Adding upstream version 5.10.209.upstream/5.10.209

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 6989 insertions, 0 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
new file mode 100644
index 000000000..0f9fe5eda
--- /dev/null
+++ b/net/ipv4/tcp_input.c
@@ -0,0 +1,6989 @@
+// 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);
+		/* If the skb has a len of exactly 1*MSS and has the PSH bit
+		 * set then it is likely the end of an application write. So
+		 * more data may not be arriving soon, and yet the data sender
+		 * may be waiting for an ACK if cwnd-bound or using TX zero
+		 * copy. So we set ICSK_ACK_PUSHED here so that
+		 * tcp_cleanup_rbuf() will send an ACK immediately if the app
+		 * reads all of the data and is not ping-pong. If len > MSS
+		 * then this logic does not matter (and does not hurt) because
+		 * tcp_cleanup_rbuf() will always ACK immediately if the app
+		 * reads data and there is more than an MSS of unACKed data.
+		 */
+		if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_PSH)
+			icsk->icsk_ack.pending |= ICSK_ACK_PUSHED;
+	} 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;
+}
+
+static 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;
+}
+
+/* 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, READ_ONCE(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,
+			     unsigned int skbtruesize)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	/* Optimize this! */
+	int truesize = tcp_win_from_space(sk, skbtruesize) >> 1;
+	int window = tcp_win_from_space(sk, READ_ONCE(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;
+}
+
+/* Even if skb appears to have a bad len/truesize ratio, TCP coalescing
+ * can play nice with us, as sk_buff and skb->head might be either
+ * freed or shared with up to MAX_SKB_FRAGS segments.
+ * Only give a boost to drivers using page frag(s) to hold the frame(s),
+ * and if no payload was pulled in skb->head before reaching us.
+ */
+static u32 truesize_adjust(bool adjust, const struct sk_buff *skb)
+{
+	u32 truesize = skb->truesize;
+
+	if (adjust && !skb_headlen(skb)) {
+		truesize -= SKB_TRUESIZE(skb_end_offset(skb));
+		/* paranoid check, some drivers might be buggy */
+		if (unlikely((int)truesize < (int)skb->len))
+			truesize = skb->truesize;
+	}
+	return truesize;
+}
+
+static void tcp_grow_window(struct sock *sk, const struct sk_buff *skb,
+			    bool adjust)
+{
+	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)) {
+		unsigned int truesize = truesize_adjust(adjust, skb);
+		int incr;
+
+		/* Check #2. Increase window, if skb with such overhead
+		 * will fit to rcvbuf in future.
+		 */
+		if (tcp_win_from_space(sk, truesize) <= skb->len)
+			incr = 2 * tp->advmss;
+		else
+			incr = __tcp_grow_window(sk, skb, truesize);
+
+		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 = READ_ONCE(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);
+	int rmem2;
+
+	icsk->icsk_ack.quick = 0;
+	rmem2 = READ_ONCE(net->ipv4.sysctl_tcp_rmem[2]);
+
+	if (sk->sk_rcvbuf < rmem2 &&
+	    !(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), rmem2));
+	}
+	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 (READ_ONCE(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,
+			       READ_ONCE(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, true);
+}
+
+/* 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,
+				       READ_ONCE(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 = max_t(int, 0, tp->undo_retrans - pcount);
+		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;
+
+	if (!tcp_skb_can_collapse(prev, 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,
+			       READ_ONCE(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) {
+		struct tcp_sock *tp = tcp_sk(sk);
+		u32 prior_sacked = tp->sacked_out;
+		s32 delivered;
+
+		tp->sacked_out += num_dupack;
+		tcp_check_reno_reordering(sk, 0);
+		delivered = tp->sacked_out - prior_sacked;
+		if (delivered > 0)
+			tcp_count_delivered(tp, delivered, ece_ack);
+		tcp_verify_left_out(tp);
+	}
+}
+
+/* Account for ACK, ACKing some data in Reno Recovery phase. */
+
+static void tcp_remove_reno_sacks(struct sock *sk, int acked, bool ece_ack)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (acked > 0) {
+		/* One ACK acked hole. The rest eat duplicate ACKs. */
+		tcp_count_delivered(tp, max_t(int, acked - tp->sacked_out, 1),
+				    ece_ack);
+		if (acked - 1 >= tp->sacked_out)
+			tp->sacked_out = 0;
+		else
+			tp->sacked_out -= acked - 1;
+	}
+	tcp_check_reno_reordering(sk, acked);
+	tcp_verify_left_out(tp);
+}
+
+static inline void tcp_reset_reno_sack(struct tcp_sock *tp)
+{
+	tp->sacked_out = 0;
+}
+
+void tcp_clear_retrans(struct tcp_sock *tp)
+{
+	tp->retrans_out = 0;
+	tp->lost_out = 0;
+	tp->undo_marker = 0;
+	tp->undo_retrans = -1;
+	tp->sacked_out = 0;
+}
+
+static inline void tcp_init_undo(struct tcp_sock *tp)
+{
+	tp->undo_marker = tp->snd_una;
+	/* Retransmission still in flight may cause DSACKs later. */
+	tp->undo_retrans = tp->retrans_out ? : -1;
+}
+
+static bool tcp_is_rack(const struct sock *sk)
+{
+	return READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_recovery) &
+		TCP_RACK_LOSS_DETECTION;
+}
+
+/* If we detect SACK reneging, forget all SACK information
+ * and reset tags completely, otherwise preserve SACKs. If receiver
+ * dropped its ofo queue, we will know this due to reneging detection.
+ */
+static void tcp_timeout_mark_lost(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *skb, *head;
+	bool is_reneg;			/* is receiver reneging on SACKs? */
+
+	head = tcp_rtx_queue_head(sk);
+	is_reneg = head && (TCP_SKB_CB(head)->sacked & TCPCB_SACKED_ACKED);
+	if (is_reneg) {
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSACKRENEGING);
+		tp->sacked_out = 0;
+		/* Mark SACK reneging until we recover from this loss event. */
+		tp->is_sack_reneg = 1;
+	} else if (tcp_is_reno(tp)) {
+		tcp_reset_reno_sack(tp);
+	}
+
+	skb = head;
+	skb_rbtree_walk_from(skb) {
+		if (is_reneg)
+			TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_ACKED;
+		else if (tcp_is_rack(sk) && skb != head &&
+			 tcp_rack_skb_timeout(tp, skb, 0) > 0)
+			continue; /* Don't mark recently sent ones lost yet */
+		tcp_mark_skb_lost(sk, skb);
+	}
+	tcp_verify_left_out(tp);
+	tcp_clear_all_retrans_hints(tp);
+}
+
+/* Enter Loss state. */
+void tcp_enter_loss(struct sock *sk)
+{
+	const struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct net *net = sock_net(sk);
+	bool new_recovery = icsk->icsk_ca_state < TCP_CA_Recovery;
+	u8 reordering;
+
+	tcp_timeout_mark_lost(sk);
+
+	/* Reduce ssthresh if it has not yet been made inside this window. */
+	if (icsk->icsk_ca_state <= TCP_CA_Disorder ||
+	    !after(tp->high_seq, tp->snd_una) ||
+	    (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)) {
+		tp->prior_ssthresh = tcp_current_ssthresh(sk);
+		tp->prior_cwnd = tp->snd_cwnd;
+		tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
+		tcp_ca_event(sk, CA_EVENT_LOSS);
+		tcp_init_undo(tp);
+	}
+	tp->snd_cwnd	   = tcp_packets_in_flight(tp) + 1;
+	tp->snd_cwnd_cnt   = 0;
+	tp->snd_cwnd_stamp = tcp_jiffies32;
+
+	/* Timeout in disordered state after receiving substantial DUPACKs
+	 * suggests that the degree of reordering is over-estimated.
+	 */
+	reordering = READ_ONCE(net->ipv4.sysctl_tcp_reordering);
+	if (icsk->icsk_ca_state <= TCP_CA_Disorder &&
+	    tp->sacked_out >= reordering)
+		tp->reordering = min_t(unsigned int, tp->reordering,
+				       reordering);
+
+	tcp_set_ca_state(sk, TCP_CA_Loss);
+	tp->high_seq = tp->snd_nxt;
+	tcp_ecn_queue_cwr(tp);
+
+	/* F-RTO RFC5682 sec 3.1 step 1: retransmit SND.UNA if no previous
+	 * loss recovery is underway except recurring timeout(s) on
+	 * the same SND.UNA (sec 3.2). Disable F-RTO on path MTU probing
+	 */
+	tp->frto = READ_ONCE(net->ipv4.sysctl_tcp_frto) &&
+		   (new_recovery || icsk->icsk_retransmits) &&
+		   !inet_csk(sk)->icsk_mtup.probe_size;
+}
+
+/* If ACK arrived pointing to a remembered SACK, it means that our
+ * remembered SACKs do not reflect real state of receiver i.e.
+ * receiver _host_ is heavily congested (or buggy).
+ *
+ * To avoid big spurious retransmission bursts due to transient SACK
+ * scoreboard oddities that look like reneging, we give the receiver a
+ * little time (max(RTT/2, 10ms)) to send us some more ACKs that will
+ * restore sanity to the SACK scoreboard. If the apparent reneging
+ * persists until this RTO then we'll clear the SACK scoreboard.
+ */
+static bool tcp_check_sack_reneging(struct sock *sk, int *ack_flag)
+{
+	if (*ack_flag & FLAG_SACK_RENEGING &&
+	    *ack_flag & FLAG_SND_UNA_ADVANCED) {
+		struct tcp_sock *tp = tcp_sk(sk);
+		unsigned long delay = max(usecs_to_jiffies(tp->srtt_us >> 4),
+					  msecs_to_jiffies(10));
+
+		inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
+					  delay, TCP_RTO_MAX);
+		*ack_flag &= ~FLAG_SET_XMIT_TIMER;
+		return true;
+	}
+	return false;
+}
+
+/* Heurestics to calculate number of duplicate ACKs. There's no dupACKs
+ * counter when SACK is enabled (without SACK, sacked_out is used for
+ * that purpose).
+ *
+ * With reordering, holes may still be in flight, so RFC3517 recovery
+ * uses pure sacked_out (total number of SACKed segments) even though
+ * it violates the RFC that uses duplicate ACKs, often these are equal
+ * but when e.g. out-of-window ACKs or packet duplication occurs,
+ * they differ. Since neither occurs due to loss, TCP should really
+ * ignore them.
+ */
+static inline int tcp_dupack_heuristics(const struct tcp_sock *tp)
+{
+	return tp->sacked_out + 1;
+}
+
+/* Linux NewReno/SACK/ECN state machine.
+ * --------------------------------------
+ *
+ * "Open"	Normal state, no dubious events, fast path.
+ * "Disorder"   In all the respects it is "Open",
+ *		but requires a bit more attention. It is entered when
+ *		we see some SACKs or dupacks. It is split of "Open"
+ *		mainly to move some processing from fast path to slow one.
+ * "CWR"	CWND was reduced due to some Congestion Notification event.
+ *		It can be ECN, ICMP source quench, local device congestion.
+ * "Recovery"	CWND was reduced, we are fast-retransmitting.
+ * "Loss"	CWND was reduced due to RTO timeout or SACK reneging.
+ *
+ * tcp_fastretrans_alert() is entered:
+ * - each incoming ACK, if state is not "Open"
+ * - when arrived ACK is unusual, namely:
+ *	* SACK
+ *	* Duplicate ACK.
+ *	* ECN ECE.
+ *
+ * Counting packets in flight is pretty simple.
+ *
+ *	in_flight = packets_out - left_out + retrans_out
+ *
+ *	packets_out is SND.NXT-SND.UNA counted in packets.
+ *
+ *	retrans_out is number of retransmitted segments.
+ *
+ *	left_out is number of segments left network, but not ACKed yet.
+ *
+ *		left_out = sacked_out + lost_out
+ *
+ *     sacked_out: Packets, which arrived to receiver out of order
+ *		   and hence not ACKed. With SACKs this number is simply
+ *		   amount of SACKed data. Even without SACKs
+ *		   it is easy to give pretty reliable estimate of this number,
+ *		   counting duplicate ACKs.
+ *
+ *       lost_out: Packets lost by network. TCP has no explicit
+ *		   "loss notification" feedback from network (for now).
+ *		   It means that this number can be only _guessed_.
+ *		   Actually, it is the heuristics to predict lossage that
+ *		   distinguishes different algorithms.
+ *
+ *	F.e. after RTO, when all the queue is considered as lost,
+ *	lost_out = packets_out and in_flight = retrans_out.
+ *
+ *		Essentially, we have now a few algorithms detecting
+ *		lost packets.
+ *
+ *		If the receiver supports SACK:
+ *
+ *		RFC6675/3517: It is the conventional algorithm. A packet is
+ *		considered lost if the number of higher sequence packets
+ *		SACKed is greater than or equal the DUPACK thoreshold
+ *		(reordering). This is implemented in tcp_mark_head_lost and
+ *		tcp_update_scoreboard.
+ *
+ *		RACK (draft-ietf-tcpm-rack-01): it is a newer algorithm
+ *		(2017-) that checks timing instead of counting DUPACKs.
+ *		Essentially a packet is considered lost if it's not S/ACKed
+ *		after RTT + reordering_window, where both metrics are
+ *		dynamically measured and adjusted. This is implemented in
+ *		tcp_rack_mark_lost.
+ *
+ *		If the receiver does not support SACK:
+ *
+ *		NewReno (RFC6582): in Recovery we assume that one segment
+ *		is lost (classic Reno). While we are in Recovery and
+ *		a partial ACK arrives, we assume that one more packet
+ *		is lost (NewReno). This heuristics are the same in NewReno
+ *		and SACK.
+ *
+ * Really tricky (and requiring careful tuning) part of algorithm
+ * is hidden in functions tcp_time_to_recover() and tcp_xmit_retransmit_queue().
+ * The first determines the moment _when_ we should reduce CWND and,
+ * hence, slow down forward transmission. In fact, it determines the moment
+ * when we decide that hole is caused by loss, rather than by a reorder.
+ *
+ * tcp_xmit_retransmit_queue() decides, _what_ we should retransmit to fill
+ * holes, caused by lost packets.
+ *
+ * And the most logically complicated part of algorithm is undo
+ * heuristics. We detect false retransmits due to both too early
+ * fast retransmit (reordering) and underestimated RTO, analyzing
+ * timestamps and D-SACKs. When we detect that some segments were
+ * retransmitted by mistake and CWND reduction was wrong, we undo
+ * window reduction and abort recovery phase. This logic is hidden
+ * inside several functions named tcp_try_undo_<something>.
+ */
+
+/* This function decides, when we should leave Disordered state
+ * and enter Recovery phase, reducing congestion window.
+ *
+ * Main question: may we further continue forward transmission
+ * with the same cwnd?
+ */
+static bool tcp_time_to_recover(struct sock *sk, int flag)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	/* Trick#1: The loss is proven. */
+	if (tp->lost_out)
+		return true;
+
+	/* Not-A-Trick#2 : Classic rule... */
+	if (!tcp_is_rack(sk) && tcp_dupack_heuristics(tp) > tp->reordering)
+		return true;
+
+	return false;
+}
+
+/* Detect loss in event "A" above by marking head of queue up as lost.
+ * For RFC3517 SACK, a segment is considered lost if it
+ * has at least tp->reordering SACKed seqments above it; "packets" refers to
+ * the maximum SACKed segments to pass before reaching this limit.
+ */
+static void tcp_mark_head_lost(struct sock *sk, int packets, int mark_head)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *skb;
+	int cnt;
+	/* Use SACK to deduce losses of new sequences sent during recovery */
+	const u32 loss_high = tp->snd_nxt;
+
+	WARN_ON(packets > tp->packets_out);
+	skb = tp->lost_skb_hint;
+	if (skb) {
+		/* Head already handled? */
+		if (mark_head && after(TCP_SKB_CB(skb)->seq, tp->snd_una))
+			return;
+		cnt = tp->lost_cnt_hint;
+	} else {
+		skb = tcp_rtx_queue_head(sk);
+		cnt = 0;
+	}
+
+	skb_rbtree_walk_from(skb) {
+		/* TODO: do this better */
+		/* this is not the most efficient way to do this... */
+		tp->lost_skb_hint = skb;
+		tp->lost_cnt_hint = cnt;
+
+		if (after(TCP_SKB_CB(skb)->end_seq, loss_high))
+			break;
+
+		if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
+			cnt += tcp_skb_pcount(skb);
+
+		if (cnt > packets)
+			break;
+
+		if (!(TCP_SKB_CB(skb)->sacked & TCPCB_LOST))
+			tcp_mark_skb_lost(sk, skb);
+
+		if (mark_head)
+			break;
+	}
+	tcp_verify_left_out(tp);
+}
+
+/* Account newly detected lost packet(s) */
+
+static void tcp_update_scoreboard(struct sock *sk, int fast_rexmit)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (tcp_is_sack(tp)) {
+		int sacked_upto = tp->sacked_out - tp->reordering;
+		if (sacked_upto >= 0)
+			tcp_mark_head_lost(sk, sacked_upto, 0);
+		else if (fast_rexmit)
+			tcp_mark_head_lost(sk, 1, 1);
+	}
+}
+
+static bool tcp_tsopt_ecr_before(const struct tcp_sock *tp, u32 when)
+{
+	return tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
+	       before(tp->rx_opt.rcv_tsecr, when);
+}
+
+/* skb is spurious retransmitted if the returned timestamp echo
+ * reply is prior to the skb transmission time
+ */
+static bool tcp_skb_spurious_retrans(const struct tcp_sock *tp,
+				     const struct sk_buff *skb)
+{
+	return (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS) &&
+	       tcp_tsopt_ecr_before(tp, tcp_skb_timestamp(skb));
+}
+
+/* Nothing was retransmitted or returned timestamp is less
+ * than timestamp of the first retransmission.
+ */
+static inline bool tcp_packet_delayed(const struct tcp_sock *tp)
+{
+	return tp->retrans_stamp &&
+	       tcp_tsopt_ecr_before(tp, tp->retrans_stamp);
+}
+
+/* Undo procedures. */
+
+/* We can clear retrans_stamp when there are no retransmissions in the
+ * window. It would seem that it is trivially available for us in
+ * tp->retrans_out, however, that kind of assumptions doesn't consider
+ * what will happen if errors occur when sending retransmission for the
+ * second time. ...It could the that such segment has only
+ * TCPCB_EVER_RETRANS set at the present time. It seems that checking
+ * the head skb is enough except for some reneging corner cases that
+ * are not worth the effort.
+ *
+ * Main reason for all this complexity is the fact that connection dying
+ * time now depends on the validity of the retrans_stamp, in particular,
+ * that successive retransmissions of a segment must not advance
+ * retrans_stamp under any conditions.
+ */
+static bool tcp_any_retrans_done(const struct sock *sk)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *skb;
+
+	if (tp->retrans_out)
+		return true;
+
+	skb = tcp_rtx_queue_head(sk);
+	if (unlikely(skb && TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS))
+		return true;
+
+	return false;
+}
+
+static void DBGUNDO(struct sock *sk, const char *msg)
+{
+#if FASTRETRANS_DEBUG > 1
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct inet_sock *inet = inet_sk(sk);
+
+	if (sk->sk_family == AF_INET) {
+		pr_debug("Undo %s %pI4/%u c%u l%u ss%u/%u p%u\n",
+			 msg,
+			 &inet->inet_daddr, ntohs(inet->inet_dport),
+			 tp->snd_cwnd, tcp_left_out(tp),
+			 tp->snd_ssthresh, tp->prior_ssthresh,
+			 tp->packets_out);
+	}
+#if IS_ENABLED(CONFIG_IPV6)
+	else if (sk->sk_family == AF_INET6) {
+		pr_debug("Undo %s %pI6/%u c%u l%u ss%u/%u p%u\n",
+			 msg,
+			 &sk->sk_v6_daddr, ntohs(inet->inet_dport),
+			 tp->snd_cwnd, tcp_left_out(tp),
+			 tp->snd_ssthresh, tp->prior_ssthresh,
+			 tp->packets_out);
+	}
+#endif
+#endif
+}
+
+static void tcp_undo_cwnd_reduction(struct sock *sk, bool unmark_loss)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (unmark_loss) {
+		struct sk_buff *skb;
+
+		skb_rbtree_walk(skb, &sk->tcp_rtx_queue) {
+			TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
+		}
+		tp->lost_out = 0;
+		tcp_clear_all_retrans_hints(tp);
+	}
+
+	if (tp->prior_ssthresh) {
+		const struct inet_connection_sock *icsk = inet_csk(sk);
+
+		tp->snd_cwnd = icsk->icsk_ca_ops->undo_cwnd(sk);
+
+		if (tp->prior_ssthresh > tp->snd_ssthresh) {
+			tp->snd_ssthresh = tp->prior_ssthresh;
+			tcp_ecn_withdraw_cwr(tp);
+		}
+	}
+	tp->snd_cwnd_stamp = tcp_jiffies32;
+	tp->undo_marker = 0;
+	tp->rack.advanced = 1; /* Force RACK to re-exam losses */
+}
+
+static inline bool tcp_may_undo(const struct tcp_sock *tp)
+{
+	return tp->undo_marker && (!tp->undo_retrans || tcp_packet_delayed(tp));
+}
+
+static bool tcp_is_non_sack_preventing_reopen(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (tp->snd_una == tp->high_seq && tcp_is_reno(tp)) {
+		/* Hold old state until something *above* high_seq
+		 * is ACKed. For Reno it is MUST to prevent false
+		 * fast retransmits (RFC2582). SACK TCP is safe. */
+		if (!tcp_any_retrans_done(sk))
+			tp->retrans_stamp = 0;
+		return true;
+	}
+	return false;
+}
+
+/* People celebrate: "We love our President!" */
+static bool tcp_try_undo_recovery(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (tcp_may_undo(tp)) {
+		int mib_idx;
+
+		/* Happy end! We did not retransmit anything
+		 * or our original transmission succeeded.
+		 */
+		DBGUNDO(sk, inet_csk(sk)->icsk_ca_state == TCP_CA_Loss ? "loss" : "retrans");
+		tcp_undo_cwnd_reduction(sk, false);
+		if (inet_csk(sk)->icsk_ca_state == TCP_CA_Loss)
+			mib_idx = LINUX_MIB_TCPLOSSUNDO;
+		else
+			mib_idx = LINUX_MIB_TCPFULLUNDO;
+
+		NET_INC_STATS(sock_net(sk), mib_idx);
+	} else if (tp->rack.reo_wnd_persist) {
+		tp->rack.reo_wnd_persist--;
+	}
+	if (tcp_is_non_sack_preventing_reopen(sk))
+		return true;
+	tcp_set_ca_state(sk, TCP_CA_Open);
+	tp->is_sack_reneg = 0;
+	return false;
+}
+
+/* Try to undo cwnd reduction, because D-SACKs acked all retransmitted data */
+static bool tcp_try_undo_dsack(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (tp->undo_marker && !tp->undo_retrans) {
+		tp->rack.reo_wnd_persist = min(TCP_RACK_RECOVERY_THRESH,
+					       tp->rack.reo_wnd_persist + 1);
+		DBGUNDO(sk, "D-SACK");
+		tcp_undo_cwnd_reduction(sk, false);
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDSACKUNDO);
+		return true;
+	}
+	return false;
+}
+
+/* Undo during loss recovery after partial ACK or using F-RTO. */
+static bool tcp_try_undo_loss(struct sock *sk, bool frto_undo)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (frto_undo || tcp_may_undo(tp)) {
+		tcp_undo_cwnd_reduction(sk, true);
+
+		DBGUNDO(sk, "partial loss");
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPLOSSUNDO);
+		if (frto_undo)
+			NET_INC_STATS(sock_net(sk),
+					LINUX_MIB_TCPSPURIOUSRTOS);
+		inet_csk(sk)->icsk_retransmits = 0;
+		if (tcp_is_non_sack_preventing_reopen(sk))
+			return true;
+		if (frto_undo || tcp_is_sack(tp)) {
+			tcp_set_ca_state(sk, TCP_CA_Open);
+			tp->is_sack_reneg = 0;
+		}
+		return true;
+	}
+	return false;
+}
+
+/* The cwnd reduction in CWR and Recovery uses the PRR algorithm in RFC 6937.
+ * It computes the number of packets to send (sndcnt) based on packets newly
+ * delivered:
+ *   1) If the packets in flight is larger than ssthresh, PRR spreads the
+ *	cwnd reductions across a full RTT.
+ *   2) Otherwise PRR uses packet conservation to send as much as delivered.
+ *      But when the retransmits are acked without further losses, PRR
+ *      slow starts cwnd up to ssthresh to speed up the recovery.
+ */
+static void tcp_init_cwnd_reduction(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	tp->high_seq = tp->snd_nxt;
+	tp->tlp_high_seq = 0;
+	tp->snd_cwnd_cnt = 0;
+	tp->prior_cwnd = tp->snd_cwnd;
+	tp->prr_delivered = 0;
+	tp->prr_out = 0;
+	tp->snd_ssthresh = inet_csk(sk)->icsk_ca_ops->ssthresh(sk);
+	tcp_ecn_queue_cwr(tp);
+}
+
+void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked, int flag)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	int sndcnt = 0;
+	int delta = tp->snd_ssthresh - tcp_packets_in_flight(tp);
+
+	if (newly_acked_sacked <= 0 || WARN_ON_ONCE(!tp->prior_cwnd))
+		return;
+
+	tp->prr_delivered += newly_acked_sacked;
+	if (delta < 0) {
+		u64 dividend = (u64)tp->snd_ssthresh * tp->prr_delivered +
+			       tp->prior_cwnd - 1;
+		sndcnt = div_u64(dividend, tp->prior_cwnd) - tp->prr_out;
+	} else if ((flag & (FLAG_RETRANS_DATA_ACKED | FLAG_LOST_RETRANS)) ==
+		   FLAG_RETRANS_DATA_ACKED) {
+		sndcnt = min_t(int, delta,
+			       max_t(int, tp->prr_delivered - tp->prr_out,
+				     newly_acked_sacked) + 1);
+	} else {
+		sndcnt = min(delta, newly_acked_sacked);
+	}
+	/* Force a fast retransmit upon entering fast recovery */
+	sndcnt = max(sndcnt, (tp->prr_out ? 0 : 1));
+	tp->snd_cwnd = tcp_packets_in_flight(tp) + sndcnt;
+}
+
+static inline void tcp_end_cwnd_reduction(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (inet_csk(sk)->icsk_ca_ops->cong_control)
+		return;
+
+	/* Reset cwnd to ssthresh in CWR or Recovery (unless it's undone) */
+	if (tp->snd_ssthresh < TCP_INFINITE_SSTHRESH &&
+	    (inet_csk(sk)->icsk_ca_state == TCP_CA_CWR || tp->undo_marker)) {
+		tp->snd_cwnd = tp->snd_ssthresh;
+		tp->snd_cwnd_stamp = tcp_jiffies32;
+	}
+	tcp_ca_event(sk, CA_EVENT_COMPLETE_CWR);
+}
+
+/* Enter CWR state. Disable cwnd undo since congestion is proven with ECN */
+void tcp_enter_cwr(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	tp->prior_ssthresh = 0;
+	if (inet_csk(sk)->icsk_ca_state < TCP_CA_CWR) {
+		tp->undo_marker = 0;
+		tcp_init_cwnd_reduction(sk);
+		tcp_set_ca_state(sk, TCP_CA_CWR);
+	}
+}
+EXPORT_SYMBOL(tcp_enter_cwr);
+
+static void tcp_try_keep_open(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	int state = TCP_CA_Open;
+
+	if (tcp_left_out(tp) || tcp_any_retrans_done(sk))
+		state = TCP_CA_Disorder;
+
+	if (inet_csk(sk)->icsk_ca_state != state) {
+		tcp_set_ca_state(sk, state);
+		tp->high_seq = tp->snd_nxt;
+	}
+}
+
+static void tcp_try_to_open(struct sock *sk, int flag)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	tcp_verify_left_out(tp);
+
+	if (!tcp_any_retrans_done(sk))
+		tp->retrans_stamp = 0;
+
+	if (flag & FLAG_ECE)
+		tcp_enter_cwr(sk);
+
+	if (inet_csk(sk)->icsk_ca_state != TCP_CA_CWR) {
+		tcp_try_keep_open(sk);
+	}
+}
+
+static void tcp_mtup_probe_failed(struct sock *sk)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+
+	icsk->icsk_mtup.search_high = icsk->icsk_mtup.probe_size - 1;
+	icsk->icsk_mtup.probe_size = 0;
+	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPMTUPFAIL);
+}
+
+static void tcp_mtup_probe_success(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	u64 val;
+
+	tp->prior_ssthresh = tcp_current_ssthresh(sk);
+
+	val = (u64)tp->snd_cwnd * tcp_mss_to_mtu(sk, tp->mss_cache);
+	do_div(val, icsk->icsk_mtup.probe_size);
+	WARN_ON_ONCE((u32)val != val);
+	tp->snd_cwnd = max_t(u32, 1U, val);
+
+	tp->snd_cwnd_cnt = 0;
+	tp->snd_cwnd_stamp = tcp_jiffies32;
+	tp->snd_ssthresh = tcp_current_ssthresh(sk);
+
+	icsk->icsk_mtup.search_low = icsk->icsk_mtup.probe_size;
+	icsk->icsk_mtup.probe_size = 0;
+	tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
+	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPMTUPSUCCESS);
+}
+
+/* Do a simple retransmit without using the backoff mechanisms in
+ * tcp_timer. This is used for path mtu discovery.
+ * The socket is already locked here.
+ */
+void tcp_simple_retransmit(struct sock *sk)
+{
+	const struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *skb;
+	unsigned int mss = tcp_current_mss(sk);
+
+	skb_rbtree_walk(skb, &sk->tcp_rtx_queue) {
+		if (tcp_skb_seglen(skb) > mss)
+			tcp_mark_skb_lost(sk, skb);
+	}
+
+	tcp_clear_retrans_hints_partial(tp);
+
+	if (!tp->lost_out)
+		return;
+
+	if (tcp_is_reno(tp))
+		tcp_limit_reno_sacked(tp);
+
+	tcp_verify_left_out(tp);
+
+	/* Don't muck with the congestion window here.
+	 * Reason is that we do not increase amount of _data_
+	 * in network, but units changed and effective
+	 * cwnd/ssthresh really reduced now.
+	 */
+	if (icsk->icsk_ca_state != TCP_CA_Loss) {
+		tp->high_seq = tp->snd_nxt;
+		tp->snd_ssthresh = tcp_current_ssthresh(sk);
+		tp->prior_ssthresh = 0;
+		tp->undo_marker = 0;
+		tcp_set_ca_state(sk, TCP_CA_Loss);
+	}
+	tcp_xmit_retransmit_queue(sk);
+}
+EXPORT_SYMBOL(tcp_simple_retransmit);
+
+void tcp_enter_recovery(struct sock *sk, bool ece_ack)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	int mib_idx;
+
+	if (tcp_is_reno(tp))
+		mib_idx = LINUX_MIB_TCPRENORECOVERY;
+	else
+		mib_idx = LINUX_MIB_TCPSACKRECOVERY;
+
+	NET_INC_STATS(sock_net(sk), mib_idx);
+
+	tp->prior_ssthresh = 0;
+	tcp_init_undo(tp);
+
+	if (!tcp_in_cwnd_reduction(sk)) {
+		if (!ece_ack)
+			tp->prior_ssthresh = tcp_current_ssthresh(sk);
+		tcp_init_cwnd_reduction(sk);
+	}
+	tcp_set_ca_state(sk, TCP_CA_Recovery);
+}
+
+/* Process an ACK in CA_Loss state. Move to CA_Open if lost data are
+ * recovered or spurious. Otherwise retransmits more on partial ACKs.
+ */
+static void tcp_process_loss(struct sock *sk, int flag, int num_dupack,
+			     int *rexmit)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	bool recovered = !before(tp->snd_una, tp->high_seq);
+
+	if ((flag & FLAG_SND_UNA_ADVANCED || rcu_access_pointer(tp->fastopen_rsk)) &&
+	    tcp_try_undo_loss(sk, false))
+		return;
+
+	if (tp->frto) { /* F-RTO RFC5682 sec 3.1 (sack enhanced version). */
+		/* Step 3.b. A timeout is spurious if not all data are
+		 * lost, i.e., never-retransmitted data are (s)acked.
+		 */
+		if ((flag & FLAG_ORIG_SACK_ACKED) &&
+		    tcp_try_undo_loss(sk, true))
+			return;
+
+		if (after(tp->snd_nxt, tp->high_seq)) {
+			if (flag & FLAG_DATA_SACKED || num_dupack)
+				tp->frto = 0; /* Step 3.a. loss was real */
+		} else if (flag & FLAG_SND_UNA_ADVANCED && !recovered) {
+			tp->high_seq = tp->snd_nxt;
+			/* Step 2.b. Try send new data (but deferred until cwnd
+			 * is updated in tcp_ack()). Otherwise fall back to
+			 * the conventional recovery.
+			 */
+			if (!tcp_write_queue_empty(sk) &&
+			    after(tcp_wnd_end(tp), tp->snd_nxt)) {
+				*rexmit = REXMIT_NEW;
+				return;
+			}
+			tp->frto = 0;
+		}
+	}
+
+	if (recovered) {
+		/* F-RTO RFC5682 sec 3.1 step 2.a and 1st part of step 3.a */
+		tcp_try_undo_recovery(sk);
+		return;
+	}
+	if (tcp_is_reno(tp)) {
+		/* A Reno DUPACK means new data in F-RTO step 2.b above are
+		 * delivered. Lower inflight to clock out (re)tranmissions.
+		 */
+		if (after(tp->snd_nxt, tp->high_seq) && num_dupack)
+			tcp_add_reno_sack(sk, num_dupack, flag & FLAG_ECE);
+		else if (flag & FLAG_SND_UNA_ADVANCED)
+			tcp_reset_reno_sack(tp);
+	}
+	*rexmit = REXMIT_LOST;
+}
+
+static bool tcp_force_fast_retransmit(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	return after(tcp_highest_sack_seq(tp),
+		     tp->snd_una + tp->reordering * tp->mss_cache);
+}
+
+/* Undo during fast recovery after partial ACK. */
+static bool tcp_try_undo_partial(struct sock *sk, u32 prior_snd_una,
+				 bool *do_lost)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (tp->undo_marker && tcp_packet_delayed(tp)) {
+		/* Plain luck! Hole if filled with delayed
+		 * packet, rather than with a retransmit. Check reordering.
+		 */
+		tcp_check_sack_reordering(sk, prior_snd_una, 1);
+
+		/* We are getting evidence that the reordering degree is higher
+		 * than we realized. If there are no retransmits out then we
+		 * can undo. Otherwise we clock out new packets but do not
+		 * mark more packets lost or retransmit more.
+		 */
+		if (tp->retrans_out)
+			return true;
+
+		if (!tcp_any_retrans_done(sk))
+			tp->retrans_stamp = 0;
+
+		DBGUNDO(sk, "partial recovery");
+		tcp_undo_cwnd_reduction(sk, true);
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPARTIALUNDO);
+		tcp_try_keep_open(sk);
+	} else {
+		/* Partial ACK arrived. Force fast retransmit. */
+		*do_lost = tcp_force_fast_retransmit(sk);
+	}
+	return false;
+}
+
+static void tcp_identify_packet_loss(struct sock *sk, int *ack_flag)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (tcp_rtx_queue_empty(sk))
+		return;
+
+	if (unlikely(tcp_is_reno(tp))) {
+		tcp_newreno_mark_lost(sk, *ack_flag & FLAG_SND_UNA_ADVANCED);
+	} else if (tcp_is_rack(sk)) {
+		u32 prior_retrans = tp->retrans_out;
+
+		if (tcp_rack_mark_lost(sk))
+			*ack_flag &= ~FLAG_SET_XMIT_TIMER;
+		if (prior_retrans > tp->retrans_out)
+			*ack_flag |= FLAG_LOST_RETRANS;
+	}
+}
+
+/* Process an event, which can update packets-in-flight not trivially.
+ * Main goal of this function is to calculate new estimate for left_out,
+ * taking into account both packets sitting in receiver's buffer and
+ * packets lost by network.
+ *
+ * Besides that it updates the congestion state when packet loss or ECN
+ * is detected. But it does not reduce the cwnd, it is done by the
+ * congestion control later.
+ *
+ * It does _not_ decide what to send, it is made in function
+ * tcp_xmit_retransmit_queue().
+ */
+static void tcp_fastretrans_alert(struct sock *sk, const u32 prior_snd_una,
+				  int num_dupack, int *ack_flag, int *rexmit)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	int fast_rexmit = 0, flag = *ack_flag;
+	bool ece_ack = flag & FLAG_ECE;
+	bool do_lost = num_dupack || ((flag & FLAG_DATA_SACKED) &&
+				      tcp_force_fast_retransmit(sk));
+
+	if (!tp->packets_out && tp->sacked_out)
+		tp->sacked_out = 0;
+
+	/* Now state machine starts.
+	 * A. ECE, hence prohibit cwnd undoing, the reduction is required. */
+	if (ece_ack)
+		tp->prior_ssthresh = 0;
+
+	/* B. In all the states check for reneging SACKs. */
+	if (tcp_check_sack_reneging(sk, ack_flag))
+		return;
+
+	/* C. Check consistency of the current state. */
+	tcp_verify_left_out(tp);
+
+	/* D. Check state exit conditions. State can be terminated
+	 *    when high_seq is ACKed. */
+	if (icsk->icsk_ca_state == TCP_CA_Open) {
+		WARN_ON(tp->retrans_out != 0);
+		tp->retrans_stamp = 0;
+	} else if (!before(tp->snd_una, tp->high_seq)) {
+		switch (icsk->icsk_ca_state) {
+		case TCP_CA_CWR:
+			/* CWR is to be held something *above* high_seq
+			 * is ACKed for CWR bit to reach receiver. */
+			if (tp->snd_una != tp->high_seq) {
+				tcp_end_cwnd_reduction(sk);
+				tcp_set_ca_state(sk, TCP_CA_Open);
+			}
+			break;
+
+		case TCP_CA_Recovery:
+			if (tcp_is_reno(tp))
+				tcp_reset_reno_sack(tp);
+			if (tcp_try_undo_recovery(sk))
+				return;
+			tcp_end_cwnd_reduction(sk);
+			break;
+		}
+	}
+
+	/* E. Process state. */
+	switch (icsk->icsk_ca_state) {
+	case TCP_CA_Recovery:
+		if (!(flag & FLAG_SND_UNA_ADVANCED)) {
+			if (tcp_is_reno(tp))
+				tcp_add_reno_sack(sk, num_dupack, ece_ack);
+		} else if (tcp_try_undo_partial(sk, prior_snd_una, &do_lost))
+			return;
+
+		if (tcp_try_undo_dsack(sk))
+			tcp_try_keep_open(sk);
+
+		tcp_identify_packet_loss(sk, ack_flag);
+		if (icsk->icsk_ca_state != TCP_CA_Recovery) {
+			if (!tcp_time_to_recover(sk, flag))
+				return;
+			/* Undo reverts the recovery state. If loss is evident,
+			 * starts a new recovery (e.g. reordering then loss);
+			 */
+			tcp_enter_recovery(sk, ece_ack);
+		}
+		break;
+	case TCP_CA_Loss:
+		tcp_process_loss(sk, flag, num_dupack, rexmit);
+		tcp_identify_packet_loss(sk, ack_flag);
+		if (!(icsk->icsk_ca_state == TCP_CA_Open ||
+		      (*ack_flag & FLAG_LOST_RETRANS)))
+			return;
+		/* Change state if cwnd is undone or retransmits are lost */
+		fallthrough;
+	default:
+		if (tcp_is_reno(tp)) {
+			if (flag & FLAG_SND_UNA_ADVANCED)
+				tcp_reset_reno_sack(tp);
+			tcp_add_reno_sack(sk, num_dupack, ece_ack);
+		}
+
+		if (icsk->icsk_ca_state <= TCP_CA_Disorder)
+			tcp_try_undo_dsack(sk);
+
+		tcp_identify_packet_loss(sk, ack_flag);
+		if (!tcp_time_to_recover(sk, flag)) {
+			tcp_try_to_open(sk, flag);
+			return;
+		}
+
+		/* MTU probe failure: don't reduce cwnd */
+		if (icsk->icsk_ca_state < TCP_CA_CWR &&
+		    icsk->icsk_mtup.probe_size &&
+		    tp->snd_una == tp->mtu_probe.probe_seq_start) {
+			tcp_mtup_probe_failed(sk);
+			/* Restores the reduction we did in tcp_mtup_probe() */
+			tp->snd_cwnd++;
+			tcp_simple_retransmit(sk);
+			return;
+		}
+
+		/* Otherwise enter Recovery state */
+		tcp_enter_recovery(sk, ece_ack);
+		fast_rexmit = 1;
+	}
+
+	if (!tcp_is_rack(sk) && do_lost)
+		tcp_update_scoreboard(sk, fast_rexmit);
+	*rexmit = REXMIT_LOST;
+}
+
+static void tcp_update_rtt_min(struct sock *sk, u32 rtt_us, const int flag)
+{
+	u32 wlen = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_min_rtt_wlen) * HZ;
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if ((flag & FLAG_ACK_MAYBE_DELAYED) && rtt_us > tcp_min_rtt(tp)) {
+		/* If the remote keeps returning delayed ACKs, eventually
+		 * the min filter would pick it up and overestimate the
+		 * prop. delay when it expires. Skip suspected delayed ACKs.
+		 */
+		return;
+	}
+	minmax_running_min(&tp->rtt_min, wlen, tcp_jiffies32,
+			   rtt_us ? : jiffies_to_usecs(1));
+}
+
+static bool tcp_ack_update_rtt(struct sock *sk, const int flag,
+			       long seq_rtt_us, long sack_rtt_us,
+			       long ca_rtt_us, struct rate_sample *rs)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+
+	/* Prefer RTT measured from ACK's timing to TS-ECR. This is because
+	 * broken middle-boxes or peers may corrupt TS-ECR fields. But
+	 * Karn's algorithm forbids taking RTT if some retransmitted data
+	 * is acked (RFC6298).
+	 */
+	if (seq_rtt_us < 0)
+		seq_rtt_us = sack_rtt_us;
+
+	/* RTTM Rule: A TSecr value received in a segment is used to
+	 * update the averaged RTT measurement only if the segment
+	 * acknowledges some new data, i.e., only if it advances the
+	 * left edge of the send window.
+	 * See draft-ietf-tcplw-high-performance-00, section 3.3.
+	 */
+	if (seq_rtt_us < 0 && tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
+	    flag & FLAG_ACKED) {
+		u32 delta = tcp_time_stamp(tp) - tp->rx_opt.rcv_tsecr;
+
+		if (likely(delta < INT_MAX / (USEC_PER_SEC / TCP_TS_HZ))) {
+			if (!delta)
+				delta = 1;
+			seq_rtt_us = delta * (USEC_PER_SEC / TCP_TS_HZ);
+			ca_rtt_us = seq_rtt_us;
+		}
+	}
+	rs->rtt_us = ca_rtt_us; /* RTT of last (S)ACKed packet (or -1) */
+	if (seq_rtt_us < 0)
+		return false;
+
+	/* ca_rtt_us >= 0 is counting on the invariant that ca_rtt_us is
+	 * always taken together with ACK, SACK, or TS-opts. Any negative
+	 * values will be skipped with the seq_rtt_us < 0 check above.
+	 */
+	tcp_update_rtt_min(sk, ca_rtt_us, flag);
+	tcp_rtt_estimator(sk, seq_rtt_us);
+	tcp_set_rto(sk);
+
+	/* RFC6298: only reset backoff on valid RTT measurement. */
+	inet_csk(sk)->icsk_backoff = 0;
+	return true;
+}
+
+/* Compute time elapsed between (last) SYNACK and the ACK completing 3WHS. */
+void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req)
+{
+	struct rate_sample rs;
+	long rtt_us = -1L;
+
+	if (req && !req->num_retrans && tcp_rsk(req)->snt_synack)
+		rtt_us = tcp_stamp_us_delta(tcp_clock_us(), tcp_rsk(req)->snt_synack);
+
+	tcp_ack_update_rtt(sk, FLAG_SYN_ACKED, rtt_us, -1L, rtt_us, &rs);
+}
+
+
+static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
+{
+	const struct inet_connection_sock *icsk = inet_csk(sk);
+
+	icsk->icsk_ca_ops->cong_avoid(sk, ack, acked);
+	tcp_sk(sk)->snd_cwnd_stamp = tcp_jiffies32;
+}
+
+/* Restart timer after forward progress on connection.
+ * RFC2988 recommends to restart timer to now+rto.
+ */
+void tcp_rearm_rto(struct sock *sk)
+{
+	const struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	/* If the retrans timer is currently being used by Fast Open
+	 * for SYN-ACK retrans purpose, stay put.
+	 */
+	if (rcu_access_pointer(tp->fastopen_rsk))
+		return;
+
+	if (!tp->packets_out) {
+		inet_csk_clear_xmit_timer(sk, ICSK_TIME_RETRANS);
+	} else {
+		u32 rto = inet_csk(sk)->icsk_rto;
+		/* Offset the time elapsed after installing regular RTO */
+		if (icsk->icsk_pending == ICSK_TIME_REO_TIMEOUT ||
+		    icsk->icsk_pending == ICSK_TIME_LOSS_PROBE) {
+			s64 delta_us = tcp_rto_delta_us(sk);
+			/* delta_us may not be positive if the socket is locked
+			 * when the retrans timer fires and is rescheduled.
+			 */
+			rto = usecs_to_jiffies(max_t(int, delta_us, 1));
+		}
+		tcp_reset_xmit_timer(sk, ICSK_TIME_RETRANS, rto,
+				     TCP_RTO_MAX);
+	}
+}
+
+/* Try to schedule a loss probe; if that doesn't work, then schedule an RTO. */
+static void tcp_set_xmit_timer(struct sock *sk)
+{
+	if (!tcp_schedule_loss_probe(sk, true))
+		tcp_rearm_rto(sk);
+}
+
+/* If we get here, the whole TSO packet has not been acked. */
+static u32 tcp_tso_acked(struct sock *sk, struct sk_buff *skb)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	u32 packets_acked;
+
+	BUG_ON(!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una));
+
+	packets_acked = tcp_skb_pcount(skb);
+	if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
+		return 0;
+	packets_acked -= tcp_skb_pcount(skb);
+
+	if (packets_acked) {
+		BUG_ON(tcp_skb_pcount(skb) == 0);
+		BUG_ON(!before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq));
+	}
+
+	return packets_acked;
+}
+
+static void tcp_ack_tstamp(struct sock *sk, struct sk_buff *skb,
+			   u32 prior_snd_una)
+{
+	const struct skb_shared_info *shinfo;
+
+	/* Avoid cache line misses to get skb_shinfo() and shinfo->tx_flags */
+	if (likely(!TCP_SKB_CB(skb)->txstamp_ack))
+		return;
+
+	shinfo = skb_shinfo(skb);
+	if (!before(shinfo->tskey, prior_snd_una) &&
+	    before(shinfo->tskey, tcp_sk(sk)->snd_una)) {
+		tcp_skb_tsorted_save(skb) {
+			__skb_tstamp_tx(skb, NULL, sk, SCM_TSTAMP_ACK);
+		} tcp_skb_tsorted_restore(skb);
+	}
+}
+
+/* Remove acknowledged frames from the retransmission queue. If our packet
+ * is before the ack sequence we can discard it as it's confirmed to have
+ * arrived at the other end.
+ */
+static int tcp_clean_rtx_queue(struct sock *sk, u32 prior_fack,
+			       u32 prior_snd_una,
+			       struct tcp_sacktag_state *sack, bool ece_ack)
+{
+	const struct inet_connection_sock *icsk = inet_csk(sk);
+	u64 first_ackt, last_ackt;
+	struct tcp_sock *tp = tcp_sk(sk);
+	u32 prior_sacked = tp->sacked_out;
+	u32 reord = tp->snd_nxt; /* lowest acked un-retx un-sacked seq */
+	struct sk_buff *skb, *next;
+	bool fully_acked = true;
+	long sack_rtt_us = -1L;
+	long seq_rtt_us = -1L;
+	long ca_rtt_us = -1L;
+	u32 pkts_acked = 0;
+	u32 last_in_flight = 0;
+	bool rtt_update;
+	int flag = 0;
+
+	first_ackt = 0;
+
+	for (skb = skb_rb_first(&sk->tcp_rtx_queue); skb; skb = next) {
+		struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
+		const u32 start_seq = scb->seq;
+		u8 sacked = scb->sacked;
+		u32 acked_pcount;
+
+		/* Determine how many packets and what bytes were acked, tso and else */
+		if (after(scb->end_seq, tp->snd_una)) {
+			if (tcp_skb_pcount(skb) == 1 ||
+			    !after(tp->snd_una, scb->seq))
+				break;
+
+			acked_pcount = tcp_tso_acked(sk, skb);
+			if (!acked_pcount)
+				break;
+			fully_acked = false;
+		} else {
+			acked_pcount = tcp_skb_pcount(skb);
+		}
+
+		if (unlikely(sacked & TCPCB_RETRANS)) {
+			if (sacked & TCPCB_SACKED_RETRANS)
+				tp->retrans_out -= acked_pcount;
+			flag |= FLAG_RETRANS_DATA_ACKED;
+		} else if (!(sacked & TCPCB_SACKED_ACKED)) {
+			last_ackt = tcp_skb_timestamp_us(skb);
+			WARN_ON_ONCE(last_ackt == 0);
+			if (!first_ackt)
+				first_ackt = last_ackt;
+
+			last_in_flight = TCP_SKB_CB(skb)->tx.in_flight;
+			if (before(start_seq, reord))
+				reord = start_seq;
+			if (!after(scb->end_seq, tp->high_seq))
+				flag |= FLAG_ORIG_SACK_ACKED;
+		}
+
+		if (sacked & TCPCB_SACKED_ACKED) {
+			tp->sacked_out -= acked_pcount;
+		} else if (tcp_is_sack(tp)) {
+			tcp_count_delivered(tp, acked_pcount, ece_ack);
+			if (!tcp_skb_spurious_retrans(tp, skb))
+				tcp_rack_advance(tp, sacked, scb->end_seq,
+						 tcp_skb_timestamp_us(skb));
+		}
+		if (sacked & TCPCB_LOST)
+			tp->lost_out -= acked_pcount;
+
+		tp->packets_out -= acked_pcount;
+		pkts_acked += acked_pcount;
+		tcp_rate_skb_delivered(sk, skb, sack->rate);
+
+		/* Initial outgoing SYN's get put onto the write_queue
+		 * just like anything else we transmit.  It is not
+		 * true data, and if we misinform our callers that
+		 * this ACK acks real data, we will erroneously exit
+		 * connection startup slow start one packet too
+		 * quickly.  This is severely frowned upon behavior.
+		 */
+		if (likely(!(scb->tcp_flags & TCPHDR_SYN))) {
+			flag |= FLAG_DATA_ACKED;
+		} else {
+			flag |= FLAG_SYN_ACKED;
+			tp->retrans_stamp = 0;
+		}
+
+		if (!fully_acked)
+			break;
+
+		tcp_ack_tstamp(sk, skb, prior_snd_una);
+
+		next = skb_rb_next(skb);
+		if (unlikely(skb == tp->retransmit_skb_hint))
+			tp->retransmit_skb_hint = NULL;
+		if (unlikely(skb == tp->lost_skb_hint))
+			tp->lost_skb_hint = NULL;
+		tcp_highest_sack_replace(sk, skb, next);
+		tcp_rtx_queue_unlink_and_free(skb, sk);
+	}
+
+	if (!skb)
+		tcp_chrono_stop(sk, TCP_CHRONO_BUSY);
+
+	if (likely(between(tp->snd_up, prior_snd_una, tp->snd_una)))
+		tp->snd_up = tp->snd_una;
+
+	if (skb) {
+		tcp_ack_tstamp(sk, skb, prior_snd_una);
+		if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
+			flag |= FLAG_SACK_RENEGING;
+	}
+
+	if (likely(first_ackt) && !(flag & FLAG_RETRANS_DATA_ACKED)) {
+		seq_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, first_ackt);
+		ca_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, last_ackt);
+
+		if (pkts_acked == 1 && last_in_flight < tp->mss_cache &&
+		    last_in_flight && !prior_sacked && fully_acked &&
+		    sack->rate->prior_delivered + 1 == tp->delivered &&
+		    !(flag & (FLAG_CA_ALERT | FLAG_SYN_ACKED))) {
+			/* Conservatively mark a delayed ACK. It's typically
+			 * from a lone runt packet over the round trip to
+			 * a receiver w/o out-of-order or CE events.
+			 */
+			flag |= FLAG_ACK_MAYBE_DELAYED;
+		}
+	}
+	if (sack->first_sackt) {
+		sack_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, sack->first_sackt);
+		ca_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, sack->last_sackt);
+	}
+	rtt_update = tcp_ack_update_rtt(sk, flag, seq_rtt_us, sack_rtt_us,
+					ca_rtt_us, sack->rate);
+
+	if (flag & FLAG_ACKED) {
+		flag |= FLAG_SET_XMIT_TIMER;  /* set TLP or RTO timer */
+		if (unlikely(icsk->icsk_mtup.probe_size &&
+			     !after(tp->mtu_probe.probe_seq_end, tp->snd_una))) {
+			tcp_mtup_probe_success(sk);
+		}
+
+		if (tcp_is_reno(tp)) {
+			tcp_remove_reno_sacks(sk, pkts_acked, ece_ack);
+
+			/* If any of the cumulatively ACKed segments was
+			 * retransmitted, non-SACK case cannot confirm that
+			 * progress was due to original transmission due to
+			 * lack of TCPCB_SACKED_ACKED bits even if some of
+			 * the packets may have been never retransmitted.
+			 */
+			if (flag & FLAG_RETRANS_DATA_ACKED)
+				flag &= ~FLAG_ORIG_SACK_ACKED;
+		} else {
+			int delta;
+
+			/* Non-retransmitted hole got filled? That's reordering */
+			if (before(reord, prior_fack))
+				tcp_check_sack_reordering(sk, reord, 0);
+
+			delta = prior_sacked - tp->sacked_out;
+			tp->lost_cnt_hint -= min(tp->lost_cnt_hint, delta);
+		}
+	} else if (skb && rtt_update && sack_rtt_us >= 0 &&
+		   sack_rtt_us > tcp_stamp_us_delta(tp->tcp_mstamp,
+						    tcp_skb_timestamp_us(skb))) {
+		/* Do not re-arm RTO if the sack RTT is measured from data sent
+		 * after when the head was last (re)transmitted. Otherwise the
+		 * timeout may continue to extend in loss recovery.
+		 */
+		flag |= FLAG_SET_XMIT_TIMER;  /* set TLP or RTO timer */
+	}
+
+	if (icsk->icsk_ca_ops->pkts_acked) {
+		struct ack_sample sample = { .pkts_acked = pkts_acked,
+					     .rtt_us = sack->rate->rtt_us,
+					     .in_flight = last_in_flight };
+
+		icsk->icsk_ca_ops->pkts_acked(sk, &sample);
+	}
+
+#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);
+	if (!tp->packets_out && tcp_is_sack(tp)) {
+		icsk = inet_csk(sk);
+		if (tp->lost_out) {
+			pr_debug("Leak l=%u %d\n",
+				 tp->lost_out, icsk->icsk_ca_state);
+			tp->lost_out = 0;
+		}
+		if (tp->sacked_out) {
+			pr_debug("Leak s=%u %d\n",
+				 tp->sacked_out, icsk->icsk_ca_state);
+			tp->sacked_out = 0;
+		}
+		if (tp->retrans_out) {
+			pr_debug("Leak r=%u %d\n",
+				 tp->retrans_out, icsk->icsk_ca_state);
+			tp->retrans_out = 0;
+		}
+	}
+#endif
+	return flag;
+}
+
+static void tcp_ack_probe(struct sock *sk)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct sk_buff *head = tcp_send_head(sk);
+	const struct tcp_sock *tp = tcp_sk(sk);
+
+	/* Was it a usable window open? */
+	if (!head)
+		return;
+	if (!after(TCP_SKB_CB(head)->end_seq, tcp_wnd_end(tp))) {
+		icsk->icsk_backoff = 0;
+		icsk->icsk_probes_tstamp = 0;
+		inet_csk_clear_xmit_timer(sk, ICSK_TIME_PROBE0);
+		/* Socket must be waked up by subsequent tcp_data_snd_check().
+		 * This function is not for random using!
+		 */
+	} else {
+		unsigned long when = tcp_probe0_when(sk, TCP_RTO_MAX);
+
+		when = tcp_clamp_probe0_to_user_timeout(sk, when);
+		tcp_reset_xmit_timer(sk, ICSK_TIME_PROBE0, when, TCP_RTO_MAX);
+	}
+}
+
+static inline bool tcp_ack_is_dubious(const struct sock *sk, const int flag)
+{
+	return !(flag & FLAG_NOT_DUP) || (flag & FLAG_CA_ALERT) ||
+		inet_csk(sk)->icsk_ca_state != TCP_CA_Open;
+}
+
+/* Decide wheather to run the increase function of congestion control. */
+static inline bool tcp_may_raise_cwnd(const struct sock *sk, const int flag)
+{
+	/* If reordering is high then always grow cwnd whenever data is
+	 * delivered regardless of its ordering. Otherwise stay conservative
+	 * and only grow cwnd on in-order delivery (RFC5681). A stretched ACK w/
+	 * new SACK or ECE mark may first advance cwnd here and later reduce
+	 * cwnd in tcp_fastretrans_alert() based on more states.
+	 */
+	if (tcp_sk(sk)->reordering >
+	    READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_reordering))
+		return flag & FLAG_FORWARD_PROGRESS;
+
+	return flag & FLAG_DATA_ACKED;
+}
+
+/* The "ultimate" congestion control function that aims to replace the rigid
+ * cwnd increase and decrease control (tcp_cong_avoid,tcp_*cwnd_reduction).
+ * It's called toward the end of processing an ACK with precise rate
+ * information. All transmission or retransmission are delayed afterwards.
+ */
+static void tcp_cong_control(struct sock *sk, u32 ack, u32 acked_sacked,
+			     int flag, const struct rate_sample *rs)
+{
+	const struct inet_connection_sock *icsk = inet_csk(sk);
+
+	if (icsk->icsk_ca_ops->cong_control) {
+		icsk->icsk_ca_ops->cong_control(sk, rs);
+		return;
+	}
+
+	if (tcp_in_cwnd_reduction(sk)) {
+		/* Reduce cwnd if state mandates */
+		tcp_cwnd_reduction(sk, acked_sacked, flag);
+	} else if (tcp_may_raise_cwnd(sk, flag)) {
+		/* Advance cwnd if state allows */
+		tcp_cong_avoid(sk, ack, acked_sacked);
+	}
+	tcp_update_pacing_rate(sk);
+}
+
+/* Check that window update is acceptable.
+ * The function assumes that snd_una<=ack<=snd_next.
+ */
+static inline bool tcp_may_update_window(const struct tcp_sock *tp,
+					const u32 ack, const u32 ack_seq,
+					const u32 nwin)
+{
+	return	after(ack, tp->snd_una) ||
+		after(ack_seq, tp->snd_wl1) ||
+		(ack_seq == tp->snd_wl1 && nwin > tp->snd_wnd);
+}
+
+/* If we update tp->snd_una, also update tp->bytes_acked */
+static void tcp_snd_una_update(struct tcp_sock *tp, u32 ack)
+{
+	u32 delta = ack - tp->snd_una;
+
+	sock_owned_by_me((struct sock *)tp);
+	tp->bytes_acked += delta;
+	tp->snd_una = ack;
+}
+
+/* If we update tp->rcv_nxt, also update tp->bytes_received */
+static void tcp_rcv_nxt_update(struct tcp_sock *tp, u32 seq)
+{
+	u32 delta = seq - tp->rcv_nxt;
+
+	sock_owned_by_me((struct sock *)tp);
+	tp->bytes_received += delta;
+	WRITE_ONCE(tp->rcv_nxt, seq);
+}
+
+/* Update our send window.
+ *
+ * Window update algorithm, described in RFC793/RFC1122 (used in linux-2.2
+ * and in FreeBSD. NetBSD's one is even worse.) is wrong.
+ */
+static int tcp_ack_update_window(struct sock *sk, const struct sk_buff *skb, u32 ack,
+				 u32 ack_seq)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	int flag = 0;
+	u32 nwin = ntohs(tcp_hdr(skb)->window);
+
+	if (likely(!tcp_hdr(skb)->syn))
+		nwin <<= tp->rx_opt.snd_wscale;
+
+	if (tcp_may_update_window(tp, ack, ack_seq, nwin)) {
+		flag |= FLAG_WIN_UPDATE;
+		tcp_update_wl(tp, ack_seq);
+
+		if (tp->snd_wnd != nwin) {
+			tp->snd_wnd = nwin;
+
+			/* Note, it is the only place, where
+			 * fast path is recovered for sending TCP.
+			 */
+			tp->pred_flags = 0;
+			tcp_fast_path_check(sk);
+
+			if (!tcp_write_queue_empty(sk))
+				tcp_slow_start_after_idle_check(sk);
+
+			if (nwin > tp->max_window) {
+				tp->max_window = nwin;
+				tcp_sync_mss(sk, inet_csk(sk)->icsk_pmtu_cookie);
+			}
+		}
+	}
+
+	tcp_snd_una_update(tp, ack);
+
+	return flag;
+}
+
+static bool __tcp_oow_rate_limited(struct net *net, int mib_idx,
+				   u32 *last_oow_ack_time)
+{
+	/* Paired with the WRITE_ONCE() in this function. */
+	u32 val = READ_ONCE(*last_oow_ack_time);
+
+	if (val) {
+		s32 elapsed = (s32)(tcp_jiffies32 - val);
+
+		if (0 <= elapsed &&
+		    elapsed < READ_ONCE(net->ipv4.sysctl_tcp_invalid_ratelimit)) {
+			NET_INC_STATS(net, mib_idx);
+			return true;	/* rate-limited: don't send yet! */
+		}
+	}
+
+	/* Paired with the prior READ_ONCE() and with itself,
+	 * as we might be lockless.
+	 */
+	WRITE_ONCE(*last_oow_ack_time, tcp_jiffies32);
+
+	return false;	/* not rate-limited: go ahead, send dupack now! */
+}
+
+/* Return true if we're currently rate-limiting out-of-window ACKs and
+ * thus shouldn't send a dupack right now. We rate-limit dupacks in
+ * response to out-of-window SYNs or ACKs to mitigate ACK loops or DoS
+ * attacks that send repeated SYNs or ACKs for the same connection. To
+ * do this, we do not send a duplicate SYNACK or ACK if the remote
+ * endpoint is sending out-of-window SYNs or pure ACKs at a high rate.
+ */
+bool tcp_oow_rate_limited(struct net *net, const struct sk_buff *skb,
+			  int mib_idx, u32 *last_oow_ack_time)
+{
+	/* Data packets without SYNs are not likely part of an ACK loop. */
+	if ((TCP_SKB_CB(skb)->seq != TCP_SKB_CB(skb)->end_seq) &&
+	    !tcp_hdr(skb)->syn)
+		return false;
+
+	return __tcp_oow_rate_limited(net, mib_idx, last_oow_ack_time);
+}
+
+/* RFC 5961 7 [ACK Throttling] */
+static void tcp_send_challenge_ack(struct sock *sk, const struct sk_buff *skb)
+{
+	/* unprotected vars, we dont care of overwrites */
+	static u32 challenge_timestamp;
+	static unsigned int challenge_count;
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct net *net = sock_net(sk);
+	u32 count, now;
+
+	/* First check our per-socket dupack rate limit. */
+	if (__tcp_oow_rate_limited(net,
+				   LINUX_MIB_TCPACKSKIPPEDCHALLENGE,
+				   &tp->last_oow_ack_time))
+		return;
+
+	/* Then check host-wide RFC 5961 rate limit. */
+	now = jiffies / HZ;
+	if (now != READ_ONCE(challenge_timestamp)) {
+		u32 ack_limit = READ_ONCE(net->ipv4.sysctl_tcp_challenge_ack_limit);
+		u32 half = (ack_limit + 1) >> 1;
+
+		WRITE_ONCE(challenge_timestamp, now);
+		WRITE_ONCE(challenge_count, half + prandom_u32_max(ack_limit));
+	}
+	count = READ_ONCE(challenge_count);
+	if (count > 0) {
+		WRITE_ONCE(challenge_count, count - 1);
+		NET_INC_STATS(net, LINUX_MIB_TCPCHALLENGEACK);
+		tcp_send_ack(sk);
+	}
+}
+
+static void tcp_store_ts_recent(struct tcp_sock *tp)
+{
+	tp->rx_opt.ts_recent = tp->rx_opt.rcv_tsval;
+	tp->rx_opt.ts_recent_stamp = ktime_get_seconds();
+}
+
+static void tcp_replace_ts_recent(struct tcp_sock *tp, u32 seq)
+{
+	if (tp->rx_opt.saw_tstamp && !after(seq, tp->rcv_wup)) {
+		/* PAWS bug workaround wrt. ACK frames, the PAWS discard
+		 * extra check below makes sure this can only happen
+		 * for pure ACK frames.  -DaveM
+		 *
+		 * Not only, also it occurs for expired timestamps.
+		 */
+
+		if (tcp_paws_check(&tp->rx_opt, 0))
+			tcp_store_ts_recent(tp);
+	}
+}
+
+/* This routine deals with acks during a TLP episode and ends an episode by
+ * resetting tlp_high_seq. Ref: TLP algorithm in draft-ietf-tcpm-rack
+ */
+static void tcp_process_tlp_ack(struct sock *sk, u32 ack, int flag)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (before(ack, tp->tlp_high_seq))
+		return;
+
+	if (!tp->tlp_retrans) {
+		/* TLP of new data has been acknowledged */
+		tp->tlp_high_seq = 0;
+	} else if (flag & FLAG_DSACKING_ACK) {
+		/* This DSACK means original and TLP probe arrived; no loss */
+		tp->tlp_high_seq = 0;
+	} else if (after(ack, tp->tlp_high_seq)) {
+		/* ACK advances: there was a loss, so reduce cwnd. Reset
+		 * tlp_high_seq in tcp_init_cwnd_reduction()
+		 */
+		tcp_init_cwnd_reduction(sk);
+		tcp_set_ca_state(sk, TCP_CA_CWR);
+		tcp_end_cwnd_reduction(sk);
+		tcp_try_keep_open(sk);
+		NET_INC_STATS(sock_net(sk),
+				LINUX_MIB_TCPLOSSPROBERECOVERY);
+	} else if (!(flag & (FLAG_SND_UNA_ADVANCED |
+			     FLAG_NOT_DUP | FLAG_DATA_SACKED))) {
+		/* Pure dupack: original and TLP probe arrived; no loss */
+		tp->tlp_high_seq = 0;
+	}
+}
+
+static inline void tcp_in_ack_event(struct sock *sk, u32 flags)
+{
+	const struct inet_connection_sock *icsk = inet_csk(sk);
+
+	if (icsk->icsk_ca_ops->in_ack_event)
+		icsk->icsk_ca_ops->in_ack_event(sk, flags);
+}
+
+/* Congestion control has updated the cwnd already. So if we're in
+ * loss recovery then now we do any new sends (for FRTO) or
+ * retransmits (for CA_Loss or CA_recovery) that make sense.
+ */
+static void tcp_xmit_recovery(struct sock *sk, int rexmit)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (rexmit == REXMIT_NONE || sk->sk_state == TCP_SYN_SENT)
+		return;
+
+	if (unlikely(rexmit == REXMIT_NEW)) {
+		__tcp_push_pending_frames(sk, tcp_current_mss(sk),
+					  TCP_NAGLE_OFF);
+		if (after(tp->snd_nxt, tp->high_seq))
+			return;
+		tp->frto = 0;
+	}
+	tcp_xmit_retransmit_queue(sk);
+}
+
+/* Returns the number of packets newly acked or sacked by the current ACK */
+static u32 tcp_newly_delivered(struct sock *sk, u32 prior_delivered, int flag)
+{
+	const struct net *net = sock_net(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	u32 delivered;
+
+	delivered = tp->delivered - prior_delivered;
+	NET_ADD_STATS(net, LINUX_MIB_TCPDELIVERED, delivered);
+	if (flag & FLAG_ECE)
+		NET_ADD_STATS(net, LINUX_MIB_TCPDELIVEREDCE, delivered);
+
+	return delivered;
+}
+
+/* This routine deals with incoming acks, but not outgoing ones. */
+static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct tcp_sacktag_state sack_state;
+	struct rate_sample rs = { .prior_delivered = 0 };
+	u32 prior_snd_una = tp->snd_una;
+	bool is_sack_reneg = tp->is_sack_reneg;
+	u32 ack_seq = TCP_SKB_CB(skb)->seq;
+	u32 ack = TCP_SKB_CB(skb)->ack_seq;
+	int num_dupack = 0;
+	int prior_packets = tp->packets_out;
+	u32 delivered = tp->delivered;
+	u32 lost = tp->lost;
+	int rexmit = REXMIT_NONE; /* Flag to (re)transmit to recover losses */
+	u32 prior_fack;
+
+	sack_state.first_sackt = 0;
+	sack_state.rate = &rs;
+	sack_state.sack_delivered = 0;
+
+	/* We very likely will need to access rtx queue. */
+	prefetch(sk->tcp_rtx_queue.rb_node);
+
+	/* If the ack is older than previous acks
+	 * then we can probably ignore it.
+	 */
+	if (before(ack, prior_snd_una)) {
+		u32 max_window;
+
+		/* do not accept ACK for bytes we never sent. */
+		max_window = min_t(u64, tp->max_window, tp->bytes_acked);
+		/* RFC 5961 5.2 [Blind Data Injection Attack].[Mitigation] */
+		if (before(ack, prior_snd_una - max_window)) {
+			if (!(flag & FLAG_NO_CHALLENGE_ACK))
+				tcp_send_challenge_ack(sk, skb);
+			return -1;
+		}
+		goto old_ack;
+	}
+
+	/* If the ack includes data we haven't sent yet, discard
+	 * this segment (RFC793 Section 3.9).
+	 */
+	if (after(ack, tp->snd_nxt))
+		return -1;
+
+	if (after(ack, prior_snd_una)) {
+		flag |= FLAG_SND_UNA_ADVANCED;
+		icsk->icsk_retransmits = 0;
+
+#if IS_ENABLED(CONFIG_TLS_DEVICE)
+		if (static_branch_unlikely(&clean_acked_data_enabled.key))
+			if (icsk->icsk_clean_acked)
+				icsk->icsk_clean_acked(sk, ack);
+#endif
+	}
+
+	prior_fack = tcp_is_sack(tp) ? tcp_highest_sack_seq(tp) : tp->snd_una;
+	rs.prior_in_flight = tcp_packets_in_flight(tp);
+
+	/* ts_recent update must be made after we are sure that the packet
+	 * is in window.
+	 */
+	if (flag & FLAG_UPDATE_TS_RECENT)
+		tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq);
+
+	if ((flag & (FLAG_SLOWPATH | FLAG_SND_UNA_ADVANCED)) ==
+	    FLAG_SND_UNA_ADVANCED) {
+		/* Window is constant, pure forward advance.
+		 * No more checks are required.
+		 * Note, we use the fact that SND.UNA>=SND.WL2.
+		 */
+		tcp_update_wl(tp, ack_seq);
+		tcp_snd_una_update(tp, ack);
+		flag |= FLAG_WIN_UPDATE;
+
+		tcp_in_ack_event(sk, CA_ACK_WIN_UPDATE);
+
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPHPACKS);
+	} else {
+		u32 ack_ev_flags = CA_ACK_SLOWPATH;
+
+		if (ack_seq != TCP_SKB_CB(skb)->end_seq)
+			flag |= FLAG_DATA;
+		else
+			NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPUREACKS);
+
+		flag |= tcp_ack_update_window(sk, skb, ack, ack_seq);
+
+		if (TCP_SKB_CB(skb)->sacked)
+			flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
+							&sack_state);
+
+		if (tcp_ecn_rcv_ecn_echo(tp, tcp_hdr(skb))) {
+			flag |= FLAG_ECE;
+			ack_ev_flags |= CA_ACK_ECE;
+		}
+
+		if (sack_state.sack_delivered)
+			tcp_count_delivered(tp, sack_state.sack_delivered,
+					    flag & FLAG_ECE);
+
+		if (flag & FLAG_WIN_UPDATE)
+			ack_ev_flags |= CA_ACK_WIN_UPDATE;
+
+		tcp_in_ack_event(sk, ack_ev_flags);
+	}
+
+	/* This is a deviation from RFC3168 since it states that:
+	 * "When the TCP data sender is ready to set the CWR bit after reducing
+	 * the congestion window, it SHOULD set the CWR bit only on the first
+	 * new data packet that it transmits."
+	 * We accept CWR on pure ACKs to be more robust
+	 * with widely-deployed TCP implementations that do this.
+	 */
+	tcp_ecn_accept_cwr(sk, skb);
+
+	/* We passed data and got it acked, remove any soft error
+	 * log. Something worked...
+	 */
+	sk->sk_err_soft = 0;
+	icsk->icsk_probes_out = 0;
+	tp->rcv_tstamp = tcp_jiffies32;
+	if (!prior_packets)
+		goto no_queue;
+
+	/* See if we can take anything off of the retransmit queue. */
+	flag |= tcp_clean_rtx_queue(sk, prior_fack, prior_snd_una, &sack_state,
+				    flag & FLAG_ECE);
+
+	tcp_rack_update_reo_wnd(sk, &rs);
+
+	if (tp->tlp_high_seq)
+		tcp_process_tlp_ack(sk, ack, flag);
+
+	if (tcp_ack_is_dubious(sk, flag)) {
+		if (!(flag & (FLAG_SND_UNA_ADVANCED |
+			      FLAG_NOT_DUP | FLAG_DSACKING_ACK))) {
+			num_dupack = 1;
+			/* Consider if pure acks were aggregated in tcp_add_backlog() */
+			if (!(flag & FLAG_DATA))
+				num_dupack = max_t(u16, 1, skb_shinfo(skb)->gso_segs);
+		}
+		tcp_fastretrans_alert(sk, prior_snd_una, num_dupack, &flag,
+				      &rexmit);
+	}
+
+	/* If needed, reset TLP/RTO timer when RACK doesn't set. */
+	if (flag & FLAG_SET_XMIT_TIMER)
+		tcp_set_xmit_timer(sk);
+
+	if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP))
+		sk_dst_confirm(sk);
+
+	delivered = tcp_newly_delivered(sk, delivered, flag);
+	lost = tp->lost - lost;			/* freshly marked lost */
+	rs.is_ack_delayed = !!(flag & FLAG_ACK_MAYBE_DELAYED);
+	tcp_rate_gen(sk, delivered, lost, is_sack_reneg, sack_state.rate);
+	tcp_cong_control(sk, ack, delivered, flag, sack_state.rate);
+	tcp_xmit_recovery(sk, rexmit);
+	return 1;
+
+no_queue:
+	/* If data was DSACKed, see if we can undo a cwnd reduction. */
+	if (flag & FLAG_DSACKING_ACK) {
+		tcp_fastretrans_alert(sk, prior_snd_una, num_dupack, &flag,
+				      &rexmit);
+		tcp_newly_delivered(sk, delivered, flag);
+	}
+	/* If this ack opens up a zero window, clear backoff.  It was
+	 * being used to time the probes, and is probably far higher than
+	 * it needs to be for normal retransmission.
+	 */
+	tcp_ack_probe(sk);
+
+	if (tp->tlp_high_seq)
+		tcp_process_tlp_ack(sk, ack, flag);
+	return 1;
+
+old_ack:
+	/* If data was SACKed, tag it and see if we should send more data.
+	 * If data was DSACKed, see if we can undo a cwnd reduction.
+	 */
+	if (TCP_SKB_CB(skb)->sacked) {
+		flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
+						&sack_state);
+		tcp_fastretrans_alert(sk, prior_snd_una, num_dupack, &flag,
+				      &rexmit);
+		tcp_newly_delivered(sk, delivered, flag);
+		tcp_xmit_recovery(sk, rexmit);
+	}
+
+	return 0;
+}
+
+static void tcp_parse_fastopen_option(int len, const unsigned char *cookie,
+				      bool syn, struct tcp_fastopen_cookie *foc,
+				      bool exp_opt)
+{
+	/* Valid only in SYN or SYN-ACK with an even length.  */
+	if (!foc || !syn || len < 0 || (len & 1))
+		return;
+
+	if (len >= TCP_FASTOPEN_COOKIE_MIN &&
+	    len <= TCP_FASTOPEN_COOKIE_MAX)
+		memcpy(foc->val, cookie, len);
+	else if (len != 0)
+		len = -1;
+	foc->len = len;
+	foc->exp = exp_opt;
+}
+
+static bool smc_parse_options(const struct tcphdr *th,
+			      struct tcp_options_received *opt_rx,
+			      const unsigned char *ptr,
+			      int opsize)
+{
+#if IS_ENABLED(CONFIG_SMC)
+	if (static_branch_unlikely(&tcp_have_smc)) {
+		if (th->syn && !(opsize & 1) &&
+		    opsize >= TCPOLEN_EXP_SMC_BASE &&
+		    get_unaligned_be32(ptr) == TCPOPT_SMC_MAGIC) {
+			opt_rx->smc_ok = 1;
+			return true;
+		}
+	}
+#endif
+	return false;
+}
+
+/* Try to parse the MSS option from the TCP header. Return 0 on failure, clamped
+ * value on success.
+ */
+static u16 tcp_parse_mss_option(const struct tcphdr *th, u16 user_mss)
+{
+	const unsigned char *ptr = (const unsigned char *)(th + 1);
+	int length = (th->doff * 4) - sizeof(struct tcphdr);
+	u16 mss = 0;
+
+	while (length > 0) {
+		int opcode = *ptr++;
+		int opsize;
+
+		switch (opcode) {
+		case TCPOPT_EOL:
+			return mss;
+		case TCPOPT_NOP:	/* Ref: RFC 793 section 3.1 */
+			length--;
+			continue;
+		default:
+			if (length < 2)
+				return mss;
+			opsize = *ptr++;
+			if (opsize < 2) /* "silly options" */
+				return mss;
+			if (opsize > length)
+				return mss;	/* fail on partial options */
+			if (opcode == TCPOPT_MSS && opsize == TCPOLEN_MSS) {
+				u16 in_mss = get_unaligned_be16(ptr);
+
+				if (in_mss) {
+					if (user_mss && user_mss < in_mss)
+						in_mss = user_mss;
+					mss = in_mss;
+				}
+			}
+			ptr += opsize - 2;
+			length -= opsize;
+		}
+	}
+	return mss;
+}
+
+/* Look for tcp options. Normally only called on SYN and SYNACK packets.
+ * But, this can also be called on packets in the established flow when
+ * the fast version below fails.
+ */
+void tcp_parse_options(const struct net *net,
+		       const struct sk_buff *skb,
+		       struct tcp_options_received *opt_rx, int estab,
+		       struct tcp_fastopen_cookie *foc)
+{
+	const unsigned char *ptr;
+	const struct tcphdr *th = tcp_hdr(skb);
+	int length = (th->doff * 4) - sizeof(struct tcphdr);
+
+	ptr = (const unsigned char *)(th + 1);
+	opt_rx->saw_tstamp = 0;
+	opt_rx->saw_unknown = 0;
+
+	while (length > 0) {
+		int opcode = *ptr++;
+		int opsize;
+
+		switch (opcode) {
+		case TCPOPT_EOL:
+			return;
+		case TCPOPT_NOP:	/* Ref: RFC 793 section 3.1 */
+			length--;
+			continue;
+		default:
+			if (length < 2)
+				return;
+			opsize = *ptr++;
+			if (opsize < 2) /* "silly options" */
+				return;
+			if (opsize > length)
+				return;	/* don't parse partial options */
+			switch (opcode) {
+			case TCPOPT_MSS:
+				if (opsize == TCPOLEN_MSS && th->syn && !estab) {
+					u16 in_mss = get_unaligned_be16(ptr);
+					if (in_mss) {
+						if (opt_rx->user_mss &&
+						    opt_rx->user_mss < in_mss)
+							in_mss = opt_rx->user_mss;
+						opt_rx->mss_clamp = in_mss;
+					}
+				}
+				break;
+			case TCPOPT_WINDOW:
+				if (opsize == TCPOLEN_WINDOW && th->syn &&
+				    !estab && READ_ONCE(net->ipv4.sysctl_tcp_window_scaling)) {
+					__u8 snd_wscale = *(__u8 *)ptr;
+					opt_rx->wscale_ok = 1;
+					if (snd_wscale > TCP_MAX_WSCALE) {
+						net_info_ratelimited("%s: Illegal window scaling value %d > %u received\n",
+								     __func__,
+								     snd_wscale,
+								     TCP_MAX_WSCALE);
+						snd_wscale = TCP_MAX_WSCALE;
+					}
+					opt_rx->snd_wscale = snd_wscale;
+				}
+				break;
+			case TCPOPT_TIMESTAMP:
+				if ((opsize == TCPOLEN_TIMESTAMP) &&
+				    ((estab && opt_rx->tstamp_ok) ||
+				     (!estab && READ_ONCE(net->ipv4.sysctl_tcp_timestamps)))) {
+					opt_rx->saw_tstamp = 1;
+					opt_rx->rcv_tsval = get_unaligned_be32(ptr);
+					opt_rx->rcv_tsecr = get_unaligned_be32(ptr + 4);
+				}
+				break;
+			case TCPOPT_SACK_PERM:
+				if (opsize == TCPOLEN_SACK_PERM && th->syn &&
+				    !estab && READ_ONCE(net->ipv4.sysctl_tcp_sack)) {
+					opt_rx->sack_ok = TCP_SACK_SEEN;
+					tcp_sack_reset(opt_rx);
+				}
+				break;
+
+			case TCPOPT_SACK:
+				if ((opsize >= (TCPOLEN_SACK_BASE + TCPOLEN_SACK_PERBLOCK)) &&
+				   !((opsize - TCPOLEN_SACK_BASE) % TCPOLEN_SACK_PERBLOCK) &&
+				   opt_rx->sack_ok) {
+					TCP_SKB_CB(skb)->sacked = (ptr - 2) - (unsigned char *)th;
+				}
+				break;
+#ifdef CONFIG_TCP_MD5SIG
+			case TCPOPT_MD5SIG:
+				/*
+				 * The MD5 Hash has already been
+				 * checked (see tcp_v{4,6}_do_rcv()).
+				 */
+				break;
+#endif
+			case TCPOPT_FASTOPEN:
+				tcp_parse_fastopen_option(
+					opsize - TCPOLEN_FASTOPEN_BASE,
+					ptr, th->syn, foc, false);
+				break;
+
+			case TCPOPT_EXP:
+				/* Fast Open option shares code 254 using a
+				 * 16 bits magic number.
+				 */
+				if (opsize >= TCPOLEN_EXP_FASTOPEN_BASE &&
+				    get_unaligned_be16(ptr) ==
+				    TCPOPT_FASTOPEN_MAGIC) {
+					tcp_parse_fastopen_option(opsize -
+						TCPOLEN_EXP_FASTOPEN_BASE,
+						ptr + 2, th->syn, foc, true);
+					break;
+				}
+
+				if (smc_parse_options(th, opt_rx, ptr, opsize))
+					break;
+
+				opt_rx->saw_unknown = 1;
+				break;
+
+			default:
+				opt_rx->saw_unknown = 1;
+			}
+			ptr += opsize-2;
+			length -= opsize;
+		}
+	}
+}
+EXPORT_SYMBOL(tcp_parse_options);
+
+static bool tcp_parse_aligned_timestamp(struct tcp_sock *tp, const struct tcphdr *th)
+{
+	const __be32 *ptr = (const __be32 *)(th + 1);
+
+	if (*ptr == htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16)
+			  | (TCPOPT_TIMESTAMP << 8) | TCPOLEN_TIMESTAMP)) {
+		tp->rx_opt.saw_tstamp = 1;
+		++ptr;
+		tp->rx_opt.rcv_tsval = ntohl(*ptr);
+		++ptr;
+		if (*ptr)
+			tp->rx_opt.rcv_tsecr = ntohl(*ptr) - tp->tsoffset;
+		else
+			tp->rx_opt.rcv_tsecr = 0;
+		return true;
+	}
+	return false;
+}
+
+/* Fast parse options. This hopes to only see timestamps.
+ * If it is wrong it falls back on tcp_parse_options().
+ */
+static bool tcp_fast_parse_options(const struct net *net,
+				   const struct sk_buff *skb,
+				   const struct tcphdr *th, struct tcp_sock *tp)
+{
+	/* In the spirit of fast parsing, compare doff directly to constant
+	 * values.  Because equality is used, short doff can be ignored here.
+	 */
+	if (th->doff == (sizeof(*th) / 4)) {
+		tp->rx_opt.saw_tstamp = 0;
+		return false;
+	} else if (tp->rx_opt.tstamp_ok &&
+		   th->doff == ((sizeof(*th) + TCPOLEN_TSTAMP_ALIGNED) / 4)) {
+		if (tcp_parse_aligned_timestamp(tp, th))
+			return true;
+	}
+
+	tcp_parse_options(net, skb, &tp->rx_opt, 1, NULL);
+	if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
+		tp->rx_opt.rcv_tsecr -= tp->tsoffset;
+
+	return true;
+}
+
+#ifdef CONFIG_TCP_MD5SIG
+/*
+ * Parse MD5 Signature option
+ */
+const u8 *tcp_parse_md5sig_option(const struct tcphdr *th)
+{
+	int length = (th->doff << 2) - sizeof(*th);
+	const u8 *ptr = (const u8 *)(th + 1);
+
+	/* If not enough data remaining, we can short cut */
+	while (length >= TCPOLEN_MD5SIG) {
+		int opcode = *ptr++;
+		int opsize;
+
+		switch (opcode) {
+		case TCPOPT_EOL:
+			return NULL;
+		case TCPOPT_NOP:
+			length--;
+			continue;
+		default:
+			opsize = *ptr++;
+			if (opsize < 2 || opsize > length)
+				return NULL;
+			if (opcode == TCPOPT_MD5SIG)
+				return opsize == TCPOLEN_MD5SIG ? ptr : NULL;
+		}
+		ptr += opsize - 2;
+		length -= opsize;
+	}
+	return NULL;
+}
+EXPORT_SYMBOL(tcp_parse_md5sig_option);
+#endif
+
+/* Sorry, PAWS as specified is broken wrt. pure-ACKs -DaveM
+ *
+ * It is not fatal. If this ACK does _not_ change critical state (seqs, window)
+ * it can pass through stack. So, the following predicate verifies that
+ * this segment is not used for anything but congestion avoidance or
+ * fast retransmit. Moreover, we even are able to eliminate most of such
+ * second order effects, if we apply some small "replay" window (~RTO)
+ * to timestamp space.
+ *
+ * All these measures still do not guarantee that we reject wrapped ACKs
+ * on networks with high bandwidth, when sequence space is recycled fastly,
+ * but it guarantees that such events will be very rare and do not affect
+ * connection seriously. This doesn't look nice, but alas, PAWS is really
+ * buggy extension.
+ *
+ * [ Later note. Even worse! It is buggy for segments _with_ data. RFC
+ * states that events when retransmit arrives after original data are rare.
+ * It is a blatant lie. VJ forgot about fast retransmit! 8)8) It is
+ * the biggest problem on large power networks even with minor reordering.
+ * OK, let's give it small replay window. If peer clock is even 1hz, it is safe
+ * up to bandwidth of 18Gigabit/sec. 8) ]
+ */
+
+static int tcp_disordered_ack(const struct sock *sk, const struct sk_buff *skb)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+	const struct tcphdr *th = tcp_hdr(skb);
+	u32 seq = TCP_SKB_CB(skb)->seq;
+	u32 ack = TCP_SKB_CB(skb)->ack_seq;
+
+	return (/* 1. Pure ACK with correct sequence number. */
+		(th->ack && seq == TCP_SKB_CB(skb)->end_seq && seq == tp->rcv_nxt) &&
+
+		/* 2. ... and duplicate ACK. */
+		ack == tp->snd_una &&
+
+		/* 3. ... and does not update window. */
+		!tcp_may_update_window(tp, ack, seq, ntohs(th->window) << tp->rx_opt.snd_wscale) &&
+
+		/* 4. ... and sits in replay window. */
+		(s32)(tp->rx_opt.ts_recent - tp->rx_opt.rcv_tsval) <= (inet_csk(sk)->icsk_rto * 1024) / HZ);
+}
+
+static inline bool tcp_paws_discard(const struct sock *sk,
+				   const struct sk_buff *skb)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+
+	return !tcp_paws_check(&tp->rx_opt, TCP_PAWS_WINDOW) &&
+	       !tcp_disordered_ack(sk, skb);
+}
+
+/* Check segment sequence number for validity.
+ *
+ * Segment controls are considered valid, if the segment
+ * fits to the window after truncation to the window. Acceptability
+ * of data (and SYN, FIN, of course) is checked separately.
+ * See tcp_data_queue(), for example.
+ *
+ * Also, controls (RST is main one) are accepted using RCV.WUP instead
+ * of RCV.NXT. Peer still did not advance his SND.UNA when we
+ * delayed ACK, so that hisSND.UNA<=ourRCV.WUP.
+ * (borrowed from freebsd)
+ */
+
+static inline bool tcp_sequence(const struct tcp_sock *tp, u32 seq, u32 end_seq)
+{
+	return	!before(end_seq, tp->rcv_wup) &&
+		!after(seq, tp->rcv_nxt + tcp_receive_window(tp));
+}
+
+/* When we get a reset we do this. */
+void tcp_reset(struct sock *sk)
+{
+	trace_tcp_receive_reset(sk);
+
+	/* We want the right error as BSD sees it (and indeed as we do). */
+	switch (sk->sk_state) {
+	case TCP_SYN_SENT:
+		sk->sk_err = ECONNREFUSED;
+		break;
+	case TCP_CLOSE_WAIT:
+		sk->sk_err = EPIPE;
+		break;
+	case TCP_CLOSE:
+		return;
+	default:
+		sk->sk_err = ECONNRESET;
+	}
+	/* This barrier is coupled with smp_rmb() in tcp_poll() */
+	smp_wmb();
+
+	tcp_write_queue_purge(sk);
+	tcp_done(sk);
+
+	if (!sock_flag(sk, SOCK_DEAD))
+		sk->sk_error_report(sk);
+}
+
+/*
+ * 	Process the FIN bit. This now behaves as it is supposed to work
+ *	and the FIN takes effect when it is validly part of sequence
+ *	space. Not before when we get holes.
+ *
+ *	If we are ESTABLISHED, a received fin moves us to CLOSE-WAIT
+ *	(and thence onto LAST-ACK and finally, CLOSE, we never enter
+ *	TIME-WAIT)
+ *
+ *	If we are in FINWAIT-1, a received FIN indicates simultaneous
+ *	close and we go into CLOSING (and later onto TIME-WAIT)
+ *
+ *	If we are in FINWAIT-2, a received FIN moves us to TIME-WAIT.
+ */
+void tcp_fin(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	inet_csk_schedule_ack(sk);
+
+	WRITE_ONCE(sk->sk_shutdown, sk->sk_shutdown | RCV_SHUTDOWN);
+	sock_set_flag(sk, SOCK_DONE);
+
+	switch (sk->sk_state) {
+	case TCP_SYN_RECV:
+	case TCP_ESTABLISHED:
+		/* Move to CLOSE_WAIT */
+		tcp_set_state(sk, TCP_CLOSE_WAIT);
+		inet_csk_enter_pingpong_mode(sk);
+		break;
+
+	case TCP_CLOSE_WAIT:
+	case TCP_CLOSING:
+		/* Received a retransmission of the FIN, do
+		 * nothing.
+		 */
+		break;
+	case TCP_LAST_ACK:
+		/* RFC793: Remain in the LAST-ACK state. */
+		break;
+
+	case TCP_FIN_WAIT1:
+		/* This case occurs when a simultaneous close
+		 * happens, we must ack the received FIN and
+		 * enter the CLOSING state.
+		 */
+		tcp_send_ack(sk);
+		tcp_set_state(sk, TCP_CLOSING);
+		break;
+	case TCP_FIN_WAIT2:
+		/* Received a FIN -- send ACK and enter TIME_WAIT. */
+		tcp_send_ack(sk);
+		tcp_time_wait(sk, TCP_TIME_WAIT, 0);
+		break;
+	default:
+		/* Only TCP_LISTEN and TCP_CLOSE are left, in these
+		 * cases we should never reach this piece of code.
+		 */
+		pr_err("%s: Impossible, sk->sk_state=%d\n",
+		       __func__, sk->sk_state);
+		break;
+	}
+
+	/* It _is_ possible, that we have something out-of-order _after_ FIN.
+	 * Probably, we should reset in this case. For now drop them.
+	 */
+	skb_rbtree_purge(&tp->out_of_order_queue);
+	if (tcp_is_sack(tp))
+		tcp_sack_reset(&tp->rx_opt);
+	sk_mem_reclaim(sk);
+
+	if (!sock_flag(sk, SOCK_DEAD)) {
+		sk->sk_state_change(sk);
+
+		/* Do not send POLL_HUP for half duplex close. */
+		if (sk->sk_shutdown == SHUTDOWN_MASK ||
+		    sk->sk_state == TCP_CLOSE)
+			sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_HUP);
+		else
+			sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_IN);
+	}
+}
+
+static inline bool tcp_sack_extend(struct tcp_sack_block *sp, u32 seq,
+				  u32 end_seq)
+{
+	if (!after(seq, sp->end_seq) && !after(sp->start_seq, end_seq)) {
+		if (before(seq, sp->start_seq))
+			sp->start_seq = seq;
+		if (after(end_seq, sp->end_seq))
+			sp->end_seq = end_seq;
+		return true;
+	}
+	return false;
+}
+
+static void tcp_dsack_set(struct sock *sk, u32 seq, u32 end_seq)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (tcp_is_sack(tp) && READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_dsack)) {
+		int mib_idx;
+
+		if (before(seq, tp->rcv_nxt))
+			mib_idx = LINUX_MIB_TCPDSACKOLDSENT;
+		else
+			mib_idx = LINUX_MIB_TCPDSACKOFOSENT;
+
+		NET_INC_STATS(sock_net(sk), mib_idx);
+
+		tp->rx_opt.dsack = 1;
+		tp->duplicate_sack[0].start_seq = seq;
+		tp->duplicate_sack[0].end_seq = end_seq;
+	}
+}
+
+static void tcp_dsack_extend(struct sock *sk, u32 seq, u32 end_seq)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (!tp->rx_opt.dsack)
+		tcp_dsack_set(sk, seq, end_seq);
+	else
+		tcp_sack_extend(tp->duplicate_sack, seq, end_seq);
+}
+
+static void tcp_rcv_spurious_retrans(struct sock *sk, const struct sk_buff *skb)
+{
+	/* When the ACK path fails or drops most ACKs, the sender would
+	 * timeout and spuriously retransmit the same segment repeatedly.
+	 * The receiver remembers and reflects via DSACKs. Leverage the
+	 * DSACK state and change the txhash to re-route speculatively.
+	 */
+	if (TCP_SKB_CB(skb)->seq == tcp_sk(sk)->duplicate_sack[0].start_seq &&
+	    sk_rethink_txhash(sk))
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDUPLICATEDATAREHASH);
+}
+
+static void tcp_send_dupack(struct sock *sk, const struct sk_buff *skb)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
+	    before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
+		tcp_enter_quickack_mode(sk, TCP_MAX_QUICKACKS);
+
+		if (tcp_is_sack(tp) && READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_dsack)) {
+			u32 end_seq = TCP_SKB_CB(skb)->end_seq;
+
+			tcp_rcv_spurious_retrans(sk, skb);
+			if (after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt))
+				end_seq = tp->rcv_nxt;
+			tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, end_seq);
+		}
+	}
+
+	tcp_send_ack(sk);
+}
+
+/* These routines update the SACK block as out-of-order packets arrive or
+ * in-order packets close up the sequence space.
+ */
+static void tcp_sack_maybe_coalesce(struct tcp_sock *tp)
+{
+	int this_sack;
+	struct tcp_sack_block *sp = &tp->selective_acks[0];
+	struct tcp_sack_block *swalk = sp + 1;
+
+	/* See if the recent change to the first SACK eats into
+	 * or hits the sequence space of other SACK blocks, if so coalesce.
+	 */
+	for (this_sack = 1; this_sack < tp->rx_opt.num_sacks;) {
+		if (tcp_sack_extend(sp, swalk->start_seq, swalk->end_seq)) {
+			int i;
+
+			/* Zap SWALK, by moving every further SACK up by one slot.
+			 * Decrease num_sacks.
+			 */
+			tp->rx_opt.num_sacks--;
+			for (i = this_sack; i < tp->rx_opt.num_sacks; i++)
+				sp[i] = sp[i + 1];
+			continue;
+		}
+		this_sack++;
+		swalk++;
+	}
+}
+
+static void tcp_sack_compress_send_ack(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (!tp->compressed_ack)
+		return;
+
+	if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1)
+		__sock_put(sk);
+
+	/* Since we have to send one ack finally,
+	 * substract one from tp->compressed_ack to keep
+	 * LINUX_MIB_TCPACKCOMPRESSED accurate.
+	 */
+	NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
+		      tp->compressed_ack - 1);
+
+	tp->compressed_ack = 0;
+	tcp_send_ack(sk);
+}
+
+/* Reasonable amount of sack blocks included in TCP SACK option
+ * The max is 4, but this becomes 3 if TCP timestamps are there.
+ * Given that SACK packets might be lost, be conservative and use 2.
+ */
+#define TCP_SACK_BLOCKS_EXPECTED 2
+
+static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct tcp_sack_block *sp = &tp->selective_acks[0];
+	int cur_sacks = tp->rx_opt.num_sacks;
+	int this_sack;
+
+	if (!cur_sacks)
+		goto new_sack;
+
+	for (this_sack = 0; this_sack < cur_sacks; this_sack++, sp++) {
+		if (tcp_sack_extend(sp, seq, end_seq)) {
+			if (this_sack >= TCP_SACK_BLOCKS_EXPECTED)
+				tcp_sack_compress_send_ack(sk);
+			/* Rotate this_sack to the first one. */
+			for (; this_sack > 0; this_sack--, sp--)
+				swap(*sp, *(sp - 1));
+			if (cur_sacks > 1)
+				tcp_sack_maybe_coalesce(tp);
+			return;
+		}
+	}
+
+	if (this_sack >= TCP_SACK_BLOCKS_EXPECTED)
+		tcp_sack_compress_send_ack(sk);
+
+	/* Could not find an adjacent existing SACK, build a new one,
+	 * put it at the front, and shift everyone else down.  We
+	 * always know there is at least one SACK present already here.
+	 *
+	 * If the sack array is full, forget about the last one.
+	 */
+	if (this_sack >= TCP_NUM_SACKS) {
+		this_sack--;
+		tp->rx_opt.num_sacks--;
+		sp--;
+	}
+	for (; this_sack > 0; this_sack--, sp--)
+		*sp = *(sp - 1);
+
+new_sack:
+	/* Build the new head SACK, and we're done. */
+	sp->start_seq = seq;
+	sp->end_seq = end_seq;
+	tp->rx_opt.num_sacks++;
+}
+
+/* RCV.NXT advances, some SACKs should be eaten. */
+
+static void tcp_sack_remove(struct tcp_sock *tp)
+{
+	struct tcp_sack_block *sp = &tp->selective_acks[0];
+	int num_sacks = tp->rx_opt.num_sacks;
+	int this_sack;
+
+	/* Empty ofo queue, hence, all the SACKs are eaten. Clear. */
+	if (RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
+		tp->rx_opt.num_sacks = 0;
+		return;
+	}
+
+	for (this_sack = 0; this_sack < num_sacks;) {
+		/* Check if the start of the sack is covered by RCV.NXT. */
+		if (!before(tp->rcv_nxt, sp->start_seq)) {
+			int i;
+
+			/* RCV.NXT must cover all the block! */
+			WARN_ON(before(tp->rcv_nxt, sp->end_seq));
+
+			/* Zap this SACK, by moving forward any other SACKS. */
+			for (i = this_sack+1; i < num_sacks; i++)
+				tp->selective_acks[i-1] = tp->selective_acks[i];
+			num_sacks--;
+			continue;
+		}
+		this_sack++;
+		sp++;
+	}
+	tp->rx_opt.num_sacks = num_sacks;
+}
+
+/**
+ * tcp_try_coalesce - try to merge skb to prior one
+ * @sk: socket
+ * @to: prior buffer
+ * @from: buffer to add in queue
+ * @fragstolen: pointer to boolean
+ *
+ * Before queueing skb @from after @to, try to merge them
+ * to reduce overall memory use and queue lengths, if cost is small.
+ * Packets in ofo or receive queues can stay a long time.
+ * Better try to coalesce them right now to avoid future collapses.
+ * Returns true if caller should free @from instead of queueing it
+ */
+static bool tcp_try_coalesce(struct sock *sk,
+			     struct sk_buff *to,
+			     struct sk_buff *from,
+			     bool *fragstolen)
+{
+	int delta;
+
+	*fragstolen = false;
+
+	/* Its possible this segment overlaps with prior segment in queue */
+	if (TCP_SKB_CB(from)->seq != TCP_SKB_CB(to)->end_seq)
+		return false;
+
+	if (!mptcp_skb_can_collapse(to, from))
+		return false;
+
+#ifdef CONFIG_TLS_DEVICE
+	if (from->decrypted != to->decrypted)
+		return false;
+#endif
+
+	if (!skb_try_coalesce(to, from, fragstolen, &delta))
+		return false;
+
+	atomic_add(delta, &sk->sk_rmem_alloc);
+	sk_mem_charge(sk, delta);
+	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVCOALESCE);
+	TCP_SKB_CB(to)->end_seq = TCP_SKB_CB(from)->end_seq;
+	TCP_SKB_CB(to)->ack_seq = TCP_SKB_CB(from)->ack_seq;
+	TCP_SKB_CB(to)->tcp_flags |= TCP_SKB_CB(from)->tcp_flags;
+
+	if (TCP_SKB_CB(from)->has_rxtstamp) {
+		TCP_SKB_CB(to)->has_rxtstamp = true;
+		to->tstamp = from->tstamp;
+		skb_hwtstamps(to)->hwtstamp = skb_hwtstamps(from)->hwtstamp;
+	}
+
+	return true;
+}
+
+static bool tcp_ooo_try_coalesce(struct sock *sk,
+			     struct sk_buff *to,
+			     struct sk_buff *from,
+			     bool *fragstolen)
+{
+	bool res = tcp_try_coalesce(sk, to, from, fragstolen);
+
+	/* In case tcp_drop() is called later, update to->gso_segs */
+	if (res) {
+		u32 gso_segs = max_t(u16, 1, skb_shinfo(to)->gso_segs) +
+			       max_t(u16, 1, skb_shinfo(from)->gso_segs);
+
+		skb_shinfo(to)->gso_segs = min_t(u32, gso_segs, 0xFFFF);
+	}
+	return res;
+}
+
+static void tcp_drop(struct sock *sk, struct sk_buff *skb)
+{
+	sk_drops_add(sk, skb);
+	__kfree_skb(skb);
+}
+
+/* This one checks to see if we can put data from the
+ * out_of_order queue into the receive_queue.
+ */
+static void tcp_ofo_queue(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	__u32 dsack_high = tp->rcv_nxt;
+	bool fin, fragstolen, eaten;
+	struct sk_buff *skb, *tail;
+	struct rb_node *p;
+
+	p = rb_first(&tp->out_of_order_queue);
+	while (p) {
+		skb = rb_to_skb(p);
+		if (after(TCP_SKB_CB(skb)->seq, tp->rcv_nxt))
+			break;
+
+		if (before(TCP_SKB_CB(skb)->seq, dsack_high)) {
+			__u32 dsack = dsack_high;
+			if (before(TCP_SKB_CB(skb)->end_seq, dsack_high))
+				dsack_high = TCP_SKB_CB(skb)->end_seq;
+			tcp_dsack_extend(sk, TCP_SKB_CB(skb)->seq, dsack);
+		}
+		p = rb_next(p);
+		rb_erase(&skb->rbnode, &tp->out_of_order_queue);
+
+		if (unlikely(!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt))) {
+			tcp_drop(sk, skb);
+			continue;
+		}
+
+		tail = skb_peek_tail(&sk->sk_receive_queue);
+		eaten = tail && tcp_try_coalesce(sk, tail, skb, &fragstolen);
+		tcp_rcv_nxt_update(tp, TCP_SKB_CB(skb)->end_seq);
+		fin = TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN;
+		if (!eaten)
+			__skb_queue_tail(&sk->sk_receive_queue, skb);
+		else
+			kfree_skb_partial(skb, fragstolen);
+
+		if (unlikely(fin)) {
+			tcp_fin(sk);
+			/* tcp_fin() purges tp->out_of_order_queue,
+			 * so we must end this loop right now.
+			 */
+			break;
+		}
+	}
+}
+
+static bool tcp_prune_ofo_queue(struct sock *sk);
+static int tcp_prune_queue(struct sock *sk);
+
+static int tcp_try_rmem_schedule(struct sock *sk, struct sk_buff *skb,
+				 unsigned int size)
+{
+	if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf ||
+	    !sk_rmem_schedule(sk, skb, size)) {
+
+		if (tcp_prune_queue(sk) < 0)
+			return -1;
+
+		while (!sk_rmem_schedule(sk, skb, size)) {
+			if (!tcp_prune_ofo_queue(sk))
+				return -1;
+		}
+	}
+	return 0;
+}
+
+static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct rb_node **p, *parent;
+	struct sk_buff *skb1;
+	u32 seq, end_seq;
+	bool fragstolen;
+
+	tcp_ecn_check_ce(sk, skb);
+
+	if (unlikely(tcp_try_rmem_schedule(sk, skb, skb->truesize))) {
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFODROP);
+		sk->sk_data_ready(sk);
+		tcp_drop(sk, skb);
+		return;
+	}
+
+	/* Disable header prediction. */
+	tp->pred_flags = 0;
+	inet_csk_schedule_ack(sk);
+
+	tp->rcv_ooopack += max_t(u16, 1, skb_shinfo(skb)->gso_segs);
+	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFOQUEUE);
+	seq = TCP_SKB_CB(skb)->seq;
+	end_seq = TCP_SKB_CB(skb)->end_seq;
+
+	p = &tp->out_of_order_queue.rb_node;
+	if (RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
+		/* Initial out of order segment, build 1 SACK. */
+		if (tcp_is_sack(tp)) {
+			tp->rx_opt.num_sacks = 1;
+			tp->selective_acks[0].start_seq = seq;
+			tp->selective_acks[0].end_seq = end_seq;
+		}
+		rb_link_node(&skb->rbnode, NULL, p);
+		rb_insert_color(&skb->rbnode, &tp->out_of_order_queue);
+		tp->ooo_last_skb = skb;
+		goto end;
+	}
+
+	/* In the typical case, we are adding an skb to the end of the list.
+	 * Use of ooo_last_skb avoids the O(Log(N)) rbtree lookup.
+	 */
+	if (tcp_ooo_try_coalesce(sk, tp->ooo_last_skb,
+				 skb, &fragstolen)) {
+coalesce_done:
+		/* For non sack flows, do not grow window to force DUPACK
+		 * and trigger fast retransmit.
+		 */
+		if (tcp_is_sack(tp))
+			tcp_grow_window(sk, skb, true);
+		kfree_skb_partial(skb, fragstolen);
+		skb = NULL;
+		goto add_sack;
+	}
+	/* Can avoid an rbtree lookup if we are adding skb after ooo_last_skb */
+	if (!before(seq, TCP_SKB_CB(tp->ooo_last_skb)->end_seq)) {
+		parent = &tp->ooo_last_skb->rbnode;
+		p = &parent->rb_right;
+		goto insert;
+	}
+
+	/* Find place to insert this segment. Handle overlaps on the way. */
+	parent = NULL;
+	while (*p) {
+		parent = *p;
+		skb1 = rb_to_skb(parent);
+		if (before(seq, TCP_SKB_CB(skb1)->seq)) {
+			p = &parent->rb_left;
+			continue;
+		}
+		if (before(seq, TCP_SKB_CB(skb1)->end_seq)) {
+			if (!after(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
+				/* All the bits are present. Drop. */
+				NET_INC_STATS(sock_net(sk),
+					      LINUX_MIB_TCPOFOMERGE);
+				tcp_drop(sk, skb);
+				skb = NULL;
+				tcp_dsack_set(sk, seq, end_seq);
+				goto add_sack;
+			}
+			if (after(seq, TCP_SKB_CB(skb1)->seq)) {
+				/* Partial overlap. */
+				tcp_dsack_set(sk, seq, TCP_SKB_CB(skb1)->end_seq);
+			} else {
+				/* skb's seq == skb1's seq and skb covers skb1.
+				 * Replace skb1 with skb.
+				 */
+				rb_replace_node(&skb1->rbnode, &skb->rbnode,
+						&tp->out_of_order_queue);
+				tcp_dsack_extend(sk,
+						 TCP_SKB_CB(skb1)->seq,
+						 TCP_SKB_CB(skb1)->end_seq);
+				NET_INC_STATS(sock_net(sk),
+					      LINUX_MIB_TCPOFOMERGE);
+				tcp_drop(sk, skb1);
+				goto merge_right;
+			}
+		} else if (tcp_ooo_try_coalesce(sk, skb1,
+						skb, &fragstolen)) {
+			goto coalesce_done;
+		}
+		p = &parent->rb_right;
+	}
+insert:
+	/* Insert segment into RB tree. */
+	rb_link_node(&skb->rbnode, parent, p);
+	rb_insert_color(&skb->rbnode, &tp->out_of_order_queue);
+
+merge_right:
+	/* Remove other segments covered by skb. */
+	while ((skb1 = skb_rb_next(skb)) != NULL) {
+		if (!after(end_seq, TCP_SKB_CB(skb1)->seq))
+			break;
+		if (before(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
+			tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
+					 end_seq);
+			break;
+		}
+		rb_erase(&skb1->rbnode, &tp->out_of_order_queue);
+		tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
+				 TCP_SKB_CB(skb1)->end_seq);
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFOMERGE);
+		tcp_drop(sk, skb1);
+	}
+	/* If there is no skb after us, we are the last_skb ! */
+	if (!skb1)
+		tp->ooo_last_skb = skb;
+
+add_sack:
+	if (tcp_is_sack(tp))
+		tcp_sack_new_ofo_skb(sk, seq, end_seq);
+end:
+	if (skb) {
+		/* For non sack flows, do not grow window to force DUPACK
+		 * and trigger fast retransmit.
+		 */
+		if (tcp_is_sack(tp))
+			tcp_grow_window(sk, skb, false);
+		skb_condense(skb);
+		skb_set_owner_r(skb, sk);
+	}
+}
+
+static int __must_check tcp_queue_rcv(struct sock *sk, struct sk_buff *skb,
+				      bool *fragstolen)
+{
+	int eaten;
+	struct sk_buff *tail = skb_peek_tail(&sk->sk_receive_queue);
+
+	eaten = (tail &&
+		 tcp_try_coalesce(sk, tail,
+				  skb, fragstolen)) ? 1 : 0;
+	tcp_rcv_nxt_update(tcp_sk(sk), TCP_SKB_CB(skb)->end_seq);
+	if (!eaten) {
+		__skb_queue_tail(&sk->sk_receive_queue, skb);
+		skb_set_owner_r(skb, sk);
+	}
+	return eaten;
+}
+
+int tcp_send_rcvq(struct sock *sk, struct msghdr *msg, size_t size)
+{
+	struct sk_buff *skb;
+	int err = -ENOMEM;
+	int data_len = 0;
+	bool fragstolen;
+
+	if (size == 0)
+		return 0;
+
+	if (size > PAGE_SIZE) {
+		int npages = min_t(size_t, size >> PAGE_SHIFT, MAX_SKB_FRAGS);
+
+		data_len = npages << PAGE_SHIFT;
+		size = data_len + (size & ~PAGE_MASK);
+	}
+	skb = alloc_skb_with_frags(size - data_len, data_len,
+				   PAGE_ALLOC_COSTLY_ORDER,
+				   &err, sk->sk_allocation);
+	if (!skb)
+		goto err;
+
+	skb_put(skb, size - data_len);
+	skb->data_len = data_len;
+	skb->len = size;
+
+	if (tcp_try_rmem_schedule(sk, skb, skb->truesize)) {
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVQDROP);
+		goto err_free;
+	}
+
+	err = skb_copy_datagram_from_iter(skb, 0, &msg->msg_iter, size);
+	if (err)
+		goto err_free;
+
+	TCP_SKB_CB(skb)->seq = tcp_sk(sk)->rcv_nxt;
+	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq + size;
+	TCP_SKB_CB(skb)->ack_seq = tcp_sk(sk)->snd_una - 1;
+
+	if (tcp_queue_rcv(sk, skb, &fragstolen)) {
+		WARN_ON_ONCE(fragstolen); /* should not happen */
+		__kfree_skb(skb);
+	}
+	return size;
+
+err_free:
+	kfree_skb(skb);
+err:
+	return err;
+
+}
+
+void tcp_data_ready(struct sock *sk)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+	int avail = tp->rcv_nxt - tp->copied_seq;
+
+	if (avail < sk->sk_rcvlowat && !tcp_rmem_pressure(sk) &&
+	    !sock_flag(sk, SOCK_DONE) &&
+	    tcp_receive_window(tp) > inet_csk(sk)->icsk_ack.rcv_mss)
+		return;
+
+	sk->sk_data_ready(sk);
+}
+
+static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	bool fragstolen;
+	int eaten;
+
+	if (sk_is_mptcp(sk))
+		mptcp_incoming_options(sk, skb);
+
+	if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq) {
+		__kfree_skb(skb);
+		return;
+	}
+	skb_dst_drop(skb);
+	__skb_pull(skb, tcp_hdr(skb)->doff * 4);
+
+	tp->rx_opt.dsack = 0;
+
+	/*  Queue data for delivery to the user.
+	 *  Packets in sequence go to the receive queue.
+	 *  Out of sequence packets to the out_of_order_queue.
+	 */
+	if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt) {
+		if (tcp_receive_window(tp) == 0) {
+			NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPZEROWINDOWDROP);
+			goto out_of_window;
+		}
+
+		/* Ok. In sequence. In window. */
+queue_and_out:
+		if (skb_queue_len(&sk->sk_receive_queue) == 0)
+			sk_forced_mem_schedule(sk, skb->truesize);
+		else if (tcp_try_rmem_schedule(sk, skb, skb->truesize)) {
+			NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVQDROP);
+			sk->sk_data_ready(sk);
+			goto drop;
+		}
+
+		eaten = tcp_queue_rcv(sk, skb, &fragstolen);
+		if (skb->len)
+			tcp_event_data_recv(sk, skb);
+		if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
+			tcp_fin(sk);
+
+		if (!RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
+			tcp_ofo_queue(sk);
+
+			/* RFC5681. 4.2. SHOULD send immediate ACK, when
+			 * gap in queue is filled.
+			 */
+			if (RB_EMPTY_ROOT(&tp->out_of_order_queue))
+				inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOW;
+		}
+
+		if (tp->rx_opt.num_sacks)
+			tcp_sack_remove(tp);
+
+		tcp_fast_path_check(sk);
+
+		if (eaten > 0)
+			kfree_skb_partial(skb, fragstolen);
+		if (!sock_flag(sk, SOCK_DEAD))
+			tcp_data_ready(sk);
+		return;
+	}
+
+	if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
+		tcp_rcv_spurious_retrans(sk, skb);
+		/* A retransmit, 2nd most common case.  Force an immediate ack. */
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
+		tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);
+
+out_of_window:
+		tcp_enter_quickack_mode(sk, TCP_MAX_QUICKACKS);
+		inet_csk_schedule_ack(sk);
+drop:
+		tcp_drop(sk, skb);
+		return;
+	}
+
+	/* Out of window. F.e. zero window probe. */
+	if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt + tcp_receive_window(tp)))
+		goto out_of_window;
+
+	if (before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
+		/* Partial packet, seq < rcv_next < end_seq */
+		tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, tp->rcv_nxt);
+
+		/* If window is closed, drop tail of packet. But after
+		 * remembering D-SACK for its head made in previous line.
+		 */
+		if (!tcp_receive_window(tp)) {
+			NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPZEROWINDOWDROP);
+			goto out_of_window;
+		}
+		goto queue_and_out;
+	}
+
+	tcp_data_queue_ofo(sk, skb);
+}
+
+static struct sk_buff *tcp_skb_next(struct sk_buff *skb, struct sk_buff_head *list)
+{
+	if (list)
+		return !skb_queue_is_last(list, skb) ? skb->next : NULL;
+
+	return skb_rb_next(skb);
+}
+
+static struct sk_buff *tcp_collapse_one(struct sock *sk, struct sk_buff *skb,
+					struct sk_buff_head *list,
+					struct rb_root *root)
+{
+	struct sk_buff *next = tcp_skb_next(skb, list);
+
+	if (list)
+		__skb_unlink(skb, list);
+	else
+		rb_erase(&skb->rbnode, root);
+
+	__kfree_skb(skb);
+	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVCOLLAPSED);
+
+	return next;
+}
+
+/* Insert skb into rb tree, ordered by TCP_SKB_CB(skb)->seq */
+void tcp_rbtree_insert(struct rb_root *root, struct sk_buff *skb)
+{
+	struct rb_node **p = &root->rb_node;
+	struct rb_node *parent = NULL;
+	struct sk_buff *skb1;
+
+	while (*p) {
+		parent = *p;
+		skb1 = rb_to_skb(parent);
+		if (before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb1)->seq))
+			p = &parent->rb_left;
+		else
+			p = &parent->rb_right;
+	}
+	rb_link_node(&skb->rbnode, parent, p);
+	rb_insert_color(&skb->rbnode, root);
+}
+
+/* Collapse contiguous sequence of skbs head..tail with
+ * sequence numbers start..end.
+ *
+ * If tail is NULL, this means until the end of the queue.
+ *
+ * Segments with FIN/SYN are not collapsed (only because this
+ * simplifies code)
+ */
+static void
+tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root,
+	     struct sk_buff *head, struct sk_buff *tail, u32 start, u32 end)
+{
+	struct sk_buff *skb = head, *n;
+	struct sk_buff_head tmp;
+	bool end_of_skbs;
+
+	/* First, check that queue is collapsible and find
+	 * the point where collapsing can be useful.
+	 */
+restart:
+	for (end_of_skbs = true; skb != NULL && skb != tail; skb = n) {
+		n = tcp_skb_next(skb, list);
+
+		/* No new bits? It is possible on ofo queue. */
+		if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
+			skb = tcp_collapse_one(sk, skb, list, root);
+			if (!skb)
+				break;
+			goto restart;
+		}
+
+		/* The first skb to collapse is:
+		 * - not SYN/FIN and
+		 * - bloated or contains data before "start" or
+		 *   overlaps to the next one and mptcp allow collapsing.
+		 */
+		if (!(TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN)) &&
+		    (tcp_win_from_space(sk, skb->truesize) > skb->len ||
+		     before(TCP_SKB_CB(skb)->seq, start))) {
+			end_of_skbs = false;
+			break;
+		}
+
+		if (n && n != tail && mptcp_skb_can_collapse(skb, n) &&
+		    TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(n)->seq) {
+			end_of_skbs = false;
+			break;
+		}
+
+		/* Decided to skip this, advance start seq. */
+		start = TCP_SKB_CB(skb)->end_seq;
+	}
+	if (end_of_skbs ||
+	    (TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN)))
+		return;
+
+	__skb_queue_head_init(&tmp);
+
+	while (before(start, end)) {
+		int copy = min_t(int, SKB_MAX_ORDER(0, 0), end - start);
+		struct sk_buff *nskb;
+
+		nskb = alloc_skb(copy, GFP_ATOMIC);
+		if (!nskb)
+			break;
+
+		memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
+#ifdef CONFIG_TLS_DEVICE
+		nskb->decrypted = skb->decrypted;
+#endif
+		TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(nskb)->end_seq = start;
+		if (list)
+			__skb_queue_before(list, skb, nskb);
+		else
+			__skb_queue_tail(&tmp, nskb); /* defer rbtree insertion */
+		skb_set_owner_r(nskb, sk);
+		mptcp_skb_ext_move(nskb, skb);
+
+		/* Copy data, releasing collapsed skbs. */
+		while (copy > 0) {
+			int offset = start - TCP_SKB_CB(skb)->seq;
+			int size = TCP_SKB_CB(skb)->end_seq - start;
+
+			BUG_ON(offset < 0);
+			if (size > 0) {
+				size = min(copy, size);
+				if (skb_copy_bits(skb, offset, skb_put(nskb, size), size))
+					BUG();
+				TCP_SKB_CB(nskb)->end_seq += size;
+				copy -= size;
+				start += size;
+			}
+			if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
+				skb = tcp_collapse_one(sk, skb, list, root);
+				if (!skb ||
+				    skb == tail ||
+				    !mptcp_skb_can_collapse(nskb, skb) ||
+				    (TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN)))
+					goto end;
+#ifdef CONFIG_TLS_DEVICE
+				if (skb->decrypted != nskb->decrypted)
+					goto end;
+#endif
+			}
+		}
+	}
+end:
+	skb_queue_walk_safe(&tmp, skb, n)
+		tcp_rbtree_insert(root, skb);
+}
+
+/* Collapse ofo queue. Algorithm: select contiguous sequence of skbs
+ * and tcp_collapse() them until all the queue is collapsed.
+ */
+static void tcp_collapse_ofo_queue(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	u32 range_truesize, sum_tiny = 0;
+	struct sk_buff *skb, *head;
+	u32 start, end;
+
+	skb = skb_rb_first(&tp->out_of_order_queue);
+new_range:
+	if (!skb) {
+		tp->ooo_last_skb = skb_rb_last(&tp->out_of_order_queue);
+		return;
+	}
+	start = TCP_SKB_CB(skb)->seq;
+	end = TCP_SKB_CB(skb)->end_seq;
+	range_truesize = skb->truesize;
+
+	for (head = skb;;) {
+		skb = skb_rb_next(skb);
+
+		/* Range is terminated when we see a gap or when
+		 * we are at the queue end.
+		 */
+		if (!skb ||
+		    after(TCP_SKB_CB(skb)->seq, end) ||
+		    before(TCP_SKB_CB(skb)->end_seq, start)) {
+			/* Do not attempt collapsing tiny skbs */
+			if (range_truesize != head->truesize ||
+			    end - start >= SKB_WITH_OVERHEAD(SK_MEM_QUANTUM)) {
+				tcp_collapse(sk, NULL, &tp->out_of_order_queue,
+					     head, skb, start, end);
+			} else {
+				sum_tiny += range_truesize;
+				if (sum_tiny > sk->sk_rcvbuf >> 3)
+					return;
+			}
+			goto new_range;
+		}
+
+		range_truesize += skb->truesize;
+		if (unlikely(before(TCP_SKB_CB(skb)->seq, start)))
+			start = TCP_SKB_CB(skb)->seq;
+		if (after(TCP_SKB_CB(skb)->end_seq, end))
+			end = TCP_SKB_CB(skb)->end_seq;
+	}
+}
+
+/*
+ * Clean the out-of-order queue to make room.
+ * We drop high sequences packets to :
+ * 1) Let a chance for holes to be filled.
+ * 2) not add too big latencies if thousands of packets sit there.
+ *    (But if application shrinks SO_RCVBUF, we could still end up
+ *     freeing whole queue here)
+ * 3) Drop at least 12.5 % of sk_rcvbuf to avoid malicious attacks.
+ *
+ * Return true if queue has shrunk.
+ */
+static bool tcp_prune_ofo_queue(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct rb_node *node, *prev;
+	int goal;
+
+	if (RB_EMPTY_ROOT(&tp->out_of_order_queue))
+		return false;
+
+	NET_INC_STATS(sock_net(sk), LINUX_MIB_OFOPRUNED);
+	goal = sk->sk_rcvbuf >> 3;
+	node = &tp->ooo_last_skb->rbnode;
+	do {
+		prev = rb_prev(node);
+		rb_erase(node, &tp->out_of_order_queue);
+		goal -= rb_to_skb(node)->truesize;
+		tcp_drop(sk, rb_to_skb(node));
+		if (!prev || goal <= 0) {
+			sk_mem_reclaim(sk);
+			if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf &&
+			    !tcp_under_memory_pressure(sk))
+				break;
+			goal = sk->sk_rcvbuf >> 3;
+		}
+		node = prev;
+	} while (node);
+	tp->ooo_last_skb = rb_to_skb(prev);
+
+	/* Reset SACK state.  A conforming SACK implementation will
+	 * do the same at a timeout based retransmit.  When a connection
+	 * is in a sad state like this, we care only about integrity
+	 * of the connection not performance.
+	 */
+	if (tp->rx_opt.sack_ok)
+		tcp_sack_reset(&tp->rx_opt);
+	return true;
+}
+
+/* Reduce allocated memory if we can, trying to get
+ * the socket within its memory limits again.
+ *
+ * Return less than zero if we should start dropping frames
+ * until the socket owning process reads some of the data
+ * to stabilize the situation.
+ */
+static int tcp_prune_queue(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	NET_INC_STATS(sock_net(sk), LINUX_MIB_PRUNECALLED);
+
+	if (atomic_read(&sk->sk_rmem_alloc) >= sk->sk_rcvbuf)
+		tcp_clamp_window(sk);
+	else if (tcp_under_memory_pressure(sk))
+		tp->rcv_ssthresh = min(tp->rcv_ssthresh, 4U * tp->advmss);
+
+	if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
+		return 0;
+
+	tcp_collapse_ofo_queue(sk);
+	if (!skb_queue_empty(&sk->sk_receive_queue))
+		tcp_collapse(sk, &sk->sk_receive_queue, NULL,
+			     skb_peek(&sk->sk_receive_queue),
+			     NULL,
+			     tp->copied_seq, tp->rcv_nxt);
+	sk_mem_reclaim(sk);
+
+	if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
+		return 0;
+
+	/* Collapsing did not help, destructive actions follow.
+	 * This must not ever occur. */
+
+	tcp_prune_ofo_queue(sk);
+
+	if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
+		return 0;
+
+	/* If we are really being abused, tell the caller to silently
+	 * drop receive data on the floor.  It will get retransmitted
+	 * and hopefully then we'll have sufficient space.
+	 */
+	NET_INC_STATS(sock_net(sk), LINUX_MIB_RCVPRUNED);
+
+	/* Massive buffer overcommit. */
+	tp->pred_flags = 0;
+	return -1;
+}
+
+static bool tcp_should_expand_sndbuf(const struct sock *sk)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+
+	/* If the user specified a specific send buffer setting, do
+	 * not modify it.
+	 */
+	if (sk->sk_userlocks & SOCK_SNDBUF_LOCK)
+		return false;
+
+	/* If we are under global TCP memory pressure, do not expand.  */
+	if (tcp_under_memory_pressure(sk))
+		return false;
+
+	/* If we are under soft global TCP memory pressure, do not expand.  */
+	if (sk_memory_allocated(sk) >= sk_prot_mem_limits(sk, 0))
+		return false;
+
+	/* If we filled the congestion window, do not expand.  */
+	if (tcp_packets_in_flight(tp) >= tp->snd_cwnd)
+		return false;
+
+	return true;
+}
+
+static void tcp_new_space(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (tcp_should_expand_sndbuf(sk)) {
+		tcp_sndbuf_expand(sk);
+		tp->snd_cwnd_stamp = tcp_jiffies32;
+	}
+
+	sk->sk_write_space(sk);
+}
+
+/* Caller made space either from:
+ * 1) Freeing skbs in rtx queues (after tp->snd_una has advanced)
+ * 2) Sent skbs from output queue (and thus advancing tp->snd_nxt)
+ *
+ * We might be able to generate EPOLLOUT to the application if:
+ * 1) Space consumed in output/rtx queues is below sk->sk_sndbuf/2
+ * 2) notsent amount (tp->write_seq - tp->snd_nxt) became
+ *    small enough that tcp_stream_memory_free() decides it
+ *    is time to generate EPOLLOUT.
+ */
+void tcp_check_space(struct sock *sk)
+{
+	/* pairs with tcp_poll() */
+	smp_mb();
+	if (sk->sk_socket &&
+	    test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
+		tcp_new_space(sk);
+		if (!test_bit(SOCK_NOSPACE, &sk->sk_socket->flags))
+			tcp_chrono_stop(sk, TCP_CHRONO_SNDBUF_LIMITED);
+	}
+}
+
+static inline void tcp_data_snd_check(struct sock *sk)
+{
+	tcp_push_pending_frames(sk);
+	tcp_check_space(sk);
+}
+
+/*
+ * Check if sending an ack is needed.
+ */
+static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	unsigned long rtt, delay;
+
+	    /* More than one full frame received... */
+	if (((tp->rcv_nxt - tp->rcv_wup) > inet_csk(sk)->icsk_ack.rcv_mss &&
+	     /* ... and right edge of window advances far enough.
+	      * (tcp_recvmsg() will send ACK otherwise).
+	      * If application uses SO_RCVLOWAT, we want send ack now if
+	      * we have not received enough bytes to satisfy the condition.
+	      */
+	    (tp->rcv_nxt - tp->copied_seq < sk->sk_rcvlowat ||
+	     __tcp_select_window(sk) >= tp->rcv_wnd)) ||
+	    /* We ACK each frame or... */
+	    tcp_in_quickack_mode(sk) ||
+	    /* Protocol state mandates a one-time immediate ACK */
+	    inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOW) {
+send_now:
+		tcp_send_ack(sk);
+		return;
+	}
+
+	if (!ofo_possible || RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
+		tcp_send_delayed_ack(sk);
+		return;
+	}
+
+	if (!tcp_is_sack(tp) ||
+	    tp->compressed_ack >= READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_comp_sack_nr))
+		goto send_now;
+
+	if (tp->compressed_ack_rcv_nxt != tp->rcv_nxt) {
+		tp->compressed_ack_rcv_nxt = tp->rcv_nxt;
+		tp->dup_ack_counter = 0;
+	}
+	if (tp->dup_ack_counter < TCP_FASTRETRANS_THRESH) {
+		tp->dup_ack_counter++;
+		goto send_now;
+	}
+	tp->compressed_ack++;
+	if (hrtimer_is_queued(&tp->compressed_ack_timer))
+		return;
+
+	/* compress ack timer : 5 % of rtt, but no more than tcp_comp_sack_delay_ns */
+
+	rtt = tp->rcv_rtt_est.rtt_us;
+	if (tp->srtt_us && tp->srtt_us < rtt)
+		rtt = tp->srtt_us;
+
+	delay = min_t(unsigned long,
+		      READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_comp_sack_delay_ns),
+		      rtt * (NSEC_PER_USEC >> 3)/20);
+	sock_hold(sk);
+	hrtimer_start_range_ns(&tp->compressed_ack_timer, ns_to_ktime(delay),
+			       READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_comp_sack_slack_ns),
+			       HRTIMER_MODE_REL_PINNED_SOFT);
+}
+
+static inline void tcp_ack_snd_check(struct sock *sk)
+{
+	if (!inet_csk_ack_scheduled(sk)) {
+		/* We sent a data segment already. */
+		return;
+	}
+	__tcp_ack_snd_check(sk, 1);
+}
+
+/*
+ *	This routine is only called when we have urgent data
+ *	signaled. Its the 'slow' part of tcp_urg. It could be
+ *	moved inline now as tcp_urg is only called from one
+ *	place. We handle URGent data wrong. We have to - as
+ *	BSD still doesn't use the correction from RFC961.
+ *	For 1003.1g we should support a new option TCP_STDURG to permit
+ *	either form (or just set the sysctl tcp_stdurg).
+ */
+
+static void tcp_check_urg(struct sock *sk, const struct tcphdr *th)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	u32 ptr = ntohs(th->urg_ptr);
+
+	if (ptr && !READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_stdurg))
+		ptr--;
+	ptr += ntohl(th->seq);
+
+	/* Ignore urgent data that we've already seen and read. */
+	if (after(tp->copied_seq, ptr))
+		return;
+
+	/* Do not replay urg ptr.
+	 *
+	 * NOTE: interesting situation not covered by specs.
+	 * Misbehaving sender may send urg ptr, pointing to segment,
+	 * which we already have in ofo queue. We are not able to fetch
+	 * such data and will stay in TCP_URG_NOTYET until will be eaten
+	 * by recvmsg(). Seems, we are not obliged to handle such wicked
+	 * situations. But it is worth to think about possibility of some
+	 * DoSes using some hypothetical application level deadlock.
+	 */
+	if (before(ptr, tp->rcv_nxt))
+		return;
+
+	/* Do we already have a newer (or duplicate) urgent pointer? */
+	if (tp->urg_data && !after(ptr, tp->urg_seq))
+		return;
+
+	/* Tell the world about our new urgent pointer. */
+	sk_send_sigurg(sk);
+
+	/* We may be adding urgent data when the last byte read was
+	 * urgent. To do this requires some care. We cannot just ignore
+	 * tp->copied_seq since we would read the last urgent byte again
+	 * as data, nor can we alter copied_seq until this data arrives
+	 * or we break the semantics of SIOCATMARK (and thus sockatmark())
+	 *
+	 * NOTE. Double Dutch. Rendering to plain English: author of comment
+	 * above did something sort of 	send("A", MSG_OOB); send("B", MSG_OOB);
+	 * and expect that both A and B disappear from stream. This is _wrong_.
+	 * Though this happens in BSD with high probability, this is occasional.
+	 * Any application relying on this is buggy. Note also, that fix "works"
+	 * only in this artificial test. Insert some normal data between A and B and we will
+	 * decline of BSD again. Verdict: it is better to remove to trap
+	 * buggy users.
+	 */
+	if (tp->urg_seq == tp->copied_seq && tp->urg_data &&
+	    !sock_flag(sk, SOCK_URGINLINE) && tp->copied_seq != tp->rcv_nxt) {
+		struct sk_buff *skb = skb_peek(&sk->sk_receive_queue);
+		tp->copied_seq++;
+		if (skb && !before(tp->copied_seq, TCP_SKB_CB(skb)->end_seq)) {
+			__skb_unlink(skb, &sk->sk_receive_queue);
+			__kfree_skb(skb);
+		}
+	}
+
+	tp->urg_data = TCP_URG_NOTYET;
+	WRITE_ONCE(tp->urg_seq, ptr);
+
+	/* Disable header prediction. */
+	tp->pred_flags = 0;
+}
+
+/* This is the 'fast' part of urgent handling. */
+static void tcp_urg(struct sock *sk, struct sk_buff *skb, const struct tcphdr *th)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	/* Check if we get a new urgent pointer - normally not. */
+	if (th->urg)
+		tcp_check_urg(sk, th);
+
+	/* Do we wait for any urgent data? - normally not... */
+	if (tp->urg_data == TCP_URG_NOTYET) {
+		u32 ptr = tp->urg_seq - ntohl(th->seq) + (th->doff * 4) -
+			  th->syn;
+
+		/* Is the urgent pointer pointing into this packet? */
+		if (ptr < skb->len) {
+			u8 tmp;
+			if (skb_copy_bits(skb, ptr, &tmp, 1))
+				BUG();
+			tp->urg_data = TCP_URG_VALID | tmp;
+			if (!sock_flag(sk, SOCK_DEAD))
+				sk->sk_data_ready(sk);
+		}
+	}
+}
+
+/* Accept RST for rcv_nxt - 1 after a FIN.
+ * When tcp connections are abruptly terminated from Mac OSX (via ^C), a
+ * FIN is sent followed by a RST packet. The RST is sent with the same
+ * sequence number as the FIN, and thus according to RFC 5961 a challenge
+ * ACK should be sent. However, Mac OSX rate limits replies to challenge
+ * ACKs on the closed socket. In addition middleboxes can drop either the
+ * challenge ACK or a subsequent RST.
+ */
+static bool tcp_reset_check(const struct sock *sk, const struct sk_buff *skb)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	return unlikely(TCP_SKB_CB(skb)->seq == (tp->rcv_nxt - 1) &&
+			(1 << sk->sk_state) & (TCPF_CLOSE_WAIT | TCPF_LAST_ACK |
+					       TCPF_CLOSING));
+}
+
+/* Does PAWS and seqno based validation of an incoming segment, flags will
+ * play significant role here.
+ */
+static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
+				  const struct tcphdr *th, int syn_inerr)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	bool rst_seq_match = false;
+
+	/* RFC1323: H1. Apply PAWS check first. */
+	if (tcp_fast_parse_options(sock_net(sk), skb, th, tp) &&
+	    tp->rx_opt.saw_tstamp &&
+	    tcp_paws_discard(sk, skb)) {
+		if (!th->rst) {
+			NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
+			if (!tcp_oow_rate_limited(sock_net(sk), skb,
+						  LINUX_MIB_TCPACKSKIPPEDPAWS,
+						  &tp->last_oow_ack_time))
+				tcp_send_dupack(sk, skb);
+			goto discard;
+		}
+		/* Reset is accepted even if it did not pass PAWS. */
+	}
+
+	/* Step 1: check sequence number */
+	if (!tcp_sequence(tp, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq)) {
+		/* RFC793, page 37: "In all states except SYN-SENT, all reset
+		 * (RST) segments are validated by checking their SEQ-fields."
+		 * And page 69: "If an incoming segment is not acceptable,
+		 * an acknowledgment should be sent in reply (unless the RST
+		 * bit is set, if so drop the segment and return)".
+		 */
+		if (!th->rst) {
+			if (th->syn)
+				goto syn_challenge;
+			if (!tcp_oow_rate_limited(sock_net(sk), skb,
+						  LINUX_MIB_TCPACKSKIPPEDSEQ,
+						  &tp->last_oow_ack_time))
+				tcp_send_dupack(sk, skb);
+		} else if (tcp_reset_check(sk, skb)) {
+			tcp_reset(sk);
+		}
+		goto discard;
+	}
+
+	/* Step 2: check RST bit */
+	if (th->rst) {
+		/* RFC 5961 3.2 (extend to match against (RCV.NXT - 1) after a
+		 * FIN and SACK too if available):
+		 * If seq num matches RCV.NXT or (RCV.NXT - 1) after a FIN, or
+		 * the right-most SACK block,
+		 * then
+		 *     RESET the connection
+		 * else
+		 *     Send a challenge ACK
+		 */
+		if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt ||
+		    tcp_reset_check(sk, skb)) {
+			rst_seq_match = true;
+		} else if (tcp_is_sack(tp) && tp->rx_opt.num_sacks > 0) {
+			struct tcp_sack_block *sp = &tp->selective_acks[0];
+			int max_sack = sp[0].end_seq;
+			int this_sack;
+
+			for (this_sack = 1; this_sack < tp->rx_opt.num_sacks;
+			     ++this_sack) {
+				max_sack = after(sp[this_sack].end_seq,
+						 max_sack) ?
+					sp[this_sack].end_seq : max_sack;
+			}
+
+			if (TCP_SKB_CB(skb)->seq == max_sack)
+				rst_seq_match = true;
+		}
+
+		if (rst_seq_match)
+			tcp_reset(sk);
+		else {
+			/* Disable TFO if RST is out-of-order
+			 * and no data has been received
+			 * for current active TFO socket
+			 */
+			if (tp->syn_fastopen && !tp->data_segs_in &&
+			    sk->sk_state == TCP_ESTABLISHED)
+				tcp_fastopen_active_disable(sk);
+			tcp_send_challenge_ack(sk, skb);
+		}
+		goto discard;
+	}
+
+	/* step 3: check security and precedence [ignored] */
+
+	/* step 4: Check for a SYN
+	 * RFC 5961 4.2 : Send a challenge ack
+	 */
+	if (th->syn) {
+syn_challenge:
+		if (syn_inerr)
+			TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNCHALLENGE);
+		tcp_send_challenge_ack(sk, skb);
+		goto discard;
+	}
+
+	bpf_skops_parse_hdr(sk, skb);
+
+	return true;
+
+discard:
+	tcp_drop(sk, skb);
+	return false;
+}
+
+/*
+ *	TCP receive function for the ESTABLISHED state.
+ *
+ *	It is split into a fast path and a slow path. The fast path is
+ * 	disabled when:
+ *	- A zero window was announced from us - zero window probing
+ *        is only handled properly in the slow path.
+ *	- Out of order segments arrived.
+ *	- Urgent data is expected.
+ *	- There is no buffer space left
+ *	- Unexpected TCP flags/window values/header lengths are received
+ *	  (detected by checking the TCP header against pred_flags)
+ *	- Data is sent in both directions. Fast path only supports pure senders
+ *	  or pure receivers (this means either the sequence number or the ack
+ *	  value must stay constant)
+ *	- Unexpected TCP option.
+ *
+ *	When these conditions are not satisfied it drops into a standard
+ *	receive procedure patterned after RFC793 to handle all cases.
+ *	The first three cases are guaranteed by proper pred_flags setting,
+ *	the rest is checked inline. Fast processing is turned on in
+ *	tcp_data_queue when everything is OK.
+ */
+void tcp_rcv_established(struct sock *sk, struct sk_buff *skb)
+{
+	const struct tcphdr *th = (const struct tcphdr *)skb->data;
+	struct tcp_sock *tp = tcp_sk(sk);
+	unsigned int len = skb->len;
+
+	/* TCP congestion window tracking */
+	trace_tcp_probe(sk, skb);
+
+	tcp_mstamp_refresh(tp);
+	if (unlikely(!rcu_access_pointer(sk->sk_rx_dst)))
+		inet_csk(sk)->icsk_af_ops->sk_rx_dst_set(sk, skb);
+	/*
+	 *	Header prediction.
+	 *	The code loosely follows the one in the famous
+	 *	"30 instruction TCP receive" Van Jacobson mail.
+	 *
+	 *	Van's trick is to deposit buffers into socket queue
+	 *	on a device interrupt, to call tcp_recv function
+	 *	on the receive process context and checksum and copy
+	 *	the buffer to user space. smart...
+	 *
+	 *	Our current scheme is not silly either but we take the
+	 *	extra cost of the net_bh soft interrupt processing...
+	 *	We do checksum and copy also but from device to kernel.
+	 */
+
+	tp->rx_opt.saw_tstamp = 0;
+
+	/*	pred_flags is 0xS?10 << 16 + snd_wnd
+	 *	if header_prediction is to be made
+	 *	'S' will always be tp->tcp_header_len >> 2
+	 *	'?' will be 0 for the fast path, otherwise pred_flags is 0 to
+	 *  turn it off	(when there are holes in the receive
+	 *	 space for instance)
+	 *	PSH flag is ignored.
+	 */
+
+	if ((tcp_flag_word(th) & TCP_HP_BITS) == tp->pred_flags &&
+	    TCP_SKB_CB(skb)->seq == tp->rcv_nxt &&
+	    !after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
+		int tcp_header_len = tp->tcp_header_len;
+
+		/* Timestamp header prediction: tcp_header_len
+		 * is automatically equal to th->doff*4 due to pred_flags
+		 * match.
+		 */
+
+		/* Check timestamp */
+		if (tcp_header_len == sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) {
+			/* No? Slow path! */
+			if (!tcp_parse_aligned_timestamp(tp, th))
+				goto slow_path;
+
+			/* If PAWS failed, check it more carefully in slow path */
+			if ((s32)(tp->rx_opt.rcv_tsval - tp->rx_opt.ts_recent) < 0)
+				goto slow_path;
+
+			/* DO NOT update ts_recent here, if checksum fails
+			 * and timestamp was corrupted part, it will result
+			 * in a hung connection since we will drop all
+			 * future packets due to the PAWS test.
+			 */
+		}
+
+		if (len <= tcp_header_len) {
+			/* Bulk data transfer: sender */
+			if (len == tcp_header_len) {
+				/* Predicted packet is in window by definition.
+				 * seq == rcv_nxt and rcv_wup <= rcv_nxt.
+				 * Hence, check seq<=rcv_wup reduces to:
+				 */
+				if (tcp_header_len ==
+				    (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
+				    tp->rcv_nxt == tp->rcv_wup)
+					tcp_store_ts_recent(tp);
+
+				/* We know that such packets are checksummed
+				 * on entry.
+				 */
+				tcp_ack(sk, skb, 0);
+				__kfree_skb(skb);
+				tcp_data_snd_check(sk);
+				/* When receiving pure ack in fast path, update
+				 * last ts ecr directly instead of calling
+				 * tcp_rcv_rtt_measure_ts()
+				 */
+				tp->rcv_rtt_last_tsecr = tp->rx_opt.rcv_tsecr;
+				return;
+			} else { /* Header too small */
+				TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
+				goto discard;
+			}
+		} else {
+			int eaten = 0;
+			bool fragstolen = false;
+
+			if (tcp_checksum_complete(skb))
+				goto csum_error;
+
+			if ((int)skb->truesize > sk->sk_forward_alloc)
+				goto step5;
+
+			/* Predicted packet is in window by definition.
+			 * seq == rcv_nxt and rcv_wup <= rcv_nxt.
+			 * Hence, check seq<=rcv_wup reduces to:
+			 */
+			if (tcp_header_len ==
+			    (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
+			    tp->rcv_nxt == tp->rcv_wup)
+				tcp_store_ts_recent(tp);
+
+			tcp_rcv_rtt_measure_ts(sk, skb);
+
+			NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPHPHITS);
+
+			/* Bulk data transfer: receiver */
+			__skb_pull(skb, tcp_header_len);
+			eaten = tcp_queue_rcv(sk, skb, &fragstolen);
+
+			tcp_event_data_recv(sk, skb);
+
+			if (TCP_SKB_CB(skb)->ack_seq != tp->snd_una) {
+				/* Well, only one small jumplet in fast path... */
+				tcp_ack(sk, skb, FLAG_DATA);
+				tcp_data_snd_check(sk);
+				if (!inet_csk_ack_scheduled(sk))
+					goto no_ack;
+			} else {
+				tcp_update_wl(tp, TCP_SKB_CB(skb)->seq);
+			}
+
+			__tcp_ack_snd_check(sk, 0);
+no_ack:
+			if (eaten)
+				kfree_skb_partial(skb, fragstolen);
+			tcp_data_ready(sk);
+			return;
+		}
+	}
+
+slow_path:
+	if (len < (th->doff << 2) || tcp_checksum_complete(skb))
+		goto csum_error;
+
+	if (!th->ack && !th->rst && !th->syn)
+		goto discard;
+
+	/*
+	 *	Standard slow path.
+	 */
+
+	if (!tcp_validate_incoming(sk, skb, th, 1))
+		return;
+
+step5:
+	if (tcp_ack(sk, skb, FLAG_SLOWPATH | FLAG_UPDATE_TS_RECENT) < 0)
+		goto discard;
+
+	tcp_rcv_rtt_measure_ts(sk, skb);
+
+	/* Process urgent data. */
+	tcp_urg(sk, skb, th);
+
+	/* step 7: process the segment text */
+	tcp_data_queue(sk, skb);
+
+	tcp_data_snd_check(sk);
+	tcp_ack_snd_check(sk);
+	return;
+
+csum_error:
+	TCP_INC_STATS(sock_net(sk), TCP_MIB_CSUMERRORS);
+	TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
+
+discard:
+	tcp_drop(sk, skb);
+}
+EXPORT_SYMBOL(tcp_rcv_established);
+
+void tcp_init_transfer(struct sock *sk, int bpf_op, struct sk_buff *skb)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	tcp_mtup_init(sk);
+	icsk->icsk_af_ops->rebuild_header(sk);
+	tcp_init_metrics(sk);
+
+	/* Initialize the congestion window to start the transfer.
+	 * Cut cwnd down to 1 per RFC5681 if SYN or SYN-ACK has been
+	 * retransmitted. In light of RFC6298 more aggressive 1sec
+	 * initRTO, we only reset cwnd when more than 1 SYN/SYN-ACK
+	 * retransmission has occurred.
+	 */
+	if (tp->total_retrans > 1 && tp->undo_marker)
+		tp->snd_cwnd = 1;
+	else
+		tp->snd_cwnd = tcp_init_cwnd(tp, __sk_dst_get(sk));
+	tp->snd_cwnd_stamp = tcp_jiffies32;
+
+	bpf_skops_established(sk, bpf_op, skb);
+	/* Initialize congestion control unless BPF initialized it already: */
+	if (!icsk->icsk_ca_initialized)
+		tcp_init_congestion_control(sk);
+	tcp_init_buffer_space(sk);
+}
+
+void tcp_finish_connect(struct sock *sk, struct sk_buff *skb)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct inet_connection_sock *icsk = inet_csk(sk);
+
+	tcp_set_state(sk, TCP_ESTABLISHED);
+	icsk->icsk_ack.lrcvtime = tcp_jiffies32;
+
+	if (skb) {
+		icsk->icsk_af_ops->sk_rx_dst_set(sk, skb);
+		security_inet_conn_established(sk, skb);
+		sk_mark_napi_id(sk, skb);
+	}
+
+	tcp_init_transfer(sk, BPF_SOCK_OPS_ACTIVE_ESTABLISHED_CB, skb);
+
+	/* Prevent spurious tcp_cwnd_restart() on first data
+	 * packet.
+	 */
+	tp->lsndtime = tcp_jiffies32;
+
+	if (sock_flag(sk, SOCK_KEEPOPEN))
+		inet_csk_reset_keepalive_timer(sk, keepalive_time_when(tp));
+
+	if (!tp->rx_opt.snd_wscale)
+		__tcp_fast_path_on(tp, tp->snd_wnd);
+	else
+		tp->pred_flags = 0;
+}
+
+static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
+				    struct tcp_fastopen_cookie *cookie)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *data = tp->syn_data ? tcp_rtx_queue_head(sk) : NULL;
+	u16 mss = tp->rx_opt.mss_clamp, try_exp = 0;
+	bool syn_drop = false;
+
+	if (mss == tp->rx_opt.user_mss) {
+		struct tcp_options_received opt;
+
+		/* Get original SYNACK MSS value if user MSS sets mss_clamp */
+		tcp_clear_options(&opt);
+		opt.user_mss = opt.mss_clamp = 0;
+		tcp_parse_options(sock_net(sk), synack, &opt, 0, NULL);
+		mss = opt.mss_clamp;
+	}
+
+	if (!tp->syn_fastopen) {
+		/* Ignore an unsolicited cookie */
+		cookie->len = -1;
+	} else if (tp->total_retrans) {
+		/* SYN timed out and the SYN-ACK neither has a cookie nor
+		 * acknowledges data. Presumably the remote received only
+		 * the retransmitted (regular) SYNs: either the original
+		 * SYN-data or the corresponding SYN-ACK was dropped.
+		 */
+		syn_drop = (cookie->len < 0 && data);
+	} else if (cookie->len < 0 && !tp->syn_data) {
+		/* We requested a cookie but didn't get it. If we did not use
+		 * the (old) exp opt format then try so next time (try_exp=1).
+		 * Otherwise we go back to use the RFC7413 opt (try_exp=2).
+		 */
+		try_exp = tp->syn_fastopen_exp ? 2 : 1;
+	}
+
+	tcp_fastopen_cache_set(sk, mss, cookie, syn_drop, try_exp);
+
+	if (data) { /* Retransmit unacked data in SYN */
+		if (tp->total_retrans)
+			tp->fastopen_client_fail = TFO_SYN_RETRANSMITTED;
+		else
+			tp->fastopen_client_fail = TFO_DATA_NOT_ACKED;
+		skb_rbtree_walk_from(data) {
+			if (__tcp_retransmit_skb(sk, data, 1))
+				break;
+		}
+		tcp_rearm_rto(sk);
+		NET_INC_STATS(sock_net(sk),
+				LINUX_MIB_TCPFASTOPENACTIVEFAIL);
+		return true;
+	}
+	tp->syn_data_acked = tp->syn_data;
+	if (tp->syn_data_acked) {
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPFASTOPENACTIVE);
+		/* SYN-data is counted as two separate packets in tcp_ack() */
+		if (tp->delivered > 1)
+			--tp->delivered;
+	}
+
+	tcp_fastopen_add_skb(sk, synack);
+
+	return false;
+}
+
+static void smc_check_reset_syn(struct tcp_sock *tp)
+{
+#if IS_ENABLED(CONFIG_SMC)
+	if (static_branch_unlikely(&tcp_have_smc)) {
+		if (tp->syn_smc && !tp->rx_opt.smc_ok)
+			tp->syn_smc = 0;
+	}
+#endif
+}
+
+static void tcp_try_undo_spurious_syn(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	u32 syn_stamp;
+
+	/* undo_marker is set when SYN or SYNACK times out. The timeout is
+	 * spurious if the ACK's timestamp option echo value matches the
+	 * original SYN timestamp.
+	 */
+	syn_stamp = tp->retrans_stamp;
+	if (tp->undo_marker && syn_stamp && tp->rx_opt.saw_tstamp &&
+	    syn_stamp == tp->rx_opt.rcv_tsecr)
+		tp->undo_marker = 0;
+}
+
+static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
+					 const struct tcphdr *th)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct tcp_fastopen_cookie foc = { .len = -1 };
+	int saved_clamp = tp->rx_opt.mss_clamp;
+	bool fastopen_fail;
+
+	tcp_parse_options(sock_net(sk), skb, &tp->rx_opt, 0, &foc);
+	if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
+		tp->rx_opt.rcv_tsecr -= tp->tsoffset;
+
+	if (th->ack) {
+		/* rfc793:
+		 * "If the state is SYN-SENT then
+		 *    first check the ACK bit
+		 *      If the ACK bit is set
+		 *	  If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send
+		 *        a reset (unless the RST bit is set, if so drop
+		 *        the segment and return)"
+		 */
+		if (!after(TCP_SKB_CB(skb)->ack_seq, tp->snd_una) ||
+		    after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
+			/* Previous FIN/ACK or RST/ACK might be ignored. */
+			if (icsk->icsk_retransmits == 0)
+				inet_csk_reset_xmit_timer(sk,
+						ICSK_TIME_RETRANS,
+						TCP_TIMEOUT_MIN, TCP_RTO_MAX);
+			goto reset_and_undo;
+		}
+
+		if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
+		    !between(tp->rx_opt.rcv_tsecr, tp->retrans_stamp,
+			     tcp_time_stamp(tp))) {
+			NET_INC_STATS(sock_net(sk),
+					LINUX_MIB_PAWSACTIVEREJECTED);
+			goto reset_and_undo;
+		}
+
+		/* Now ACK is acceptable.
+		 *
+		 * "If the RST bit is set
+		 *    If the ACK was acceptable then signal the user "error:
+		 *    connection reset", drop the segment, enter CLOSED state,
+		 *    delete TCB, and return."
+		 */
+
+		if (th->rst) {
+			tcp_reset(sk);
+			goto discard;
+		}
+
+		/* rfc793:
+		 *   "fifth, if neither of the SYN or RST bits is set then
+		 *    drop the segment and return."
+		 *
+		 *    See note below!
+		 *                                        --ANK(990513)
+		 */
+		if (!th->syn)
+			goto discard_and_undo;
+
+		/* rfc793:
+		 *   "If the SYN bit is on ...
+		 *    are acceptable then ...
+		 *    (our SYN has been ACKed), change the connection
+		 *    state to ESTABLISHED..."
+		 */
+
+		tcp_ecn_rcv_synack(tp, th);
+
+		tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
+		tcp_try_undo_spurious_syn(sk);
+		tcp_ack(sk, skb, FLAG_SLOWPATH);
+
+		/* Ok.. it's good. Set up sequence numbers and
+		 * move to established.
+		 */
+		WRITE_ONCE(tp->rcv_nxt, TCP_SKB_CB(skb)->seq + 1);
+		tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1;
+
+		/* RFC1323: The window in SYN & SYN/ACK segments is
+		 * never scaled.
+		 */
+		tp->snd_wnd = ntohs(th->window);
+
+		if (!tp->rx_opt.wscale_ok) {
+			tp->rx_opt.snd_wscale = tp->rx_opt.rcv_wscale = 0;
+			tp->window_clamp = min(tp->window_clamp, 65535U);
+		}
+
+		if (tp->rx_opt.saw_tstamp) {
+			tp->rx_opt.tstamp_ok	   = 1;
+			tp->tcp_header_len =
+				sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
+			tp->advmss	    -= TCPOLEN_TSTAMP_ALIGNED;
+			tcp_store_ts_recent(tp);
+		} else {
+			tp->tcp_header_len = sizeof(struct tcphdr);
+		}
+
+		tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
+		tcp_initialize_rcv_mss(sk);
+
+		/* Remember, tcp_poll() does not lock socket!
+		 * Change state from SYN-SENT only after copied_seq
+		 * is initialized. */
+		WRITE_ONCE(tp->copied_seq, tp->rcv_nxt);
+
+		smc_check_reset_syn(tp);
+
+		smp_mb();
+
+		tcp_finish_connect(sk, skb);
+
+		fastopen_fail = (tp->syn_fastopen || tp->syn_data) &&
+				tcp_rcv_fastopen_synack(sk, skb, &foc);
+
+		if (!sock_flag(sk, SOCK_DEAD)) {
+			sk->sk_state_change(sk);
+			sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
+		}
+		if (fastopen_fail)
+			return -1;
+		if (sk->sk_write_pending ||
+		    icsk->icsk_accept_queue.rskq_defer_accept ||
+		    inet_csk_in_pingpong_mode(sk)) {
+			/* Save one ACK. Data will be ready after
+			 * several ticks, if write_pending is set.
+			 *
+			 * It may be deleted, but with this feature tcpdumps
+			 * look so _wonderfully_ clever, that I was not able
+			 * to stand against the temptation 8)     --ANK
+			 */
+			inet_csk_schedule_ack(sk);
+			tcp_enter_quickack_mode(sk, TCP_MAX_QUICKACKS);
+			inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
+						  TCP_DELACK_MAX, TCP_RTO_MAX);
+
+discard:
+			tcp_drop(sk, skb);
+			return 0;
+		} else {
+			tcp_send_ack(sk);
+		}
+		return -1;
+	}
+
+	/* No ACK in the segment */
+
+	if (th->rst) {
+		/* rfc793:
+		 * "If the RST bit is set
+		 *
+		 *      Otherwise (no ACK) drop the segment and return."
+		 */
+
+		goto discard_and_undo;
+	}
+
+	/* PAWS check. */
+	if (tp->rx_opt.ts_recent_stamp && tp->rx_opt.saw_tstamp &&
+	    tcp_paws_reject(&tp->rx_opt, 0))
+		goto discard_and_undo;
+
+	if (th->syn) {
+		/* We see SYN without ACK. It is attempt of
+		 * simultaneous connect with crossed SYNs.
+		 * Particularly, it can be connect to self.
+		 */
+		tcp_set_state(sk, TCP_SYN_RECV);
+
+		if (tp->rx_opt.saw_tstamp) {
+			tp->rx_opt.tstamp_ok = 1;
+			tcp_store_ts_recent(tp);
+			tp->tcp_header_len =
+				sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
+		} else {
+			tp->tcp_header_len = sizeof(struct tcphdr);
+		}
+
+		WRITE_ONCE(tp->rcv_nxt, TCP_SKB_CB(skb)->seq + 1);
+		WRITE_ONCE(tp->copied_seq, tp->rcv_nxt);
+		tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1;
+
+		/* RFC1323: The window in SYN & SYN/ACK segments is
+		 * never scaled.
+		 */
+		tp->snd_wnd    = ntohs(th->window);
+		tp->snd_wl1    = TCP_SKB_CB(skb)->seq;
+		tp->max_window = tp->snd_wnd;
+
+		tcp_ecn_rcv_syn(tp, th);
+
+		tcp_mtup_init(sk);
+		tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
+		tcp_initialize_rcv_mss(sk);
+
+		tcp_send_synack(sk);
+#if 0
+		/* Note, we could accept data and URG from this segment.
+		 * There are no obstacles to make this (except that we must
+		 * either change tcp_recvmsg() to prevent it from returning data
+		 * before 3WHS completes per RFC793, or employ TCP Fast Open).
+		 *
+		 * However, if we ignore data in ACKless segments sometimes,
+		 * we have no reasons to accept it sometimes.
+		 * Also, seems the code doing it in step6 of tcp_rcv_state_process
+		 * is not flawless. So, discard packet for sanity.
+		 * Uncomment this return to process the data.
+		 */
+		return -1;
+#else
+		goto discard;
+#endif
+	}
+	/* "fifth, if neither of the SYN or RST bits is set then
+	 * drop the segment and return."
+	 */
+
+discard_and_undo:
+	tcp_clear_options(&tp->rx_opt);
+	tp->rx_opt.mss_clamp = saved_clamp;
+	goto discard;
+
+reset_and_undo:
+	tcp_clear_options(&tp->rx_opt);
+	tp->rx_opt.mss_clamp = saved_clamp;
+	return 1;
+}
+
+static void tcp_rcv_synrecv_state_fastopen(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct request_sock *req;
+
+	/* If we are still handling the SYNACK RTO, see if timestamp ECR allows
+	 * undo. If peer SACKs triggered fast recovery, we can't undo here.
+	 */
+	if (inet_csk(sk)->icsk_ca_state == TCP_CA_Loss && !tp->packets_out)
+		tcp_try_undo_recovery(sk);
+
+	/* Reset rtx states to prevent spurious retransmits_timed_out() */
+	tp->retrans_stamp = 0;
+	inet_csk(sk)->icsk_retransmits = 0;
+
+	/* Once we leave TCP_SYN_RECV or TCP_FIN_WAIT_1,
+	 * we no longer need req so release it.
+	 */
+	req = rcu_dereference_protected(tp->fastopen_rsk,
+					lockdep_sock_is_held(sk));
+	reqsk_fastopen_remove(sk, req, false);
+
+	/* Re-arm the timer because data may have been sent out.
+	 * This is similar to the regular data transmission case
+	 * when new data has just been ack'ed.
+	 *
+	 * (TFO) - we could try to be more aggressive and
+	 * retransmitting any data sooner based on when they
+	 * are sent out.
+	 */
+	tcp_rearm_rto(sk);
+}
+
+/*
+ *	This function implements the receiving procedure of RFC 793 for
+ *	all states except ESTABLISHED and TIME_WAIT.
+ *	It's called from both tcp_v4_rcv and tcp_v6_rcv and should be
+ *	address independent.
+ */
+
+int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	const struct tcphdr *th = tcp_hdr(skb);
+	struct request_sock *req;
+	int queued = 0;
+	bool acceptable;
+
+	switch (sk->sk_state) {
+	case TCP_CLOSE:
+		goto discard;
+
+	case TCP_LISTEN:
+		if (th->ack)
+			return 1;
+
+		if (th->rst)
+			goto discard;
+
+		if (th->syn) {
+			if (th->fin)
+				goto discard;
+			/* It is possible that we process SYN packets from backlog,
+			 * so we need to make sure to disable BH and RCU right there.
+			 */
+			rcu_read_lock();
+			local_bh_disable();
+			acceptable = icsk->icsk_af_ops->conn_request(sk, skb) >= 0;
+			local_bh_enable();
+			rcu_read_unlock();
+
+			if (!acceptable)
+				return 1;
+			consume_skb(skb);
+			return 0;
+		}
+		goto discard;
+
+	case TCP_SYN_SENT:
+		tp->rx_opt.saw_tstamp = 0;
+		tcp_mstamp_refresh(tp);
+		queued = tcp_rcv_synsent_state_process(sk, skb, th);
+		if (queued >= 0)
+			return queued;
+
+		/* Do step6 onward by hand. */
+		tcp_urg(sk, skb, th);
+		__kfree_skb(skb);
+		tcp_data_snd_check(sk);
+		return 0;
+	}
+
+	tcp_mstamp_refresh(tp);
+	tp->rx_opt.saw_tstamp = 0;
+	req = rcu_dereference_protected(tp->fastopen_rsk,
+					lockdep_sock_is_held(sk));
+	if (req) {
+		bool req_stolen;
+
+		WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
+		    sk->sk_state != TCP_FIN_WAIT1);
+
+		if (!tcp_check_req(sk, skb, req, true, &req_stolen))
+			goto discard;
+	}
+
+	if (!th->ack && !th->rst && !th->syn)
+		goto discard;
+
+	if (!tcp_validate_incoming(sk, skb, th, 0))
+		return 0;
+
+	/* step 5: check the ACK field */
+	acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH |
+				      FLAG_UPDATE_TS_RECENT |
+				      FLAG_NO_CHALLENGE_ACK) > 0;
+
+	if (!acceptable) {
+		if (sk->sk_state == TCP_SYN_RECV)
+			return 1;	/* send one RST */
+		tcp_send_challenge_ack(sk, skb);
+		goto discard;
+	}
+	switch (sk->sk_state) {
+	case TCP_SYN_RECV:
+		tp->delivered++; /* SYN-ACK delivery isn't tracked in tcp_ack */
+		if (!tp->srtt_us)
+			tcp_synack_rtt_meas(sk, req);
+
+		if (req) {
+			tcp_rcv_synrecv_state_fastopen(sk);
+		} else {
+			tcp_try_undo_spurious_syn(sk);
+			tp->retrans_stamp = 0;
+			tcp_init_transfer(sk, BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB,
+					  skb);
+			WRITE_ONCE(tp->copied_seq, tp->rcv_nxt);
+		}
+		smp_mb();
+		tcp_set_state(sk, TCP_ESTABLISHED);
+		sk->sk_state_change(sk);
+
+		/* Note, that this wakeup is only for marginal crossed SYN case.
+		 * Passively open sockets are not waked up, because
+		 * sk->sk_sleep == NULL and sk->sk_socket == NULL.
+		 */
+		if (sk->sk_socket)
+			sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
+
+		tp->snd_una = TCP_SKB_CB(skb)->ack_seq;
+		tp->snd_wnd = ntohs(th->window) << tp->rx_opt.snd_wscale;
+		tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
+
+		if (tp->rx_opt.tstamp_ok)
+			tp->advmss -= TCPOLEN_TSTAMP_ALIGNED;
+
+		if (!inet_csk(sk)->icsk_ca_ops->cong_control)
+			tcp_update_pacing_rate(sk);
+
+		/* Prevent spurious tcp_cwnd_restart() on first data packet */
+		tp->lsndtime = tcp_jiffies32;
+
+		tcp_initialize_rcv_mss(sk);
+		tcp_fast_path_on(tp);
+		break;
+
+	case TCP_FIN_WAIT1: {
+		int tmo;
+
+		if (req)
+			tcp_rcv_synrecv_state_fastopen(sk);
+
+		if (tp->snd_una != tp->write_seq)
+			break;
+
+		tcp_set_state(sk, TCP_FIN_WAIT2);
+		WRITE_ONCE(sk->sk_shutdown, sk->sk_shutdown | SEND_SHUTDOWN);
+
+		sk_dst_confirm(sk);
+
+		if (!sock_flag(sk, SOCK_DEAD)) {
+			/* Wake up lingering close() */
+			sk->sk_state_change(sk);
+			break;
+		}
+
+		if (tp->linger2 < 0) {
+			tcp_done(sk);
+			NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
+			return 1;
+		}
+		if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
+		    after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) {
+			/* Receive out of order FIN after close() */
+			if (tp->syn_fastopen && th->fin)
+				tcp_fastopen_active_disable(sk);
+			tcp_done(sk);
+			NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
+			return 1;
+		}
+
+		tmo = tcp_fin_time(sk);
+		if (tmo > TCP_TIMEWAIT_LEN) {
+			inet_csk_reset_keepalive_timer(sk, tmo - TCP_TIMEWAIT_LEN);
+		} else if (th->fin || sock_owned_by_user(sk)) {
+			/* Bad case. We could lose such FIN otherwise.
+			 * It is not a big problem, but it looks confusing
+			 * and not so rare event. We still can lose it now,
+			 * if it spins in bh_lock_sock(), but it is really
+			 * marginal case.
+			 */
+			inet_csk_reset_keepalive_timer(sk, tmo);
+		} else {
+			tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
+			goto discard;
+		}
+		break;
+	}
+
+	case TCP_CLOSING:
+		if (tp->snd_una == tp->write_seq) {
+			tcp_time_wait(sk, TCP_TIME_WAIT, 0);
+			goto discard;
+		}
+		break;
+
+	case TCP_LAST_ACK:
+		if (tp->snd_una == tp->write_seq) {
+			tcp_update_metrics(sk);
+			tcp_done(sk);
+			goto discard;
+		}
+		break;
+	}
+
+	/* step 6: check the URG bit */
+	tcp_urg(sk, skb, th);
+
+	/* step 7: process the segment text */
+	switch (sk->sk_state) {
+	case TCP_CLOSE_WAIT:
+	case TCP_CLOSING:
+	case TCP_LAST_ACK:
+		if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
+			if (sk_is_mptcp(sk))
+				mptcp_incoming_options(sk, skb);
+			break;
+		}
+		fallthrough;
+	case TCP_FIN_WAIT1:
+	case TCP_FIN_WAIT2:
+		/* RFC 793 says to queue data in these states,
+		 * RFC 1122 says we MUST send a reset.
+		 * BSD 4.4 also does reset.
+		 */
+		if (sk->sk_shutdown & RCV_SHUTDOWN) {
+			if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
+			    after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) {
+				NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
+				tcp_reset(sk);
+				return 1;
+			}
+		}
+		fallthrough;
+	case TCP_ESTABLISHED:
+		tcp_data_queue(sk, skb);
+		queued = 1;
+		break;
+	}
+
+	/* tcp_data could move socket to TIME-WAIT */
+	if (sk->sk_state != TCP_CLOSE) {
+		tcp_data_snd_check(sk);
+		tcp_ack_snd_check(sk);
+	}
+
+	if (!queued) {
+discard:
+		tcp_drop(sk, skb);
+	}
+	return 0;
+}
+EXPORT_SYMBOL(tcp_rcv_state_process);
+
+static inline void pr_drop_req(struct request_sock *req, __u16 port, int family)
+{
+	struct inet_request_sock *ireq = inet_rsk(req);
+
+	if (family == AF_INET)
+		net_dbg_ratelimited("drop open request from %pI4/%u\n",
+				    &ireq->ir_rmt_addr, port);
+#if IS_ENABLED(CONFIG_IPV6)
+	else if (family == AF_INET6)
+		net_dbg_ratelimited("drop open request from %pI6/%u\n",
+				    &ireq->ir_v6_rmt_addr, port);
+#endif
+}
+
+/* RFC3168 : 6.1.1 SYN packets must not have ECT/ECN bits set
+ *
+ * If we receive a SYN packet with these bits set, it means a
+ * network is playing bad games with TOS bits. In order to
+ * avoid possible false congestion notifications, we disable
+ * TCP ECN negotiation.
+ *
+ * Exception: tcp_ca wants ECN. This is required for DCTCP
+ * congestion control: Linux DCTCP asserts ECT on all packets,
+ * including SYN, which is most optimal solution; however,
+ * others, such as FreeBSD do not.
+ *
+ * Exception: At least one of the reserved bits of the TCP header (th->res1) is
+ * set, indicating the use of a future TCP extension (such as AccECN). See
+ * RFC8311 §4.3 which updates RFC3168 to allow the development of such
+ * extensions.
+ */
+static void tcp_ecn_create_request(struct request_sock *req,
+				   const struct sk_buff *skb,
+				   const struct sock *listen_sk,
+				   const struct dst_entry *dst)
+{
+	const struct tcphdr *th = tcp_hdr(skb);
+	const struct net *net = sock_net(listen_sk);
+	bool th_ecn = th->ece && th->cwr;
+	bool ect, ecn_ok;
+	u32 ecn_ok_dst;
+
+	if (!th_ecn)
+		return;
+
+	ect = !INET_ECN_is_not_ect(TCP_SKB_CB(skb)->ip_dsfield);
+	ecn_ok_dst = dst_feature(dst, DST_FEATURE_ECN_MASK);
+	ecn_ok = net->ipv4.sysctl_tcp_ecn || ecn_ok_dst;
+
+	if (((!ect || th->res1) && ecn_ok) || tcp_ca_needs_ecn(listen_sk) ||
+	    (ecn_ok_dst & DST_FEATURE_ECN_CA) ||
+	    tcp_bpf_ca_needs_ecn((struct sock *)req))
+		inet_rsk(req)->ecn_ok = 1;
+}
+
+static void tcp_openreq_init(struct request_sock *req,
+			     const struct tcp_options_received *rx_opt,
+			     struct sk_buff *skb, const struct sock *sk)
+{
+	struct inet_request_sock *ireq = inet_rsk(req);
+
+	req->rsk_rcv_wnd = 0;		/* So that tcp_send_synack() knows! */
+	tcp_rsk(req)->rcv_isn = TCP_SKB_CB(skb)->seq;
+	tcp_rsk(req)->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
+	tcp_rsk(req)->snt_synack = 0;
+	tcp_rsk(req)->last_oow_ack_time = 0;
+	req->mss = rx_opt->mss_clamp;
+	req->ts_recent = rx_opt->saw_tstamp ? rx_opt->rcv_tsval : 0;
+	ireq->tstamp_ok = rx_opt->tstamp_ok;
+	ireq->sack_ok = rx_opt->sack_ok;
+	ireq->snd_wscale = rx_opt->snd_wscale;
+	ireq->wscale_ok = rx_opt->wscale_ok;
+	ireq->acked = 0;
+	ireq->ecn_ok = 0;
+	ireq->ir_rmt_port = tcp_hdr(skb)->source;
+	ireq->ir_num = ntohs(tcp_hdr(skb)->dest);
+	ireq->ir_mark = inet_request_mark(sk, skb);
+#if IS_ENABLED(CONFIG_SMC)
+	ireq->smc_ok = rx_opt->smc_ok;
+#endif
+}
+
+struct request_sock *inet_reqsk_alloc(const struct request_sock_ops *ops,
+				      struct sock *sk_listener,
+				      bool attach_listener)
+{
+	struct request_sock *req = reqsk_alloc(ops, sk_listener,
+					       attach_listener);
+
+	if (req) {
+		struct inet_request_sock *ireq = inet_rsk(req);
+
+		ireq->ireq_opt = NULL;
+#if IS_ENABLED(CONFIG_IPV6)
+		ireq->pktopts = NULL;
+#endif
+		atomic64_set(&ireq->ir_cookie, 0);
+		ireq->ireq_state = TCP_NEW_SYN_RECV;
+		write_pnet(&ireq->ireq_net, sock_net(sk_listener));
+		ireq->ireq_family = sk_listener->sk_family;
+	}
+
+	return req;
+}
+EXPORT_SYMBOL(inet_reqsk_alloc);
+
+/*
+ * Return true if a syncookie should be sent
+ */
+static bool tcp_syn_flood_action(const struct sock *sk, const char *proto)
+{
+	struct request_sock_queue *queue = &inet_csk(sk)->icsk_accept_queue;
+	const char *msg = "Dropping request";
+	struct net *net = sock_net(sk);
+	bool want_cookie = false;
+	u8 syncookies;
+
+	syncookies = READ_ONCE(net->ipv4.sysctl_tcp_syncookies);
+
+#ifdef CONFIG_SYN_COOKIES
+	if (syncookies) {
+		msg = "Sending cookies";
+		want_cookie = true;
+		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPREQQFULLDOCOOKIES);
+	} else
+#endif
+		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPREQQFULLDROP);
+
+	if (!queue->synflood_warned && syncookies != 2 &&
+	    xchg(&queue->synflood_warned, 1) == 0)
+		net_info_ratelimited("%s: Possible SYN flooding on port %d. %s.  Check SNMP counters.\n",
+				     proto, sk->sk_num, msg);
+
+	return want_cookie;
+}
+
+static void tcp_reqsk_record_syn(const struct sock *sk,
+				 struct request_sock *req,
+				 const struct sk_buff *skb)
+{
+	if (tcp_sk(sk)->save_syn) {
+		u32 len = skb_network_header_len(skb) + tcp_hdrlen(skb);
+		struct saved_syn *saved_syn;
+		u32 mac_hdrlen;
+		void *base;
+
+		if (tcp_sk(sk)->save_syn == 2) {  /* Save full header. */
+			base = skb_mac_header(skb);
+			mac_hdrlen = skb_mac_header_len(skb);
+			len += mac_hdrlen;
+		} else {
+			base = skb_network_header(skb);
+			mac_hdrlen = 0;
+		}
+
+		saved_syn = kmalloc(struct_size(saved_syn, data, len),
+				    GFP_ATOMIC);
+		if (saved_syn) {
+			saved_syn->mac_hdrlen = mac_hdrlen;
+			saved_syn->network_hdrlen = skb_network_header_len(skb);
+			saved_syn->tcp_hdrlen = tcp_hdrlen(skb);
+			memcpy(saved_syn->data, base, len);
+			req->saved_syn = saved_syn;
+		}
+	}
+}
+
+/* If a SYN cookie is required and supported, returns a clamped MSS value to be
+ * used for SYN cookie generation.
+ */
+u16 tcp_get_syncookie_mss(struct request_sock_ops *rsk_ops,
+			  const struct tcp_request_sock_ops *af_ops,
+			  struct sock *sk, struct tcphdr *th)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	u16 mss;
+
+	if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_syncookies) != 2 &&
+	    !inet_csk_reqsk_queue_is_full(sk))
+		return 0;
+
+	if (!tcp_syn_flood_action(sk, rsk_ops->slab_name))
+		return 0;
+
+	if (sk_acceptq_is_full(sk)) {
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
+		return 0;
+	}
+
+	mss = tcp_parse_mss_option(th, tp->rx_opt.user_mss);
+	if (!mss)
+		mss = af_ops->mss_clamp;
+
+	return mss;
+}
+EXPORT_SYMBOL_GPL(tcp_get_syncookie_mss);
+
+int tcp_conn_request(struct request_sock_ops *rsk_ops,
+		     const struct tcp_request_sock_ops *af_ops,
+		     struct sock *sk, struct sk_buff *skb)
+{
+	struct tcp_fastopen_cookie foc = { .len = -1 };
+	__u32 isn = TCP_SKB_CB(skb)->tcp_tw_isn;
+	struct tcp_options_received tmp_opt;
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct net *net = sock_net(sk);
+	struct sock *fastopen_sk = NULL;
+	struct request_sock *req;
+	bool want_cookie = false;
+	struct dst_entry *dst;
+	struct flowi fl;
+	u8 syncookies;
+
+	syncookies = READ_ONCE(net->ipv4.sysctl_tcp_syncookies);
+
+	/* TW buckets are converted to open requests without
+	 * limitations, they conserve resources and peer is
+	 * evidently real one.
+	 */
+	if ((syncookies == 2 || inet_csk_reqsk_queue_is_full(sk)) && !isn) {
+		want_cookie = tcp_syn_flood_action(sk, rsk_ops->slab_name);
+		if (!want_cookie)
+			goto drop;
+	}
+
+	if (sk_acceptq_is_full(sk)) {
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
+		goto drop;
+	}
+
+	req = inet_reqsk_alloc(rsk_ops, sk, !want_cookie);
+	if (!req)
+		goto drop;
+
+	req->syncookie = want_cookie;
+	tcp_rsk(req)->af_specific = af_ops;
+	tcp_rsk(req)->ts_off = 0;
+#if IS_ENABLED(CONFIG_MPTCP)
+	tcp_rsk(req)->is_mptcp = 0;
+#endif
+
+	tcp_clear_options(&tmp_opt);
+	tmp_opt.mss_clamp = af_ops->mss_clamp;
+	tmp_opt.user_mss  = tp->rx_opt.user_mss;
+	tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0,
+			  want_cookie ? NULL : &foc);
+
+	if (want_cookie && !tmp_opt.saw_tstamp)
+		tcp_clear_options(&tmp_opt);
+
+	if (IS_ENABLED(CONFIG_SMC) && want_cookie)
+		tmp_opt.smc_ok = 0;
+
+	tmp_opt.tstamp_ok = tmp_opt.saw_tstamp;
+	tcp_openreq_init(req, &tmp_opt, skb, sk);
+	inet_rsk(req)->no_srccheck = inet_sk(sk)->transparent;
+
+	/* Note: tcp_v6_init_req() might override ir_iif for link locals */
+	inet_rsk(req)->ir_iif = inet_request_bound_dev_if(sk, skb);
+
+	af_ops->init_req(req, sk, skb);
+
+	if (security_inet_conn_request(sk, skb, req))
+		goto drop_and_free;
+
+	if (tmp_opt.tstamp_ok)
+		tcp_rsk(req)->ts_off = af_ops->init_ts_off(net, skb);
+
+	dst = af_ops->route_req(sk, &fl, req);
+	if (!dst)
+		goto drop_and_free;
+
+	if (!want_cookie && !isn) {
+		int max_syn_backlog = READ_ONCE(net->ipv4.sysctl_max_syn_backlog);
+
+		/* Kill the following clause, if you dislike this way. */
+		if (!syncookies &&
+		    (max_syn_backlog - inet_csk_reqsk_queue_len(sk) <
+		     (max_syn_backlog >> 2)) &&
+		    !tcp_peer_is_proven(req, dst)) {
+			/* Without syncookies last quarter of
+			 * backlog is filled with destinations,
+			 * proven to be alive.
+			 * It means that we continue to communicate
+			 * to destinations, already remembered
+			 * to the moment of synflood.
+			 */
+			pr_drop_req(req, ntohs(tcp_hdr(skb)->source),
+				    rsk_ops->family);
+			goto drop_and_release;
+		}
+
+		isn = af_ops->init_seq(skb);
+	}
+
+	tcp_ecn_create_request(req, skb, sk, dst);
+
+	if (want_cookie) {
+		isn = cookie_init_sequence(af_ops, sk, skb, &req->mss);
+		if (!tmp_opt.tstamp_ok)
+			inet_rsk(req)->ecn_ok = 0;
+	}
+
+	tcp_rsk(req)->snt_isn = isn;
+	tcp_rsk(req)->txhash = net_tx_rndhash();
+	tcp_rsk(req)->syn_tos = TCP_SKB_CB(skb)->ip_dsfield;
+	tcp_openreq_init_rwin(req, sk, dst);
+	sk_rx_queue_set(req_to_sk(req), skb);
+	if (!want_cookie) {
+		tcp_reqsk_record_syn(sk, req, skb);
+		fastopen_sk = tcp_try_fastopen(sk, skb, req, &foc, dst);
+	}
+	if (fastopen_sk) {
+		af_ops->send_synack(fastopen_sk, dst, &fl, req,
+				    &foc, TCP_SYNACK_FASTOPEN, skb);
+		/* Add the child socket directly into the accept queue */
+		if (!inet_csk_reqsk_queue_add(sk, req, fastopen_sk)) {
+			reqsk_fastopen_remove(fastopen_sk, req, false);
+			bh_unlock_sock(fastopen_sk);
+			sock_put(fastopen_sk);
+			goto drop_and_free;
+		}
+		sk->sk_data_ready(sk);
+		bh_unlock_sock(fastopen_sk);
+		sock_put(fastopen_sk);
+	} else {
+		tcp_rsk(req)->tfo_listener = false;
+		if (!want_cookie)
+			inet_csk_reqsk_queue_hash_add(sk, req,
+				tcp_timeout_init((struct sock *)req));
+		af_ops->send_synack(sk, dst, &fl, req, &foc,
+				    !want_cookie ? TCP_SYNACK_NORMAL :
+						   TCP_SYNACK_COOKIE,
+				    skb);
+		if (want_cookie) {
+			reqsk_free(req);
+			return 0;
+		}
+	}
+	reqsk_put(req);
+	return 0;
+
+drop_and_release:
+	dst_release(dst);
+drop_and_free:
+	__reqsk_free(req);
+drop:
+	tcp_listendrop(sk);
+	return 0;
+}
+EXPORT_SYMBOL(tcp_conn_request);