dn_nsp_in.c

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/*
 * DECnet       An implementation of the DECnet protocol suite for the LINUX
 *              operating system.  DECnet is implemented using the  BSD Socket
 *              interface as the means of communication with the user level.
 *
 *              DECnet Network Services Protocol (Input)
 *
 * Author:      Eduardo Marcelo Serrat <[email protected]>
 *
 * Changes:
 *
 *    Steve Whitehouse:  Split into dn_nsp_in.c and dn_nsp_out.c from
 *                       original dn_nsp.c.
 *    Steve Whitehouse:  Updated to work with my new routing architecture.
 *    Steve Whitehouse:  Add changes from Eduardo Serrat's patches.
 *    Steve Whitehouse:  Put all ack handling code in a common routine.
 *    Steve Whitehouse:  Put other common bits into dn_nsp_rx()
 *    Steve Whitehouse:  More checks on skb->len to catch bogus packets
 *                       Fixed various race conditions and possible nasties.
 *    Steve Whitehouse:  Now handles returned conninit frames.
 *     David S. Miller:  New socket locking
 *    Steve Whitehouse:  Fixed lockup when socket filtering was enabled.
 *         Paul Koning:  Fix to push CC sockets into RUN when acks are
 *                       received.
 *    Steve Whitehouse:
 *   Patrick Caulfield:  Checking conninits for correctness & sending of error
 *                       responses.
 *    Steve Whitehouse:  Added backlog congestion level return codes.
 *   Patrick Caulfield:
 *    Steve Whitehouse:  Added flow control support (outbound)
 *    Steve Whitehouse:  Prepare for nonlinear skbs
 */

/******************************************************************************
    (c) 1995-1998 E.M. Serrat       [email protected]

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
*******************************************************************************/

#include <linux/errno.h>
#include <linux/types.h>
#include <linux/socket.h>
#include <linux/in.h>
#include <linux/kernel.h>
#include <linux/timer.h>
#include <linux/string.h>
#include <linux/sockios.h>
#include <linux/net.h>
#include <linux/netdevice.h>
#include <linux/inet.h>
#include <linux/route.h>
#include <linux/slab.h>
#include <net/sock.h>
#include <net/tcp_states.h>
#include <linux/fcntl.h>
#include <linux/mm.h>
#include <linux/termios.h>
#include <linux/interrupt.h>
#include <linux/proc_fs.h>
#include <linux/stat.h>
#include <linux/init.h>
#include <linux/poll.h>
#include <linux/netfilter_decnet.h>
#include <net/neighbour.h>
#include <net/dst.h>
#include <net/dn.h>
#include <net/dn_nsp.h>
#include <net/dn_dev.h>
#include <net/dn_route.h>

extern int decnet_log_martians;

static void dn_log_martian(struct sk_buff *skb, const char *msg)
{
    if (decnet_log_martians) {
        char *devname = skb->dev ? skb->dev->name : "???";
        struct dn_skb_cb *cb = DN_SKB_CB(skb);
        net_info_ratelimited("DECnet: Martian packet (%s) dev=%s src=0x%04hx dst=0x%04hx srcport=0x%04hx dstport=0x%04hx\n",
                     msg, devname,
                     le16_to_cpu(cb->src),
                     le16_to_cpu(cb->dst),
                     le16_to_cpu(cb->src_port),
                     le16_to_cpu(cb->dst_port));
    }
}

/*
 * For this function we've flipped the cross-subchannel bit
 * if the message is an otherdata or linkservice message. Thus
 * we can use it to work out what to update.
 */
static void dn_ack(struct sock *sk, struct sk_buff *skb, unsigned short ack)
{
    struct dn_scp *scp = DN_SK(sk);
    unsigned short type = ((ack >> 12) & 0x0003);
    int wakeup = 0;

    switch (type) {
    case 0: /* ACK - Data */
        if (dn_after(ack, scp->ackrcv_dat)) {
            scp->ackrcv_dat = ack & 0x0fff;
            wakeup |= dn_nsp_check_xmit_queue(sk, skb,
                              &scp->data_xmit_queue,
                              ack);
        }
        break;
    case 1: /* NAK - Data */
        break;
    case 2: /* ACK - OtherData */
        if (dn_after(ack, scp->ackrcv_oth)) {
            scp->ackrcv_oth = ack & 0x0fff;
            wakeup |= dn_nsp_check_xmit_queue(sk, skb,
                              &scp->other_xmit_queue,
                              ack);
        }
        break;
    case 3: /* NAK - OtherData */
        break;
    }

    if (wakeup && !sock_flag(sk, SOCK_DEAD))
        sk->sk_state_change(sk);
}

/*
 * This function is a universal ack processor.
 */
static int dn_process_ack(struct sock *sk, struct sk_buff *skb, int oth)
{
    __le16 *ptr = (__le16 *)skb->data;
    int len = 0;
    unsigned short ack;

    if (skb->len < 2)
        return len;

    if ((ack = le16_to_cpu(*ptr)) & 0x8000) {
        skb_pull(skb, 2);
        ptr++;
        len += 2;
        if ((ack & 0x4000) == 0) {
            if (oth)
                ack ^= 0x2000;
            dn_ack(sk, skb, ack);
        }
    }

    if (skb->len < 2)
        return len;

    if ((ack = le16_to_cpu(*ptr)) & 0x8000) {
        skb_pull(skb, 2);
        len += 2;
        if ((ack & 0x4000) == 0) {
            if (oth)
                ack ^= 0x2000;
            dn_ack(sk, skb, ack);
        }
    }

    return len;
}


static inline int dn_check_idf(unsigned char **pptr, int *len, unsigned char max, unsigned char follow_on)
{
    unsigned char *ptr = *pptr;
    unsigned char flen = *ptr++;

    (*len)--;
    if (flen > max)
        return -1;
    if ((flen + follow_on) > *len)
        return -1;

    *len -= flen;
    *pptr = ptr + flen;
    return 0;
}

/*
 * Table of reason codes to pass back to node which sent us a badly
 * formed message, plus text messages for the log. A zero entry in
 * the reason field means "don't reply" otherwise a disc init is sent with
 * the specified reason code.
 */
static struct {
    unsigned short reason;
    const char *text;
} ci_err_table[] = {
 { 0,             "CI: Truncated message" },
 { NSP_REASON_ID, "CI: Destination username error" },
 { NSP_REASON_ID, "CI: Destination username type" },
 { NSP_REASON_US, "CI: Source username error" },
 { 0,             "CI: Truncated at menuver" },
 { 0,             "CI: Truncated before access or user data" },
 { NSP_REASON_IO, "CI: Access data format error" },
 { NSP_REASON_IO, "CI: User data format error" }
};

/*
 * This function uses a slightly different lookup method
 * to find its sockets, since it searches on object name/number
 * rather than port numbers. Various tests are done to ensure that
 * the incoming data is in the correct format before it is queued to
 * a socket.
 */
static struct sock *dn_find_listener(struct sk_buff *skb, unsigned short *reason)
{
    struct dn_skb_cb *cb = DN_SKB_CB(skb);
    struct nsp_conn_init_msg *msg = (struct nsp_conn_init_msg *)skb->data;
    struct sockaddr_dn dstaddr;
    struct sockaddr_dn srcaddr;
    unsigned char type = 0;
    int dstlen;
    int srclen;
    unsigned char *ptr;
    int len;
    int err = 0;
    unsigned char menuver;

    memset(&dstaddr, 0, sizeof(struct sockaddr_dn));
    memset(&srcaddr, 0, sizeof(struct sockaddr_dn));

    /*
     * 1. Decode & remove message header
     */
    cb->src_port = msg->srcaddr;
    cb->dst_port = msg->dstaddr;
    cb->services = msg->services;
    cb->info     = msg->info;
    cb->segsize  = le16_to_cpu(msg->segsize);

    if (!pskb_may_pull(skb, sizeof(*msg)))
        goto err_out;

    skb_pull(skb, sizeof(*msg));

    len = skb->len;
    ptr = skb->data;

    /*
     * 2. Check destination end username format
     */
    dstlen = dn_username2sockaddr(ptr, len, &dstaddr, &type);
    err++;
    if (dstlen < 0)
        goto err_out;

    err++;
    if (type > 1)
        goto err_out;

    len -= dstlen;
    ptr += dstlen;

    /*
     * 3. Check source end username format
     */
    srclen = dn_username2sockaddr(ptr, len, &srcaddr, &type);
    err++;
    if (srclen < 0)
        goto err_out;

    len -= srclen;
    ptr += srclen;
    err++;
    if (len < 1)
        goto err_out;

    menuver = *ptr;
    ptr++;
    len--;

    /*
     * 4. Check that optional data actually exists if menuver says it does
     */
    err++;
    if ((menuver & (DN_MENUVER_ACC | DN_MENUVER_USR)) && (len < 1))
        goto err_out;

    /*
     * 5. Check optional access data format
     */
    err++;
    if (menuver & DN_MENUVER_ACC) {
        if (dn_check_idf(&ptr, &len, 39, 1))
            goto err_out;
        if (dn_check_idf(&ptr, &len, 39, 1))
            goto err_out;
        if (dn_check_idf(&ptr, &len, 39, (menuver & DN_MENUVER_USR) ? 1 : 0))
            goto err_out;
    }

    /*
     * 6. Check optional user data format
     */
    err++;
    if (menuver & DN_MENUVER_USR) {
        if (dn_check_idf(&ptr, &len, 16, 0))
            goto err_out;
    }

    /*
     * 7. Look up socket based on destination end username
     */
    return dn_sklist_find_listener(&dstaddr);
err_out:
    dn_log_martian(skb, ci_err_table[err].text);
    *reason = ci_err_table[err].reason;
    return NULL;
}


static void dn_nsp_conn_init(struct sock *sk, struct sk_buff *skb)
{
    if (sk_acceptq_is_full(sk)) {
        kfree_skb(skb);
        return;
    }

    sk->sk_ack_backlog++;
    skb_queue_tail(&sk->sk_receive_queue, skb);
    sk->sk_state_change(sk);
}

static void dn_nsp_conn_conf(struct sock *sk, struct sk_buff *skb)
{
    struct dn_skb_cb *cb = DN_SKB_CB(skb);
    struct dn_scp *scp = DN_SK(sk);
    unsigned char *ptr;

    if (skb->len < 4)
        goto out;

    ptr = skb->data;
    cb->services = *ptr++;
    cb->info = *ptr++;
    cb->segsize = le16_to_cpu(*(__le16 *)ptr);

    if ((scp->state == DN_CI) || (scp->state == DN_CD)) {
        scp->persist = 0;
        scp->addrrem = cb->src_port;
        sk->sk_state = TCP_ESTABLISHED;
        scp->state = DN_RUN;
        scp->services_rem = cb->services;
        scp->info_rem = cb->info;
        scp->segsize_rem = cb->segsize;

        if ((scp->services_rem & NSP_FC_MASK) == NSP_FC_NONE)
            scp->max_window = decnet_no_fc_max_cwnd;

        if (skb->len > 0) {
            u16 dlen = *skb->data;
            if ((dlen <= 16) && (dlen <= skb->len)) {
                scp->conndata_in.opt_optl = cpu_to_le16(dlen);
                skb_copy_from_linear_data_offset(skb, 1,
                          scp->conndata_in.opt_data, dlen);
            }
        }
        dn_nsp_send_link(sk, DN_NOCHANGE, 0);
        if (!sock_flag(sk, SOCK_DEAD))
            sk->sk_state_change(sk);
    }

out:
    kfree_skb(skb);
}

static void dn_nsp_conn_ack(struct sock *sk, struct sk_buff *skb)
{
    struct dn_scp *scp = DN_SK(sk);

    if (scp->state == DN_CI) {
        scp->state = DN_CD;
        scp->persist = 0;
    }

    kfree_skb(skb);
}

static void dn_nsp_disc_init(struct sock *sk, struct sk_buff *skb)
{
    struct dn_scp *scp = DN_SK(sk);
    struct dn_skb_cb *cb = DN_SKB_CB(skb);
    unsigned short reason;

    if (skb->len < 2)
        goto out;

    reason = le16_to_cpu(*(__le16 *)skb->data);
    skb_pull(skb, 2);

    scp->discdata_in.opt_status = cpu_to_le16(reason);
    scp->discdata_in.opt_optl   = 0;
    memset(scp->discdata_in.opt_data, 0, 16);

    if (skb->len > 0) {
        u16 dlen = *skb->data;
        if ((dlen <= 16) && (dlen <= skb->len)) {
            scp->discdata_in.opt_optl = cpu_to_le16(dlen);
            skb_copy_from_linear_data_offset(skb, 1, scp->discdata_in.opt_data, dlen);
        }
    }

    scp->addrrem = cb->src_port;
    sk->sk_state = TCP_CLOSE;

    switch (scp->state) {
    case DN_CI:
    case DN_CD:
        scp->state = DN_RJ;
        sk->sk_err = ECONNREFUSED;
        break;
    case DN_RUN:
        sk->sk_shutdown |= SHUTDOWN_MASK;
        scp->state = DN_DN;
        break;
    case DN_DI:
        scp->state = DN_DIC;
        break;
    }

    if (!sock_flag(sk, SOCK_DEAD)) {
        if (sk->sk_socket->state != SS_UNCONNECTED)
            sk->sk_socket->state = SS_DISCONNECTING;
        sk->sk_state_change(sk);
    }

    /*
     * It appears that its possible for remote machines to send disc
     * init messages with no port identifier if we are in the CI and
     * possibly also the CD state. Obviously we shouldn't reply with
     * a message if we don't know what the end point is.
     */
    if (scp->addrrem) {
        dn_nsp_send_disc(sk, NSP_DISCCONF, NSP_REASON_DC, GFP_ATOMIC);
    }
    scp->persist_fxn = dn_destroy_timer;
    scp->persist = dn_nsp_persist(sk);

out:
    kfree_skb(skb);
}

/*
 * disc_conf messages are also called no_resources or no_link
 * messages depending upon the "reason" field.
 */
static void dn_nsp_disc_conf(struct sock *sk, struct sk_buff *skb)
{
    struct dn_scp *scp = DN_SK(sk);
    unsigned short reason;

    if (skb->len != 2)
        goto out;

    reason = le16_to_cpu(*(__le16 *)skb->data);

    sk->sk_state = TCP_CLOSE;

    switch (scp->state) {
    case DN_CI:
        scp->state = DN_NR;
        break;
    case DN_DR:
        if (reason == NSP_REASON_DC)
            scp->state = DN_DRC;
        if (reason == NSP_REASON_NL)
            scp->state = DN_CN;
        break;
    case DN_DI:
        scp->state = DN_DIC;
        break;
    case DN_RUN:
        sk->sk_shutdown |= SHUTDOWN_MASK;
    case DN_CC:
        scp->state = DN_CN;
    }

    if (!sock_flag(sk, SOCK_DEAD)) {
        if (sk->sk_socket->state != SS_UNCONNECTED)
            sk->sk_socket->state = SS_DISCONNECTING;
        sk->sk_state_change(sk);
    }

    scp->persist_fxn = dn_destroy_timer;
    scp->persist = dn_nsp_persist(sk);

out:
    kfree_skb(skb);
}

static void dn_nsp_linkservice(struct sock *sk, struct sk_buff *skb)
{
    struct dn_scp *scp = DN_SK(sk);
    unsigned short segnum;
    unsigned char lsflags;
    signed char fcval;
    int wake_up = 0;
    char *ptr = skb->data;
    unsigned char fctype = scp->services_rem & NSP_FC_MASK;

    if (skb->len != 4)
        goto out;

    segnum = le16_to_cpu(*(__le16 *)ptr);
    ptr += 2;
    lsflags = *(unsigned char *)ptr++;
    fcval = *ptr;

    /*
     * Here we ignore erronous packets which should really
     * should cause a connection abort. It is not critical
     * for now though.
     */
    if (lsflags & 0xf8)
        goto out;

    if (seq_next(scp->numoth_rcv, segnum)) {
        seq_add(&scp->numoth_rcv, 1);
        switch(lsflags & 0x04) { /* FCVAL INT */
        case 0x00: /* Normal Request */
            switch(lsflags & 0x03) { /* FCVAL MOD */
            case 0x00: /* Request count */
                if (fcval < 0) {
                    unsigned char p_fcval = -fcval;
                    if ((scp->flowrem_dat > p_fcval) &&
                        (fctype == NSP_FC_SCMC)) {
                        scp->flowrem_dat -= p_fcval;
                    }
                } else if (fcval > 0) {
                    scp->flowrem_dat += fcval;
                    wake_up = 1;
                }
                break;
            case 0x01: /* Stop outgoing data */
                scp->flowrem_sw = DN_DONTSEND;
                break;
            case 0x02: /* Ok to start again */
                scp->flowrem_sw = DN_SEND;
                dn_nsp_output(sk);
                wake_up = 1;
            }
            break;
        case 0x04: /* Interrupt Request */
            if (fcval > 0) {
                scp->flowrem_oth += fcval;
                wake_up = 1;
            }
            break;
        }
        if (wake_up && !sock_flag(sk, SOCK_DEAD))
            sk->sk_state_change(sk);
    }

    dn_nsp_send_oth_ack(sk);

out:
    kfree_skb(skb);
}

/*
 * Copy of sock_queue_rcv_skb (from sock.h) without
 * bh_lock_sock() (its already held when this is called) which
 * also allows data and other data to be queued to a socket.
 */
static __inline__ int dn_queue_skb(struct sock *sk, struct sk_buff *skb, int sig, struct sk_buff_head *queue)
{
    int err;
    int skb_len;

    /* Cast skb->rcvbuf to unsigned... It's pointless, but reduces
       number of warnings when compiling with -W --ANK
     */
    if (atomic_read(&sk->sk_rmem_alloc) + skb->truesize >=
        (unsigned int)sk->sk_rcvbuf) {
        err = -ENOMEM;
        goto out;
    }

    err = sk_filter(sk, skb);
    if (err)
        goto out;

    skb_len = skb->len;
    skb_set_owner_r(skb, sk);
    skb_queue_tail(queue, skb);

    if (!sock_flag(sk, SOCK_DEAD))
        sk->sk_data_ready(sk, skb_len);
out:
    return err;
}

static void dn_nsp_otherdata(struct sock *sk, struct sk_buff *skb)
{
    struct dn_scp *scp = DN_SK(sk);
    unsigned short segnum;
    struct dn_skb_cb *cb = DN_SKB_CB(skb);
    int queued = 0;

    if (skb->len < 2)
        goto out;

    cb->segnum = segnum = le16_to_cpu(*(__le16 *)skb->data);
    skb_pull(skb, 2);

    if (seq_next(scp->numoth_rcv, segnum)) {

        if (dn_queue_skb(sk, skb, SIGURG, &scp->other_receive_queue) == 0) {
            seq_add(&scp->numoth_rcv, 1);
            scp->other_report = 0;
            queued = 1;
        }
    }

    dn_nsp_send_oth_ack(sk);
out:
    if (!queued)
        kfree_skb(skb);
}

static void dn_nsp_data(struct sock *sk, struct sk_buff *skb)
{
    int queued = 0;
    unsigned short segnum;
    struct dn_skb_cb *cb = DN_SKB_CB(skb);
    struct dn_scp *scp = DN_SK(sk);

    if (skb->len < 2)
        goto out;

    cb->segnum = segnum = le16_to_cpu(*(__le16 *)skb->data);
    skb_pull(skb, 2);

    if (seq_next(scp->numdat_rcv, segnum)) {
        if (dn_queue_skb(sk, skb, SIGIO, &sk->sk_receive_queue) == 0) {
            seq_add(&scp->numdat_rcv, 1);
            queued = 1;
        }

        if ((scp->flowloc_sw == DN_SEND) && dn_congested(sk)) {
            scp->flowloc_sw = DN_DONTSEND;
            dn_nsp_send_link(sk, DN_DONTSEND, 0);
        }
    }

    dn_nsp_send_data_ack(sk);
out:
    if (!queued)
        kfree_skb(skb);
}

/*
 * If one of our conninit messages is returned, this function
 * deals with it. It puts the socket into the NO_COMMUNICATION
 * state.
 */
static void dn_returned_conn_init(struct sock *sk, struct sk_buff *skb)
{
    struct dn_scp *scp = DN_SK(sk);

    if (scp->state == DN_CI) {
        scp->state = DN_NC;
        sk->sk_state = TCP_CLOSE;
        if (!sock_flag(sk, SOCK_DEAD))
            sk->sk_state_change(sk);
    }

    kfree_skb(skb);
}

static int dn_nsp_no_socket(struct sk_buff *skb, unsigned short reason)
{
    struct dn_skb_cb *cb = DN_SKB_CB(skb);
    int ret = NET_RX_DROP;

    /* Must not reply to returned packets */
    if (cb->rt_flags & DN_RT_F_RTS)
        goto out;

    if ((reason != NSP_REASON_OK) && ((cb->nsp_flags & 0x0c) == 0x08)) {
        switch (cb->nsp_flags & 0x70) {
        case 0x10:
        case 0x60: /* (Retransmitted) Connect Init */
            dn_nsp_return_disc(skb, NSP_DISCINIT, reason);
            ret = NET_RX_SUCCESS;
            break;
        case 0x20: /* Connect Confirm */
            dn_nsp_return_disc(skb, NSP_DISCCONF, reason);
            ret = NET_RX_SUCCESS;
            break;
        }
    }

out:
    kfree_skb(skb);
    return ret;
}

static int dn_nsp_rx_packet(struct sk_buff *skb)
{
    struct dn_skb_cb *cb = DN_SKB_CB(skb);
    struct sock *sk = NULL;
    unsigned char *ptr = (unsigned char *)skb->data;
    unsigned short reason = NSP_REASON_NL;

    if (!pskb_may_pull(skb, 2))
        goto free_out;

    skb_reset_transport_header(skb);
    cb->nsp_flags = *ptr++;

    if (decnet_debug_level & 2)
        printk(KERN_DEBUG "dn_nsp_rx: Message type 0x%02x\n", (int)cb->nsp_flags);

    if (cb->nsp_flags & 0x83)
        goto free_out;

    /*
     * Filter out conninits and useless packet types
     */
    if ((cb->nsp_flags & 0x0c) == 0x08) {
        switch (cb->nsp_flags & 0x70) {
        case 0x00: /* NOP */
        case 0x70: /* Reserved */
        case 0x50: /* Reserved, Phase II node init */
            goto free_out;
        case 0x10:
        case 0x60:
            if (unlikely(cb->rt_flags & DN_RT_F_RTS))
                goto free_out;
            sk = dn_find_listener(skb, &reason);
            goto got_it;
        }
    }

    if (!pskb_may_pull(skb, 3))
        goto free_out;

    /*
     * Grab the destination address.
     */
    cb->dst_port = *(__le16 *)ptr;
    cb->src_port = 0;
    ptr += 2;

    /*
     * If not a connack, grab the source address too.
     */
    if (pskb_may_pull(skb, 5)) {
        cb->src_port = *(__le16 *)ptr;
        ptr += 2;
        skb_pull(skb, 5);
    }

    /*
     * Returned packets...
     * Swap src & dst and look up in the normal way.
     */
    if (unlikely(cb->rt_flags & DN_RT_F_RTS)) {
        __le16 tmp = cb->dst_port;
        cb->dst_port = cb->src_port;
        cb->src_port = tmp;
        tmp = cb->dst;
        cb->dst = cb->src;
        cb->src = tmp;
    }

    /*
     * Find the socket to which this skb is destined.
     */
    sk = dn_find_by_skb(skb);
got_it:
    if (sk != NULL) {
        struct dn_scp *scp = DN_SK(sk);

        /* Reset backoff */
        scp->nsp_rxtshift = 0;

        /*
         * We linearize everything except data segments here.
         */
        if (cb->nsp_flags & ~0x60) {
            if (unlikely(skb_linearize(skb)))
                goto free_out;
        }

        return sk_receive_skb(sk, skb, 0);
    }

    return dn_nsp_no_socket(skb, reason);

free_out:
    kfree_skb(skb);
    return NET_RX_DROP;
}

int dn_nsp_rx(struct sk_buff *skb)
{
    return NF_HOOK(NFPROTO_DECNET, NF_DN_LOCAL_IN, skb, skb->dev, NULL,
               dn_nsp_rx_packet);
}

/*
 * This is the main receive routine for sockets. It is called
 * from the above when the socket is not busy, and also from
 * sock_release() when there is a backlog queued up.
 */
int dn_nsp_backlog_rcv(struct sock *sk, struct sk_buff *skb)
{
    struct dn_scp *scp = DN_SK(sk);
    struct dn_skb_cb *cb = DN_SKB_CB(skb);

    if (cb->rt_flags & DN_RT_F_RTS) {
        if (cb->nsp_flags == 0x18 || cb->nsp_flags == 0x68)
            dn_returned_conn_init(sk, skb);
        else
            kfree_skb(skb);
        return NET_RX_SUCCESS;
    }

    /*
     * Control packet.
     */
    if ((cb->nsp_flags & 0x0c) == 0x08) {
        switch (cb->nsp_flags & 0x70) {
        case 0x10:
        case 0x60:
            dn_nsp_conn_init(sk, skb);
            break;
        case 0x20:
            dn_nsp_conn_conf(sk, skb);
            break;
        case 0x30:
            dn_nsp_disc_init(sk, skb);
            break;
        case 0x40:
            dn_nsp_disc_conf(sk, skb);
            break;
        }

    } else if (cb->nsp_flags == 0x24) {
        /*
         * Special for connacks, 'cos they don't have
         * ack data or ack otherdata info.
         */
        dn_nsp_conn_ack(sk, skb);
    } else {
        int other = 1;

        /* both data and ack frames can kick a CC socket into RUN */
        if ((scp->state == DN_CC) && !sock_flag(sk, SOCK_DEAD)) {
            scp->state = DN_RUN;
            sk->sk_state = TCP_ESTABLISHED;
            sk->sk_state_change(sk);
        }

        if ((cb->nsp_flags & 0x1c) == 0)
            other = 0;
        if (cb->nsp_flags == 0x04)
            other = 0;

        /*
         * Read out ack data here, this applies equally
         * to data, other data, link serivce and both
         * ack data and ack otherdata.
         */
        dn_process_ack(sk, skb, other);

        /*
         * If we've some sort of data here then call a
         * suitable routine for dealing with it, otherwise
         * the packet is an ack and can be discarded.
         */
        if ((cb->nsp_flags & 0x0c) == 0) {

            if (scp->state != DN_RUN)
                goto free_out;

            switch (cb->nsp_flags) {
            case 0x10: /* LS */
                dn_nsp_linkservice(sk, skb);
                break;
            case 0x30: /* OD */
                dn_nsp_otherdata(sk, skb);
                break;
            default:
                dn_nsp_data(sk, skb);
            }

        } else { /* Ack, chuck it out here */
free_out:
            kfree_skb(skb);
        }
    }

    return NET_RX_SUCCESS;
}