// SPDX-License-Identifier: GPL-2.0-only
/*
 * 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>
#include <net/tcp_ecn.h>
#include <net/rstreason.h>

static u32 tcp_clamp_rto_to_user_timeout(const struct sock *sk)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        const struct tcp_sock *tp = tcp_sk(sk);
        u32 elapsed, user_timeout;
        s32 remaining;

        user_timeout = READ_ONCE(icsk->icsk_user_timeout);
        if (!user_timeout)
                return icsk->icsk_rto;

        elapsed = tcp_time_stamp_ts(tp) - tp->retrans_stamp;
        if (tp->tcp_usec_ts)
                elapsed /= USEC_PER_MSEC;

        remaining = user_timeout - elapsed;
        if (remaining <= 0)
                return 1; /* user timeout has passed; fire ASAP */

        return min_t(u32, icsk->icsk_rto, msecs_to_jiffies(remaining));
}

u32 tcp_clamp_probe0_to_user_timeout(const struct sock *sk, u32 when)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        u32 user_timeout;
        s32 remaining;
        s32 elapsed;

        user_timeout = READ_ONCE(icsk->icsk_user_timeout);
        if (!user_timeout || !icsk->icsk_probes_tstamp)
                return when;

        elapsed = tcp_jiffies32 - icsk->icsk_probes_tstamp;
        if (unlikely(elapsed < 0))
                elapsed = 0;
        remaining = msecs_to_jiffies(user_timeout) - elapsed;
        remaining = max_t(int, remaining, TCP_TIMEOUT_MIN);

        return min_t(u32, remaining, when);
}

/**
 *  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)
{
        tcp_done_with_error(sk, READ_ONCE(sk->sk_err_soft) ? : ETIMEDOUT);
        __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(sk) || !do_reset)
                shift++;

        /* If some dubious ICMP arrived, penalize even more. */
        if (READ_ONCE(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,
                                              SK_RST_REASON_TCP_ABORT_ON_MEMORY);
                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 = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_orphan_retries); /* May be zero. */

        /* We know from an ICMP that something is wrong. */
        if (READ_ONCE(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 (!READ_ONCE(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(READ_ONCE(net->ipv4.sysctl_tcp_base_mss), mss);
                mss = max(mss, READ_ONCE(net->ipv4.sysctl_tcp_mtu_probe_floor));
                mss = max(mss, READ_ONCE(net->ipv4.sysctl_tcp_min_snd_mss));
                icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
        }
        tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
}

static unsigned int tcp_model_timeout(struct sock *sk,
                                      unsigned int boundary,
                                      unsigned int rto_base)
{
        unsigned int linear_backoff_thresh, timeout;

        linear_backoff_thresh = ilog2(tcp_rto_max(sk) / 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(sk);
        return jiffies_to_msecs(timeout);
}
/**
 *  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)
{
        struct tcp_sock *tp = tcp_sk(sk);
        unsigned int start_ts, delta;

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

        start_ts = tp->retrans_stamp;
        if (likely(timeout == 0)) {
                unsigned int rto_base = TCP_RTO_MIN;

                if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV))
                        rto_base = tcp_timeout_init(sk);
                timeout = tcp_model_timeout(sk, boundary, rto_base);
        }

        if (tp->tcp_usec_ts) {
                /* delta maybe off up to a jiffy due to timer granularity. */
                delta = tp->tcp_mstamp - start_ts + jiffies_to_usecs(1);
                return (s32)(delta - timeout * USEC_PER_MSEC) >= 0;
        }
        return (s32)(tcp_time_stamp_ts(tp) - start_ts - timeout) >= 0;
}

/* 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 = false, do_reset;
        int retry_until, max_retransmits;

        if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
                if (icsk->icsk_retransmits)
                        __dst_negative_advice(sk);
                /* Paired with WRITE_ONCE() in tcp_sock_set_syncnt() */
                retry_until = READ_ONCE(icsk->icsk_syn_retries) ? :
                        READ_ONCE(net->ipv4.sysctl_tcp_syn_retries);

                max_retransmits = retry_until;
                if (sk->sk_state == TCP_SYN_SENT)
                        max_retransmits += READ_ONCE(net->ipv4.sysctl_tcp_syn_linear_timeouts);

                expired = icsk->icsk_retransmits >= max_retransmits;
        } else {
                if (retransmits_timed_out(sk, READ_ONCE(net->ipv4.sysctl_tcp_retries1), 0)) {
                        /* Black hole detection */
                        tcp_mtu_probing(icsk, sk);

                        __dst_negative_advice(sk);
                }

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

                        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;
                }
        }
        if (!expired)
                expired = retransmits_timed_out(sk, retry_until,
                                                READ_ONCE(icsk->icsk_user_timeout));
        tcp_fastopen_active_detect_blackhole(sk, expired);
        mptcp_active_detect_blackhole(sk, expired);

        if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RTO_CB_FLAG))
                tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RTO_CB,
                                  icsk->icsk_retransmits,
                                  icsk->icsk_rto, (int)expired);

        if (expired) {
                /* Has it gone just too far? */
                tcp_write_err(sk);
                return 1;
        }

        if (__sk_rethink_txhash_reset_dst(sk)) {
                WRITE_ONCE(tp->timeout_rehash, tp->timeout_rehash + 1);
                __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEOUTREHASH);
        }

        return 0;
}

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

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

        /* Handling the sack compression case */
        if (tp->compressed_ack) {
                tcp_mstamp_refresh(tp);
                tcp_sack_compress_send_ack(sk);
                return;
        }

        if (!(icsk->icsk_ack.pending & ICSK_ACK_TIMER))
                return;

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

        if (inet_csk_ack_scheduled(sk)) {
                if (!inet_csk_in_pingpong_mode(sk)) {
                        /* Delayed ACK missed: inflate ATO. */
                        icsk->icsk_ack.ato = min_t(u32, icsk->icsk_ack.ato << 1, icsk->icsk_rto);
                } else {
                        /* Delayed ACK missed: leave pingpong mode and
                         * deflate ATO.
                         */
                        inet_csk_exit_pingpong_mode(sk);
                        icsk->icsk_ack.ato      = TCP_ATO_MIN;
                }
                tcp_mstamp_refresh(tp);
                tcp_send_ack(sk);
                __NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKS);
        }
}


/**
 *  tcp_delack_timer() - The TCP delayed ACK timeout handler
 *  @t:  Pointer to the timer. (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 =
                        timer_container_of(icsk, t, icsk_delack_timer);
        struct sock *sk = &icsk->icsk_inet.sk;

        /* Avoid taking socket spinlock if there is no ACK to send.
         * The compressed_ack check is racy, but a separate hrtimer
         * will take care of it eventually.
         */
        if (!(smp_load_acquire(&icsk->icsk_ack.pending) & ICSK_ACK_TIMER) &&
            !READ_ONCE(tcp_sk(sk)->compressed_ack))
                goto out;

        bh_lock_sock(sk);
        if (!sock_owned_by_user(sk)) {
                tcp_delack_timer_handler(sk);
        } else {
                __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);
out:
        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;

        if (tp->packets_out || !skb) {
                WRITE_ONCE(icsk->icsk_probes_out, 0);
                icsk->icsk_probes_tstamp = 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().
         */
        if (!icsk->icsk_probes_tstamp) {
                icsk->icsk_probes_tstamp = tcp_jiffies32;
        } else {
                u32 user_timeout = READ_ONCE(icsk->icsk_user_timeout);

                if (user_timeout &&
                    (s32)(tcp_jiffies32 - icsk->icsk_probes_tstamp) >=
                     msecs_to_jiffies(user_timeout))
                        goto abort;
        }
        max_probes = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_retries2);
        if (sock_flag(sk, SOCK_DEAD)) {
                unsigned int rto_max = tcp_rto_max(sk);
                const bool alive = inet_csk_rto_backoff(icsk, rto_max) < 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);
        }
}

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

        if (!icsk->icsk_retransmits) {
                tp->total_rto_recoveries++;
                tp->rto_stamp = tcp_time_stamp_ms(tp);
        }
        WRITE_ONCE(icsk->icsk_retransmits, icsk->icsk_retransmits + 1);
        tp->total_rto++;
}

/*
 *      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 request_sock *req)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        int max_retries;

        tcp_syn_ack_timeout(req);

        /* Add one more retry for fastopen.
         * Paired with WRITE_ONCE() in tcp_sock_set_syncnt()
         */
        max_retries = READ_ONCE(icsk->icsk_syn_retries) ? :
                READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_synack_retries) + 1;

        if (req->num_timeout >= max_retries) {
                tcp_write_err(sk);
                return;
        }
        /* Lower cwnd after certain SYNACK timeout like tcp_init_transfer() */
        if (icsk->icsk_retransmits == 1)
                tcp_enter_loss(sk);
        /* 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.
         */
        tcp_rtx_synack(sk, req);
        if (req->num_retrans > 1 && tcp_rsk(req)->accecn_ok)
                tcp_rsk(req)->accecn_fail_mode |= TCP_ACCECN_ACE_FAIL_SEND;
        req->num_timeout++;
        tcp_update_rto_stats(sk);
        if (!tp->retrans_stamp)
                tp->retrans_stamp = tcp_time_stamp_ts(tp);
        tcp_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                          req->timeout << req->num_timeout, false);
}

static bool tcp_rtx_probe0_timed_out(const struct sock *sk,
                                     const struct sk_buff *skb,
                                     u32 rtx_delta)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        u32 user_timeout = READ_ONCE(icsk->icsk_user_timeout);
        const struct tcp_sock *tp = tcp_sk(sk);
        int timeout = tcp_rto_max(sk) * 2;
        s32 rcv_delta;

        if (user_timeout) {
                /* If user application specified a TCP_USER_TIMEOUT,
                 * it does not want win 0 packets to 'reset the timer'
                 * while retransmits are not making progress.
                 */
                if (rtx_delta > user_timeout)
                        return true;
                timeout = min_t(u32, timeout, msecs_to_jiffies(user_timeout));
        }
        /* Note: timer interrupt might have been delayed by at least one jiffy,
         * and tp->rcv_tstamp might very well have been written recently.
         * rcv_delta can thus be negative.
         */
        rcv_delta = tcp_timeout_expires(sk) - tp->rcv_tstamp;
        if (rcv_delta <= timeout)
                return false;

        return msecs_to_jiffies(rtx_delta) > timeout;
}

/**
 *  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 necessary 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);
        struct request_sock *req;
        struct sk_buff *skb;

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

        if (!tp->packets_out)
                return;

        skb = tcp_rtx_queue_head(sk);
        if (WARN_ON_ONCE(!skb))
                return;

        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);
                u32 rtx_delta;

                rtx_delta = tcp_time_stamp_ts(tp) - (tp->retrans_stamp ?: 
                                tcp_skb_timestamp_ts(tp->tcp_usec_ts, skb));
                if (tp->tcp_usec_ts)
                        rtx_delta /= USEC_PER_MSEC;

                if (sk->sk_family == AF_INET) {
                        net_dbg_ratelimited("Probing zero-window on %pI4:%u/%u, seq=%u:%u, recv %ums ago, lasting %ums\n",
                                &inet->inet_daddr, ntohs(inet->inet_dport),
                                inet->inet_num, tp->snd_una, tp->snd_nxt,
                                jiffies_to_msecs(jiffies - tp->rcv_tstamp),
                                rtx_delta);
                }
#if IS_ENABLED(CONFIG_IPV6)
                else if (sk->sk_family == AF_INET6) {
                        net_dbg_ratelimited("Probing zero-window on %pI6:%u/%u, seq=%u:%u, recv %ums ago, lasting %ums\n",
                                &sk->sk_v6_daddr, ntohs(inet->inet_dport),
                                inet->inet_num, tp->snd_una, tp->snd_nxt,
                                jiffies_to_msecs(jiffies - tp->rcv_tstamp),
                                rtx_delta);
                }
#endif
                if (tcp_rtx_probe0_timed_out(sk, skb, rtx_delta)) {
                        tcp_write_err(sk);
                        goto out;
                }
                tcp_enter_loss(sk);
                tcp_retransmit_skb(sk, skb, 1);
                __sk_dst_reset(sk);
                goto out_reset_timer;
        }

        __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEOUTS);
        if (tcp_write_timeout(sk))
                goto out;

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

                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;
                }
                if (mib_idx)
                        __NET_INC_STATS(sock_net(sk), mib_idx);
        }

        tcp_enter_loss(sk);

        tcp_update_rto_stats(sk);
        if (tcp_retransmit_skb(sk, tcp_rtx_queue_head(sk), 1) > 0) {
                /* Retransmission failed because of local congestion,
                 * Let senders fight for local resources conservatively.
                 */
                tcp_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                                     TCP_RESOURCE_PROBE_INTERVAL,
                                     false);
                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!
         */

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 || READ_ONCE(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 = clamp(__tcp_set_rto(tp),
                                       tcp_rto_min(sk),
                                       tcp_rto_max(sk));
        } else if (sk->sk_state != TCP_SYN_SENT ||
                   tp->total_rto >
                   READ_ONCE(net->ipv4.sysctl_tcp_syn_linear_timeouts)) {
                /* Use normal (exponential) backoff unless linear timeouts are
                 * activated.
                 */
                icsk->icsk_backoff++;
                icsk->icsk_rto = min(icsk->icsk_rto << 1, tcp_rto_max(sk));
        }
        tcp_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                             tcp_clamp_rto_to_user_timeout(sk), false);
        if (retransmits_timed_out(sk, READ_ONCE(net->ipv4.sysctl_tcp_retries1) + 1, 0))
                __sk_dst_reset(sk);

out:;
}

/* Called with bottom-half processing disabled.
 * Called by tcp_write_timer() and tcp_release_cb().
 */
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)
                return;

        if (time_after(tcp_timeout_expires(sk), jiffies)) {
                sk_reset_timer(sk, &sk->tcp_retransmit_timer,
                               tcp_timeout_expires(sk));
                return;
        }
        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:
                smp_store_release(&icsk->icsk_pending, 0);
                tcp_retransmit_timer(sk);
                break;
        case ICSK_TIME_PROBE0:
                smp_store_release(&icsk->icsk_pending, 0);
                tcp_probe_timer(sk);
                break;
        }
}

static void tcp_write_timer(struct timer_list *t)
{
        struct sock *sk = timer_container_of(sk, t, tcp_retransmit_timer);

        /* Avoid locking the socket when there is no pending event. */
        if (!smp_load_acquire(&inet_csk(sk)->icsk_pending))
                goto out;

        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);
out:
        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);
}

void tcp_reset_keepalive_timer(struct sock *sk, unsigned long len)
{
        sk_reset_timer(sk, &inet_csk(sk)->icsk_keepalive_timer, jiffies + len);
}

static void tcp_delete_keepalive_timer(struct sock *sk)
{
        sk_stop_timer(sk, &inet_csk(sk)->icsk_keepalive_timer);
}

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))
                tcp_reset_keepalive_timer(sk, keepalive_time_when(tcp_sk(sk)));
        else if (!val)
                tcp_delete_keepalive_timer(sk);
}

static void tcp_keepalive_timer(struct timer_list *t)
{
        struct inet_connection_sock *icsk =
                timer_container_of(icsk, t, icsk_keepalive_timer);
        struct sock *sk = &icsk->icsk_inet.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. */
                tcp_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 (READ_ONCE(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, SK_RST_REASON_TCP_STATE);
                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)) {
                u32 user_timeout = READ_ONCE(icsk->icsk_user_timeout);

                /* If the TCP_USER_TIMEOUT option is enabled, use that
                 * to determine when to timeout instead.
                 */
                if ((user_timeout != 0 &&
                    elapsed >= msecs_to_jiffies(user_timeout) &&
                    icsk->icsk_probes_out > 0) ||
                    (user_timeout == 0 &&
                    icsk->icsk_probes_out >= keepalive_probes(tp))) {
                        tcp_send_active_reset(sk, GFP_ATOMIC,
                                              SK_RST_REASON_TCP_KEEPALIVE_TIMEOUT);
                        tcp_write_err(sk);
                        goto out;
                }
                if (tcp_write_wakeup(sk, LINUX_MIB_TCPKEEPALIVE) <= 0) {
                        WRITE_ONCE(icsk->icsk_probes_out, icsk->icsk_probes_out + 1);
                        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;
        }

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

death:
        tcp_done(sk);

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

static enum hrtimer_restart tcp_compressed_ack_kick(struct hrtimer *timer)
{
        struct tcp_sock *tp = container_of(timer, struct tcp_sock, compressed_ack_timer);
        struct sock *sk = (struct sock *)tp;

        bh_lock_sock(sk);
        if (!sock_owned_by_user(sk)) {
                if (tp->compressed_ack) {
                        /* Since we have to send one ack finally,
                         * subtract one from tp->compressed_ack to keep
                         * LINUX_MIB_TCPACKCOMPRESSED accurate.
                         */
                        tp->compressed_ack--;
                        tcp_mstamp_refresh(tp);
                        tcp_send_ack(sk);
                }
        } else {
                if (!test_and_set_bit(TCP_DELACK_TIMER_DEFERRED,
                                      &sk->sk_tsq_flags))
                        sock_hold(sk);
        }
        bh_unlock_sock(sk);

        sock_put(sk);

        return HRTIMER_NORESTART;
}

void tcp_init_xmit_timers(struct sock *sk)
{
        inet_csk_init_xmit_timers(sk, &tcp_write_timer, &tcp_delack_timer,
                                  &tcp_keepalive_timer);
        hrtimer_setup(&tcp_sk(sk)->pacing_timer, tcp_pace_kick, CLOCK_MONOTONIC,
                      HRTIMER_MODE_ABS_PINNED_SOFT);

        hrtimer_setup(&tcp_sk(sk)->compressed_ack_timer, tcp_compressed_ack_kick, CLOCK_MONOTONIC,
                      HRTIMER_MODE_REL_PINNED_SOFT);
}