// SPDX-License-Identifier: GPL-2.0
/*
 * 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>
 */

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

#define pr_fmt(fmt) "TCP: " fmt

#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/module.h>
#include <linux/sysctl.h>
#include <linux/kernel.h>
#include <linux/prefetch.h>
#include <net/dst.h>
#include <net/tcp.h>
#include <net/inet_common.h>
#include <linux/ipsec.h>
#include <asm/unaligned.h>
#include <linux/errqueue.h>
#include <trace/events/tcp.h>
#include <linux/jump_label_ratelimit.h>
#include <net/busy_poll.h>
#include <net/mptcp.h>

int sysctl_tcp_max_orphans __read_mostly = NR_FILE;

#define FLAG_DATA               0x01 /* Incoming frame contained data.          */
#define FLAG_WIN_UPDATE         0x02 /* Incoming ACK was a window update.       */
#define FLAG_DATA_ACKED         0x04 /* This ACK acknowledged new data.         */
#define FLAG_RETRANS_DATA_ACKED 0x08 /* "" "" some of which was retransmitted.  */
#define FLAG_SYN_ACKED          0x10 /* This ACK acknowledged SYN.              */
#define FLAG_DATA_SACKED        0x20 /* New SACK.                               */
#define FLAG_ECE                0x40 /* ECE in this ACK                         */
#define FLAG_LOST_RETRANS       0x80 /* This ACK marks some retransmission lost */
#define FLAG_SLOWPATH           0x100 /* Do not skip RFC checks for window update.*/
#define FLAG_ORIG_SACK_ACKED    0x200 /* Never retransmitted data are (s)acked  */
#define FLAG_SND_UNA_ADVANCED   0x400 /* Snd_una was changed (!= FLAG_DATA_ACKED) */
#define FLAG_DSACKING_ACK       0x800 /* SACK blocks contained D-SACK info */
#define FLAG_SET_XMIT_TIMER     0x1000 /* Set TLP or RTO timer */
#define FLAG_SACK_RENEGING      0x2000 /* snd_una advanced to a sacked seq */
#define FLAG_UPDATE_TS_RECENT   0x4000 /* tcp_replace_ts_recent() */
#define FLAG_NO_CHALLENGE_ACK   0x8000 /* do not call tcp_send_challenge_ack()  */
#define FLAG_ACK_MAYBE_DELAYED  0x10000 /* Likely a delayed ACK */

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

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

#define REXMIT_NONE     0 /* no loss recovery to do */
#define REXMIT_LOST     1 /* retransmit packets marked lost */
#define REXMIT_NEW      2 /* FRTO-style transmit of unsent/new packets */

#if IS_ENABLED(CONFIG_TLS_DEVICE)
static DEFINE_STATIC_KEY_DEFERRED_FALSE(clean_acked_data_enabled, HZ);

void clean_acked_data_enable(struct inet_connection_sock *icsk,
                             void (*cad)(struct sock *sk, u32 ack_seq))
{
        icsk->icsk_clean_acked = cad;
        static_branch_deferred_inc(&clean_acked_data_enabled);
}
EXPORT_SYMBOL_GPL(clean_acked_data_enable);

void clean_acked_data_disable(struct inet_connection_sock *icsk)
{
        static_branch_slow_dec_deferred(&clean_acked_data_enabled);
        icsk->icsk_clean_acked = NULL;
}
EXPORT_SYMBOL_GPL(clean_acked_data_disable);

void clean_acked_data_flush(void)
{
        static_key_deferred_flush(&clean_acked_data_enabled);
}
EXPORT_SYMBOL_GPL(clean_acked_data_flush);
#endif

#ifdef CONFIG_CGROUP_BPF
static void bpf_skops_parse_hdr(struct sock *sk, struct sk_buff *skb)
{
        bool unknown_opt = tcp_sk(sk)->rx_opt.saw_unknown &&
                BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk),
                                       BPF_SOCK_OPS_PARSE_UNKNOWN_HDR_OPT_CB_FLAG);
        bool parse_all_opt = BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk),
                                                    BPF_SOCK_OPS_PARSE_ALL_HDR_OPT_CB_FLAG);
        struct bpf_sock_ops_kern sock_ops;

        if (likely(!unknown_opt && !parse_all_opt))
                return;

        /* The skb will be handled in the
         * bpf_skops_established() or
         * bpf_skops_write_hdr_opt().
         */
        switch (sk->sk_state) {
        case TCP_SYN_RECV:
        case TCP_SYN_SENT:
        case TCP_LISTEN:
                return;
        }

        sock_owned_by_me(sk);

        memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
        sock_ops.op = BPF_SOCK_OPS_PARSE_HDR_OPT_CB;
        sock_ops.is_fullsock = 1;
        sock_ops.sk = sk;
        bpf_skops_init_skb(&sock_ops, skb, tcp_hdrlen(skb));

        BPF_CGROUP_RUN_PROG_SOCK_OPS(&sock_ops);
}

static void bpf_skops_established(struct sock *sk, int bpf_op,
                                  struct sk_buff *skb)
{
        struct bpf_sock_ops_kern sock_ops;

        sock_owned_by_me(sk);

        memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
        sock_ops.op = bpf_op;
        sock_ops.is_fullsock = 1;
        sock_ops.sk = sk;
        /* sk with TCP_REPAIR_ON does not have skb in tcp_finish_connect */
        if (skb)
                bpf_skops_init_skb(&sock_ops, skb, tcp_hdrlen(skb));

        BPF_CGROUP_RUN_PROG_SOCK_OPS(&sock_ops);
}
#else
static void bpf_skops_parse_hdr(struct sock *sk, struct sk_buff *skb)
{
}

static void bpf_skops_established(struct sock *sk, int bpf_op,
                                  struct sk_buff *skb)
{
}
#endif

static void tcp_gro_dev_warn(struct sock *sk, const struct sk_buff *skb,
                             unsigned int len)
{
        static bool __once __read_mostly;

        if (!__once) {
                struct net_device *dev;

                __once = true;

                rcu_read_lock();
                dev = dev_get_by_index_rcu(sock_net(sk), skb->skb_iif);
                if (!dev || len >= dev->mtu)
                        pr_warn("%s: Driver has suspect GRO implementation, TCP performance may be compromised.\n",
                                dev ? dev->name : "Unknown driver");
                rcu_read_unlock();
        }
}

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

        icsk->icsk_ack.last_seg_size = 0;

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

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

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

void tcp_enter_quickack_mode(struct sock *sk, unsigned int max_quickacks)
{
        struct inet_connection_sock *icsk = inet_csk(sk);

        tcp_incr_quickack(sk, max_quickacks);
        inet_csk_exit_pingpong_mode(sk);
        icsk->icsk_ack.ato = TCP_ATO_MIN;
}
EXPORT_SYMBOL(tcp_enter_quickack_mode);

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

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

        return (dst && dst_metric(dst, RTAX_QUICKACK)) ||
                (icsk->icsk_ack.quick && !inet_csk_in_pingpong_mode(sk));
}

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

static void tcp_ecn_accept_cwr(struct sock *sk, const struct sk_buff *skb)
{
        if (tcp_hdr(skb)->cwr) {
                tcp_sk(sk)->ecn_flags &= ~TCP_ECN_DEMAND_CWR;

                /* If the sender is telling us it has entered CWR, then its
                 * cwnd may be very low (even just 1 packet), so we should ACK
                 * immediately.
                 */
                if (TCP_SKB_CB(skb)->seq != TCP_SKB_CB(skb)->end_seq)
                        inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOW;
        }
}

static void tcp_ecn_withdraw_cwr(struct tcp_sock *tp)
{
        tp->ecn_flags &= ~TCP_ECN_QUEUE_CWR;
}

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

        switch (TCP_SKB_CB(skb)->ip_dsfield & INET_ECN_MASK) {
        case INET_ECN_NOT_ECT:
                /* Funny extension: if ECT is not set on a segment,
                 * and we already seen ECT on a previous segment,
                 * it is probably a retransmit.
                 */
                if (tp->ecn_flags & TCP_ECN_SEEN)
                        tcp_enter_quickack_mode(sk, 2);
                break;
        case INET_ECN_CE:
                if (tcp_ca_needs_ecn(sk))
                        tcp_ca_event(sk, CA_EVENT_ECN_IS_CE);

                if (!(tp->ecn_flags & TCP_ECN_DEMAND_CWR)) {
                        /* Better not delay acks, sender can have a very low cwnd */
                        tcp_enter_quickack_mode(sk, 2);
                        tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
                }
                tp->ecn_flags |= TCP_ECN_SEEN;
                break;
        default:
                if (tcp_ca_needs_ecn(sk))
                        tcp_ca_event(sk, CA_EVENT_ECN_NO_CE);
                tp->ecn_flags |= TCP_ECN_SEEN;
                break;
        }
}

static void tcp_ecn_check_ce(struct sock *sk, const struct sk_buff *skb)
{
        if (tcp_sk(sk)->ecn_flags & TCP_ECN_OK)
                __tcp_ecn_check_ce(sk, skb);
}

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

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

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

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

static void tcp_sndbuf_expand(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
        int sndmem, per_mss;
        u32 nr_segs;

        /* Worst case is non GSO/TSO : each frame consumes one skb
         * and skb->head is kmalloced using power of two area of memory
         */
        per_mss = max_t(u32, tp->rx_opt.mss_clamp, tp->mss_cache) +
                  MAX_TCP_HEADER +
                  SKB_DATA_ALIGN(sizeof(struct skb_shared_info));

        per_mss = roundup_pow_of_two(per_mss) +
                  SKB_DATA_ALIGN(sizeof(struct sk_buff));

        nr_segs = max_t(u32, TCP_INIT_CWND, tp->snd_cwnd);
        nr_segs = max_t(u32, nr_segs, tp->reordering + 1);

        /* Fast Recovery (RFC 5681 3.2) :
         * Cubic needs 1.7 factor, rounded to 2 to include
         * extra cushion (application might react slowly to EPOLLOUT)
         */
        sndmem = ca_ops->sndbuf_expand ? ca_ops->sndbuf_expand(sk) : 2;
        sndmem *= nr_segs * per_mss;

        if (sk->sk_sndbuf < sndmem)
                WRITE_ONCE(sk->sk_sndbuf,
                           min(sndmem, sock_net(sk)->ipv4.sysctl_tcp_wmem[2]));
}

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

/* Slow part of check#2. */
static int __tcp_grow_window(const struct sock *sk, const struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);
        /* Optimize this! */
        int truesize = tcp_win_from_space(sk, skb->truesize) >> 1;
        int window = tcp_win_from_space(sk, sock_net(sk)->ipv4.sysctl_tcp_rmem[2]) >> 1;

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

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

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

        room = min_t(int, tp->window_clamp, tcp_space(sk)) - tp->rcv_ssthresh;

        /* Check #1 */
        if (room > 0 && !tcp_under_memory_pressure(sk)) {
                int incr;

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

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

/* 3. Try to fixup all. It is made immediately after connection enters
 *    established state.
 */
static void tcp_init_buffer_space(struct sock *sk)
{
        int tcp_app_win = sock_net(sk)->ipv4.sysctl_tcp_app_win;
        struct tcp_sock *tp = tcp_sk(sk);
        int maxwin;

        if (!(sk->sk_userlocks & SOCK_SNDBUF_LOCK))
                tcp_sndbuf_expand(sk);

        tcp_mstamp_refresh(tp);
        tp->rcvq_space.time = tp->tcp_mstamp;
        tp->rcvq_space.seq = tp->copied_seq;

        maxwin = tcp_full_space(sk);

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

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

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

        tp->rcv_ssthresh = min(tp->rcv_ssthresh, tp->window_clamp);
        tp->snd_cwnd_stamp = tcp_jiffies32;
        tp->rcvq_space.space = min3(tp->rcv_ssthresh, tp->rcv_wnd,
                                    (u32)TCP_INIT_CWND * tp->advmss);
}

/* 4. Recalculate window clamp after socket hit its memory bounds. */
static void tcp_clamp_window(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct inet_connection_sock *icsk = inet_csk(sk);
        struct net *net = sock_net(sk);

        icsk->icsk_ack.quick = 0;

        if (sk->sk_rcvbuf < net->ipv4.sysctl_tcp_rmem[2] &&
            !(sk->sk_userlocks & SOCK_RCVBUF_LOCK) &&
            !tcp_under_memory_pressure(sk) &&
            sk_memory_allocated(sk) < sk_prot_mem_limits(sk, 0)) {
                WRITE_ONCE(sk->sk_rcvbuf,
                           min(atomic_read(&sk->sk_rmem_alloc),
                               net->ipv4.sysctl_tcp_rmem[2]));
        }
        if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf)
                tp->rcv_ssthresh = min(tp->window_clamp, 2U * tp->advmss);
}

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

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

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

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

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

        tp->rcv_rtt_est.rtt_us = new_sample;
}

static inline void tcp_rcv_rtt_measure(struct tcp_sock *tp)
{
        u32 delta_us;

        if (tp->rcv_rtt_est.time == 0)
                goto new_measure;
        if (before(tp->rcv_nxt, tp->rcv_rtt_est.seq))
                return;
        delta_us = tcp_stamp_us_delta(tp->tcp_mstamp, tp->rcv_rtt_est.time);
        if (!delta_us)
                delta_us = 1;
        tcp_rcv_rtt_update(tp, delta_us, 1);

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

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

        if (tp->rx_opt.rcv_tsecr == tp->rcv_rtt_last_tsecr)
                return;
        tp->rcv_rtt_last_tsecr = tp->rx_opt.rcv_tsecr;

        if (TCP_SKB_CB(skb)->end_seq -
            TCP_SKB_CB(skb)->seq >= inet_csk(sk)->icsk_ack.rcv_mss) {
                u32 delta = tcp_time_stamp(tp) - tp->rx_opt.rcv_tsecr;
                u32 delta_us;

                if (likely(delta < INT_MAX / (USEC_PER_SEC / TCP_TS_HZ))) {
                        if (!delta)
                                delta = 1;
                        delta_us = delta * (USEC_PER_SEC / TCP_TS_HZ);
                        tcp_rcv_rtt_update(tp, delta_us, 0);
                }
        }
}

/*
 * This function should be called every time data is copied to user space.
 * It calculates the appropriate TCP receive buffer space.
 */
void tcp_rcv_space_adjust(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        u32 copied;
        int time;

        trace_tcp_rcv_space_adjust(sk);

        tcp_mstamp_refresh(tp);
        time = tcp_stamp_us_delta(tp->tcp_mstamp, tp->rcvq_space.time);
        if (time < (tp->rcv_rtt_est.rtt_us >> 3) || tp->rcv_rtt_est.rtt_us == 0)
                return;

        /* Number of bytes copied to user in last RTT */
        copied = tp->copied_seq - tp->rcvq_space.seq;
        if (copied <= tp->rcvq_space.space)
                goto new_measure;

        /* A bit of theory :
         * copied = bytes received in previous RTT, our base window
         * To cope with packet losses, we need a 2x factor
         * To cope with slow start, and sender growing its cwin by 100 %
         * every RTT, we need a 4x factor, because the ACK we are sending
         * now is for the next RTT, not the current one :
         * <prev RTT . ><current RTT .. ><next RTT .... >
         */

        if (sock_net(sk)->ipv4.sysctl_tcp_moderate_rcvbuf &&
            !(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
                int rcvmem, rcvbuf;
                u64 rcvwin, grow;

                /* minimal window to cope with packet losses, assuming
                 * steady state. Add some cushion because of small variations.
                 */
                rcvwin = ((u64)copied << 1) + 16 * tp->advmss;

                /* Accommodate for sender rate increase (eg. slow start) */
                grow = rcvwin * (copied - tp->rcvq_space.space);
                do_div(grow, tp->rcvq_space.space);
                rcvwin += (grow << 1);

                rcvmem = SKB_TRUESIZE(tp->advmss + MAX_TCP_HEADER);
                while (tcp_win_from_space(sk, rcvmem) < tp->advmss)
                        rcvmem += 128;

                do_div(rcvwin, tp->advmss);
                rcvbuf = min_t(u64, rcvwin * rcvmem,
                               sock_net(sk)->ipv4.sysctl_tcp_rmem[2]);
                if (rcvbuf > sk->sk_rcvbuf) {
                        WRITE_ONCE(sk->sk_rcvbuf, rcvbuf);

                        /* Make the window clamp follow along.  */
                        tp->window_clamp = tcp_win_from_space(sk, rcvbuf);
                }
        }
        tp->rcvq_space.space = copied;

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

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

        inet_csk_schedule_ack(sk);

        tcp_measure_rcv_mss(sk, skb);

        tcp_rcv_rtt_measure(tp);

        now = tcp_jiffies32;

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

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

        tcp_ecn_check_ce(sk, skb);

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

/* Called to compute a smoothed rtt estimate. The data fed to this
 * routine either comes from timestamps, or from segments that were
 * known _not_ to have been retransmitted [see Karn/Partridge
 * Proceedings SIGCOMM 87]. The algorithm is from the SIGCOMM 88
 * piece by Van Jacobson.
 * NOTE: the next three routines used to be one big routine.
 * To save cycles in the RFC 1323 implementation it was better to break
 * it up into three procedures. -- erics
 */
static void tcp_rtt_estimator(struct sock *sk, long mrtt_us)
{
        struct tcp_sock *tp = tcp_sk(sk);
        long m = mrtt_us; /* RTT */
        u32 srtt = tp->srtt_us;

        /*      The following amusing code comes from Jacobson's
         *      article in SIGCOMM '88.  Note that rtt and mdev
         *      are scaled versions of rtt and mean deviation.
         *      This is designed to be as fast as possible
         *      m stands for "measurement".
         *
         *      On a 1990 paper the rto value is changed to:
         *      RTO = rtt + 4 * mdev
         *
         * Funny. This algorithm seems to be very broken.
         * These formulae increase RTO, when it should be decreased, increase
         * too slowly, when it should be increased quickly, decrease too quickly
         * etc. I guess in BSD RTO takes ONE value, so that it is absolutely
         * does not matter how to _calculate_ it. Seems, it was trap
         * that VJ failed to avoid. 8)
         */
        if (srtt != 0) {
                m -= (srtt >> 3);       /* m is now error in rtt est */
                srtt += m;              /* rtt = 7/8 rtt + 1/8 new */
                if (m < 0) {
                        m = -m;         /* m is now abs(error) */
                        m -= (tp->mdev_us >> 2);   /* similar update on mdev */
                        /* This is similar to one of Eifel findings.
                         * Eifel blocks mdev updates when rtt decreases.
                         * This solution is a bit different: we use finer gain
                         * for mdev in this case (alpha*beta).
                         * Like Eifel it also prevents growth of rto,
                         * but also it limits too fast rto decreases,
                         * happening in pure Eifel.
                         */
                        if (m > 0)
                                m >>= 3;
                } else {
                        m -= (tp->mdev_us >> 2);   /* similar update on mdev */
                }
                tp->mdev_us += m;               /* mdev = 3/4 mdev + 1/4 new */
                if (tp->mdev_us > tp->mdev_max_us) {
                        tp->mdev_max_us = tp->mdev_us;
                        if (tp->mdev_max_us > tp->rttvar_us)
                                tp->rttvar_us = tp->mdev_max_us;
                }
                if (after(tp->snd_una, tp->rtt_seq)) {
                        if (tp->mdev_max_us < tp->rttvar_us)
                                tp->rttvar_us -= (tp->rttvar_us - tp->mdev_max_us) >> 2;
                        tp->rtt_seq = tp->snd_nxt;
                        tp->mdev_max_us = tcp_rto_min_us(sk);

                        tcp_bpf_rtt(sk);
                }
        } else {
                /* no previous measure. */
                srtt = m << 3;          /* take the measured time to be rtt */
                tp->mdev_us = m << 1;   /* make sure rto = 3*rtt */
                tp->rttvar_us = max(tp->mdev_us, tcp_rto_min_us(sk));
                tp->mdev_max_us = tp->rttvar_us;
                tp->rtt_seq = tp->snd_nxt;

                tcp_bpf_rtt(sk);
        }
        tp->srtt_us = max(1U, srtt);
}

static void tcp_update_pacing_rate(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        u64 rate;

        /* set sk_pacing_rate to 200 % of current rate (mss * cwnd / srtt) */
        rate = (u64)tp->mss_cache * ((USEC_PER_SEC / 100) << 3);

        /* current rate is (cwnd * mss) / srtt
         * In Slow Start [1], set sk_pacing_rate to 200 % the current rate.
         * In Congestion Avoidance phase, set it to 120 % the current rate.
         *
         * [1] : Normal Slow Start condition is (tp->snd_cwnd < tp->snd_ssthresh)
         *       If snd_cwnd >= (tp->snd_ssthresh / 2), we are approaching
         *       end of slow start and should slow down.
         */
        if (tp->snd_cwnd < tp->snd_ssthresh / 2)
                rate *= sock_net(sk)->ipv4.sysctl_tcp_pacing_ss_ratio;
        else
                rate *= sock_net(sk)->ipv4.sysctl_tcp_pacing_ca_ratio;

        rate *= max(tp->snd_cwnd, tp->packets_out);

        if (likely(tp->srtt_us))
                do_div(rate, tp->srtt_us);

        /* WRITE_ONCE() is needed because sch_fq fetches sk_pacing_rate
         * without any lock. We want to make sure compiler wont store
         * intermediate values in this location.
         */
        WRITE_ONCE(sk->sk_pacing_rate, min_t(u64, rate,
                                             sk->sk_max_pacing_rate));
}

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

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

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

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

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

struct tcp_sacktag_state {
        /* Timestamps for earliest and latest never-retransmitted segment
         * that was SACKed. RTO needs the earliest RTT to stay conservative,
         * but congestion control should still get an accurate delay signal.
         */
        u64     first_sackt;
        u64     last_sackt;
        u32     reord;
        u32     sack_delivered;
        int     flag;
        unsigned int mss_now;
        struct rate_sample *rate;
};

/* Take a notice that peer is sending D-SACKs. Skip update of data delivery
 * and spurious retransmission information if this DSACK is unlikely caused by
 * sender's action:
 * - DSACKed sequence range is larger than maximum receiver's window.
 * - Total no. of DSACKed segments exceed the total no. of retransmitted segs.
 */
static u32 tcp_dsack_seen(struct tcp_sock *tp, u32 start_seq,
                          u32 end_seq, struct tcp_sacktag_state *state)
{
        u32 seq_len, dup_segs = 1;

        if (!before(start_seq, end_seq))
                return 0;

        seq_len = end_seq - start_seq;
        /* Dubious DSACK: DSACKed range greater than maximum advertised rwnd */
        if (seq_len > tp->max_window)
                return 0;
        if (seq_len > tp->mss_cache)
                dup_segs = DIV_ROUND_UP(seq_len, tp->mss_cache);

        tp->dsack_dups += dup_segs;
        /* Skip the DSACK if dup segs weren't retransmitted by sender */
        if (tp->dsack_dups > tp->total_retrans)
                return 0;

        tp->rx_opt.sack_ok |= TCP_DSACK_SEEN;
        tp->rack.dsack_seen = 1;

        state->flag |= FLAG_DSACKING_ACK;
        /* A spurious retransmission is delivered */
        state->sack_delivered += dup_segs;

        return dup_segs;
}

/* It's reordering when higher sequence was delivered (i.e. sacked) before
 * some lower never-retransmitted sequence ("low_seq"). The maximum reordering
 * distance is approximated in full-mss packet distance ("reordering").
 */
static void tcp_check_sack_reordering(struct sock *sk, const u32 low_seq,
                                      const int ts)
{
        struct tcp_sock *tp = tcp_sk(sk);
        const u32 mss = tp->mss_cache;
        u32 fack, metric;

        fack = tcp_highest_sack_seq(tp);
        if (!before(low_seq, fack))

                return;

        metric = fack - low_seq;
        if ((metric > tp->reordering * mss) && mss) {
#if FASTRETRANS_DEBUG > 1
                pr_debug("Disorder%d %d %u f%u s%u rr%d\n",
                         tp->rx_opt.sack_ok, inet_csk(sk)->icsk_ca_state,
                         tp->reordering,
                         0,
                         tp->sacked_out,
                         tp->undo_marker ? tp->undo_retrans : 0);
#endif
                tp->reordering = min_t(u32, (metric + mss - 1) / mss,
                                       sock_net(sk)->ipv4.sysctl_tcp_max_reordering);
        }

        /* This exciting event is worth to be remembered. 8) */
        tp->reord_seen++;
        NET_INC_STATS(sock_net(sk),
                      ts ? LINUX_MIB_TCPTSREORDER : LINUX_MIB_TCPSACKREORDER);
}

 /* This must be called before lost_out or retrans_out are updated
  * on a new loss, because we want to know if all skbs previously
  * known to be lost have already been retransmitted, indicating
  * that this newly lost skb is our next skb to retransmit.
  */
static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb)
{
        if ((!tp->retransmit_skb_hint && tp->retrans_out >= tp->lost_out) ||
            (tp->retransmit_skb_hint &&
             before(TCP_SKB_CB(skb)->seq,
                    TCP_SKB_CB(tp->retransmit_skb_hint)->seq)))
                tp->retransmit_skb_hint = skb;
}

/* Sum the number of packets on the wire we have marked as lost, and
 * notify the congestion control module that the given skb was marked lost.
 */
static void tcp_notify_skb_loss_event(struct tcp_sock *tp, const struct sk_buff *skb)
{
        tp->lost += tcp_skb_pcount(skb);
}

void tcp_mark_skb_lost(struct sock *sk, struct sk_buff *skb)
{
        __u8 sacked = TCP_SKB_CB(skb)->sacked;
        struct tcp_sock *tp = tcp_sk(sk);

        if (sacked & TCPCB_SACKED_ACKED)
                return;

        tcp_verify_retransmit_hint(tp, skb);
        if (sacked & TCPCB_LOST) {
                if (sacked & TCPCB_SACKED_RETRANS) {
                        /* Account for retransmits that are lost again */
                        TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
                        tp->retrans_out -= tcp_skb_pcount(skb);
                        NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT,
                                      tcp_skb_pcount(skb));
                        tcp_notify_skb_loss_event(tp, skb);
                }
        } else {
                tp->lost_out += tcp_skb_pcount(skb);
                TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                tcp_notify_skb_loss_event(tp, skb);
        }
}

/* Updates the delivered and delivered_ce counts */
static void tcp_count_delivered(struct tcp_sock *tp, u32 delivered,
                                bool ece_ack)
{
        tp->delivered += delivered;
        if (ece_ack)
                tp->delivered_ce += delivered;
}

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

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

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

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

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

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

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

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

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

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

                if (after(end_seq_0, end_seq_1) || before(start_seq_0, start_seq_1))
                        return false;
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDSACKOFORECV);
        } else {
                return false;
        }

        dup_segs = tcp_dsack_seen(tp, start_seq_0, end_seq_0, state);
        if (!dup_segs) {        /* Skip dubious DSACK */
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDSACKIGNOREDDUBIOUS);
                return false;
        }

        NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPDSACKRECVSEGS, dup_segs);

        /* D-SACK for already forgotten data... Do dumb counting. */
        if (tp->undo_marker && tp->undo_retrans > 0 &&
            !after(end_seq_0, prior_snd_una) &&
            after(end_seq_0, tp->undo_marker))
                tp->undo_retrans = max_t(int, 0, tp->undo_retrans - dup_segs);

        return true;
}

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

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

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

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

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

                if (pkt_len >= skb->len && !in_sack)
                        return 0;

                err = tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
                                   pkt_len, mss, GFP_ATOMIC);
                if (err < 0)
                        return err;
        }

        return in_sack;
}

/* Mark the given newly-SACKed range as such, adjusting counters and hints. */
static u8 tcp_sacktag_one(struct sock *sk,
                          struct tcp_sacktag_state *state, u8 sacked,
                          u32 start_seq, u32 end_seq,
                          int dup_sack, int pcount,
                          u64 xmit_time)
{
        struct tcp_sock *tp = tcp_sk(sk);

        /* Account D-SACK for retransmitted packet. */
        if (dup_sack && (sacked & TCPCB_RETRANS)) {
                if (tp->undo_marker && tp->undo_retrans > 0 &&
                    after(end_seq, tp->undo_marker))
                        tp->undo_retrans--;
                if ((sacked & TCPCB_SACKED_ACKED) &&
                    before(start_seq, state->reord))
                                state->reord = start_seq;
        }

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

        if (!(sacked & TCPCB_SACKED_ACKED)) {
                tcp_rack_advance(tp, sacked, end_seq, xmit_time);

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

                                if (!after(end_seq, tp->high_seq))
                                        state->flag |= FLAG_ORIG_SACK_ACKED;
                                if (state->first_sackt == 0)
                                        state->first_sackt = xmit_time;
                                state->last_sackt = xmit_time;
                        }

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

                sacked |= TCPCB_SACKED_ACKED;
                state->flag |= FLAG_DATA_SACKED;
                tp->sacked_out += pcount;
                /* Out-of-order packets delivered */
                state->sack_delivered += pcount;

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

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

        return sacked;
}

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

        BUG_ON(!pcount);

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

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

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

        tcp_skb_pcount_add(prev, pcount);
        WARN_ON_ONCE(tcp_skb_pcount(skb) < pcount);
        tcp_skb_pcount_add(skb, -pcount);

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

        /* CHECKME: To clear or not to clear? Mimics normal skb currently */
        if (tcp_skb_pcount(skb) <= 1)
                TCP_SKB_CB(skb)->tcp_gso_size = 0;

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

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

        /* Whole SKB was eaten :-) */

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

        TCP_SKB_CB(prev)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags;
        TCP_SKB_CB(prev)->eor = TCP_SKB_CB(skb)->eor;
        if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
                TCP_SKB_CB(prev)->end_seq++;

        if (skb == tcp_highest_sack(sk))
                tcp_advance_highest_sack(sk, skb);

        tcp_skb_collapse_tstamp(prev, skb);
        if (unlikely(TCP_SKB_CB(prev)->tx.delivered_mstamp))
                TCP_SKB_CB(prev)->tx.delivered_mstamp = 0;

        tcp_rtx_queue_unlink_and_free(skb, sk);

        NET_INC_STATS(sock_net(sk), LINUX_MIB_SACKMERGED);

        return true;
}

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

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

int tcp_skb_shift(struct sk_buff *to, struct sk_buff *from,
                  int pcount, int shiftlen)
{
        /* TCP min gso_size is 8 bytes (TCP_MIN_GSO_SIZE)
         * Since TCP_SKB_CB(skb)->tcp_gso_segs is 16 bits, we need
         * to make sure not storing more than 65535 * 8 bytes per skb,
         * even if current MSS is bigger.
         */
        if (unlikely(to->len + shiftlen >= 65535 * TCP_MIN_GSO_SIZE))
                return 0;
        if (unlikely(tcp_skb_pcount(to) + pcount > 65535))
                return 0;
        return skb_shift(to, from, shiftlen);
}

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

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

        /* Can only happen with delayed DSACK + discard craziness */
        prev = skb_rb_prev(skb);
        if (!prev)
                goto fallback;

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

        if (!tcp_skb_can_collapse(prev, skb))
                goto fallback;

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

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

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

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

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

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

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

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

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

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

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

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

        len = skb->len;
        pcount = tcp_skb_pcount(skb);
        if (tcp_skb_shift(prev, skb, pcount, len))
                tcp_shifted_skb(sk, prev, skb, state, pcount,
                                len, mss, 0);

out:
        return prev;

noop:
        return skb;

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

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

        skb_rbtree_walk_from(skb) {
                int in_sack = 0;
                bool dup_sack = dup_sack_in;

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

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

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

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

                if (unlikely(in_sack < 0))
                        break;

                if (in_sack) {
                        TCP_SKB_CB(skb)->sacked =
                                tcp_sacktag_one(sk,
                                                state,
                                                TCP_SKB_CB(skb)->sacked,
                                                TCP_SKB_CB(skb)->seq,
                                                TCP_SKB_CB(skb)->end_seq,
                                                dup_sack,
                                                tcp_skb_pcount(skb),
                                                tcp_skb_timestamp_us(skb));
                        tcp_rate_skb_delivered(sk, skb, state->rate);
                        if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
                                list_del_init(&skb->tcp_tsorted_anchor);

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

static struct sk_buff *tcp_sacktag_bsearch(struct sock *sk, u32 seq)
{
        struct rb_node *parent, **p = &sk->tcp_rtx_queue.rb_node;
        struct sk_buff *skb;

        while (*p) {
                parent = *p;
                skb = rb_to_skb(parent);
                if (before(seq, TCP_SKB_CB(skb)->seq)) {
                        p = &parent->rb_left;
                        continue;
                }
                if (!before(seq, TCP_SKB_CB(skb)->end_seq)) {
                        p = &parent->rb_right;
                        continue;
                }
                return skb;
        }
        return NULL;
}

static struct sk_buff *tcp_sacktag_skip(struct sk_buff *skb, struct sock *sk,
                                        u32 skip_to_seq)
{
        if (skb && after(TCP_SKB_CB(skb)->seq, skip_to_seq))
                return skb;

        return tcp_sacktag_bsearch(sk, skip_to_seq);
}

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

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

        return skb;
}

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

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

        state->flag = 0;
        state->reord = tp->snd_nxt;

        if (!tp->sacked_out)
                tcp_highest_sack_reset(sk);

        found_dup_sack = tcp_check_dsack(sk, ack_skb, sp_wire,
                                         num_sacks, prior_snd_una, state);

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

        if (!tp->packets_out)
                goto out;

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

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

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

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

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

                /* Ignore very old stuff early */
                if (!after(sp[used_sacks].end_seq, prior_snd_una)) {
                        if (i == 0)
                                first_sack_index = -1;
                        continue;
                }

                used_sacks++;
        }

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

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

        state->mss_now = tcp_current_mss(sk);
        skb = NULL;
        i = 0;

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

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

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

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

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

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

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

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

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

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

                if (!before(start_seq, tcp_highest_sack_seq(tp))) {
                        skb = tcp_highest_sack(sk);
                        if (!skb)
                                break;
                }
                skb = tcp_sacktag_skip(skb, sk, start_seq);

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

advance_sp:
                i++;
        }

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

        if (inet_csk(sk)->icsk_ca_state != TCP_CA_Loss || tp->undo_marker)
                tcp_check_sack_reordering(sk, state->reord, 0);

        tcp_verify_left_out(tp);
out:

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

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

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

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

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

        if (!tcp_limit_reno_sacked(tp))
                return;

        tp->reordering = min_t(u32, tp->packets_out + addend,
                               sock_net(sk)->ipv4.sysctl_tcp_max_reordering);
        tp->reord_seen++;
        NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRENOREORDER);
}

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

static void tcp_add_reno_sack(struct sock *sk, int num_dupack, bool ece_ack)
{
        if (num_dupack) {