
             */
            inet_csk_schedule_ack(sk);
            icsk->icsk_ack.lrcvtime = tcp_time_stamp;
            tcp_enter_quickack_mode(sk);
            inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
                          TCP_DELACK_MAX, TCP_RTO_MAX);

discard:
            __kfree_skb(skb);
            return 0;
        } else {
            tcp_send_ack(sk);
        }
        return -1;
    }

    /* No ACK in the segment */

    if (th->rst) {
        /* rfc793:
         * "If the RST bit is set
         *
         *      Otherwise (no ACK) drop the segment and return."
         */

        goto discard_and_undo;
    }

    /* PAWS check. */
    if (tp->rx_opt.ts_recent_stamp && tp->rx_opt.saw_tstamp &&
        tcp_paws_reject(&tp->rx_opt, 0))
        goto discard_and_undo;

    if (th->syn) {
        /* We see SYN without ACK. It is attempt of
         * simultaneous connect with crossed SYNs.
         * Particularly, it can be connect to self.
         */
        tcp_set_state(sk, TCP_SYN_RECV);

        if (tp->rx_opt.saw_tstamp) {
            tp->rx_opt.tstamp_ok = 1;
            tcp_store_ts_recent(tp);
            tp->tcp_header_len =
                sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
        } else {
            tp->tcp_header_len = sizeof(struct tcphdr);
        }

        tp->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
        tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1;

        /* RFC1323: The window in SYN & SYN/ACK segments is
         * never scaled.
         */
        tp->snd_wnd    = ntohs(th->window);
        tp->snd_wl1    = TCP_SKB_CB(skb)->seq;
        tp->max_window = tp->snd_wnd;

        TCP_ECN_rcv_syn(tp, th);

        tcp_mtup_init(sk);
        tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
        tcp_initialize_rcv_mss(sk);

        tcp_send_synack(sk);
#if 0
        /* Note, we could accept data and URG from this segment.
         * There are no obstacles to make this (except that we must
         * either change tcp_recvmsg() to prevent it from returning data
         * before 3WHS completes per RFC793, or employ TCP Fast Open).
         *
         * However, if we ignore data in ACKless segments sometimes,
         * we have no reasons to accept it sometimes.
         * Also, seems the code doing it in step6 of tcp_rcv_state_process
         * is not flawless. So, discard packet for sanity.
         * Uncomment this return to process the data.
         */
        return -1;
#else
        goto discard;
#endif
    }
    /* "fifth, if neither of the SYN or RST bits is set then
     * drop the segment and return."
     */

discard_and_undo:
    tcp_clear_options(&tp->rx_opt);
    tp->rx_opt.mss_clamp = saved_clamp;
    goto discard;

reset_and_undo:
    tcp_clear_options(&tp->rx_opt);
    tp->rx_opt.mss_clamp = saved_clamp;
    return 1;
}

/*
 *  This function implements the receiving procedure of RFC 793 for
 *  all states except ESTABLISHED and TIME_WAIT.
 *  It's called from both tcp_v4_rcv and tcp_v6_rcv and should be
 *  address independent.
 */

int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
              const struct tcphdr *th, unsigned int len)
{
    struct tcp_sock *tp = tcp_sk(sk);
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct request_sock *req;
    int queued = 0;

    tp->rx_opt.saw_tstamp = 0;

    switch (sk->sk_state) {
    case TCP_CLOSE:
        goto discard;

    case TCP_LISTEN:
        if (th->ack)
            return 1;

        if (th->rst)
            goto discard;

        if (th->syn) {
            if (th->fin)
                goto discard;
            if (icsk->icsk_af_ops->conn_request(sk, skb) < 0)
                return 1;

            /* Now we have several options: In theory there is
             * nothing else in the frame. KA9Q has an option to
             * send data with the syn, BSD accepts data with the
             * syn up to the [to be] advertised window and
             * Solaris 2.1 gives you a protocol error. For now
             * we just ignore it, that fits the spec precisely
             * and avoids incompatibilities. It would be nice in
             * future to drop through and process the data.
             *
             * Now that TTCP is starting to be used we ought to
             * queue this data.
             * But, this leaves one open to an easy denial of
             * service attack, and SYN cookies can't defend
             * against this problem. So, we drop the data
             * in the interest of security over speed unless
             * it's still in use.
             */
            kfree_skb(skb);
            return 0;
        }
        goto discard;

    case TCP_SYN_SENT:
        queued = tcp_rcv_synsent_state_process(sk, skb, th, len);
        if (queued >= 0)
            return queued;

        /* Do step6 onward by hand. */
        tcp_urg(sk, skb, th);
        __kfree_skb(skb);
        tcp_data_snd_check(sk);
        return 0;
    }

    req = tp->fastopen_rsk;
    if (req != NULL) {
        WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
            sk->sk_state != TCP_FIN_WAIT1);

        if (tcp_check_req(sk, skb, req, NULL, true) == NULL)
            goto discard;
    }
    if (!tcp_validate_incoming(sk, skb, th, 0))
        return 0;

    /* step 5: check the ACK field */
    if (th->ack) {
        int acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH) > 0;

        switch (sk->sk_state) {
        case TCP_SYN_RECV:
            if (acceptable) {
                /* Once we leave TCP_SYN_RECV, we no longer
                 * need req so release it.
                 */
                if (req) {
                    tcp_synack_rtt_meas(sk, req);
                    tp->total_retrans = req->retrans;

                    reqsk_fastopen_remove(sk, req, false);
                } else {
                    /* Make sure socket is routed, for
                     * correct metrics.
                     */
                    icsk->icsk_af_ops->rebuild_header(sk);
                    tcp_init_congestion_control(sk);

                    tcp_mtup_init(sk);
                    tcp_init_buffer_space(sk);
                    tp->copied_seq = tp->rcv_nxt;
                }
                smp_mb();
                tcp_set_state(sk, TCP_ESTABLISHED);
                sk->sk_state_change(sk);

                /* Note, that this wakeup is only for marginal
                 * crossed SYN case. Passively open sockets
                 * are not waked up, because sk->sk_sleep ==
                 * NULL and sk->sk_socket == NULL.
                 */
                if (sk->sk_socket)
                    sk_wake_async(sk,
                              SOCK_WAKE_IO, POLL_OUT);

                tp->snd_una = TCP_SKB_CB(skb)->ack_seq;
                tp->snd_wnd = ntohs(th->window) <<
                          tp->rx_opt.snd_wscale;
                tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);

                if (tp->rx_opt.tstamp_ok)
                    tp->advmss -= TCPOLEN_TSTAMP_ALIGNED;

                if (req) {
                    /* Re-arm the timer because data may
                     * have been sent out. This is similar
                     * to the regular data transmission case
                     * when new data has just been ack'ed.
                     *
                     * (TFO) - we could try to be more
                     * aggressive and retranmitting any data
                     * sooner based on when they were sent
                     * out.
                     */
                    tcp_rearm_rto(sk);
                } else
                    tcp_init_metrics(sk);

                /* Prevent spurious tcp_cwnd_restart() on
                 * first data packet.
                 */
                tp->lsndtime = tcp_time_stamp;

                tcp_initialize_rcv_mss(sk);
                tcp_fast_path_on(tp);
            } else {
                return 1;
            }
            break;

        case TCP_FIN_WAIT1:
            /* If we enter the TCP_FIN_WAIT1 state and we are a
             * Fast Open socket and this is the first acceptable
             * ACK we have received, this would have acknowledged
             * our SYNACK so stop the SYNACK timer.
             */
            if (req != NULL) {
                /* Return RST if ack_seq is invalid.
                 * Note that RFC793 only says to generate a
                 * DUPACK for it but for TCP Fast Open it seems
                 * better to treat this case like TCP_SYN_RECV
                 * above.
                 */
                if (!acceptable)
                    return 1;
                /* We no longer need the request sock. */
                reqsk_fastopen_remove(sk, req, false);
                tcp_rearm_rto(sk);
            }
            if (tp->snd_una == tp->write_seq) {
                struct dst_entry *dst;

                tcp_set_state(sk, TCP_FIN_WAIT2);
                sk->sk_shutdown |= SEND_SHUTDOWN;

                dst = __sk_dst_get(sk);
                if (dst)
                    dst_confirm(dst);

                if (!sock_flag(sk, SOCK_DEAD))
                    /* Wake up lingering close() */
                    sk->sk_state_change(sk);
                else {
                    int tmo;

                    if (tp->linger2 < 0 ||
                        (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
                         after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt))) {
                        tcp_done(sk);
                        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
                        return 1;
                    }

                    tmo = tcp_fin_time(sk);
                    if (tmo > TCP_TIMEWAIT_LEN) {
                        inet_csk_reset_keepalive_timer(sk, tmo - TCP_TIMEWAIT_LEN);
                    } else if (th->fin || sock_owned_by_user(sk)) {
                        /* Bad case. We could lose such FIN otherwise.
                         * It is not a big problem, but it looks confusing
                         * and not so rare event. We still can lose it now,
                         * if it spins in bh_lock_sock(), but it is really
                         * marginal case.
                         */
                        inet_csk_reset_keepalive_timer(sk, tmo);
                    } else {
                        tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
                        goto discard;
                    }
                }
            }
            break;

        case TCP_CLOSING:
            if (tp->snd_una == tp->write_seq) {
                tcp_time_wait(sk, TCP_TIME_WAIT, 0);
                goto discard;
            }
            break;

        case TCP_LAST_ACK:
            if (tp->snd_una == tp->write_seq) {
                tcp_update_metrics(sk);
                tcp_done(sk);
                goto discard;
            }
            break;
        }
    } else
        goto discard;

    /* ts_recent update must be made after we are sure that the packet
     * is in window.
     */
    tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq);

    /* step 6: check the URG bit */
    tcp_urg(sk, skb, th);

    /* step 7: process the segment text */
    switch (sk->sk_state) {
    case TCP_CLOSE_WAIT:
    case TCP_CLOSING:
    case TCP_LAST_ACK:
        if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt))
            break;
    case TCP_FIN_WAIT1:
    case TCP_FIN_WAIT2:
        /* RFC 793 says to queue data in these states,
         * RFC 1122 says we MUST send a reset.
         * BSD 4.4 also does reset.
         */
        if (sk->sk_shutdown & RCV_SHUTDOWN) {
            if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
                after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) {
                NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
                tcp_reset(sk);
                return 1;
            }
        }
        /* Fall through */
    case TCP_ESTABLISHED:
        tcp_data_queue(sk, skb);
        queued = 1;
        break;
    }

    /* tcp_data could move socket to TIME-WAIT */
    if (sk->sk_state != TCP_CLOSE) {
        tcp_data_snd_check(sk);
        tcp_ack_snd_check(sk);
    }

    if (!queued) {
discard:
        __kfree_skb(skb);
    }
    return 0;
}
EXPORT_SYMBOL(tcp_rcv_state_process);