tcp_timer.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]>
 */

#include <linux/module.h>
#include <linux/gfp.h>
#include <net/tcp.h>

int sysctl_tcp_syn_retries __read_mostly = TCP_SYN_RETRIES;
int sysctl_tcp_synack_retries __read_mostly = TCP_SYNACK_RETRIES;
int sysctl_tcp_keepalive_time __read_mostly = TCP_KEEPALIVE_TIME;
int sysctl_tcp_keepalive_probes __read_mostly = TCP_KEEPALIVE_PROBES;
int sysctl_tcp_keepalive_intvl __read_mostly = TCP_KEEPALIVE_INTVL;
int sysctl_tcp_retries1 __read_mostly = TCP_RETR1;
int sysctl_tcp_retries2 __read_mostly = TCP_RETR2;
int sysctl_tcp_orphan_retries __read_mostly;
int sysctl_tcp_thin_linear_timeouts __read_mostly;

static void tcp_write_err(struct sock *sk)
{
    sk->sk_err = sk->sk_err_soft ? : ETIMEDOUT;
    sk->sk_error_report(sk);

    tcp_done(sk);
    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONTIMEOUT);
}

/* Do not allow orphaned sockets to eat all our resources.
 * This is direct violation of TCP specs, but it is required
 * to prevent DoS attacks. It is called when a retransmission timeout
 * or zero probe timeout occurs on orphaned socket.
 *
 * Criteria is still not confirmed experimentally and may change.
 * We kill the socket, if:
 * 1. If number of orphaned sockets exceeds an administratively configured
 *    limit.
 * 2. If we have strong memory pressure.
 */
static int tcp_out_of_resources(struct sock *sk, int do_reset)
{
    struct tcp_sock *tp = tcp_sk(sk);
    int shift = 0;

    /* If peer does not open window for long time, or did not transmit
     * anything for long time, penalize it. */
    if ((s32)(tcp_time_stamp - tp->lsndtime) > 2*TCP_RTO_MAX || !do_reset)
        shift++;

    /* If some dubious ICMP arrived, penalize even more. */
    if (sk->sk_err_soft)
        shift++;

    if (tcp_check_oom(sk, shift)) {
        /* Catch exceptional cases, when connection requires reset.
         *      1. Last segment was sent recently. */
        if ((s32)(tcp_time_stamp - tp->lsndtime) <= TCP_TIMEWAIT_LEN ||
            /*  2. Window is closed. */
            (!tp->snd_wnd && !tp->packets_out))
            do_reset = 1;
        if (do_reset)
            tcp_send_active_reset(sk, GFP_ATOMIC);
        tcp_done(sk);
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONMEMORY);
        return 1;
    }
    return 0;
}

/* Calculate maximal number or retries on an orphaned socket. */
static int tcp_orphan_retries(struct sock *sk, int alive)
{
    int retries = sysctl_tcp_orphan_retries; /* May be zero. */

    /* We know from an ICMP that something is wrong. */
    if (sk->sk_err_soft && !alive)
        retries = 0;

    /* However, if socket sent something recently, select some safe
     * number of retries. 8 corresponds to >100 seconds with minimal
     * RTO of 200msec. */
    if (retries == 0 && alive)
        retries = 8;
    return retries;
}

static void tcp_mtu_probing(struct inet_connection_sock *icsk, struct sock *sk)
{
    /* Black hole detection */
    if (sysctl_tcp_mtu_probing) {
        if (!icsk->icsk_mtup.enabled) {
            icsk->icsk_mtup.enabled = 1;
            tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
        } else {
            struct tcp_sock *tp = tcp_sk(sk);
            int mss;

            mss = tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low) >> 1;
            mss = min(sysctl_tcp_base_mss, mss);
            mss = max(mss, 68 - tp->tcp_header_len);
            icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
            tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
        }
    }
}

/* This function calculates a "timeout" which is equivalent to the timeout of a
 * TCP connection after "boundary" unsuccessful, exponentially backed-off
 * retransmissions with an initial RTO of TCP_RTO_MIN or TCP_TIMEOUT_INIT if
 * syn_set flag is set.
 */
static bool retransmits_timed_out(struct sock *sk,
                  unsigned int boundary,
                  unsigned int timeout,
                  bool syn_set)
{
    unsigned int linear_backoff_thresh, start_ts;
    unsigned int rto_base = syn_set ? TCP_TIMEOUT_INIT : TCP_RTO_MIN;

    if (!inet_csk(sk)->icsk_retransmits)
        return false;

    if (unlikely(!tcp_sk(sk)->retrans_stamp))
        start_ts = TCP_SKB_CB(tcp_write_queue_head(sk))->when;
    else
        start_ts = tcp_sk(sk)->retrans_stamp;

    if (likely(timeout == 0)) {
        linear_backoff_thresh = ilog2(TCP_RTO_MAX/rto_base);

        if (boundary <= linear_backoff_thresh)
            timeout = ((2 << boundary) - 1) * rto_base;
        else
            timeout = ((2 << linear_backoff_thresh) - 1) * rto_base +
                (boundary - linear_backoff_thresh) * TCP_RTO_MAX;
    }
    return (tcp_time_stamp - start_ts) >= timeout;
}

/* A write timeout has occurred. Process the after effects. */
static int tcp_write_timeout(struct sock *sk)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    int retry_until;
    bool do_reset, syn_set = false;

    if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
        if (icsk->icsk_retransmits)
            dst_negative_advice(sk);
        retry_until = icsk->icsk_syn_retries ? : sysctl_tcp_syn_retries;
        syn_set = true;
    } else {
        if (retransmits_timed_out(sk, sysctl_tcp_retries1, 0, 0)) {
            /* Black hole detection */
            tcp_mtu_probing(icsk, sk);

            dst_negative_advice(sk);
        }

        retry_until = sysctl_tcp_retries2;
        if (sock_flag(sk, SOCK_DEAD)) {
            const int alive = (icsk->icsk_rto < TCP_RTO_MAX);

            retry_until = tcp_orphan_retries(sk, alive);
            do_reset = alive ||
                !retransmits_timed_out(sk, retry_until, 0, 0);

            if (tcp_out_of_resources(sk, do_reset))
                return 1;
        }
    }

    if (retransmits_timed_out(sk, retry_until,
                  syn_set ? 0 : icsk->icsk_user_timeout, syn_set)) {
        /* Has it gone just too far? */
        tcp_write_err(sk);
        return 1;
    }
    return 0;
}

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

    sk_mem_reclaim_partial(sk);

    if (sk->sk_state == TCP_CLOSE || !(icsk->icsk_ack.pending & ICSK_ACK_TIMER))
        goto out;

    if (time_after(icsk->icsk_ack.timeout, jiffies)) {
        sk_reset_timer(sk, &icsk->icsk_delack_timer, icsk->icsk_ack.timeout);
        goto out;
    }
    icsk->icsk_ack.pending &= ~ICSK_ACK_TIMER;

    if (!skb_queue_empty(&tp->ucopy.prequeue)) {
        struct sk_buff *skb;

        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSCHEDULERFAILED);

        while ((skb = __skb_dequeue(&tp->ucopy.prequeue)) != NULL)
            sk_backlog_rcv(sk, skb);

        tp->ucopy.memory = 0;
    }

    if (inet_csk_ack_scheduled(sk)) {
        if (!icsk->icsk_ack.pingpong) {
            /* Delayed ACK missed: inflate ATO. */
            icsk->icsk_ack.ato = min(icsk->icsk_ack.ato << 1, icsk->icsk_rto);
        } else {
            /* Delayed ACK missed: leave pingpong mode and
             * deflate ATO.
             */
            icsk->icsk_ack.pingpong = 0;
            icsk->icsk_ack.ato      = TCP_ATO_MIN;
        }
        tcp_send_ack(sk);
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKS);
    }

out:
    if (sk_under_memory_pressure(sk))
        sk_mem_reclaim(sk);
}

static void tcp_delack_timer(unsigned long data)
{
    struct sock *sk = (struct sock *)data;

    bh_lock_sock(sk);
    if (!sock_owned_by_user(sk)) {
        tcp_delack_timer_handler(sk);
    } else {
        inet_csk(sk)->icsk_ack.blocked = 1;
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOCKED);
        /* deleguate our work to tcp_release_cb() */
        if (!test_and_set_bit(TCP_DELACK_TIMER_DEFERRED, &tcp_sk(sk)->tsq_flags))
            sock_hold(sk);
    }
    bh_unlock_sock(sk);
    sock_put(sk);
}

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

    if (tp->packets_out || !tcp_send_head(sk)) {
        icsk->icsk_probes_out = 0;
        return;
    }

    /* *WARNING* RFC 1122 forbids this
     *
     * It doesn't AFAIK, because we kill the retransmit timer -AK
     *
     * FIXME: We ought not to do it, Solaris 2.5 actually has fixing
     * this behaviour in Solaris down as a bug fix. [AC]
     *
     * Let me to explain. icsk_probes_out is zeroed by incoming ACKs
     * even if they advertise zero window. Hence, connection is killed only
     * if we received no ACKs for normal connection timeout. It is not killed
     * only because window stays zero for some time, window may be zero
     * until armageddon and even later. We are in full accordance
     * with RFCs, only probe timer combines both retransmission timeout
     * and probe timeout in one bottle.             --ANK
     */
    max_probes = sysctl_tcp_retries2;

    if (sock_flag(sk, SOCK_DEAD)) {
        const int alive = ((icsk->icsk_rto << icsk->icsk_backoff) < TCP_RTO_MAX);

        max_probes = tcp_orphan_retries(sk, alive);

        if (tcp_out_of_resources(sk, alive || icsk->icsk_probes_out <= max_probes))
            return;
    }

    if (icsk->icsk_probes_out > max_probes) {
        tcp_write_err(sk);
    } else {
        /* Only send another probe if we didn't close things up. */
        tcp_send_probe0(sk);
    }
}

/*
 *  Timer for Fast Open socket to retransmit SYNACK. Note that the
 *  sk here is the child socket, not the parent (listener) socket.
 */
static void tcp_fastopen_synack_timer(struct sock *sk)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    int max_retries = icsk->icsk_syn_retries ? :
        sysctl_tcp_synack_retries + 1; /* add one more retry for fastopen */
    struct request_sock *req;

    req = tcp_sk(sk)->fastopen_rsk;
    req->rsk_ops->syn_ack_timeout(sk, req);

    if (req->retrans >= max_retries) {
        tcp_write_err(sk);
        return;
    }
    /* XXX (TFO) - Unlike regular SYN-ACK retransmit, we ignore error
     * returned from rtx_syn_ack() to make it more persistent like
     * regular retransmit because if the child socket has been accepted
     * it's not good to give up too easily.
     */
    req->rsk_ops->rtx_syn_ack(sk, req, NULL);
    req->retrans++;
    inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
              TCP_TIMEOUT_INIT << req->retrans, TCP_RTO_MAX);
}

/*
 *  The TCP retransmit timer.
 */

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

    if (tp->early_retrans_delayed) {
        tcp_resume_early_retransmit(sk);
        return;
    }
    if (tp->fastopen_rsk) {
        WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
                 sk->sk_state != TCP_FIN_WAIT1);
        tcp_fastopen_synack_timer(sk);
        /* Before we receive ACK to our SYN-ACK don't retransmit
         * anything else (e.g., data or FIN segments).
         */
        return;
    }
    if (!tp->packets_out)
        goto out;

    WARN_ON(tcp_write_queue_empty(sk));

    if (!tp->snd_wnd && !sock_flag(sk, SOCK_DEAD) &&
        !((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV))) {
        /* Receiver dastardly shrinks window. Our retransmits
         * become zero probes, but we should not timeout this
         * connection. If the socket is an orphan, time it out,
         * we cannot allow such beasts to hang infinitely.
         */
        struct inet_sock *inet = inet_sk(sk);
        if (sk->sk_family == AF_INET) {
            LIMIT_NETDEBUG(KERN_DEBUG pr_fmt("Peer %pI4:%u/%u unexpectedly shrunk window %u:%u (repaired)\n"),
                       &inet->inet_daddr,
                       ntohs(inet->inet_dport), inet->inet_num,
                       tp->snd_una, tp->snd_nxt);
        }
#if IS_ENABLED(CONFIG_IPV6)
        else if (sk->sk_family == AF_INET6) {
            struct ipv6_pinfo *np = inet6_sk(sk);
            LIMIT_NETDEBUG(KERN_DEBUG pr_fmt("Peer %pI6:%u/%u unexpectedly shrunk window %u:%u (repaired)\n"),
                       &np->daddr,
                       ntohs(inet->inet_dport), inet->inet_num,
                       tp->snd_una, tp->snd_nxt);
        }
#endif
        if (tcp_time_stamp - tp->rcv_tstamp > TCP_RTO_MAX) {
            tcp_write_err(sk);
            goto out;
        }
        tcp_enter_loss(sk, 0);
        tcp_retransmit_skb(sk, tcp_write_queue_head(sk));
        __sk_dst_reset(sk);
        goto out_reset_timer;
    }

    if (tcp_write_timeout(sk))
        goto out;

    if (icsk->icsk_retransmits == 0) {
        int mib_idx;

        if (icsk->icsk_ca_state == TCP_CA_Recovery) {
            if (tcp_is_sack(tp))
                mib_idx = LINUX_MIB_TCPSACKRECOVERYFAIL;
            else
                mib_idx = LINUX_MIB_TCPRENORECOVERYFAIL;
        } else if (icsk->icsk_ca_state == TCP_CA_Loss) {
            mib_idx = LINUX_MIB_TCPLOSSFAILURES;
        } else if ((icsk->icsk_ca_state == TCP_CA_Disorder) ||
               tp->sacked_out) {
            if (tcp_is_sack(tp))
                mib_idx = LINUX_MIB_TCPSACKFAILURES;
            else
                mib_idx = LINUX_MIB_TCPRENOFAILURES;
        } else {
            mib_idx = LINUX_MIB_TCPTIMEOUTS;
        }
        NET_INC_STATS_BH(sock_net(sk), mib_idx);
    }

    if (tcp_use_frto(sk)) {
        tcp_enter_frto(sk);
    } else {
        tcp_enter_loss(sk, 0);
    }

    if (tcp_retransmit_skb(sk, tcp_write_queue_head(sk)) > 0) {
        /* Retransmission failed because of local congestion,
         * do not backoff.
         */
        if (!icsk->icsk_retransmits)
            icsk->icsk_retransmits = 1;
        inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                      min(icsk->icsk_rto, TCP_RESOURCE_PROBE_INTERVAL),
                      TCP_RTO_MAX);
        goto out;
    }

    /* Increase the timeout each time we retransmit.  Note that
     * we do not increase the rtt estimate.  rto is initialized
     * from rtt, but increases here.  Jacobson (SIGCOMM 88) suggests
     * that doubling rto each time is the least we can get away with.
     * In KA9Q, Karn uses this for the first few times, and then
     * goes to quadratic.  netBSD doubles, but only goes up to *64,
     * and clamps at 1 to 64 sec afterwards.  Note that 120 sec is
     * defined in the protocol as the maximum possible RTT.  I guess
     * we'll have to use something other than TCP to talk to the
     * University of Mars.
     *
     * PAWS allows us longer timeouts and large windows, so once
     * implemented ftp to mars will work nicely. We will have to fix
     * the 120 second clamps though!
     */
    icsk->icsk_backoff++;
    icsk->icsk_retransmits++;

out_reset_timer:
    /* If stream is thin, use linear timeouts. Since 'icsk_backoff' is
     * used to reset timer, set to 0. Recalculate 'icsk_rto' as this
     * might be increased if the stream oscillates between thin and thick,
     * thus the old value might already be too high compared to the value
     * set by 'tcp_set_rto' in tcp_input.c which resets the rto without
     * backoff. Limit to TCP_THIN_LINEAR_RETRIES before initiating
     * exponential backoff behaviour to avoid continue hammering
     * linear-timeout retransmissions into a black hole
     */
    if (sk->sk_state == TCP_ESTABLISHED &&
        (tp->thin_lto || sysctl_tcp_thin_linear_timeouts) &&
        tcp_stream_is_thin(tp) &&
        icsk->icsk_retransmits <= TCP_THIN_LINEAR_RETRIES) {
        icsk->icsk_backoff = 0;
        icsk->icsk_rto = min(__tcp_set_rto(tp), TCP_RTO_MAX);
    } else {
        /* Use normal (exponential) backoff */
        icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
    }
    inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto, TCP_RTO_MAX);
    if (retransmits_timed_out(sk, sysctl_tcp_retries1 + 1, 0, 0))
        __sk_dst_reset(sk);

out:;
}

void tcp_write_timer_handler(struct sock *sk)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    int event;

    if (sk->sk_state == TCP_CLOSE || !icsk->icsk_pending)
        goto out;

    if (time_after(icsk->icsk_timeout, jiffies)) {
        sk_reset_timer(sk, &icsk->icsk_retransmit_timer, icsk->icsk_timeout);
        goto out;
    }

    event = icsk->icsk_pending;
    icsk->icsk_pending = 0;

    switch (event) {
    case ICSK_TIME_RETRANS:
        tcp_retransmit_timer(sk);
        break;
    case ICSK_TIME_PROBE0:
        tcp_probe_timer(sk);
        break;
    }

out:
    sk_mem_reclaim(sk);
}

static void tcp_write_timer(unsigned long data)
{
    struct sock *sk = (struct sock *)data;

    bh_lock_sock(sk);
    if (!sock_owned_by_user(sk)) {
        tcp_write_timer_handler(sk);
    } else {
        /* deleguate our work to tcp_release_cb() */
        if (!test_and_set_bit(TCP_WRITE_TIMER_DEFERRED, &tcp_sk(sk)->tsq_flags))
            sock_hold(sk);
    }
    bh_unlock_sock(sk);
    sock_put(sk);
}

/*
 *  Timer for listening sockets
 */

static void tcp_synack_timer(struct sock *sk)
{
    inet_csk_reqsk_queue_prune(sk, TCP_SYNQ_INTERVAL,
                   TCP_TIMEOUT_INIT, TCP_RTO_MAX);
}

void tcp_syn_ack_timeout(struct sock *sk, struct request_sock *req)
{
    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPTIMEOUTS);
}
EXPORT_SYMBOL(tcp_syn_ack_timeout);

void tcp_set_keepalive(struct sock *sk, int val)
{
    if ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN))
        return;

    if (val && !sock_flag(sk, SOCK_KEEPOPEN))
        inet_csk_reset_keepalive_timer(sk, keepalive_time_when(tcp_sk(sk)));
    else if (!val)
        inet_csk_delete_keepalive_timer(sk);
}


static void tcp_keepalive_timer (unsigned long data)
{
    struct sock *sk = (struct sock *) data;
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    u32 elapsed;

    /* Only process if socket is not in use. */
    bh_lock_sock(sk);
    if (sock_owned_by_user(sk)) {
        /* Try again later. */
        inet_csk_reset_keepalive_timer (sk, HZ/20);
        goto out;
    }

    if (sk->sk_state == TCP_LISTEN) {
        tcp_synack_timer(sk);
        goto out;
    }

    if (sk->sk_state == TCP_FIN_WAIT2 && sock_flag(sk, SOCK_DEAD)) {
        if (tp->linger2 >= 0) {
            const int tmo = tcp_fin_time(sk) - TCP_TIMEWAIT_LEN;

            if (tmo > 0) {
                tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
                goto out;
            }
        }
        tcp_send_active_reset(sk, GFP_ATOMIC);
        goto death;
    }

    if (!sock_flag(sk, SOCK_KEEPOPEN) || sk->sk_state == TCP_CLOSE)
        goto out;

    elapsed = keepalive_time_when(tp);

    /* It is alive without keepalive 8) */
    if (tp->packets_out || tcp_send_head(sk))
        goto resched;

    elapsed = keepalive_time_elapsed(tp);

    if (elapsed >= keepalive_time_when(tp)) {
        /* If the TCP_USER_TIMEOUT option is enabled, use that
         * to determine when to timeout instead.
         */
        if ((icsk->icsk_user_timeout != 0 &&
            elapsed >= icsk->icsk_user_timeout &&
            icsk->icsk_probes_out > 0) ||
            (icsk->icsk_user_timeout == 0 &&
            icsk->icsk_probes_out >= keepalive_probes(tp))) {
            tcp_send_active_reset(sk, GFP_ATOMIC);
            tcp_write_err(sk);
            goto out;
        }
        if (tcp_write_wakeup(sk) <= 0) {
            icsk->icsk_probes_out++;
            elapsed = keepalive_intvl_when(tp);
        } else {
            /* If keepalive was lost due to local congestion,
             * try harder.
             */
            elapsed = TCP_RESOURCE_PROBE_INTERVAL;
        }
    } else {
        /* It is tp->rcv_tstamp + keepalive_time_when(tp) */
        elapsed = keepalive_time_when(tp) - elapsed;
    }

    sk_mem_reclaim(sk);

resched:
    inet_csk_reset_keepalive_timer (sk, elapsed);
    goto out;

death:
    tcp_done(sk);

out:
    bh_unlock_sock(sk);
    sock_put(sk);
}

void tcp_init_xmit_timers(struct sock *sk)
{
    inet_csk_init_xmit_timers(sk, &tcp_write_timer, &tcp_delack_timer,
                  &tcp_keepalive_timer);
}
EXPORT_SYMBOL(tcp_init_xmit_timers);