/*
 * 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, <waltje@uWalt.NL.Mugnet.ORG>
 *              Mark Evans, <evansmp@uhura.aston.ac.uk>
 *              Corey Minyard <wf-rch!minyard@relay.EU.net>
 *              Florian La Roche, <flla@stud.uni-sb.de>
 *              Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
 *              Linus Torvalds, <torvalds@cs.helsinki.fi>
 *              Alan Cox, <gw4pts@gw4pts.ampr.org>
 *              Matthew Dillon, <dillon@apollo.west.oic.com>
 *              Arnt Gulbrandsen, <agulbra@nvg.unit.no>
 *              Jorge Cwik, <jorge@laser.satlink.net>
 */

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

/**
 *  tcp_write_err() - close socket and save error info
 *  @sk:  The socket the error has appeared on.
 *
 *  Returns: Nothing (void)
 */

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(sock_net(sk), LINUX_MIB_TCPABORTONTIMEOUT);
}

/**
 *  tcp_out_of_resources() - Close socket if out of resources
 *  @sk:        pointer to current socket
 *  @do_reset:  send a last packet with reset flag
 *
 *  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.
 *
 *  Also close if our net namespace is exiting; in that case there is no
 *  hope of ever communicating again since all netns interfaces are already
 *  down (or about to be down), and we need to release our dst references,
 *  which have been moved to the netns loopback interface, so the namespace
 *  can finish exiting.  This condition is only possible if we are a kernel
 *  socket, as those do not hold references to the namespace.
 *
 *  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.
 *  3. If our net namespace is exiting.
 */
static int tcp_out_of_resources(struct sock *sk, bool 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_jiffies32 - 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_jiffies32 - tp->lsndtime) <= TCP_TIMEWAIT_LEN ||
                    /*  2. Window is closed. */
                    (!tp->snd_wnd && !tp->packets_out))
                        do_reset = true;
                if (do_reset)
                        tcp_send_active_reset(sk, GFP_ATOMIC);
                tcp_done(sk);
                __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONMEMORY);
                return 1;
        }

        if (!check_net(sock_net(sk))) {
                /* Not possible to send reset; just close */
                tcp_done(sk);
                return 1;
        }

        return 0;
}

/**
 *  tcp_orphan_retries() - Returns maximal number of retries on an orphaned socket
 *  @sk:    Pointer to the current socket.
 *  @alive: bool, socket alive state
 */
static int tcp_orphan_retries(struct sock *sk, bool alive)
{
        int retries = sock_net(sk)->ipv4.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)
{
        const struct net *net = sock_net(sk);
        int mss;

        /* Black hole detection */
        if (!net->ipv4.sysctl_tcp_mtu_probing)
                return;

        if (!icsk->icsk_mtup.enabled) {
                icsk->icsk_mtup.enabled = 1;
                icsk->icsk_mtup.probe_timestamp = tcp_jiffies32;
        } else {
                mss = tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low) >> 1;
                mss = min(net->ipv4.sysctl_tcp_base_mss, mss);
                mss = max(mss, 68 - tcp_sk(sk)->tcp_header_len);
                icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
        }
        tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
}


/**
 *  retransmits_timed_out() - returns true if this connection has timed out
 *  @sk:       The current socket
 *  @boundary: max number of retransmissions
 *  @timeout:  A custom timeout value.
 *             If set to 0 the default timeout is calculated and used.
 *             Using TCP_RTO_MIN and the number of unsuccessful retransmits.
 *
 * The default "timeout" value this function can calculate and use
 * is equivalent to the timeout of a TCP Connection
 * after "boundary" unsuccessful, exponentially backed-off
 * retransmissions with an initial RTO of TCP_RTO_MIN.
 */
static bool retransmits_timed_out(struct sock *sk,
                                  unsigned int boundary,
                                  unsigned int timeout)
{
        const unsigned int rto_base = TCP_RTO_MIN;
        unsigned int linear_backoff_thresh, start_ts;

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

        start_ts = tcp_sk(sk)->retrans_stamp;
        if (unlikely(!start_ts)) {
                struct sk_buff *head = tcp_rtx_queue_head(sk);

                if (!head)
                        return false;
                start_ts = tcp_skb_timestamp(head);
        }

        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(tcp_sk(sk)) - start_ts) >= jiffies_to_msecs(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);
        struct tcp_sock *tp = tcp_sk(sk);
        struct net *net = sock_net(sk);
        bool expired, do_reset;
        int retry_until;

        if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
                if (icsk->icsk_retransmits) {
                        dst_negative_advice(sk);
                        if (tp->syn_fastopen || tp->syn_data)
                                tcp_fastopen_cache_set(sk, 0, NULL, true, 0);
                        if (tp->syn_data && icsk->icsk_retransmits == 1)
                                NET_INC_STATS(sock_net(sk),
                                              LINUX_MIB_TCPFASTOPENACTIVEFAIL);
                } else if (!tp->syn_data && !tp->syn_fastopen) {
                        sk_rethink_txhash(sk);
                }
                retry_until = icsk->icsk_syn_retries ? : net->ipv4.sysctl_tcp_syn_retries;
                expired = icsk->icsk_retransmits >= retry_until;
        } else {
                if (retransmits_timed_out(sk, net->ipv4.sysctl_tcp_retries1, 0)) {
                        /* Some middle-boxes may black-hole Fast Open _after_
                         * the handshake. Therefore we conservatively disable
                         * Fast Open on this path on recurring timeouts after
                         * successful Fast Open.
                         */
                        if (tp->syn_data_acked) {
                                tcp_fastopen_cache_set(sk, 0, NULL, true, 0);
                                if (icsk->icsk_retransmits == net->ipv4.sysctl_tcp_retries1)
                                        NET_INC_STATS(sock_net(sk),
                                                      LINUX_MIB_TCPFASTOPENACTIVEFAIL);
                        }
                        /* Black hole detection */
                        tcp_mtu_probing(icsk, sk);

                        dst_negative_advice(sk);
                } else {
                        sk_rethink_txhash(sk);
                }

                retry_until = net->ipv4.sysctl_tcp_retries2;
                if (sock_flag(sk, SOCK_DEAD)) {
                        const bool alive = icsk->icsk_rto < TCP_RTO_MAX;

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

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

/* Called with BH disabled */
void tcp_delack_timer_handler(struct sock *sk)
{
        struct inet_connection_sock *icsk = inet_csk(sk);

        sk_mem_reclaim_partial(sk);

        if (((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)) ||
            !(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 (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_mstamp_refresh(tcp_sk(sk));
                tcp_send_ack(sk);
                __NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKS);
        }

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


/**
 *  tcp_delack_timer() - The TCP delayed ACK timeout handler
 *  @data:  Pointer to the current socket. (gets casted to struct sock *)
 *
 *  This function gets (indirectly) called when the kernel timer for a TCP packet
 *  of this socket expires. Calls tcp_delack_timer_handler() to do the actual work.
 *
 *  Returns: Nothing (void)
 */
static void tcp_delack_timer(struct timer_list *t)
{
        struct inet_connection_sock *icsk =
                        from_timer(icsk, t, icsk_delack_timer);
        struct sock *sk = &icsk->icsk_inet.sk;

        bh_lock_sock(sk);
        if (!sock_owned_by_user(sk)) {
                tcp_delack_timer_handler(sk);
        } else {
                icsk->icsk_ack.blocked = 1;
                __NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKLOCKED);
                /* deleguate our work to tcp_release_cb() */
                if (!test_and_set_bit(TCP_DELACK_TIMER_DEFERRED, &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 sk_buff *skb = tcp_send_head(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        int max_probes;
        u32 start_ts;

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

        /* RFC 1122 4.2.2.17 requires the sender to stay open indefinitely as
         * long as the receiver continues to respond probes. We support this by
         * default and reset icsk_probes_out with incoming ACKs. But if the
         * socket is orphaned or the user specifies TCP_USER_TIMEOUT, we
         * kill the socket when the retry count and the time exceeds the
         * corresponding system limit. We also implement similar policy when
         * we use RTO to probe window in tcp_retransmit_timer().
         */
        start_ts = tcp_skb_timestamp(skb);
        if (!start_ts)
                skb->skb_mstamp = tp->tcp_mstamp;
        else if (icsk->icsk_user_timeout &&
                 (s32)(tcp_time_stamp(tp) - start_ts) >
                 jiffies_to_msecs(icsk->icsk_user_timeout))
                goto abort;

        max_probes = sock_net(sk)->ipv4.sysctl_tcp_retries2;
        if (sock_flag(sk, SOCK_DEAD)) {
                const bool alive = inet_csk_rto_backoff(icsk, TCP_RTO_MAX) < TCP_RTO_MAX;

                max_probes = tcp_orphan_retries(sk, alive);
                if (!alive && icsk->icsk_backoff >= max_probes)
                        goto abort;
                if (tcp_out_of_resources(sk, true))
                        return;
        }

        if (icsk->icsk_probes_out > max_probes) {
abort:          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 ? :
            sock_net(sk)->ipv4.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(req);

        if (req->num_timeout >= 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.
         */
        inet_rtx_syn_ack(sk, req);
        req->num_timeout++;
        icsk->icsk_retransmits++;
        inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                          TCP_TIMEOUT_INIT << req->num_timeout, TCP_RTO_MAX);
}


/**
 *  tcp_retransmit_timer() - The TCP retransmit timeout handler
 *  @sk:  Pointer to the current socket.
 *
 *  This function gets called when the kernel timer for a TCP packet
 *  of this socket expires.
 *
 *  It handles retransmission, timer adjustment and other necesarry measures.
 *
 *  Returns: Nothing (void)
 */
void tcp_retransmit_timer(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct net *net = sock_net(sk);
        struct inet_connection_sock *icsk = inet_csk(sk);

        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_rtx_queue_empty(sk));

        tp->tlp_high_seq = 0;

        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) {
                        net_dbg_ratelimited("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) {
                        net_dbg_ratelimited("Peer %pI6:%u/%u unexpectedly shrunk window %u:%u (repaired)\n",
                                            &sk->sk_v6_daddr,
                                            ntohs(inet->inet_dport),
                                            inet->inet_num,
                                            tp->snd_una, tp->snd_nxt);
                }
#endif
                if (tcp_jiffies32 - tp->rcv_tstamp > TCP_RTO_MAX) {
                        tcp_write_err(sk);
                        goto out;
                }
                tcp_enter_loss(sk);
                tcp_retransmit_skb(sk, tcp_rtx_queue_head(sk), 1);
                __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(sock_net(sk), mib_idx);
        }

        tcp_enter_loss(sk);

        if (tcp_retransmit_skb(sk, tcp_rtx_queue_head(sk), 1) > 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 || net->ipv4.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, net->ipv4.sysctl_tcp_retries1 + 1, 0))
                __sk_dst_reset(sk);

out:;
}

/* Called with bottom-half processing disabled.
   Called by tcp_write_timer() */
void tcp_write_timer_handler(struct sock *sk)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        int event;

        if (((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)) ||
            !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;
        }

        tcp_mstamp_refresh(tcp_sk(sk));
        event = icsk->icsk_pending;

        switch (event) {
        case ICSK_TIME_REO_TIMEOUT:
                tcp_rack_reo_timeout(sk);
                break;
        case ICSK_TIME_LOSS_PROBE:
                tcp_send_loss_probe(sk);
                break;
        case ICSK_TIME_RETRANS:
                icsk->icsk_pending = 0;
                tcp_retransmit_timer(sk);
                break;
        case ICSK_TIME_PROBE0:
                icsk->icsk_pending = 0;
                tcp_probe_timer(sk);
                break;
        }

out:
        sk_mem_reclaim(sk);
}

static void tcp_write_timer(struct timer_list *t)
{
        struct inet_connection_sock *icsk =
                        from_timer(icsk, t, icsk_retransmit_timer);
        struct sock *sk = &icsk->icsk_inet.sk;

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

void tcp_syn_ack_timeout(const struct request_sock *req)
{
        struct net *net = read_pnet(&inet_rsk(req)->ireq_net);

        __NET_INC_STATS(net, 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);
}
EXPORT_SYMBOL_GPL(tcp_set_keepalive);


static void tcp_keepalive_timer (struct timer_list *t)
{
        struct sock *sk = from_timer(sk, t, sk_timer);
        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) {
                pr_err("Hmm... keepalive on a LISTEN ???\n");
                goto out;
        }

        tcp_mstamp_refresh(tp);
        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) ||
            ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_SYN_SENT)))
                goto out;

        elapsed = keepalive_time_when(tp);

        /* It is alive without keepalive 8) */
        if (tp->packets_out || !tcp_write_queue_empty(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, LINUX_MIB_TCPKEEPALIVE) <= 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);
        hrtimer_init(&tcp_sk(sk)->pacing_timer, CLOCK_MONOTONIC,
                     HRTIMER_MODE_ABS_PINNED);
        tcp_sk(sk)->pacing_timer.function = tcp_pace_kick;
}