Adding upstream version 6.6.15.upstream/6.6.15

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

1 files changed, 4273 insertions, 0 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
new file mode 100644
index 0000000000..ab3b7b4b44
--- /dev/null
+++ b/net/ipv4/tcp_output.c
@@ -0,0 +1,4273 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * INET		An implementation of the TCP/IP protocol suite for the LINUX
+ *		operating system.  INET is implemented using the  BSD Socket
+ *		interface as the means of communication with the user level.
+ *
+ *		Implementation of the Transmission Control Protocol(TCP).
+ *
+ * Authors:	Ross Biro
+ *		Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
+ *		Mark Evans, <evansmp@uhura.aston.ac.uk>
+ *		Corey Minyard <wf-rch!minyard@relay.EU.net>
+ *		Florian La Roche, <flla@stud.uni-sb.de>
+ *		Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
+ *		Linus Torvalds, <torvalds@cs.helsinki.fi>
+ *		Alan Cox, <gw4pts@gw4pts.ampr.org>
+ *		Matthew Dillon, <dillon@apollo.west.oic.com>
+ *		Arnt Gulbrandsen, <agulbra@nvg.unit.no>
+ *		Jorge Cwik, <jorge@laser.satlink.net>
+ */
+
+/*
+ * Changes:	Pedro Roque	:	Retransmit queue handled by TCP.
+ *				:	Fragmentation on mtu decrease
+ *				:	Segment collapse on retransmit
+ *				:	AF independence
+ *
+ *		Linus Torvalds	:	send_delayed_ack
+ *		David S. Miller	:	Charge memory using the right skb
+ *					during syn/ack processing.
+ *		David S. Miller :	Output engine completely rewritten.
+ *		Andrea Arcangeli:	SYNACK carry ts_recent in tsecr.
+ *		Cacophonix Gaul :	draft-minshall-nagle-01
+ *		J Hadi Salim	:	ECN support
+ *
+ */
+
+#define pr_fmt(fmt) "TCP: " fmt
+
+#include <net/tcp.h>
+#include <net/mptcp.h>
+
+#include <linux/compiler.h>
+#include <linux/gfp.h>
+#include <linux/module.h>
+#include <linux/static_key.h>
+
+#include <trace/events/tcp.h>
+
+/* Refresh clocks of a TCP socket,
+ * ensuring monotically increasing values.
+ */
+void tcp_mstamp_refresh(struct tcp_sock *tp)
+{
+	u64 val = tcp_clock_ns();
+
+	tp->tcp_clock_cache = val;
+	tp->tcp_mstamp = div_u64(val, NSEC_PER_USEC);
+}
+
+static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
+			   int push_one, gfp_t gfp);
+
+/* Account for new data that has been sent to the network. */
+static void tcp_event_new_data_sent(struct sock *sk, struct sk_buff *skb)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	unsigned int prior_packets = tp->packets_out;
+
+	WRITE_ONCE(tp->snd_nxt, TCP_SKB_CB(skb)->end_seq);
+
+	__skb_unlink(skb, &sk->sk_write_queue);
+	tcp_rbtree_insert(&sk->tcp_rtx_queue, skb);
+
+	if (tp->highest_sack == NULL)
+		tp->highest_sack = skb;
+
+	tp->packets_out += tcp_skb_pcount(skb);
+	if (!prior_packets || icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)
+		tcp_rearm_rto(sk);
+
+	NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPORIGDATASENT,
+		      tcp_skb_pcount(skb));
+	tcp_check_space(sk);
+}
+
+/* SND.NXT, if window was not shrunk or the amount of shrunk was less than one
+ * window scaling factor due to loss of precision.
+ * If window has been shrunk, what should we make? It is not clear at all.
+ * Using SND.UNA we will fail to open window, SND.NXT is out of window. :-(
+ * Anything in between SND.UNA...SND.UNA+SND.WND also can be already
+ * invalid. OK, let's make this for now:
+ */
+static inline __u32 tcp_acceptable_seq(const struct sock *sk)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+
+	if (!before(tcp_wnd_end(tp), tp->snd_nxt) ||
+	    (tp->rx_opt.wscale_ok &&
+	     ((tp->snd_nxt - tcp_wnd_end(tp)) < (1 << tp->rx_opt.rcv_wscale))))
+		return tp->snd_nxt;
+	else
+		return tcp_wnd_end(tp);
+}
+
+/* Calculate mss to advertise in SYN segment.
+ * RFC1122, RFC1063, draft-ietf-tcpimpl-pmtud-01 state that:
+ *
+ * 1. It is independent of path mtu.
+ * 2. Ideally, it is maximal possible segment size i.e. 65535-40.
+ * 3. For IPv4 it is reasonable to calculate it from maximal MTU of
+ *    attached devices, because some buggy hosts are confused by
+ *    large MSS.
+ * 4. We do not make 3, we advertise MSS, calculated from first
+ *    hop device mtu, but allow to raise it to ip_rt_min_advmss.
+ *    This may be overridden via information stored in routing table.
+ * 5. Value 65535 for MSS is valid in IPv6 and means "as large as possible,
+ *    probably even Jumbo".
+ */
+static __u16 tcp_advertise_mss(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	const struct dst_entry *dst = __sk_dst_get(sk);
+	int mss = tp->advmss;
+
+	if (dst) {
+		unsigned int metric = dst_metric_advmss(dst);
+
+		if (metric < mss) {
+			mss = metric;
+			tp->advmss = mss;
+		}
+	}
+
+	return (__u16)mss;
+}
+
+/* RFC2861. Reset CWND after idle period longer RTO to "restart window".
+ * This is the first part of cwnd validation mechanism.
+ */
+void tcp_cwnd_restart(struct sock *sk, s32 delta)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	u32 restart_cwnd = tcp_init_cwnd(tp, __sk_dst_get(sk));
+	u32 cwnd = tcp_snd_cwnd(tp);
+
+	tcp_ca_event(sk, CA_EVENT_CWND_RESTART);
+
+	tp->snd_ssthresh = tcp_current_ssthresh(sk);
+	restart_cwnd = min(restart_cwnd, cwnd);
+
+	while ((delta -= inet_csk(sk)->icsk_rto) > 0 && cwnd > restart_cwnd)
+		cwnd >>= 1;
+	tcp_snd_cwnd_set(tp, max(cwnd, restart_cwnd));
+	tp->snd_cwnd_stamp = tcp_jiffies32;
+	tp->snd_cwnd_used = 0;
+}
+
+/* Congestion state accounting after a packet has been sent. */
+static void tcp_event_data_sent(struct tcp_sock *tp,
+				struct sock *sk)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	const u32 now = tcp_jiffies32;
+
+	if (tcp_packets_in_flight(tp) == 0)
+		tcp_ca_event(sk, CA_EVENT_TX_START);
+
+	tp->lsndtime = now;
+
+	/* If it is a reply for ato after last received
+	 * packet, enter pingpong mode.
+	 */
+	if ((u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
+		inet_csk_enter_pingpong_mode(sk);
+}
+
+/* Account for an ACK we sent. */
+static inline void tcp_event_ack_sent(struct sock *sk, u32 rcv_nxt)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (unlikely(tp->compressed_ack)) {
+		NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
+			      tp->compressed_ack);
+		tp->compressed_ack = 0;
+		if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1)
+			__sock_put(sk);
+	}
+
+	if (unlikely(rcv_nxt != tp->rcv_nxt))
+		return;  /* Special ACK sent by DCTCP to reflect ECN */
+	tcp_dec_quickack_mode(sk);
+	inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
+}
+
+/* Determine a window scaling and initial window to offer.
+ * Based on the assumption that the given amount of space
+ * will be offered. Store the results in the tp structure.
+ * NOTE: for smooth operation initial space offering should
+ * be a multiple of mss if possible. We assume here that mss >= 1.
+ * This MUST be enforced by all callers.
+ */
+void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,
+			       __u32 *rcv_wnd, __u32 *window_clamp,
+			       int wscale_ok, __u8 *rcv_wscale,
+			       __u32 init_rcv_wnd)
+{
+	unsigned int space = (__space < 0 ? 0 : __space);
+
+	/* If no clamp set the clamp to the max possible scaled window */
+	if (*window_clamp == 0)
+		(*window_clamp) = (U16_MAX << TCP_MAX_WSCALE);
+	space = min(*window_clamp, space);
+
+	/* Quantize space offering to a multiple of mss if possible. */
+	if (space > mss)
+		space = rounddown(space, mss);
+
+	/* NOTE: offering an initial window larger than 32767
+	 * will break some buggy TCP stacks. If the admin tells us
+	 * it is likely we could be speaking with such a buggy stack
+	 * we will truncate our initial window offering to 32K-1
+	 * unless the remote has sent us a window scaling option,
+	 * which we interpret as a sign the remote TCP is not
+	 * misinterpreting the window field as a signed quantity.
+	 */
+	if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_workaround_signed_windows))
+		(*rcv_wnd) = min(space, MAX_TCP_WINDOW);
+	else
+		(*rcv_wnd) = min_t(u32, space, U16_MAX);
+
+	if (init_rcv_wnd)
+		*rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
+
+	*rcv_wscale = 0;
+	if (wscale_ok) {
+		/* Set window scaling on max possible window */
+		space = max_t(u32, space, READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_rmem[2]));
+		space = max_t(u32, space, READ_ONCE(sysctl_rmem_max));
+		space = min_t(u32, space, *window_clamp);
+		*rcv_wscale = clamp_t(int, ilog2(space) - 15,
+				      0, TCP_MAX_WSCALE);
+	}
+	/* Set the clamp no higher than max representable value */
+	(*window_clamp) = min_t(__u32, U16_MAX << (*rcv_wscale), *window_clamp);
+}
+EXPORT_SYMBOL(tcp_select_initial_window);
+
+/* Chose a new window to advertise, update state in tcp_sock for the
+ * socket, and return result with RFC1323 scaling applied.  The return
+ * value can be stuffed directly into th->window for an outgoing
+ * frame.
+ */
+static u16 tcp_select_window(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct net *net = sock_net(sk);
+	u32 old_win = tp->rcv_wnd;
+	u32 cur_win, new_win;
+
+	/* Make the window 0 if we failed to queue the data because we
+	 * are out of memory. The window is temporary, so we don't store
+	 * it on the socket.
+	 */
+	if (unlikely(inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM))
+		return 0;
+
+	cur_win = tcp_receive_window(tp);
+	new_win = __tcp_select_window(sk);
+	if (new_win < cur_win) {
+		/* Danger Will Robinson!
+		 * Don't update rcv_wup/rcv_wnd here or else
+		 * we will not be able to advertise a zero
+		 * window in time.  --DaveM
+		 *
+		 * Relax Will Robinson.
+		 */
+		if (!READ_ONCE(net->ipv4.sysctl_tcp_shrink_window) || !tp->rx_opt.rcv_wscale) {
+			/* Never shrink the offered window */
+			if (new_win == 0)
+				NET_INC_STATS(net, LINUX_MIB_TCPWANTZEROWINDOWADV);
+			new_win = ALIGN(cur_win, 1 << tp->rx_opt.rcv_wscale);
+		}
+	}
+
+	tp->rcv_wnd = new_win;
+	tp->rcv_wup = tp->rcv_nxt;
+
+	/* Make sure we do not exceed the maximum possible
+	 * scaled window.
+	 */
+	if (!tp->rx_opt.rcv_wscale &&
+	    READ_ONCE(net->ipv4.sysctl_tcp_workaround_signed_windows))
+		new_win = min(new_win, MAX_TCP_WINDOW);
+	else
+		new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));
+
+	/* RFC1323 scaling applied */
+	new_win >>= tp->rx_opt.rcv_wscale;
+
+	/* If we advertise zero window, disable fast path. */
+	if (new_win == 0) {
+		tp->pred_flags = 0;
+		if (old_win)
+			NET_INC_STATS(net, LINUX_MIB_TCPTOZEROWINDOWADV);
+	} else if (old_win == 0) {
+		NET_INC_STATS(net, LINUX_MIB_TCPFROMZEROWINDOWADV);
+	}
+
+	return new_win;
+}
+
+/* Packet ECN state for a SYN-ACK */
+static void tcp_ecn_send_synack(struct sock *sk, struct sk_buff *skb)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+
+	TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_CWR;
+	if (!(tp->ecn_flags & TCP_ECN_OK))
+		TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_ECE;
+	else if (tcp_ca_needs_ecn(sk) ||
+		 tcp_bpf_ca_needs_ecn(sk))
+		INET_ECN_xmit(sk);
+}
+
+/* Packet ECN state for a SYN.  */
+static void tcp_ecn_send_syn(struct sock *sk, struct sk_buff *skb)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	bool bpf_needs_ecn = tcp_bpf_ca_needs_ecn(sk);
+	bool use_ecn = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn) == 1 ||
+		tcp_ca_needs_ecn(sk) || bpf_needs_ecn;
+
+	if (!use_ecn) {
+		const struct dst_entry *dst = __sk_dst_get(sk);
+
+		if (dst && dst_feature(dst, RTAX_FEATURE_ECN))
+			use_ecn = true;
+	}
+
+	tp->ecn_flags = 0;
+
+	if (use_ecn) {
+		TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ECE | TCPHDR_CWR;
+		tp->ecn_flags = TCP_ECN_OK;
+		if (tcp_ca_needs_ecn(sk) || bpf_needs_ecn)
+			INET_ECN_xmit(sk);
+	}
+}
+
+static void tcp_ecn_clear_syn(struct sock *sk, struct sk_buff *skb)
+{
+	if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_fallback))
+		/* tp->ecn_flags are cleared at a later point in time when
+		 * SYN ACK is ultimatively being received.
+		 */
+		TCP_SKB_CB(skb)->tcp_flags &= ~(TCPHDR_ECE | TCPHDR_CWR);
+}
+
+static void
+tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th)
+{
+	if (inet_rsk(req)->ecn_ok)
+		th->ece = 1;
+}
+
+/* Set up ECN state for a packet on a ESTABLISHED socket that is about to
+ * be sent.
+ */
+static void tcp_ecn_send(struct sock *sk, struct sk_buff *skb,
+			 struct tcphdr *th, int tcp_header_len)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (tp->ecn_flags & TCP_ECN_OK) {
+		/* Not-retransmitted data segment: set ECT and inject CWR. */
+		if (skb->len != tcp_header_len &&
+		    !before(TCP_SKB_CB(skb)->seq, tp->snd_nxt)) {
+			INET_ECN_xmit(sk);
+			if (tp->ecn_flags & TCP_ECN_QUEUE_CWR) {
+				tp->ecn_flags &= ~TCP_ECN_QUEUE_CWR;
+				th->cwr = 1;
+				skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
+			}
+		} else if (!tcp_ca_needs_ecn(sk)) {
+			/* ACK or retransmitted segment: clear ECT|CE */
+			INET_ECN_dontxmit(sk);
+		}
+		if (tp->ecn_flags & TCP_ECN_DEMAND_CWR)
+			th->ece = 1;
+	}
+}
+
+/* Constructs common control bits of non-data skb. If SYN/FIN is present,
+ * auto increment end seqno.
+ */
+static void tcp_init_nondata_skb(struct sk_buff *skb, u32 seq, u8 flags)
+{
+	skb->ip_summed = CHECKSUM_PARTIAL;
+
+	TCP_SKB_CB(skb)->tcp_flags = flags;
+
+	tcp_skb_pcount_set(skb, 1);
+
+	TCP_SKB_CB(skb)->seq = seq;
+	if (flags & (TCPHDR_SYN | TCPHDR_FIN))
+		seq++;
+	TCP_SKB_CB(skb)->end_seq = seq;
+}
+
+static inline bool tcp_urg_mode(const struct tcp_sock *tp)
+{
+	return tp->snd_una != tp->snd_up;
+}
+
+#define OPTION_SACK_ADVERTISE	BIT(0)
+#define OPTION_TS		BIT(1)
+#define OPTION_MD5		BIT(2)
+#define OPTION_WSCALE		BIT(3)
+#define OPTION_FAST_OPEN_COOKIE	BIT(8)
+#define OPTION_SMC		BIT(9)
+#define OPTION_MPTCP		BIT(10)
+
+static void smc_options_write(__be32 *ptr, u16 *options)
+{
+#if IS_ENABLED(CONFIG_SMC)
+	if (static_branch_unlikely(&tcp_have_smc)) {
+		if (unlikely(OPTION_SMC & *options)) {
+			*ptr++ = htonl((TCPOPT_NOP  << 24) |
+				       (TCPOPT_NOP  << 16) |
+				       (TCPOPT_EXP <<  8) |
+				       (TCPOLEN_EXP_SMC_BASE));
+			*ptr++ = htonl(TCPOPT_SMC_MAGIC);
+		}
+	}
+#endif
+}
+
+struct tcp_out_options {
+	u16 options;		/* bit field of OPTION_* */
+	u16 mss;		/* 0 to disable */
+	u8 ws;			/* window scale, 0 to disable */
+	u8 num_sack_blocks;	/* number of SACK blocks to include */
+	u8 hash_size;		/* bytes in hash_location */
+	u8 bpf_opt_len;		/* length of BPF hdr option */
+	__u8 *hash_location;	/* temporary pointer, overloaded */
+	__u32 tsval, tsecr;	/* need to include OPTION_TS */
+	struct tcp_fastopen_cookie *fastopen_cookie;	/* Fast open cookie */
+	struct mptcp_out_options mptcp;
+};
+
+static void mptcp_options_write(struct tcphdr *th, __be32 *ptr,
+				struct tcp_sock *tp,
+				struct tcp_out_options *opts)
+{
+#if IS_ENABLED(CONFIG_MPTCP)
+	if (unlikely(OPTION_MPTCP & opts->options))
+		mptcp_write_options(th, ptr, tp, &opts->mptcp);
+#endif
+}
+
+#ifdef CONFIG_CGROUP_BPF
+static int bpf_skops_write_hdr_opt_arg0(struct sk_buff *skb,
+					enum tcp_synack_type synack_type)
+{
+	if (unlikely(!skb))
+		return BPF_WRITE_HDR_TCP_CURRENT_MSS;
+
+	if (unlikely(synack_type == TCP_SYNACK_COOKIE))
+		return BPF_WRITE_HDR_TCP_SYNACK_COOKIE;
+
+	return 0;
+}
+
+/* req, syn_skb and synack_type are used when writing synack */
+static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
+				  struct request_sock *req,
+				  struct sk_buff *syn_skb,
+				  enum tcp_synack_type synack_type,
+				  struct tcp_out_options *opts,
+				  unsigned int *remaining)
+{
+	struct bpf_sock_ops_kern sock_ops;
+	int err;
+
+	if (likely(!BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk),
+					   BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG)) ||
+	    !*remaining)
+		return;
+
+	/* *remaining has already been aligned to 4 bytes, so *remaining >= 4 */
+
+	/* init sock_ops */
+	memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
+
+	sock_ops.op = BPF_SOCK_OPS_HDR_OPT_LEN_CB;
+
+	if (req) {
+		/* The listen "sk" cannot be passed here because
+		 * it is not locked.  It would not make too much
+		 * sense to do bpf_setsockopt(listen_sk) based
+		 * on individual connection request also.
+		 *
+		 * Thus, "req" is passed here and the cgroup-bpf-progs
+		 * of the listen "sk" will be run.
+		 *
+		 * "req" is also used here for fastopen even the "sk" here is
+		 * a fullsock "child" sk.  It is to keep the behavior
+		 * consistent between fastopen and non-fastopen on
+		 * the bpf programming side.
+		 */
+		sock_ops.sk = (struct sock *)req;
+		sock_ops.syn_skb = syn_skb;
+	} else {
+		sock_owned_by_me(sk);
+
+		sock_ops.is_fullsock = 1;
+		sock_ops.sk = sk;
+	}
+
+	sock_ops.args[0] = bpf_skops_write_hdr_opt_arg0(skb, synack_type);
+	sock_ops.remaining_opt_len = *remaining;
+	/* tcp_current_mss() does not pass a skb */
+	if (skb)
+		bpf_skops_init_skb(&sock_ops, skb, 0);
+
+	err = BPF_CGROUP_RUN_PROG_SOCK_OPS_SK(&sock_ops, sk);
+
+	if (err || sock_ops.remaining_opt_len == *remaining)
+		return;
+
+	opts->bpf_opt_len = *remaining - sock_ops.remaining_opt_len;
+	/* round up to 4 bytes */
+	opts->bpf_opt_len = (opts->bpf_opt_len + 3) & ~3;
+
+	*remaining -= opts->bpf_opt_len;
+}
+
+static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
+				    struct request_sock *req,
+				    struct sk_buff *syn_skb,
+				    enum tcp_synack_type synack_type,
+				    struct tcp_out_options *opts)
+{
+	u8 first_opt_off, nr_written, max_opt_len = opts->bpf_opt_len;
+	struct bpf_sock_ops_kern sock_ops;
+	int err;
+
+	if (likely(!max_opt_len))
+		return;
+
+	memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
+
+	sock_ops.op = BPF_SOCK_OPS_WRITE_HDR_OPT_CB;
+
+	if (req) {
+		sock_ops.sk = (struct sock *)req;
+		sock_ops.syn_skb = syn_skb;
+	} else {
+		sock_owned_by_me(sk);
+
+		sock_ops.is_fullsock = 1;
+		sock_ops.sk = sk;
+	}
+
+	sock_ops.args[0] = bpf_skops_write_hdr_opt_arg0(skb, synack_type);
+	sock_ops.remaining_opt_len = max_opt_len;
+	first_opt_off = tcp_hdrlen(skb) - max_opt_len;
+	bpf_skops_init_skb(&sock_ops, skb, first_opt_off);
+
+	err = BPF_CGROUP_RUN_PROG_SOCK_OPS_SK(&sock_ops, sk);
+
+	if (err)
+		nr_written = 0;
+	else
+		nr_written = max_opt_len - sock_ops.remaining_opt_len;
+
+	if (nr_written < max_opt_len)
+		memset(skb->data + first_opt_off + nr_written, TCPOPT_NOP,
+		       max_opt_len - nr_written);
+}
+#else
+static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
+				  struct request_sock *req,
+				  struct sk_buff *syn_skb,
+				  enum tcp_synack_type synack_type,
+				  struct tcp_out_options *opts,
+				  unsigned int *remaining)
+{
+}
+
+static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
+				    struct request_sock *req,
+				    struct sk_buff *syn_skb,
+				    enum tcp_synack_type synack_type,
+				    struct tcp_out_options *opts)
+{
+}
+#endif
+
+/* Write previously computed TCP options to the packet.
+ *
+ * Beware: Something in the Internet is very sensitive to the ordering of
+ * TCP options, we learned this through the hard way, so be careful here.
+ * Luckily we can at least blame others for their non-compliance but from
+ * inter-operability perspective it seems that we're somewhat stuck with
+ * the ordering which we have been using if we want to keep working with
+ * those broken things (not that it currently hurts anybody as there isn't
+ * particular reason why the ordering would need to be changed).
+ *
+ * At least SACK_PERM as the first option is known to lead to a disaster
+ * (but it may well be that other scenarios fail similarly).
+ */
+static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
+			      struct tcp_out_options *opts)
+{
+	__be32 *ptr = (__be32 *)(th + 1);
+	u16 options = opts->options;	/* mungable copy */
+
+	if (unlikely(OPTION_MD5 & options)) {
+		*ptr++ = htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16) |
+			       (TCPOPT_MD5SIG << 8) | TCPOLEN_MD5SIG);
+		/* overload cookie hash location */
+		opts->hash_location = (__u8 *)ptr;
+		ptr += 4;
+	}
+
+	if (unlikely(opts->mss)) {
+		*ptr++ = htonl((TCPOPT_MSS << 24) |
+			       (TCPOLEN_MSS << 16) |
+			       opts->mss);
+	}
+
+	if (likely(OPTION_TS & options)) {
+		if (unlikely(OPTION_SACK_ADVERTISE & options)) {
+			*ptr++ = htonl((TCPOPT_SACK_PERM << 24) |
+				       (TCPOLEN_SACK_PERM << 16) |
+				       (TCPOPT_TIMESTAMP << 8) |
+				       TCPOLEN_TIMESTAMP);
+			options &= ~OPTION_SACK_ADVERTISE;
+		} else {
+			*ptr++ = htonl((TCPOPT_NOP << 24) |
+				       (TCPOPT_NOP << 16) |
+				       (TCPOPT_TIMESTAMP << 8) |
+				       TCPOLEN_TIMESTAMP);
+		}
+		*ptr++ = htonl(opts->tsval);
+		*ptr++ = htonl(opts->tsecr);
+	}
+
+	if (unlikely(OPTION_SACK_ADVERTISE & options)) {
+		*ptr++ = htonl((TCPOPT_NOP << 24) |
+			       (TCPOPT_NOP << 16) |
+			       (TCPOPT_SACK_PERM << 8) |
+			       TCPOLEN_SACK_PERM);
+	}
+
+	if (unlikely(OPTION_WSCALE & options)) {
+		*ptr++ = htonl((TCPOPT_NOP << 24) |
+			       (TCPOPT_WINDOW << 16) |
+			       (TCPOLEN_WINDOW << 8) |
+			       opts->ws);
+	}
+
+	if (unlikely(opts->num_sack_blocks)) {
+		struct tcp_sack_block *sp = tp->rx_opt.dsack ?
+			tp->duplicate_sack : tp->selective_acks;
+		int this_sack;
+
+		*ptr++ = htonl((TCPOPT_NOP  << 24) |
+			       (TCPOPT_NOP  << 16) |
+			       (TCPOPT_SACK <<  8) |
+			       (TCPOLEN_SACK_BASE + (opts->num_sack_blocks *
+						     TCPOLEN_SACK_PERBLOCK)));
+
+		for (this_sack = 0; this_sack < opts->num_sack_blocks;
+		     ++this_sack) {
+			*ptr++ = htonl(sp[this_sack].start_seq);
+			*ptr++ = htonl(sp[this_sack].end_seq);
+		}
+
+		tp->rx_opt.dsack = 0;
+	}
+
+	if (unlikely(OPTION_FAST_OPEN_COOKIE & options)) {
+		struct tcp_fastopen_cookie *foc = opts->fastopen_cookie;
+		u8 *p = (u8 *)ptr;
+		u32 len; /* Fast Open option length */
+
+		if (foc->exp) {
+			len = TCPOLEN_EXP_FASTOPEN_BASE + foc->len;
+			*ptr = htonl((TCPOPT_EXP << 24) | (len << 16) |
+				     TCPOPT_FASTOPEN_MAGIC);
+			p += TCPOLEN_EXP_FASTOPEN_BASE;
+		} else {
+			len = TCPOLEN_FASTOPEN_BASE + foc->len;
+			*p++ = TCPOPT_FASTOPEN;
+			*p++ = len;
+		}
+
+		memcpy(p, foc->val, foc->len);
+		if ((len & 3) == 2) {
+			p[foc->len] = TCPOPT_NOP;
+			p[foc->len + 1] = TCPOPT_NOP;
+		}
+		ptr += (len + 3) >> 2;
+	}
+
+	smc_options_write(ptr, &options);
+
+	mptcp_options_write(th, ptr, tp, opts);
+}
+
+static void smc_set_option(const struct tcp_sock *tp,
+			   struct tcp_out_options *opts,
+			   unsigned int *remaining)
+{
+#if IS_ENABLED(CONFIG_SMC)
+	if (static_branch_unlikely(&tcp_have_smc)) {
+		if (tp->syn_smc) {
+			if (*remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
+				opts->options |= OPTION_SMC;
+				*remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
+			}
+		}
+	}
+#endif
+}
+
+static void smc_set_option_cond(const struct tcp_sock *tp,
+				const struct inet_request_sock *ireq,
+				struct tcp_out_options *opts,
+				unsigned int *remaining)
+{
+#if IS_ENABLED(CONFIG_SMC)
+	if (static_branch_unlikely(&tcp_have_smc)) {
+		if (tp->syn_smc && ireq->smc_ok) {
+			if (*remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
+				opts->options |= OPTION_SMC;
+				*remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
+			}
+		}
+	}
+#endif
+}
+
+static void mptcp_set_option_cond(const struct request_sock *req,
+				  struct tcp_out_options *opts,
+				  unsigned int *remaining)
+{
+	if (rsk_is_mptcp(req)) {
+		unsigned int size;
+
+		if (mptcp_synack_options(req, &size, &opts->mptcp)) {
+			if (*remaining >= size) {
+				opts->options |= OPTION_MPTCP;
+				*remaining -= size;
+			}
+		}
+	}
+}
+
+/* Compute TCP options for SYN packets. This is not the final
+ * network wire format yet.
+ */
+static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
+				struct tcp_out_options *opts,
+				struct tcp_md5sig_key **md5)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	unsigned int remaining = MAX_TCP_OPTION_SPACE;
+	struct tcp_fastopen_request *fastopen = tp->fastopen_req;
+
+	*md5 = NULL;
+#ifdef CONFIG_TCP_MD5SIG
+	if (static_branch_unlikely(&tcp_md5_needed.key) &&
+	    rcu_access_pointer(tp->md5sig_info)) {
+		*md5 = tp->af_specific->md5_lookup(sk, sk);
+		if (*md5) {
+			opts->options |= OPTION_MD5;
+			remaining -= TCPOLEN_MD5SIG_ALIGNED;
+		}
+	}
+#endif
+
+	/* We always get an MSS option.  The option bytes which will be seen in
+	 * normal data packets should timestamps be used, must be in the MSS
+	 * advertised.  But we subtract them from tp->mss_cache so that
+	 * calculations in tcp_sendmsg are simpler etc.  So account for this
+	 * fact here if necessary.  If we don't do this correctly, as a
+	 * receiver we won't recognize data packets as being full sized when we
+	 * should, and thus we won't abide by the delayed ACK rules correctly.
+	 * SACKs don't matter, we never delay an ACK when we have any of those
+	 * going out.  */
+	opts->mss = tcp_advertise_mss(sk);
+	remaining -= TCPOLEN_MSS_ALIGNED;
+
+	if (likely(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_timestamps) && !*md5)) {
+		opts->options |= OPTION_TS;
+		opts->tsval = tcp_skb_timestamp(skb) + tp->tsoffset;
+		opts->tsecr = tp->rx_opt.ts_recent;
+		remaining -= TCPOLEN_TSTAMP_ALIGNED;
+	}
+	if (likely(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_window_scaling))) {
+		opts->ws = tp->rx_opt.rcv_wscale;
+		opts->options |= OPTION_WSCALE;
+		remaining -= TCPOLEN_WSCALE_ALIGNED;
+	}
+	if (likely(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_sack))) {
+		opts->options |= OPTION_SACK_ADVERTISE;
+		if (unlikely(!(OPTION_TS & opts->options)))
+			remaining -= TCPOLEN_SACKPERM_ALIGNED;
+	}
+
+	if (fastopen && fastopen->cookie.len >= 0) {
+		u32 need = fastopen->cookie.len;
+
+		need += fastopen->cookie.exp ? TCPOLEN_EXP_FASTOPEN_BASE :
+					       TCPOLEN_FASTOPEN_BASE;
+		need = (need + 3) & ~3U;  /* Align to 32 bits */
+		if (remaining >= need) {
+			opts->options |= OPTION_FAST_OPEN_COOKIE;
+			opts->fastopen_cookie = &fastopen->cookie;
+			remaining -= need;
+			tp->syn_fastopen = 1;
+			tp->syn_fastopen_exp = fastopen->cookie.exp ? 1 : 0;
+		}
+	}
+
+	smc_set_option(tp, opts, &remaining);
+
+	if (sk_is_mptcp(sk)) {
+		unsigned int size;
+
+		if (mptcp_syn_options(sk, skb, &size, &opts->mptcp)) {
+			opts->options |= OPTION_MPTCP;
+			remaining -= size;
+		}
+	}
+
+	bpf_skops_hdr_opt_len(sk, skb, NULL, NULL, 0, opts, &remaining);
+
+	return MAX_TCP_OPTION_SPACE - remaining;
+}
+
+/* Set up TCP options for SYN-ACKs. */
+static unsigned int tcp_synack_options(const struct sock *sk,
+				       struct request_sock *req,
+				       unsigned int mss, struct sk_buff *skb,
+				       struct tcp_out_options *opts,
+				       const struct tcp_md5sig_key *md5,
+				       struct tcp_fastopen_cookie *foc,
+				       enum tcp_synack_type synack_type,
+				       struct sk_buff *syn_skb)
+{
+	struct inet_request_sock *ireq = inet_rsk(req);
+	unsigned int remaining = MAX_TCP_OPTION_SPACE;
+
+#ifdef CONFIG_TCP_MD5SIG
+	if (md5) {
+		opts->options |= OPTION_MD5;
+		remaining -= TCPOLEN_MD5SIG_ALIGNED;
+
+		/* We can't fit any SACK blocks in a packet with MD5 + TS
+		 * options. There was discussion about disabling SACK
+		 * rather than TS in order to fit in better with old,
+		 * buggy kernels, but that was deemed to be unnecessary.
+		 */
+		if (synack_type != TCP_SYNACK_COOKIE)
+			ireq->tstamp_ok &= !ireq->sack_ok;
+	}
+#endif
+
+	/* We always send an MSS option. */
+	opts->mss = mss;
+	remaining -= TCPOLEN_MSS_ALIGNED;
+
+	if (likely(ireq->wscale_ok)) {
+		opts->ws = ireq->rcv_wscale;
+		opts->options |= OPTION_WSCALE;
+		remaining -= TCPOLEN_WSCALE_ALIGNED;
+	}
+	if (likely(ireq->tstamp_ok)) {
+		opts->options |= OPTION_TS;
+		opts->tsval = tcp_skb_timestamp(skb) + tcp_rsk(req)->ts_off;
+		opts->tsecr = READ_ONCE(req->ts_recent);
+		remaining -= TCPOLEN_TSTAMP_ALIGNED;
+	}
+	if (likely(ireq->sack_ok)) {
+		opts->options |= OPTION_SACK_ADVERTISE;
+		if (unlikely(!ireq->tstamp_ok))
+			remaining -= TCPOLEN_SACKPERM_ALIGNED;
+	}
+	if (foc != NULL && foc->len >= 0) {
+		u32 need = foc->len;
+
+		need += foc->exp ? TCPOLEN_EXP_FASTOPEN_BASE :
+				   TCPOLEN_FASTOPEN_BASE;
+		need = (need + 3) & ~3U;  /* Align to 32 bits */
+		if (remaining >= need) {
+			opts->options |= OPTION_FAST_OPEN_COOKIE;
+			opts->fastopen_cookie = foc;
+			remaining -= need;
+		}
+	}
+
+	mptcp_set_option_cond(req, opts, &remaining);
+
+	smc_set_option_cond(tcp_sk(sk), ireq, opts, &remaining);
+
+	bpf_skops_hdr_opt_len((struct sock *)sk, skb, req, syn_skb,
+			      synack_type, opts, &remaining);
+
+	return MAX_TCP_OPTION_SPACE - remaining;
+}
+
+/* Compute TCP options for ESTABLISHED sockets. This is not the
+ * final wire format yet.
+ */
+static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb,
+					struct tcp_out_options *opts,
+					struct tcp_md5sig_key **md5)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	unsigned int size = 0;
+	unsigned int eff_sacks;
+
+	opts->options = 0;
+
+	*md5 = NULL;
+#ifdef CONFIG_TCP_MD5SIG
+	if (static_branch_unlikely(&tcp_md5_needed.key) &&
+	    rcu_access_pointer(tp->md5sig_info)) {
+		*md5 = tp->af_specific->md5_lookup(sk, sk);
+		if (*md5) {
+			opts->options |= OPTION_MD5;
+			size += TCPOLEN_MD5SIG_ALIGNED;
+		}
+	}
+#endif
+
+	if (likely(tp->rx_opt.tstamp_ok)) {
+		opts->options |= OPTION_TS;
+		opts->tsval = skb ? tcp_skb_timestamp(skb) + tp->tsoffset : 0;
+		opts->tsecr = tp->rx_opt.ts_recent;
+		size += TCPOLEN_TSTAMP_ALIGNED;
+	}
+
+	/* MPTCP options have precedence over SACK for the limited TCP
+	 * option space because a MPTCP connection would be forced to
+	 * fall back to regular TCP if a required multipath option is
+	 * missing. SACK still gets a chance to use whatever space is
+	 * left.
+	 */
+	if (sk_is_mptcp(sk)) {
+		unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
+		unsigned int opt_size = 0;
+
+		if (mptcp_established_options(sk, skb, &opt_size, remaining,
+					      &opts->mptcp)) {
+			opts->options |= OPTION_MPTCP;
+			size += opt_size;
+		}
+	}
+
+	eff_sacks = tp->rx_opt.num_sacks + tp->rx_opt.dsack;
+	if (unlikely(eff_sacks)) {
+		const unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
+		if (unlikely(remaining < TCPOLEN_SACK_BASE_ALIGNED +
+					 TCPOLEN_SACK_PERBLOCK))
+			return size;
+
+		opts->num_sack_blocks =
+			min_t(unsigned int, eff_sacks,
+			      (remaining - TCPOLEN_SACK_BASE_ALIGNED) /
+			      TCPOLEN_SACK_PERBLOCK);
+
+		size += TCPOLEN_SACK_BASE_ALIGNED +
+			opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
+	}
+
+	if (unlikely(BPF_SOCK_OPS_TEST_FLAG(tp,
+					    BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG))) {
+		unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
+
+		bpf_skops_hdr_opt_len(sk, skb, NULL, NULL, 0, opts, &remaining);
+
+		size = MAX_TCP_OPTION_SPACE - remaining;
+	}
+
+	return size;
+}
+
+
+/* TCP SMALL QUEUES (TSQ)
+ *
+ * TSQ goal is to keep small amount of skbs per tcp flow in tx queues (qdisc+dev)
+ * to reduce RTT and bufferbloat.
+ * We do this using a special skb destructor (tcp_wfree).
+ *
+ * Its important tcp_wfree() can be replaced by sock_wfree() in the event skb
+ * needs to be reallocated in a driver.
+ * The invariant being skb->truesize subtracted from sk->sk_wmem_alloc
+ *
+ * Since transmit from skb destructor is forbidden, we use a tasklet
+ * to process all sockets that eventually need to send more skbs.
+ * We use one tasklet per cpu, with its own queue of sockets.
+ */
+struct tsq_tasklet {
+	struct tasklet_struct	tasklet;
+	struct list_head	head; /* queue of tcp sockets */
+};
+static DEFINE_PER_CPU(struct tsq_tasklet, tsq_tasklet);
+
+static void tcp_tsq_write(struct sock *sk)
+{
+	if ((1 << sk->sk_state) &
+	    (TCPF_ESTABLISHED | TCPF_FIN_WAIT1 | TCPF_CLOSING |
+	     TCPF_CLOSE_WAIT  | TCPF_LAST_ACK)) {
+		struct tcp_sock *tp = tcp_sk(sk);
+
+		if (tp->lost_out > tp->retrans_out &&
+		    tcp_snd_cwnd(tp) > tcp_packets_in_flight(tp)) {
+			tcp_mstamp_refresh(tp);
+			tcp_xmit_retransmit_queue(sk);
+		}
+
+		tcp_write_xmit(sk, tcp_current_mss(sk), tp->nonagle,
+			       0, GFP_ATOMIC);
+	}
+}
+
+static void tcp_tsq_handler(struct sock *sk)
+{
+	bh_lock_sock(sk);
+	if (!sock_owned_by_user(sk))
+		tcp_tsq_write(sk);
+	else if (!test_and_set_bit(TCP_TSQ_DEFERRED, &sk->sk_tsq_flags))
+		sock_hold(sk);
+	bh_unlock_sock(sk);
+}
+/*
+ * One tasklet per cpu tries to send more skbs.
+ * We run in tasklet context but need to disable irqs when
+ * transferring tsq->head because tcp_wfree() might
+ * interrupt us (non NAPI drivers)
+ */
+static void tcp_tasklet_func(struct tasklet_struct *t)
+{
+	struct tsq_tasklet *tsq = from_tasklet(tsq,  t, tasklet);
+	LIST_HEAD(list);
+	unsigned long flags;
+	struct list_head *q, *n;
+	struct tcp_sock *tp;
+	struct sock *sk;
+
+	local_irq_save(flags);
+	list_splice_init(&tsq->head, &list);
+	local_irq_restore(flags);
+
+	list_for_each_safe(q, n, &list) {
+		tp = list_entry(q, struct tcp_sock, tsq_node);
+		list_del(&tp->tsq_node);
+
+		sk = (struct sock *)tp;
+		smp_mb__before_atomic();
+		clear_bit(TSQ_QUEUED, &sk->sk_tsq_flags);
+
+		tcp_tsq_handler(sk);
+		sk_free(sk);
+	}
+}
+
+#define TCP_DEFERRED_ALL (TCPF_TSQ_DEFERRED |		\
+			  TCPF_WRITE_TIMER_DEFERRED |	\
+			  TCPF_DELACK_TIMER_DEFERRED |	\
+			  TCPF_MTU_REDUCED_DEFERRED)
+/**
+ * tcp_release_cb - tcp release_sock() callback
+ * @sk: socket
+ *
+ * called from release_sock() to perform protocol dependent
+ * actions before socket release.
+ */
+void tcp_release_cb(struct sock *sk)
+{
+	unsigned long flags = smp_load_acquire(&sk->sk_tsq_flags);
+	unsigned long nflags;
+
+	/* perform an atomic operation only if at least one flag is set */
+	do {
+		if (!(flags & TCP_DEFERRED_ALL))
+			return;
+		nflags = flags & ~TCP_DEFERRED_ALL;
+	} while (!try_cmpxchg(&sk->sk_tsq_flags, &flags, nflags));
+
+	if (flags & TCPF_TSQ_DEFERRED) {
+		tcp_tsq_write(sk);
+		__sock_put(sk);
+	}
+	/* Here begins the tricky part :
+	 * We are called from release_sock() with :
+	 * 1) BH disabled
+	 * 2) sk_lock.slock spinlock held
+	 * 3) socket owned by us (sk->sk_lock.owned == 1)
+	 *
+	 * But following code is meant to be called from BH handlers,
+	 * so we should keep BH disabled, but early release socket ownership
+	 */
+	sock_release_ownership(sk);
+
+	if (flags & TCPF_WRITE_TIMER_DEFERRED) {
+		tcp_write_timer_handler(sk);
+		__sock_put(sk);
+	}
+	if (flags & TCPF_DELACK_TIMER_DEFERRED) {
+		tcp_delack_timer_handler(sk);
+		__sock_put(sk);
+	}
+	if (flags & TCPF_MTU_REDUCED_DEFERRED) {
+		inet_csk(sk)->icsk_af_ops->mtu_reduced(sk);
+		__sock_put(sk);
+	}
+}
+EXPORT_SYMBOL(tcp_release_cb);
+
+void __init tcp_tasklet_init(void)
+{
+	int i;
+
+	for_each_possible_cpu(i) {
+		struct tsq_tasklet *tsq = &per_cpu(tsq_tasklet, i);
+
+		INIT_LIST_HEAD(&tsq->head);
+		tasklet_setup(&tsq->tasklet, tcp_tasklet_func);
+	}
+}
+
+/*
+ * Write buffer destructor automatically called from kfree_skb.
+ * We can't xmit new skbs from this context, as we might already
+ * hold qdisc lock.
+ */
+void tcp_wfree(struct sk_buff *skb)
+{
+	struct sock *sk = skb->sk;
+	struct tcp_sock *tp = tcp_sk(sk);
+	unsigned long flags, nval, oval;
+	struct tsq_tasklet *tsq;
+	bool empty;
+
+	/* Keep one reference on sk_wmem_alloc.
+	 * Will be released by sk_free() from here or tcp_tasklet_func()
+	 */
+	WARN_ON(refcount_sub_and_test(skb->truesize - 1, &sk->sk_wmem_alloc));
+
+	/* If this softirq is serviced by ksoftirqd, we are likely under stress.
+	 * Wait until our queues (qdisc + devices) are drained.
+	 * This gives :
+	 * - less callbacks to tcp_write_xmit(), reducing stress (batches)
+	 * - chance for incoming ACK (processed by another cpu maybe)
+	 *   to migrate this flow (skb->ooo_okay will be eventually set)
+	 */
+	if (refcount_read(&sk->sk_wmem_alloc) >= SKB_TRUESIZE(1) && this_cpu_ksoftirqd() == current)
+		goto out;
+
+	oval = smp_load_acquire(&sk->sk_tsq_flags);
+	do {
+		if (!(oval & TSQF_THROTTLED) || (oval & TSQF_QUEUED))
+			goto out;
+
+		nval = (oval & ~TSQF_THROTTLED) | TSQF_QUEUED;
+	} while (!try_cmpxchg(&sk->sk_tsq_flags, &oval, nval));
+
+	/* queue this socket to tasklet queue */
+	local_irq_save(flags);
+	tsq = this_cpu_ptr(&tsq_tasklet);
+	empty = list_empty(&tsq->head);
+	list_add(&tp->tsq_node, &tsq->head);
+	if (empty)
+		tasklet_schedule(&tsq->tasklet);
+	local_irq_restore(flags);
+	return;
+out:
+	sk_free(sk);
+}
+
+/* Note: Called under soft irq.
+ * We can call TCP stack right away, unless socket is owned by user.
+ */
+enum hrtimer_restart tcp_pace_kick(struct hrtimer *timer)
+{
+	struct tcp_sock *tp = container_of(timer, struct tcp_sock, pacing_timer);
+	struct sock *sk = (struct sock *)tp;
+
+	tcp_tsq_handler(sk);
+	sock_put(sk);
+
+	return HRTIMER_NORESTART;
+}
+
+static void tcp_update_skb_after_send(struct sock *sk, struct sk_buff *skb,
+				      u64 prior_wstamp)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (sk->sk_pacing_status != SK_PACING_NONE) {
+		unsigned long rate = sk->sk_pacing_rate;
+
+		/* Original sch_fq does not pace first 10 MSS
+		 * Note that tp->data_segs_out overflows after 2^32 packets,
+		 * this is a minor annoyance.
+		 */
+		if (rate != ~0UL && rate && tp->data_segs_out >= 10) {
+			u64 len_ns = div64_ul((u64)skb->len * NSEC_PER_SEC, rate);
+			u64 credit = tp->tcp_wstamp_ns - prior_wstamp;
+
+			/* take into account OS jitter */
+			len_ns -= min_t(u64, len_ns / 2, credit);
+			tp->tcp_wstamp_ns += len_ns;
+		}
+	}
+	list_move_tail(&skb->tcp_tsorted_anchor, &tp->tsorted_sent_queue);
+}
+
+INDIRECT_CALLABLE_DECLARE(int ip_queue_xmit(struct sock *sk, struct sk_buff *skb, struct flowi *fl));
+INDIRECT_CALLABLE_DECLARE(int inet6_csk_xmit(struct sock *sk, struct sk_buff *skb, struct flowi *fl));
+INDIRECT_CALLABLE_DECLARE(void tcp_v4_send_check(struct sock *sk, struct sk_buff *skb));
+
+/* This routine actually transmits TCP packets queued in by
+ * tcp_do_sendmsg().  This is used by both the initial
+ * transmission and possible later retransmissions.
+ * All SKB's seen here are completely headerless.  It is our
+ * job to build the TCP header, and pass the packet down to
+ * IP so it can do the same plus pass the packet off to the
+ * device.
+ *
+ * We are working here with either a clone of the original
+ * SKB, or a fresh unique copy made by the retransmit engine.
+ */
+static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,
+			      int clone_it, gfp_t gfp_mask, u32 rcv_nxt)
+{
+	const struct inet_connection_sock *icsk = inet_csk(sk);
+	struct inet_sock *inet;
+	struct tcp_sock *tp;
+	struct tcp_skb_cb *tcb;
+	struct tcp_out_options opts;
+	unsigned int tcp_options_size, tcp_header_size;
+	struct sk_buff *oskb = NULL;
+	struct tcp_md5sig_key *md5;
+	struct tcphdr *th;
+	u64 prior_wstamp;
+	int err;
+
+	BUG_ON(!skb || !tcp_skb_pcount(skb));
+	tp = tcp_sk(sk);
+	prior_wstamp = tp->tcp_wstamp_ns;
+	tp->tcp_wstamp_ns = max(tp->tcp_wstamp_ns, tp->tcp_clock_cache);
+	skb_set_delivery_time(skb, tp->tcp_wstamp_ns, true);
+	if (clone_it) {
+		oskb = skb;
+
+		tcp_skb_tsorted_save(oskb) {
+			if (unlikely(skb_cloned(oskb)))
+				skb = pskb_copy(oskb, gfp_mask);
+			else
+				skb = skb_clone(oskb, gfp_mask);
+		} tcp_skb_tsorted_restore(oskb);
+
+		if (unlikely(!skb))
+			return -ENOBUFS;
+		/* retransmit skbs might have a non zero value in skb->dev
+		 * because skb->dev is aliased with skb->rbnode.rb_left
+		 */
+		skb->dev = NULL;
+	}
+
+	inet = inet_sk(sk);
+	tcb = TCP_SKB_CB(skb);
+	memset(&opts, 0, sizeof(opts));
+
+	if (unlikely(tcb->tcp_flags & TCPHDR_SYN)) {
+		tcp_options_size = tcp_syn_options(sk, skb, &opts, &md5);
+	} else {
+		tcp_options_size = tcp_established_options(sk, skb, &opts,
+							   &md5);
+		/* Force a PSH flag on all (GSO) packets to expedite GRO flush
+		 * at receiver : This slightly improve GRO performance.
+		 * Note that we do not force the PSH flag for non GSO packets,
+		 * because they might be sent under high congestion events,
+		 * and in this case it is better to delay the delivery of 1-MSS
+		 * packets and thus the corresponding ACK packet that would
+		 * release the following packet.
+		 */
+		if (tcp_skb_pcount(skb) > 1)
+			tcb->tcp_flags |= TCPHDR_PSH;
+	}
+	tcp_header_size = tcp_options_size + sizeof(struct tcphdr);
+
+	/* We set skb->ooo_okay to one if this packet can select
+	 * a different TX queue than prior packets of this flow,
+	 * to avoid self inflicted reorders.
+	 * The 'other' queue decision is based on current cpu number
+	 * if XPS is enabled, or sk->sk_txhash otherwise.
+	 * We can switch to another (and better) queue if:
+	 * 1) No packet with payload is in qdisc/device queues.
+	 *    Delays in TX completion can defeat the test
+	 *    even if packets were already sent.
+	 * 2) Or rtx queue is empty.
+	 *    This mitigates above case if ACK packets for
+	 *    all prior packets were already processed.
+	 */
+	skb->ooo_okay = sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1) ||
+			tcp_rtx_queue_empty(sk);
+
+	/* If we had to use memory reserve to allocate this skb,
+	 * this might cause drops if packet is looped back :
+	 * Other socket might not have SOCK_MEMALLOC.
+	 * Packets not looped back do not care about pfmemalloc.
+	 */
+	skb->pfmemalloc = 0;
+
+	skb_push(skb, tcp_header_size);
+	skb_reset_transport_header(skb);
+
+	skb_orphan(skb);
+	skb->sk = sk;
+	skb->destructor = skb_is_tcp_pure_ack(skb) ? __sock_wfree : tcp_wfree;
+	refcount_add(skb->truesize, &sk->sk_wmem_alloc);
+
+	skb_set_dst_pending_confirm(skb, READ_ONCE(sk->sk_dst_pending_confirm));
+
+	/* Build TCP header and checksum it. */
+	th = (struct tcphdr *)skb->data;
+	th->source		= inet->inet_sport;
+	th->dest		= inet->inet_dport;
+	th->seq			= htonl(tcb->seq);
+	th->ack_seq		= htonl(rcv_nxt);
+	*(((__be16 *)th) + 6)	= htons(((tcp_header_size >> 2) << 12) |
+					tcb->tcp_flags);
+
+	th->check		= 0;
+	th->urg_ptr		= 0;
+
+	/* The urg_mode check is necessary during a below snd_una win probe */
+	if (unlikely(tcp_urg_mode(tp) && before(tcb->seq, tp->snd_up))) {
+		if (before(tp->snd_up, tcb->seq + 0x10000)) {
+			th->urg_ptr = htons(tp->snd_up - tcb->seq);
+			th->urg = 1;
+		} else if (after(tcb->seq + 0xFFFF, tp->snd_nxt)) {
+			th->urg_ptr = htons(0xFFFF);
+			th->urg = 1;
+		}
+	}
+
+	skb_shinfo(skb)->gso_type = sk->sk_gso_type;
+	if (likely(!(tcb->tcp_flags & TCPHDR_SYN))) {
+		th->window      = htons(tcp_select_window(sk));
+		tcp_ecn_send(sk, skb, th, tcp_header_size);
+	} else {
+		/* RFC1323: The window in SYN & SYN/ACK segments
+		 * is never scaled.
+		 */
+		th->window	= htons(min(tp->rcv_wnd, 65535U));
+	}
+
+	tcp_options_write(th, tp, &opts);
+
+#ifdef CONFIG_TCP_MD5SIG
+	/* Calculate the MD5 hash, as we have all we need now */
+	if (md5) {
+		sk_gso_disable(sk);
+		tp->af_specific->calc_md5_hash(opts.hash_location,
+					       md5, sk, skb);
+	}
+#endif
+
+	/* BPF prog is the last one writing header option */
+	bpf_skops_write_hdr_opt(sk, skb, NULL, NULL, 0, &opts);
+
+	INDIRECT_CALL_INET(icsk->icsk_af_ops->send_check,
+			   tcp_v6_send_check, tcp_v4_send_check,
+			   sk, skb);
+
+	if (likely(tcb->tcp_flags & TCPHDR_ACK))
+		tcp_event_ack_sent(sk, rcv_nxt);
+
+	if (skb->len != tcp_header_size) {
+		tcp_event_data_sent(tp, sk);
+		tp->data_segs_out += tcp_skb_pcount(skb);
+		tp->bytes_sent += skb->len - tcp_header_size;
+	}
+
+	if (after(tcb->end_seq, tp->snd_nxt) || tcb->seq == tcb->end_seq)
+		TCP_ADD_STATS(sock_net(sk), TCP_MIB_OUTSEGS,
+			      tcp_skb_pcount(skb));
+
+	tp->segs_out += tcp_skb_pcount(skb);
+	skb_set_hash_from_sk(skb, sk);
+	/* OK, its time to fill skb_shinfo(skb)->gso_{segs|size} */
+	skb_shinfo(skb)->gso_segs = tcp_skb_pcount(skb);
+	skb_shinfo(skb)->gso_size = tcp_skb_mss(skb);
+
+	/* Leave earliest departure time in skb->tstamp (skb->skb_mstamp_ns) */
+
+	/* Cleanup our debris for IP stacks */
+	memset(skb->cb, 0, max(sizeof(struct inet_skb_parm),
+			       sizeof(struct inet6_skb_parm)));
+
+	tcp_add_tx_delay(skb, tp);
+
+	err = INDIRECT_CALL_INET(icsk->icsk_af_ops->queue_xmit,
+				 inet6_csk_xmit, ip_queue_xmit,
+				 sk, skb, &inet->cork.fl);
+
+	if (unlikely(err > 0)) {
+		tcp_enter_cwr(sk);
+		err = net_xmit_eval(err);
+	}
+	if (!err && oskb) {
+		tcp_update_skb_after_send(sk, oskb, prior_wstamp);
+		tcp_rate_skb_sent(sk, oskb);
+	}
+	return err;
+}
+
+static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
+			    gfp_t gfp_mask)
+{
+	return __tcp_transmit_skb(sk, skb, clone_it, gfp_mask,
+				  tcp_sk(sk)->rcv_nxt);
+}
+
+/* This routine just queues the buffer for sending.
+ *
+ * NOTE: probe0 timer is not checked, do not forget tcp_push_pending_frames,
+ * otherwise socket can stall.
+ */
+static void tcp_queue_skb(struct sock *sk, struct sk_buff *skb)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	/* Advance write_seq and place onto the write_queue. */
+	WRITE_ONCE(tp->write_seq, TCP_SKB_CB(skb)->end_seq);
+	__skb_header_release(skb);
+	tcp_add_write_queue_tail(sk, skb);
+	sk_wmem_queued_add(sk, skb->truesize);
+	sk_mem_charge(sk, skb->truesize);
+}
+
+/* Initialize TSO segments for a packet. */
+static void tcp_set_skb_tso_segs(struct sk_buff *skb, unsigned int mss_now)
+{
+	if (skb->len <= mss_now) {
+		/* Avoid the costly divide in the normal
+		 * non-TSO case.
+		 */
+		tcp_skb_pcount_set(skb, 1);
+		TCP_SKB_CB(skb)->tcp_gso_size = 0;
+	} else {
+		tcp_skb_pcount_set(skb, DIV_ROUND_UP(skb->len, mss_now));
+		TCP_SKB_CB(skb)->tcp_gso_size = mss_now;
+	}
+}
+
+/* Pcount in the middle of the write queue got changed, we need to do various
+ * tweaks to fix counters
+ */
+static void tcp_adjust_pcount(struct sock *sk, const struct sk_buff *skb, int decr)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	tp->packets_out -= decr;
+
+	if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
+		tp->sacked_out -= decr;
+	if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)
+		tp->retrans_out -= decr;
+	if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
+		tp->lost_out -= decr;
+
+	/* Reno case is special. Sigh... */
+	if (tcp_is_reno(tp) && decr > 0)
+		tp->sacked_out -= min_t(u32, tp->sacked_out, decr);
+
+	if (tp->lost_skb_hint &&
+	    before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(tp->lost_skb_hint)->seq) &&
+	    (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
+		tp->lost_cnt_hint -= decr;
+
+	tcp_verify_left_out(tp);
+}
+
+static bool tcp_has_tx_tstamp(const struct sk_buff *skb)
+{
+	return TCP_SKB_CB(skb)->txstamp_ack ||
+		(skb_shinfo(skb)->tx_flags & SKBTX_ANY_TSTAMP);
+}
+
+static void tcp_fragment_tstamp(struct sk_buff *skb, struct sk_buff *skb2)
+{
+	struct skb_shared_info *shinfo = skb_shinfo(skb);
+
+	if (unlikely(tcp_has_tx_tstamp(skb)) &&
+	    !before(shinfo->tskey, TCP_SKB_CB(skb2)->seq)) {
+		struct skb_shared_info *shinfo2 = skb_shinfo(skb2);
+		u8 tsflags = shinfo->tx_flags & SKBTX_ANY_TSTAMP;
+
+		shinfo->tx_flags &= ~tsflags;
+		shinfo2->tx_flags |= tsflags;
+		swap(shinfo->tskey, shinfo2->tskey);
+		TCP_SKB_CB(skb2)->txstamp_ack = TCP_SKB_CB(skb)->txstamp_ack;
+		TCP_SKB_CB(skb)->txstamp_ack = 0;
+	}
+}
+
+static void tcp_skb_fragment_eor(struct sk_buff *skb, struct sk_buff *skb2)
+{
+	TCP_SKB_CB(skb2)->eor = TCP_SKB_CB(skb)->eor;
+	TCP_SKB_CB(skb)->eor = 0;
+}
+
+/* Insert buff after skb on the write or rtx queue of sk.  */
+static void tcp_insert_write_queue_after(struct sk_buff *skb,
+					 struct sk_buff *buff,
+					 struct sock *sk,
+					 enum tcp_queue tcp_queue)
+{
+	if (tcp_queue == TCP_FRAG_IN_WRITE_QUEUE)
+		__skb_queue_after(&sk->sk_write_queue, skb, buff);
+	else
+		tcp_rbtree_insert(&sk->tcp_rtx_queue, buff);
+}
+
+/* Function to create two new TCP segments.  Shrinks the given segment
+ * to the specified size and appends a new segment with the rest of the
+ * packet to the list.  This won't be called frequently, I hope.
+ * Remember, these are still headerless SKBs at this point.
+ */
+int tcp_fragment(struct sock *sk, enum tcp_queue tcp_queue,
+		 struct sk_buff *skb, u32 len,
+		 unsigned int mss_now, gfp_t gfp)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *buff;
+	int old_factor;
+	long limit;
+	int nlen;
+	u8 flags;
+
+	if (WARN_ON(len > skb->len))
+		return -EINVAL;
+
+	DEBUG_NET_WARN_ON_ONCE(skb_headlen(skb));
+
+	/* tcp_sendmsg() can overshoot sk_wmem_queued by one full size skb.
+	 * We need some allowance to not penalize applications setting small
+	 * SO_SNDBUF values.
+	 * Also allow first and last skb in retransmit queue to be split.
+	 */
+	limit = sk->sk_sndbuf + 2 * SKB_TRUESIZE(GSO_LEGACY_MAX_SIZE);
+	if (unlikely((sk->sk_wmem_queued >> 1) > limit &&
+		     tcp_queue != TCP_FRAG_IN_WRITE_QUEUE &&
+		     skb != tcp_rtx_queue_head(sk) &&
+		     skb != tcp_rtx_queue_tail(sk))) {
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPWQUEUETOOBIG);
+		return -ENOMEM;
+	}
+
+	if (skb_unclone_keeptruesize(skb, gfp))
+		return -ENOMEM;
+
+	/* Get a new skb... force flag on. */
+	buff = tcp_stream_alloc_skb(sk, gfp, true);
+	if (!buff)
+		return -ENOMEM; /* We'll just try again later. */
+	skb_copy_decrypted(buff, skb);
+	mptcp_skb_ext_copy(buff, skb);
+
+	sk_wmem_queued_add(sk, buff->truesize);
+	sk_mem_charge(sk, buff->truesize);
+	nlen = skb->len - len;
+	buff->truesize += nlen;
+	skb->truesize -= nlen;
+
+	/* Correct the sequence numbers. */
+	TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
+	TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
+	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;
+
+	/* PSH and FIN should only be set in the second packet. */
+	flags = TCP_SKB_CB(skb)->tcp_flags;
+	TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
+	TCP_SKB_CB(buff)->tcp_flags = flags;
+	TCP_SKB_CB(buff)->sacked = TCP_SKB_CB(skb)->sacked;
+	tcp_skb_fragment_eor(skb, buff);
+
+	skb_split(skb, buff, len);
+
+	skb_set_delivery_time(buff, skb->tstamp, true);
+	tcp_fragment_tstamp(skb, buff);
+
+	old_factor = tcp_skb_pcount(skb);
+
+	/* Fix up tso_factor for both original and new SKB.  */
+	tcp_set_skb_tso_segs(skb, mss_now);
+	tcp_set_skb_tso_segs(buff, mss_now);
+
+	/* Update delivered info for the new segment */
+	TCP_SKB_CB(buff)->tx = TCP_SKB_CB(skb)->tx;
+
+	/* If this packet has been sent out already, we must
+	 * adjust the various packet counters.
+	 */
+	if (!before(tp->snd_nxt, TCP_SKB_CB(buff)->end_seq)) {
+		int diff = old_factor - tcp_skb_pcount(skb) -
+			tcp_skb_pcount(buff);
+
+		if (diff)
+			tcp_adjust_pcount(sk, skb, diff);
+	}
+
+	/* Link BUFF into the send queue. */
+	__skb_header_release(buff);
+	tcp_insert_write_queue_after(skb, buff, sk, tcp_queue);
+	if (tcp_queue == TCP_FRAG_IN_RTX_QUEUE)
+		list_add(&buff->tcp_tsorted_anchor, &skb->tcp_tsorted_anchor);
+
+	return 0;
+}
+
+/* This is similar to __pskb_pull_tail(). The difference is that pulled
+ * data is not copied, but immediately discarded.
+ */
+static int __pskb_trim_head(struct sk_buff *skb, int len)
+{
+	struct skb_shared_info *shinfo;
+	int i, k, eat;
+
+	DEBUG_NET_WARN_ON_ONCE(skb_headlen(skb));
+	eat = len;
+	k = 0;
+	shinfo = skb_shinfo(skb);
+	for (i = 0; i < shinfo->nr_frags; i++) {
+		int size = skb_frag_size(&shinfo->frags[i]);
+
+		if (size <= eat) {
+			skb_frag_unref(skb, i);
+			eat -= size;
+		} else {
+			shinfo->frags[k] = shinfo->frags[i];
+			if (eat) {
+				skb_frag_off_add(&shinfo->frags[k], eat);
+				skb_frag_size_sub(&shinfo->frags[k], eat);
+				eat = 0;
+			}
+			k++;
+		}
+	}
+	shinfo->nr_frags = k;
+
+	skb->data_len -= len;
+	skb->len = skb->data_len;
+	return len;
+}
+
+/* Remove acked data from a packet in the transmit queue. */
+int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
+{
+	u32 delta_truesize;
+
+	if (skb_unclone_keeptruesize(skb, GFP_ATOMIC))
+		return -ENOMEM;
+
+	delta_truesize = __pskb_trim_head(skb, len);
+
+	TCP_SKB_CB(skb)->seq += len;
+
+	skb->truesize	   -= delta_truesize;
+	sk_wmem_queued_add(sk, -delta_truesize);
+	if (!skb_zcopy_pure(skb))
+		sk_mem_uncharge(sk, delta_truesize);
+
+	/* Any change of skb->len requires recalculation of tso factor. */
+	if (tcp_skb_pcount(skb) > 1)
+		tcp_set_skb_tso_segs(skb, tcp_skb_mss(skb));
+
+	return 0;
+}
+
+/* Calculate MSS not accounting any TCP options.  */
+static inline int __tcp_mtu_to_mss(struct sock *sk, int pmtu)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+	const struct inet_connection_sock *icsk = inet_csk(sk);
+	int mss_now;
+
+	/* Calculate base mss without TCP options:
+	   It is MMS_S - sizeof(tcphdr) of rfc1122
+	 */
+	mss_now = pmtu - icsk->icsk_af_ops->net_header_len - sizeof(struct tcphdr);
+
+	/* IPv6 adds a frag_hdr in case RTAX_FEATURE_ALLFRAG is set */
+	if (icsk->icsk_af_ops->net_frag_header_len) {
+		const struct dst_entry *dst = __sk_dst_get(sk);
+
+		if (dst && dst_allfrag(dst))
+			mss_now -= icsk->icsk_af_ops->net_frag_header_len;
+	}
+
+	/* Clamp it (mss_clamp does not include tcp options) */
+	if (mss_now > tp->rx_opt.mss_clamp)
+		mss_now = tp->rx_opt.mss_clamp;
+
+	/* Now subtract optional transport overhead */
+	mss_now -= icsk->icsk_ext_hdr_len;
+
+	/* Then reserve room for full set of TCP options and 8 bytes of data */
+	mss_now = max(mss_now,
+		      READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_min_snd_mss));
+	return mss_now;
+}
+
+/* Calculate MSS. Not accounting for SACKs here.  */
+int tcp_mtu_to_mss(struct sock *sk, int pmtu)
+{
+	/* Subtract TCP options size, not including SACKs */
+	return __tcp_mtu_to_mss(sk, pmtu) -
+	       (tcp_sk(sk)->tcp_header_len - sizeof(struct tcphdr));
+}
+EXPORT_SYMBOL(tcp_mtu_to_mss);
+
+/* Inverse of above */
+int tcp_mss_to_mtu(struct sock *sk, int mss)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+	const struct inet_connection_sock *icsk = inet_csk(sk);
+	int mtu;
+
+	mtu = mss +
+	      tp->tcp_header_len +
+	      icsk->icsk_ext_hdr_len +
+	      icsk->icsk_af_ops->net_header_len;
+
+	/* IPv6 adds a frag_hdr in case RTAX_FEATURE_ALLFRAG is set */
+	if (icsk->icsk_af_ops->net_frag_header_len) {
+		const struct dst_entry *dst = __sk_dst_get(sk);
+
+		if (dst && dst_allfrag(dst))
+			mtu += icsk->icsk_af_ops->net_frag_header_len;
+	}
+	return mtu;
+}
+EXPORT_SYMBOL(tcp_mss_to_mtu);
+
+/* MTU probing init per socket */
+void tcp_mtup_init(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct net *net = sock_net(sk);
+
+	icsk->icsk_mtup.enabled = READ_ONCE(net->ipv4.sysctl_tcp_mtu_probing) > 1;
+	icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp + sizeof(struct tcphdr) +
+			       icsk->icsk_af_ops->net_header_len;
+	icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, READ_ONCE(net->ipv4.sysctl_tcp_base_mss));
+	icsk->icsk_mtup.probe_size = 0;
+	if (icsk->icsk_mtup.enabled)
+		icsk->icsk_mtup.probe_timestamp = tcp_jiffies32;
+}
+EXPORT_SYMBOL(tcp_mtup_init);
+
+/* This function synchronize snd mss to current pmtu/exthdr set.
+
+   tp->rx_opt.user_mss is mss set by user by TCP_MAXSEG. It does NOT counts
+   for TCP options, but includes only bare TCP header.
+
+   tp->rx_opt.mss_clamp is mss negotiated at connection setup.
+   It is minimum of user_mss and mss received with SYN.
+   It also does not include TCP options.
+
+   inet_csk(sk)->icsk_pmtu_cookie is last pmtu, seen by this function.
+
+   tp->mss_cache is current effective sending mss, including
+   all tcp options except for SACKs. It is evaluated,
+   taking into account current pmtu, but never exceeds
+   tp->rx_opt.mss_clamp.
+
+   NOTE1. rfc1122 clearly states that advertised MSS
+   DOES NOT include either tcp or ip options.
+
+   NOTE2. inet_csk(sk)->icsk_pmtu_cookie and tp->mss_cache
+   are READ ONLY outside this function.		--ANK (980731)
+ */
+unsigned int tcp_sync_mss(struct sock *sk, u32 pmtu)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	int mss_now;
+
+	if (icsk->icsk_mtup.search_high > pmtu)
+		icsk->icsk_mtup.search_high = pmtu;
+
+	mss_now = tcp_mtu_to_mss(sk, pmtu);
+	mss_now = tcp_bound_to_half_wnd(tp, mss_now);
+
+	/* And store cached results */
+	icsk->icsk_pmtu_cookie = pmtu;
+	if (icsk->icsk_mtup.enabled)
+		mss_now = min(mss_now, tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low));
+	tp->mss_cache = mss_now;
+
+	return mss_now;
+}
+EXPORT_SYMBOL(tcp_sync_mss);
+
+/* Compute the current effective MSS, taking SACKs and IP options,
+ * and even PMTU discovery events into account.
+ */
+unsigned int tcp_current_mss(struct sock *sk)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+	const struct dst_entry *dst = __sk_dst_get(sk);
+	u32 mss_now;
+	unsigned int header_len;
+	struct tcp_out_options opts;
+	struct tcp_md5sig_key *md5;
+
+	mss_now = tp->mss_cache;
+
+	if (dst) {
+		u32 mtu = dst_mtu(dst);
+		if (mtu != inet_csk(sk)->icsk_pmtu_cookie)
+			mss_now = tcp_sync_mss(sk, mtu);
+	}
+
+	header_len = tcp_established_options(sk, NULL, &opts, &md5) +
+		     sizeof(struct tcphdr);
+	/* The mss_cache is sized based on tp->tcp_header_len, which assumes
+	 * some common options. If this is an odd packet (because we have SACK
+	 * blocks etc) then our calculated header_len will be different, and
+	 * we have to adjust mss_now correspondingly */
+	if (header_len != tp->tcp_header_len) {
+		int delta = (int) header_len - tp->tcp_header_len;
+		mss_now -= delta;
+	}
+
+	return mss_now;
+}
+
+/* RFC2861, slow part. Adjust cwnd, after it was not full during one rto.
+ * As additional protections, we do not touch cwnd in retransmission phases,
+ * and if application hit its sndbuf limit recently.
+ */
+static void tcp_cwnd_application_limited(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (inet_csk(sk)->icsk_ca_state == TCP_CA_Open &&
+	    sk->sk_socket && !test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
+		/* Limited by application or receiver window. */
+		u32 init_win = tcp_init_cwnd(tp, __sk_dst_get(sk));
+		u32 win_used = max(tp->snd_cwnd_used, init_win);
+		if (win_used < tcp_snd_cwnd(tp)) {
+			tp->snd_ssthresh = tcp_current_ssthresh(sk);
+			tcp_snd_cwnd_set(tp, (tcp_snd_cwnd(tp) + win_used) >> 1);
+		}
+		tp->snd_cwnd_used = 0;
+	}
+	tp->snd_cwnd_stamp = tcp_jiffies32;
+}
+
+static void tcp_cwnd_validate(struct sock *sk, bool is_cwnd_limited)
+{
+	const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	/* Track the strongest available signal of the degree to which the cwnd
+	 * is fully utilized. If cwnd-limited then remember that fact for the
+	 * current window. If not cwnd-limited then track the maximum number of
+	 * outstanding packets in the current window. (If cwnd-limited then we
+	 * chose to not update tp->max_packets_out to avoid an extra else
+	 * clause with no functional impact.)
+	 */
+	if (!before(tp->snd_una, tp->cwnd_usage_seq) ||
+	    is_cwnd_limited ||
+	    (!tp->is_cwnd_limited &&
+	     tp->packets_out > tp->max_packets_out)) {
+		tp->is_cwnd_limited = is_cwnd_limited;
+		tp->max_packets_out = tp->packets_out;
+		tp->cwnd_usage_seq = tp->snd_nxt;
+	}
+
+	if (tcp_is_cwnd_limited(sk)) {
+		/* Network is feed fully. */
+		tp->snd_cwnd_used = 0;
+		tp->snd_cwnd_stamp = tcp_jiffies32;
+	} else {
+		/* Network starves. */
+		if (tp->packets_out > tp->snd_cwnd_used)
+			tp->snd_cwnd_used = tp->packets_out;
+
+		if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_slow_start_after_idle) &&
+		    (s32)(tcp_jiffies32 - tp->snd_cwnd_stamp) >= inet_csk(sk)->icsk_rto &&
+		    !ca_ops->cong_control)
+			tcp_cwnd_application_limited(sk);
+
+		/* The following conditions together indicate the starvation
+		 * is caused by insufficient sender buffer:
+		 * 1) just sent some data (see tcp_write_xmit)
+		 * 2) not cwnd limited (this else condition)
+		 * 3) no more data to send (tcp_write_queue_empty())
+		 * 4) application is hitting buffer limit (SOCK_NOSPACE)
+		 */
+		if (tcp_write_queue_empty(sk) && sk->sk_socket &&
+		    test_bit(SOCK_NOSPACE, &sk->sk_socket->flags) &&
+		    (1 << sk->sk_state) & (TCPF_ESTABLISHED | TCPF_CLOSE_WAIT))
+			tcp_chrono_start(sk, TCP_CHRONO_SNDBUF_LIMITED);
+	}
+}
+
+/* Minshall's variant of the Nagle send check. */
+static bool tcp_minshall_check(const struct tcp_sock *tp)
+{
+	return after(tp->snd_sml, tp->snd_una) &&
+		!after(tp->snd_sml, tp->snd_nxt);
+}
+
+/* Update snd_sml if this skb is under mss
+ * Note that a TSO packet might end with a sub-mss segment
+ * The test is really :
+ * if ((skb->len % mss) != 0)
+ *        tp->snd_sml = TCP_SKB_CB(skb)->end_seq;
+ * But we can avoid doing the divide again given we already have
+ *  skb_pcount = skb->len / mss_now
+ */
+static void tcp_minshall_update(struct tcp_sock *tp, unsigned int mss_now,
+				const struct sk_buff *skb)
+{
+	if (skb->len < tcp_skb_pcount(skb) * mss_now)
+		tp->snd_sml = TCP_SKB_CB(skb)->end_seq;
+}
+
+/* Return false, if packet can be sent now without violation Nagle's rules:
+ * 1. It is full sized. (provided by caller in %partial bool)
+ * 2. Or it contains FIN. (already checked by caller)
+ * 3. Or TCP_CORK is not set, and TCP_NODELAY is set.
+ * 4. Or TCP_CORK is not set, and all sent packets are ACKed.
+ *    With Minshall's modification: all sent small packets are ACKed.
+ */
+static bool tcp_nagle_check(bool partial, const struct tcp_sock *tp,
+			    int nonagle)
+{
+	return partial &&
+		((nonagle & TCP_NAGLE_CORK) ||
+		 (!nonagle && tp->packets_out && tcp_minshall_check(tp)));
+}
+
+/* Return how many segs we'd like on a TSO packet,
+ * depending on current pacing rate, and how close the peer is.
+ *
+ * Rationale is:
+ * - For close peers, we rather send bigger packets to reduce
+ *   cpu costs, because occasional losses will be repaired fast.
+ * - For long distance/rtt flows, we would like to get ACK clocking
+ *   with 1 ACK per ms.
+ *
+ * Use min_rtt to help adapt TSO burst size, with smaller min_rtt resulting
+ * in bigger TSO bursts. We we cut the RTT-based allowance in half
+ * for every 2^9 usec (aka 512 us) of RTT, so that the RTT-based allowance
+ * is below 1500 bytes after 6 * ~500 usec = 3ms.
+ */
+static u32 tcp_tso_autosize(const struct sock *sk, unsigned int mss_now,
+			    int min_tso_segs)
+{
+	unsigned long bytes;
+	u32 r;
+
+	bytes = sk->sk_pacing_rate >> READ_ONCE(sk->sk_pacing_shift);
+
+	r = tcp_min_rtt(tcp_sk(sk)) >> READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_tso_rtt_log);
+	if (r < BITS_PER_TYPE(sk->sk_gso_max_size))
+		bytes += sk->sk_gso_max_size >> r;
+
+	bytes = min_t(unsigned long, bytes, sk->sk_gso_max_size);
+
+	return max_t(u32, bytes / mss_now, min_tso_segs);
+}
+
+/* Return the number of segments we want in the skb we are transmitting.
+ * See if congestion control module wants to decide; otherwise, autosize.
+ */
+static u32 tcp_tso_segs(struct sock *sk, unsigned int mss_now)
+{
+	const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
+	u32 min_tso, tso_segs;
+
+	min_tso = ca_ops->min_tso_segs ?
+			ca_ops->min_tso_segs(sk) :
+			READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_min_tso_segs);
+
+	tso_segs = tcp_tso_autosize(sk, mss_now, min_tso);
+	return min_t(u32, tso_segs, sk->sk_gso_max_segs);
+}
+
+/* Returns the portion of skb which can be sent right away */
+static unsigned int tcp_mss_split_point(const struct sock *sk,
+					const struct sk_buff *skb,
+					unsigned int mss_now,
+					unsigned int max_segs,
+					int nonagle)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+	u32 partial, needed, window, max_len;
+
+	window = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
+	max_len = mss_now * max_segs;
+
+	if (likely(max_len <= window && skb != tcp_write_queue_tail(sk)))
+		return max_len;
+
+	needed = min(skb->len, window);
+
+	if (max_len <= needed)
+		return max_len;
+
+	partial = needed % mss_now;
+	/* If last segment is not a full MSS, check if Nagle rules allow us
+	 * to include this last segment in this skb.
+	 * Otherwise, we'll split the skb at last MSS boundary
+	 */
+	if (tcp_nagle_check(partial != 0, tp, nonagle))
+		return needed - partial;
+
+	return needed;
+}
+
+/* Can at least one segment of SKB be sent right now, according to the
+ * congestion window rules?  If so, return how many segments are allowed.
+ */
+static inline unsigned int tcp_cwnd_test(const struct tcp_sock *tp,
+					 const struct sk_buff *skb)
+{
+	u32 in_flight, cwnd, halfcwnd;
+
+	/* Don't be strict about the congestion window for the final FIN.  */
+	if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) &&
+	    tcp_skb_pcount(skb) == 1)
+		return 1;
+
+	in_flight = tcp_packets_in_flight(tp);
+	cwnd = tcp_snd_cwnd(tp);
+	if (in_flight >= cwnd)
+		return 0;
+
+	/* For better scheduling, ensure we have at least
+	 * 2 GSO packets in flight.
+	 */
+	halfcwnd = max(cwnd >> 1, 1U);
+	return min(halfcwnd, cwnd - in_flight);
+}
+
+/* Initialize TSO state of a skb.
+ * This must be invoked the first time we consider transmitting
+ * SKB onto the wire.
+ */
+static int tcp_init_tso_segs(struct sk_buff *skb, unsigned int mss_now)
+{
+	int tso_segs = tcp_skb_pcount(skb);
+
+	if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now)) {
+		tcp_set_skb_tso_segs(skb, mss_now);
+		tso_segs = tcp_skb_pcount(skb);
+	}
+	return tso_segs;
+}
+
+
+/* Return true if the Nagle test allows this packet to be
+ * sent now.
+ */
+static inline bool tcp_nagle_test(const struct tcp_sock *tp, const struct sk_buff *skb,
+				  unsigned int cur_mss, int nonagle)
+{
+	/* Nagle rule does not apply to frames, which sit in the middle of the
+	 * write_queue (they have no chances to get new data).
+	 *
+	 * This is implemented in the callers, where they modify the 'nonagle'
+	 * argument based upon the location of SKB in the send queue.
+	 */
+	if (nonagle & TCP_NAGLE_PUSH)
+		return true;
+
+	/* Don't use the nagle rule for urgent data (or for the final FIN). */
+	if (tcp_urg_mode(tp) || (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN))
+		return true;
+
+	if (!tcp_nagle_check(skb->len < cur_mss, tp, nonagle))
+		return true;
+
+	return false;
+}
+
+/* Does at least the first segment of SKB fit into the send window? */
+static bool tcp_snd_wnd_test(const struct tcp_sock *tp,
+			     const struct sk_buff *skb,
+			     unsigned int cur_mss)
+{
+	u32 end_seq = TCP_SKB_CB(skb)->end_seq;
+
+	if (skb->len > cur_mss)
+		end_seq = TCP_SKB_CB(skb)->seq + cur_mss;
+
+	return !after(end_seq, tcp_wnd_end(tp));
+}
+
+/* Trim TSO SKB to LEN bytes, put the remaining data into a new packet
+ * which is put after SKB on the list.  It is very much like
+ * tcp_fragment() except that it may make several kinds of assumptions
+ * in order to speed up the splitting operation.  In particular, we
+ * know that all the data is in scatter-gather pages, and that the
+ * packet has never been sent out before (and thus is not cloned).
+ */
+static int tso_fragment(struct sock *sk, struct sk_buff *skb, unsigned int len,
+			unsigned int mss_now, gfp_t gfp)
+{
+	int nlen = skb->len - len;
+	struct sk_buff *buff;
+	u8 flags;
+
+	/* All of a TSO frame must be composed of paged data.  */
+	DEBUG_NET_WARN_ON_ONCE(skb->len != skb->data_len);
+
+	buff = tcp_stream_alloc_skb(sk, gfp, true);
+	if (unlikely(!buff))
+		return -ENOMEM;
+	skb_copy_decrypted(buff, skb);
+	mptcp_skb_ext_copy(buff, skb);
+
+	sk_wmem_queued_add(sk, buff->truesize);
+	sk_mem_charge(sk, buff->truesize);
+	buff->truesize += nlen;
+	skb->truesize -= nlen;
+
+	/* Correct the sequence numbers. */
+	TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
+	TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
+	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;
+
+	/* PSH and FIN should only be set in the second packet. */
+	flags = TCP_SKB_CB(skb)->tcp_flags;
+	TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
+	TCP_SKB_CB(buff)->tcp_flags = flags;
+
+	tcp_skb_fragment_eor(skb, buff);
+
+	skb_split(skb, buff, len);
+	tcp_fragment_tstamp(skb, buff);
+
+	/* Fix up tso_factor for both original and new SKB.  */
+	tcp_set_skb_tso_segs(skb, mss_now);
+	tcp_set_skb_tso_segs(buff, mss_now);
+
+	/* Link BUFF into the send queue. */
+	__skb_header_release(buff);
+	tcp_insert_write_queue_after(skb, buff, sk, TCP_FRAG_IN_WRITE_QUEUE);
+
+	return 0;
+}
+
+/* Try to defer sending, if possible, in order to minimize the amount
+ * of TSO splitting we do.  View it as a kind of TSO Nagle test.
+ *
+ * This algorithm is from John Heffner.
+ */
+static bool tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb,
+				 bool *is_cwnd_limited,
+				 bool *is_rwnd_limited,
+				 u32 max_segs)
+{
+	const struct inet_connection_sock *icsk = inet_csk(sk);
+	u32 send_win, cong_win, limit, in_flight;
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *head;
+	int win_divisor;
+	s64 delta;
+
+	if (icsk->icsk_ca_state >= TCP_CA_Recovery)
+		goto send_now;
+
+	/* Avoid bursty behavior by allowing defer
+	 * only if the last write was recent (1 ms).
+	 * Note that tp->tcp_wstamp_ns can be in the future if we have
+	 * packets waiting in a qdisc or device for EDT delivery.
+	 */
+	delta = tp->tcp_clock_cache - tp->tcp_wstamp_ns - NSEC_PER_MSEC;
+	if (delta > 0)
+		goto send_now;
+
+	in_flight = tcp_packets_in_flight(tp);
+
+	BUG_ON(tcp_skb_pcount(skb) <= 1);
+	BUG_ON(tcp_snd_cwnd(tp) <= in_flight);
+
+	send_win = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
+
+	/* From in_flight test above, we know that cwnd > in_flight.  */
+	cong_win = (tcp_snd_cwnd(tp) - in_flight) * tp->mss_cache;
+
+	limit = min(send_win, cong_win);
+
+	/* If a full-sized TSO skb can be sent, do it. */
+	if (limit >= max_segs * tp->mss_cache)
+		goto send_now;
+
+	/* Middle in queue won't get any more data, full sendable already? */
+	if ((skb != tcp_write_queue_tail(sk)) && (limit >= skb->len))
+		goto send_now;
+
+	win_divisor = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_tso_win_divisor);
+	if (win_divisor) {
+		u32 chunk = min(tp->snd_wnd, tcp_snd_cwnd(tp) * tp->mss_cache);
+
+		/* If at least some fraction of a window is available,
+		 * just use it.
+		 */
+		chunk /= win_divisor;
+		if (limit >= chunk)
+			goto send_now;
+	} else {
+		/* Different approach, try not to defer past a single
+		 * ACK.  Receiver should ACK every other full sized
+		 * frame, so if we have space for more than 3 frames
+		 * then send now.
+		 */
+		if (limit > tcp_max_tso_deferred_mss(tp) * tp->mss_cache)
+			goto send_now;
+	}
+
+	/* TODO : use tsorted_sent_queue ? */
+	head = tcp_rtx_queue_head(sk);
+	if (!head)
+		goto send_now;
+	delta = tp->tcp_clock_cache - head->tstamp;
+	/* If next ACK is likely to come too late (half srtt), do not defer */
+	if ((s64)(delta - (u64)NSEC_PER_USEC * (tp->srtt_us >> 4)) < 0)
+		goto send_now;
+
+	/* Ok, it looks like it is advisable to defer.
+	 * Three cases are tracked :
+	 * 1) We are cwnd-limited
+	 * 2) We are rwnd-limited
+	 * 3) We are application limited.
+	 */
+	if (cong_win < send_win) {
+		if (cong_win <= skb->len) {
+			*is_cwnd_limited = true;
+			return true;
+		}
+	} else {
+		if (send_win <= skb->len) {
+			*is_rwnd_limited = true;
+			return true;
+		}
+	}
+
+	/* If this packet won't get more data, do not wait. */
+	if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) ||
+	    TCP_SKB_CB(skb)->eor)
+		goto send_now;
+
+	return true;
+
+send_now:
+	return false;
+}
+
+static inline void tcp_mtu_check_reprobe(struct sock *sk)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct net *net = sock_net(sk);
+	u32 interval;
+	s32 delta;
+
+	interval = READ_ONCE(net->ipv4.sysctl_tcp_probe_interval);
+	delta = tcp_jiffies32 - icsk->icsk_mtup.probe_timestamp;
+	if (unlikely(delta >= interval * HZ)) {
+		int mss = tcp_current_mss(sk);
+
+		/* Update current search range */
+		icsk->icsk_mtup.probe_size = 0;
+		icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp +
+			sizeof(struct tcphdr) +
+			icsk->icsk_af_ops->net_header_len;
+		icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
+
+		/* Update probe time stamp */
+		icsk->icsk_mtup.probe_timestamp = tcp_jiffies32;
+	}
+}
+
+static bool tcp_can_coalesce_send_queue_head(struct sock *sk, int len)
+{
+	struct sk_buff *skb, *next;
+
+	skb = tcp_send_head(sk);
+	tcp_for_write_queue_from_safe(skb, next, sk) {
+		if (len <= skb->len)
+			break;
+
+		if (unlikely(TCP_SKB_CB(skb)->eor) ||
+		    tcp_has_tx_tstamp(skb) ||
+		    !skb_pure_zcopy_same(skb, next))
+			return false;
+
+		len -= skb->len;
+	}
+
+	return true;
+}
+
+static int tcp_clone_payload(struct sock *sk, struct sk_buff *to,
+			     int probe_size)
+{
+	skb_frag_t *lastfrag = NULL, *fragto = skb_shinfo(to)->frags;
+	int i, todo, len = 0, nr_frags = 0;
+	const struct sk_buff *skb;
+
+	if (!sk_wmem_schedule(sk, to->truesize + probe_size))
+		return -ENOMEM;
+
+	skb_queue_walk(&sk->sk_write_queue, skb) {
+		const skb_frag_t *fragfrom = skb_shinfo(skb)->frags;
+
+		if (skb_headlen(skb))
+			return -EINVAL;
+
+		for (i = 0; i < skb_shinfo(skb)->nr_frags; i++, fragfrom++) {
+			if (len >= probe_size)
+				goto commit;
+			todo = min_t(int, skb_frag_size(fragfrom),
+				     probe_size - len);
+			len += todo;
+			if (lastfrag &&
+			    skb_frag_page(fragfrom) == skb_frag_page(lastfrag) &&
+			    skb_frag_off(fragfrom) == skb_frag_off(lastfrag) +
+						      skb_frag_size(lastfrag)) {
+				skb_frag_size_add(lastfrag, todo);
+				continue;
+			}
+			if (unlikely(nr_frags == MAX_SKB_FRAGS))
+				return -E2BIG;
+			skb_frag_page_copy(fragto, fragfrom);
+			skb_frag_off_copy(fragto, fragfrom);
+			skb_frag_size_set(fragto, todo);
+			nr_frags++;
+			lastfrag = fragto++;
+		}
+	}
+commit:
+	WARN_ON_ONCE(len != probe_size);
+	for (i = 0; i < nr_frags; i++)
+		skb_frag_ref(to, i);
+
+	skb_shinfo(to)->nr_frags = nr_frags;
+	to->truesize += probe_size;
+	to->len += probe_size;
+	to->data_len += probe_size;
+	__skb_header_release(to);
+	return 0;
+}
+
+/* Create a new MTU probe if we are ready.
+ * MTU probe is regularly attempting to increase the path MTU by
+ * deliberately sending larger packets.  This discovers routing
+ * changes resulting in larger path MTUs.
+ *
+ * Returns 0 if we should wait to probe (no cwnd available),
+ *         1 if a probe was sent,
+ *         -1 otherwise
+ */
+static int tcp_mtu_probe(struct sock *sk)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *skb, *nskb, *next;
+	struct net *net = sock_net(sk);
+	int probe_size;
+	int size_needed;
+	int copy, len;
+	int mss_now;
+	int interval;
+
+	/* Not currently probing/verifying,
+	 * not in recovery,
+	 * have enough cwnd, and
+	 * not SACKing (the variable headers throw things off)
+	 */
+	if (likely(!icsk->icsk_mtup.enabled ||
+		   icsk->icsk_mtup.probe_size ||
+		   inet_csk(sk)->icsk_ca_state != TCP_CA_Open ||
+		   tcp_snd_cwnd(tp) < 11 ||
+		   tp->rx_opt.num_sacks || tp->rx_opt.dsack))
+		return -1;
+
+	/* Use binary search for probe_size between tcp_mss_base,
+	 * and current mss_clamp. if (search_high - search_low)
+	 * smaller than a threshold, backoff from probing.
+	 */
+	mss_now = tcp_current_mss(sk);
+	probe_size = tcp_mtu_to_mss(sk, (icsk->icsk_mtup.search_high +
+				    icsk->icsk_mtup.search_low) >> 1);
+	size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
+	interval = icsk->icsk_mtup.search_high - icsk->icsk_mtup.search_low;
+	/* When misfortune happens, we are reprobing actively,
+	 * and then reprobe timer has expired. We stick with current
+	 * probing process by not resetting search range to its orignal.
+	 */
+	if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high) ||
+	    interval < READ_ONCE(net->ipv4.sysctl_tcp_probe_threshold)) {
+		/* Check whether enough time has elaplased for
+		 * another round of probing.
+		 */
+		tcp_mtu_check_reprobe(sk);
+		return -1;
+	}
+
+	/* Have enough data in the send queue to probe? */
+	if (tp->write_seq - tp->snd_nxt < size_needed)
+		return -1;
+
+	if (tp->snd_wnd < size_needed)
+		return -1;
+	if (after(tp->snd_nxt + size_needed, tcp_wnd_end(tp)))
+		return 0;
+
+	/* Do we need to wait to drain cwnd? With none in flight, don't stall */
+	if (tcp_packets_in_flight(tp) + 2 > tcp_snd_cwnd(tp)) {
+		if (!tcp_packets_in_flight(tp))
+			return -1;
+		else
+			return 0;
+	}
+
+	if (!tcp_can_coalesce_send_queue_head(sk, probe_size))
+		return -1;
+
+	/* We're allowed to probe.  Build it now. */
+	nskb = tcp_stream_alloc_skb(sk, GFP_ATOMIC, false);
+	if (!nskb)
+		return -1;
+
+	/* build the payload, and be prepared to abort if this fails. */
+	if (tcp_clone_payload(sk, nskb, probe_size)) {
+		tcp_skb_tsorted_anchor_cleanup(nskb);
+		consume_skb(nskb);
+		return -1;
+	}
+	sk_wmem_queued_add(sk, nskb->truesize);
+	sk_mem_charge(sk, nskb->truesize);
+
+	skb = tcp_send_head(sk);
+	skb_copy_decrypted(nskb, skb);
+	mptcp_skb_ext_copy(nskb, skb);
+
+	TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(skb)->seq;
+	TCP_SKB_CB(nskb)->end_seq = TCP_SKB_CB(skb)->seq + probe_size;
+	TCP_SKB_CB(nskb)->tcp_flags = TCPHDR_ACK;
+
+	tcp_insert_write_queue_before(nskb, skb, sk);
+	tcp_highest_sack_replace(sk, skb, nskb);
+
+	len = 0;
+	tcp_for_write_queue_from_safe(skb, next, sk) {
+		copy = min_t(int, skb->len, probe_size - len);
+
+		if (skb->len <= copy) {
+			/* We've eaten all the data from this skb.
+			 * Throw it away. */
+			TCP_SKB_CB(nskb)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags;
+			/* If this is the last SKB we copy and eor is set
+			 * we need to propagate it to the new skb.
+			 */
+			TCP_SKB_CB(nskb)->eor = TCP_SKB_CB(skb)->eor;
+			tcp_skb_collapse_tstamp(nskb, skb);
+			tcp_unlink_write_queue(skb, sk);
+			tcp_wmem_free_skb(sk, skb);
+		} else {
+			TCP_SKB_CB(nskb)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags &
+						   ~(TCPHDR_FIN|TCPHDR_PSH);
+			__pskb_trim_head(skb, copy);
+			tcp_set_skb_tso_segs(skb, mss_now);
+			TCP_SKB_CB(skb)->seq += copy;
+		}
+
+		len += copy;
+
+		if (len >= probe_size)
+			break;
+	}
+	tcp_init_tso_segs(nskb, nskb->len);
+
+	/* We're ready to send.  If this fails, the probe will
+	 * be resegmented into mss-sized pieces by tcp_write_xmit().
+	 */
+	if (!tcp_transmit_skb(sk, nskb, 1, GFP_ATOMIC)) {
+		/* Decrement cwnd here because we are sending
+		 * effectively two packets. */
+		tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) - 1);
+		tcp_event_new_data_sent(sk, nskb);
+
+		icsk->icsk_mtup.probe_size = tcp_mss_to_mtu(sk, nskb->len);
+		tp->mtu_probe.probe_seq_start = TCP_SKB_CB(nskb)->seq;
+		tp->mtu_probe.probe_seq_end = TCP_SKB_CB(nskb)->end_seq;
+
+		return 1;
+	}
+
+	return -1;
+}
+
+static bool tcp_pacing_check(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (!tcp_needs_internal_pacing(sk))
+		return false;
+
+	if (tp->tcp_wstamp_ns <= tp->tcp_clock_cache)
+		return false;
+
+	if (!hrtimer_is_queued(&tp->pacing_timer)) {
+		hrtimer_start(&tp->pacing_timer,
+			      ns_to_ktime(tp->tcp_wstamp_ns),
+			      HRTIMER_MODE_ABS_PINNED_SOFT);
+		sock_hold(sk);
+	}
+	return true;
+}
+
+static bool tcp_rtx_queue_empty_or_single_skb(const struct sock *sk)
+{
+	const struct rb_node *node = sk->tcp_rtx_queue.rb_node;
+
+	/* No skb in the rtx queue. */
+	if (!node)
+		return true;
+
+	/* Only one skb in rtx queue. */
+	return !node->rb_left && !node->rb_right;
+}
+
+/* TCP Small Queues :
+ * Control number of packets in qdisc/devices to two packets / or ~1 ms.
+ * (These limits are doubled for retransmits)
+ * This allows for :
+ *  - better RTT estimation and ACK scheduling
+ *  - faster recovery
+ *  - high rates
+ * Alas, some drivers / subsystems require a fair amount
+ * of queued bytes to ensure line rate.
+ * One example is wifi aggregation (802.11 AMPDU)
+ */
+static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb,
+				  unsigned int factor)
+{
+	unsigned long limit;
+
+	limit = max_t(unsigned long,
+		      2 * skb->truesize,
+		      sk->sk_pacing_rate >> READ_ONCE(sk->sk_pacing_shift));
+	if (sk->sk_pacing_status == SK_PACING_NONE)
+		limit = min_t(unsigned long, limit,
+			      READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_limit_output_bytes));
+	limit <<= factor;
+
+	if (static_branch_unlikely(&tcp_tx_delay_enabled) &&
+	    tcp_sk(sk)->tcp_tx_delay) {
+		u64 extra_bytes = (u64)sk->sk_pacing_rate * tcp_sk(sk)->tcp_tx_delay;
+
+		/* TSQ is based on skb truesize sum (sk_wmem_alloc), so we
+		 * approximate our needs assuming an ~100% skb->truesize overhead.
+		 * USEC_PER_SEC is approximated by 2^20.
+		 * do_div(extra_bytes, USEC_PER_SEC/2) is replaced by a right shift.
+		 */
+		extra_bytes >>= (20 - 1);
+		limit += extra_bytes;
+	}
+	if (refcount_read(&sk->sk_wmem_alloc) > limit) {
+		/* Always send skb if rtx queue is empty or has one skb.
+		 * No need to wait for TX completion to call us back,
+		 * after softirq/tasklet schedule.
+		 * This helps when TX completions are delayed too much.
+		 */
+		if (tcp_rtx_queue_empty_or_single_skb(sk))
+			return false;
+
+		set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
+		/* It is possible TX completion already happened
+		 * before we set TSQ_THROTTLED, so we must
+		 * test again the condition.
+		 */
+		smp_mb__after_atomic();
+		if (refcount_read(&sk->sk_wmem_alloc) > limit)
+			return true;
+	}
+	return false;
+}
+
+static void tcp_chrono_set(struct tcp_sock *tp, const enum tcp_chrono new)
+{
+	const u32 now = tcp_jiffies32;
+	enum tcp_chrono old = tp->chrono_type;
+
+	if (old > TCP_CHRONO_UNSPEC)
+		tp->chrono_stat[old - 1] += now - tp->chrono_start;
+	tp->chrono_start = now;
+	tp->chrono_type = new;
+}
+
+void tcp_chrono_start(struct sock *sk, const enum tcp_chrono type)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	/* If there are multiple conditions worthy of tracking in a
+	 * chronograph then the highest priority enum takes precedence
+	 * over the other conditions. So that if something "more interesting"
+	 * starts happening, stop the previous chrono and start a new one.
+	 */
+	if (type > tp->chrono_type)
+		tcp_chrono_set(tp, type);
+}
+
+void tcp_chrono_stop(struct sock *sk, const enum tcp_chrono type)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+
+	/* There are multiple conditions worthy of tracking in a
+	 * chronograph, so that the highest priority enum takes
+	 * precedence over the other conditions (see tcp_chrono_start).
+	 * If a condition stops, we only stop chrono tracking if
+	 * it's the "most interesting" or current chrono we are
+	 * tracking and starts busy chrono if we have pending data.
+	 */
+	if (tcp_rtx_and_write_queues_empty(sk))
+		tcp_chrono_set(tp, TCP_CHRONO_UNSPEC);
+	else if (type == tp->chrono_type)
+		tcp_chrono_set(tp, TCP_CHRONO_BUSY);
+}
+
+/* This routine writes packets to the network.  It advances the
+ * send_head.  This happens as incoming acks open up the remote
+ * window for us.
+ *
+ * LARGESEND note: !tcp_urg_mode is overkill, only frames between
+ * snd_up-64k-mss .. snd_up cannot be large. However, taking into
+ * account rare use of URG, this is not a big flaw.
+ *
+ * Send at most one packet when push_one > 0. Temporarily ignore
+ * cwnd limit to force at most one packet out when push_one == 2.
+
+ * Returns true, if no segments are in flight and we have queued segments,
+ * but cannot send anything now because of SWS or another problem.
+ */
+static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
+			   int push_one, gfp_t gfp)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *skb;
+	unsigned int tso_segs, sent_pkts;
+	int cwnd_quota;
+	int result;
+	bool is_cwnd_limited = false, is_rwnd_limited = false;
+	u32 max_segs;
+
+	sent_pkts = 0;
+
+	tcp_mstamp_refresh(tp);
+	if (!push_one) {
+		/* Do MTU probing. */
+		result = tcp_mtu_probe(sk);
+		if (!result) {
+			return false;
+		} else if (result > 0) {
+			sent_pkts = 1;
+		}
+	}
+
+	max_segs = tcp_tso_segs(sk, mss_now);
+	while ((skb = tcp_send_head(sk))) {
+		unsigned int limit;
+
+		if (unlikely(tp->repair) && tp->repair_queue == TCP_SEND_QUEUE) {
+			/* "skb_mstamp_ns" is used as a start point for the retransmit timer */
+			tp->tcp_wstamp_ns = tp->tcp_clock_cache;
+			skb_set_delivery_time(skb, tp->tcp_wstamp_ns, true);
+			list_move_tail(&skb->tcp_tsorted_anchor, &tp->tsorted_sent_queue);
+			tcp_init_tso_segs(skb, mss_now);
+			goto repair; /* Skip network transmission */
+		}
+
+		if (tcp_pacing_check(sk))
+			break;
+
+		tso_segs = tcp_init_tso_segs(skb, mss_now);
+		BUG_ON(!tso_segs);
+
+		cwnd_quota = tcp_cwnd_test(tp, skb);
+		if (!cwnd_quota) {
+			if (push_one == 2)
+				/* Force out a loss probe pkt. */
+				cwnd_quota = 1;
+			else
+				break;
+		}
+
+		if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now))) {
+			is_rwnd_limited = true;
+			break;
+		}
+
+		if (tso_segs == 1) {
+			if (unlikely(!tcp_nagle_test(tp, skb, mss_now,
+						     (tcp_skb_is_last(sk, skb) ?
+						      nonagle : TCP_NAGLE_PUSH))))
+				break;
+		} else {
+			if (!push_one &&
+			    tcp_tso_should_defer(sk, skb, &is_cwnd_limited,
+						 &is_rwnd_limited, max_segs))
+				break;
+		}
+
+		limit = mss_now;
+		if (tso_segs > 1 && !tcp_urg_mode(tp))
+			limit = tcp_mss_split_point(sk, skb, mss_now,
+						    min_t(unsigned int,
+							  cwnd_quota,
+							  max_segs),
+						    nonagle);
+
+		if (skb->len > limit &&
+		    unlikely(tso_fragment(sk, skb, limit, mss_now, gfp)))
+			break;
+
+		if (tcp_small_queue_check(sk, skb, 0))
+			break;
+
+		/* Argh, we hit an empty skb(), presumably a thread
+		 * is sleeping in sendmsg()/sk_stream_wait_memory().
+		 * We do not want to send a pure-ack packet and have
+		 * a strange looking rtx queue with empty packet(s).
+		 */
+		if (TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq)
+			break;
+
+		if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
+			break;
+
+repair:
+		/* Advance the send_head.  This one is sent out.
+		 * This call will increment packets_out.
+		 */
+		tcp_event_new_data_sent(sk, skb);
+
+		tcp_minshall_update(tp, mss_now, skb);
+		sent_pkts += tcp_skb_pcount(skb);
+
+		if (push_one)
+			break;
+	}
+
+	if (is_rwnd_limited)
+		tcp_chrono_start(sk, TCP_CHRONO_RWND_LIMITED);
+	else
+		tcp_chrono_stop(sk, TCP_CHRONO_RWND_LIMITED);
+
+	is_cwnd_limited |= (tcp_packets_in_flight(tp) >= tcp_snd_cwnd(tp));
+	if (likely(sent_pkts || is_cwnd_limited))
+		tcp_cwnd_validate(sk, is_cwnd_limited);
+
+	if (likely(sent_pkts)) {
+		if (tcp_in_cwnd_reduction(sk))
+			tp->prr_out += sent_pkts;
+
+		/* Send one loss probe per tail loss episode. */
+		if (push_one != 2)
+			tcp_schedule_loss_probe(sk, false);
+		return false;
+	}
+	return !tp->packets_out && !tcp_write_queue_empty(sk);
+}
+
+bool tcp_schedule_loss_probe(struct sock *sk, bool advancing_rto)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	u32 timeout, timeout_us, rto_delta_us;
+	int early_retrans;
+
+	/* Don't do any loss probe on a Fast Open connection before 3WHS
+	 * finishes.
+	 */
+	if (rcu_access_pointer(tp->fastopen_rsk))
+		return false;
+
+	early_retrans = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_early_retrans);
+	/* Schedule a loss probe in 2*RTT for SACK capable connections
+	 * not in loss recovery, that are either limited by cwnd or application.
+	 */
+	if ((early_retrans != 3 && early_retrans != 4) ||
+	    !tp->packets_out || !tcp_is_sack(tp) ||
+	    (icsk->icsk_ca_state != TCP_CA_Open &&
+	     icsk->icsk_ca_state != TCP_CA_CWR))
+		return false;
+
+	/* Probe timeout is 2*rtt. Add minimum RTO to account
+	 * for delayed ack when there's one outstanding packet. If no RTT
+	 * sample is available then probe after TCP_TIMEOUT_INIT.
+	 */
+	if (tp->srtt_us) {
+		timeout_us = tp->srtt_us >> 2;
+		if (tp->packets_out == 1)
+			timeout_us += tcp_rto_min_us(sk);
+		else
+			timeout_us += TCP_TIMEOUT_MIN_US;
+		timeout = usecs_to_jiffies(timeout_us);
+	} else {
+		timeout = TCP_TIMEOUT_INIT;
+	}
+
+	/* If the RTO formula yields an earlier time, then use that time. */
+	rto_delta_us = advancing_rto ?
+			jiffies_to_usecs(inet_csk(sk)->icsk_rto) :
+			tcp_rto_delta_us(sk);  /* How far in future is RTO? */
+	if (rto_delta_us > 0)
+		timeout = min_t(u32, timeout, usecs_to_jiffies(rto_delta_us));
+
+	tcp_reset_xmit_timer(sk, ICSK_TIME_LOSS_PROBE, timeout, TCP_RTO_MAX);
+	return true;
+}
+
+/* Thanks to skb fast clones, we can detect if a prior transmit of
+ * a packet is still in a qdisc or driver queue.
+ * In this case, there is very little point doing a retransmit !
+ */
+static bool skb_still_in_host_queue(struct sock *sk,
+				    const struct sk_buff *skb)
+{
+	if (unlikely(skb_fclone_busy(sk, skb))) {
+		set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
+		smp_mb__after_atomic();
+		if (skb_fclone_busy(sk, skb)) {
+			NET_INC_STATS(sock_net(sk),
+				      LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);
+			return true;
+		}
+	}
+	return false;
+}
+
+/* When probe timeout (PTO) fires, try send a new segment if possible, else
+ * retransmit the last segment.
+ */
+void tcp_send_loss_probe(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *skb;
+	int pcount;
+	int mss = tcp_current_mss(sk);
+
+	/* At most one outstanding TLP */
+	if (tp->tlp_high_seq)
+		goto rearm_timer;
+
+	tp->tlp_retrans = 0;
+	skb = tcp_send_head(sk);
+	if (skb && tcp_snd_wnd_test(tp, skb, mss)) {
+		pcount = tp->packets_out;
+		tcp_write_xmit(sk, mss, TCP_NAGLE_OFF, 2, GFP_ATOMIC);
+		if (tp->packets_out > pcount)
+			goto probe_sent;
+		goto rearm_timer;
+	}
+	skb = skb_rb_last(&sk->tcp_rtx_queue);
+	if (unlikely(!skb)) {
+		WARN_ONCE(tp->packets_out,
+			  "invalid inflight: %u state %u cwnd %u mss %d\n",
+			  tp->packets_out, sk->sk_state, tcp_snd_cwnd(tp), mss);
+		inet_csk(sk)->icsk_pending = 0;
+		return;
+	}
+
+	if (skb_still_in_host_queue(sk, skb))
+		goto rearm_timer;
+
+	pcount = tcp_skb_pcount(skb);
+	if (WARN_ON(!pcount))
+		goto rearm_timer;
+
+	if ((pcount > 1) && (skb->len > (pcount - 1) * mss)) {
+		if (unlikely(tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
+					  (pcount - 1) * mss, mss,
+					  GFP_ATOMIC)))
+			goto rearm_timer;
+		skb = skb_rb_next(skb);
+	}
+
+	if (WARN_ON(!skb || !tcp_skb_pcount(skb)))
+		goto rearm_timer;
+
+	if (__tcp_retransmit_skb(sk, skb, 1))
+		goto rearm_timer;
+
+	tp->tlp_retrans = 1;
+
+probe_sent:
+	/* Record snd_nxt for loss detection. */
+	tp->tlp_high_seq = tp->snd_nxt;
+
+	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPLOSSPROBES);
+	/* Reset s.t. tcp_rearm_rto will restart timer from now */
+	inet_csk(sk)->icsk_pending = 0;
+rearm_timer:
+	tcp_rearm_rto(sk);
+}
+
+/* Push out any pending frames which were held back due to
+ * TCP_CORK or attempt at coalescing tiny packets.
+ * The socket must be locked by the caller.
+ */
+void __tcp_push_pending_frames(struct sock *sk, unsigned int cur_mss,
+			       int nonagle)
+{
+	/* If we are closed, the bytes will have to remain here.
+	 * In time closedown will finish, we empty the write queue and
+	 * all will be happy.
+	 */
+	if (unlikely(sk->sk_state == TCP_CLOSE))
+		return;
+
+	if (tcp_write_xmit(sk, cur_mss, nonagle, 0,
+			   sk_gfp_mask(sk, GFP_ATOMIC)))
+		tcp_check_probe_timer(sk);
+}
+
+/* Send _single_ skb sitting at the send head. This function requires
+ * true push pending frames to setup probe timer etc.
+ */
+void tcp_push_one(struct sock *sk, unsigned int mss_now)
+{
+	struct sk_buff *skb = tcp_send_head(sk);
+
+	BUG_ON(!skb || skb->len < mss_now);
+
+	tcp_write_xmit(sk, mss_now, TCP_NAGLE_PUSH, 1, sk->sk_allocation);
+}
+
+/* This function returns the amount that we can raise the
+ * usable window based on the following constraints
+ *
+ * 1. The window can never be shrunk once it is offered (RFC 793)
+ * 2. We limit memory per socket
+ *
+ * RFC 1122:
+ * "the suggested [SWS] avoidance algorithm for the receiver is to keep
+ *  RECV.NEXT + RCV.WIN fixed until:
+ *  RCV.BUFF - RCV.USER - RCV.WINDOW >= min(1/2 RCV.BUFF, MSS)"
+ *
+ * i.e. don't raise the right edge of the window until you can raise
+ * it at least MSS bytes.
+ *
+ * Unfortunately, the recommended algorithm breaks header prediction,
+ * since header prediction assumes th->window stays fixed.
+ *
+ * Strictly speaking, keeping th->window fixed violates the receiver
+ * side SWS prevention criteria. The problem is that under this rule
+ * a stream of single byte packets will cause the right side of the
+ * window to always advance by a single byte.
+ *
+ * Of course, if the sender implements sender side SWS prevention
+ * then this will not be a problem.
+ *
+ * BSD seems to make the following compromise:
+ *
+ *	If the free space is less than the 1/4 of the maximum
+ *	space available and the free space is less than 1/2 mss,
+ *	then set the window to 0.
+ *	[ Actually, bsd uses MSS and 1/4 of maximal _window_ ]
+ *	Otherwise, just prevent the window from shrinking
+ *	and from being larger than the largest representable value.
+ *
+ * This prevents incremental opening of the window in the regime
+ * where TCP is limited by the speed of the reader side taking
+ * data out of the TCP receive queue. It does nothing about
+ * those cases where the window is constrained on the sender side
+ * because the pipeline is full.
+ *
+ * BSD also seems to "accidentally" limit itself to windows that are a
+ * multiple of MSS, at least until the free space gets quite small.
+ * This would appear to be a side effect of the mbuf implementation.
+ * Combining these two algorithms results in the observed behavior
+ * of having a fixed window size at almost all times.
+ *
+ * Below we obtain similar behavior by forcing the offered window to
+ * a multiple of the mss when it is feasible to do so.
+ *
+ * Note, we don't "adjust" for TIMESTAMP or SACK option bytes.
+ * Regular options like TIMESTAMP are taken into account.
+ */
+u32 __tcp_select_window(struct sock *sk)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct net *net = sock_net(sk);
+	/* MSS for the peer's data.  Previous versions used mss_clamp
+	 * here.  I don't know if the value based on our guesses
+	 * of peer's MSS is better for the performance.  It's more correct
+	 * but may be worse for the performance because of rcv_mss
+	 * fluctuations.  --SAW  1998/11/1
+	 */
+	int mss = icsk->icsk_ack.rcv_mss;
+	int free_space = tcp_space(sk);
+	int allowed_space = tcp_full_space(sk);
+	int full_space, window;
+
+	if (sk_is_mptcp(sk))
+		mptcp_space(sk, &free_space, &allowed_space);
+
+	full_space = min_t(int, tp->window_clamp, allowed_space);
+
+	if (unlikely(mss > full_space)) {
+		mss = full_space;
+		if (mss <= 0)
+			return 0;
+	}
+
+	/* Only allow window shrink if the sysctl is enabled and we have
+	 * a non-zero scaling factor in effect.
+	 */
+	if (READ_ONCE(net->ipv4.sysctl_tcp_shrink_window) && tp->rx_opt.rcv_wscale)
+		goto shrink_window_allowed;
+
+	/* do not allow window to shrink */
+
+	if (free_space < (full_space >> 1)) {
+		icsk->icsk_ack.quick = 0;
+
+		if (tcp_under_memory_pressure(sk))
+			tcp_adjust_rcv_ssthresh(sk);
+
+		/* free_space might become our new window, make sure we don't
+		 * increase it due to wscale.
+		 */
+		free_space = round_down(free_space, 1 << tp->rx_opt.rcv_wscale);
+
+		/* if free space is less than mss estimate, or is below 1/16th
+		 * of the maximum allowed, try to move to zero-window, else
+		 * tcp_clamp_window() will grow rcv buf up to tcp_rmem[2], and
+		 * new incoming data is dropped due to memory limits.
+		 * With large window, mss test triggers way too late in order
+		 * to announce zero window in time before rmem limit kicks in.
+		 */
+		if (free_space < (allowed_space >> 4) || free_space < mss)
+			return 0;
+	}
+
+	if (free_space > tp->rcv_ssthresh)
+		free_space = tp->rcv_ssthresh;
+
+	/* Don't do rounding if we are using window scaling, since the
+	 * scaled window will not line up with the MSS boundary anyway.
+	 */
+	if (tp->rx_opt.rcv_wscale) {
+		window = free_space;
+
+		/* Advertise enough space so that it won't get scaled away.
+		 * Import case: prevent zero window announcement if
+		 * 1<<rcv_wscale > mss.
+		 */
+		window = ALIGN(window, (1 << tp->rx_opt.rcv_wscale));
+	} else {
+		window = tp->rcv_wnd;
+		/* Get the largest window that is a nice multiple of mss.
+		 * Window clamp already applied above.
+		 * If our current window offering is within 1 mss of the
+		 * free space we just keep it. This prevents the divide
+		 * and multiply from happening most of the time.
+		 * We also don't do any window rounding when the free space
+		 * is too small.
+		 */
+		if (window <= free_space - mss || window > free_space)
+			window = rounddown(free_space, mss);
+		else if (mss == full_space &&
+			 free_space > window + (full_space >> 1))
+			window = free_space;
+	}
+
+	return window;
+
+shrink_window_allowed:
+	/* new window should always be an exact multiple of scaling factor */
+	free_space = round_down(free_space, 1 << tp->rx_opt.rcv_wscale);
+
+	if (free_space < (full_space >> 1)) {
+		icsk->icsk_ack.quick = 0;
+
+		if (tcp_under_memory_pressure(sk))
+			tcp_adjust_rcv_ssthresh(sk);
+
+		/* if free space is too low, return a zero window */
+		if (free_space < (allowed_space >> 4) || free_space < mss ||
+			free_space < (1 << tp->rx_opt.rcv_wscale))
+			return 0;
+	}
+
+	if (free_space > tp->rcv_ssthresh) {
+		free_space = tp->rcv_ssthresh;
+		/* new window should always be an exact multiple of scaling factor
+		 *
+		 * For this case, we ALIGN "up" (increase free_space) because
+		 * we know free_space is not zero here, it has been reduced from
+		 * the memory-based limit, and rcv_ssthresh is not a hard limit
+		 * (unlike sk_rcvbuf).
+		 */
+		free_space = ALIGN(free_space, (1 << tp->rx_opt.rcv_wscale));
+	}
+
+	return free_space;
+}
+
+void tcp_skb_collapse_tstamp(struct sk_buff *skb,
+			     const struct sk_buff *next_skb)
+{
+	if (unlikely(tcp_has_tx_tstamp(next_skb))) {
+		const struct skb_shared_info *next_shinfo =
+			skb_shinfo(next_skb);
+		struct skb_shared_info *shinfo = skb_shinfo(skb);
+
+		shinfo->tx_flags |= next_shinfo->tx_flags & SKBTX_ANY_TSTAMP;
+		shinfo->tskey = next_shinfo->tskey;
+		TCP_SKB_CB(skb)->txstamp_ack |=
+			TCP_SKB_CB(next_skb)->txstamp_ack;
+	}
+}
+
+/* Collapses two adjacent SKB's during retransmission. */
+static bool tcp_collapse_retrans(struct sock *sk, struct sk_buff *skb)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *next_skb = skb_rb_next(skb);
+	int next_skb_size;
+
+	next_skb_size = next_skb->len;
+
+	BUG_ON(tcp_skb_pcount(skb) != 1 || tcp_skb_pcount(next_skb) != 1);
+
+	if (next_skb_size && !tcp_skb_shift(skb, next_skb, 1, next_skb_size))
+		return false;
+
+	tcp_highest_sack_replace(sk, next_skb, skb);
+
+	/* Update sequence range on original skb. */
+	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;
+
+	/* Merge over control information. This moves PSH/FIN etc. over */
+	TCP_SKB_CB(skb)->tcp_flags |= TCP_SKB_CB(next_skb)->tcp_flags;
+
+	/* All done, get rid of second SKB and account for it so
+	 * packet counting does not break.
+	 */
+	TCP_SKB_CB(skb)->sacked |= TCP_SKB_CB(next_skb)->sacked & TCPCB_EVER_RETRANS;
+	TCP_SKB_CB(skb)->eor = TCP_SKB_CB(next_skb)->eor;
+
+	/* changed transmit queue under us so clear hints */
+	tcp_clear_retrans_hints_partial(tp);
+	if (next_skb == tp->retransmit_skb_hint)
+		tp->retransmit_skb_hint = skb;
+
+	tcp_adjust_pcount(sk, next_skb, tcp_skb_pcount(next_skb));
+
+	tcp_skb_collapse_tstamp(skb, next_skb);
+
+	tcp_rtx_queue_unlink_and_free(next_skb, sk);
+	return true;
+}
+
+/* Check if coalescing SKBs is legal. */
+static bool tcp_can_collapse(const struct sock *sk, const struct sk_buff *skb)
+{
+	if (tcp_skb_pcount(skb) > 1)
+		return false;
+	if (skb_cloned(skb))
+		return false;
+	/* Some heuristics for collapsing over SACK'd could be invented */
+	if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
+		return false;
+
+	return true;
+}
+
+/* Collapse packets in the retransmit queue to make to create
+ * less packets on the wire. This is only done on retransmission.
+ */
+static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
+				     int space)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *skb = to, *tmp;
+	bool first = true;
+
+	if (!READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_retrans_collapse))
+		return;
+	if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
+		return;
+
+	skb_rbtree_walk_from_safe(skb, tmp) {
+		if (!tcp_can_collapse(sk, skb))
+			break;
+
+		if (!tcp_skb_can_collapse(to, skb))
+			break;
+
+		space -= skb->len;
+
+		if (first) {
+			first = false;
+			continue;
+		}
+
+		if (space < 0)
+			break;
+
+		if (after(TCP_SKB_CB(skb)->end_seq, tcp_wnd_end(tp)))
+			break;
+
+		if (!tcp_collapse_retrans(sk, to))
+			break;
+	}
+}
+
+/* This retransmits one SKB.  Policy decisions and retransmit queue
+ * state updates are done by the caller.  Returns non-zero if an
+ * error occurred which prevented the send.
+ */
+int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	unsigned int cur_mss;
+	int diff, len, err;
+	int avail_wnd;
+
+	/* Inconclusive MTU probe */
+	if (icsk->icsk_mtup.probe_size)
+		icsk->icsk_mtup.probe_size = 0;
+
+	if (skb_still_in_host_queue(sk, skb))
+		return -EBUSY;
+
+start:
+	if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
+		if (unlikely(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)) {
+			TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_SYN;
+			TCP_SKB_CB(skb)->seq++;
+			goto start;
+		}
+		if (unlikely(before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))) {
+			WARN_ON_ONCE(1);
+			return -EINVAL;
+		}
+		if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
+			return -ENOMEM;
+	}
+
+	if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
+		return -EHOSTUNREACH; /* Routing failure or similar. */
+
+	cur_mss = tcp_current_mss(sk);
+	avail_wnd = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
+
+	/* If receiver has shrunk his window, and skb is out of
+	 * new window, do not retransmit it. The exception is the
+	 * case, when window is shrunk to zero. In this case
+	 * our retransmit of one segment serves as a zero window probe.
+	 */
+	if (avail_wnd <= 0) {
+		if (TCP_SKB_CB(skb)->seq != tp->snd_una)
+			return -EAGAIN;
+		avail_wnd = cur_mss;
+	}
+
+	len = cur_mss * segs;
+	if (len > avail_wnd) {
+		len = rounddown(avail_wnd, cur_mss);
+		if (!len)
+			len = avail_wnd;
+	}
+	if (skb->len > len) {
+		if (tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb, len,
+				 cur_mss, GFP_ATOMIC))
+			return -ENOMEM; /* We'll try again later. */
+	} else {
+		if (skb_unclone_keeptruesize(skb, GFP_ATOMIC))
+			return -ENOMEM;
+
+		diff = tcp_skb_pcount(skb);
+		tcp_set_skb_tso_segs(skb, cur_mss);
+		diff -= tcp_skb_pcount(skb);
+		if (diff)
+			tcp_adjust_pcount(sk, skb, diff);
+		avail_wnd = min_t(int, avail_wnd, cur_mss);
+		if (skb->len < avail_wnd)
+			tcp_retrans_try_collapse(sk, skb, avail_wnd);
+	}
+
+	/* RFC3168, section 6.1.1.1. ECN fallback */
+	if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) == TCPHDR_SYN_ECN)
+		tcp_ecn_clear_syn(sk, skb);
+
+	/* Update global and local TCP statistics. */
+	segs = tcp_skb_pcount(skb);
+	TCP_ADD_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS, segs);
+	if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
+		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
+	tp->total_retrans += segs;
+	tp->bytes_retrans += skb->len;
+
+	/* make sure skb->data is aligned on arches that require it
+	 * and check if ack-trimming & collapsing extended the headroom
+	 * beyond what csum_start can cover.
+	 */
+	if (unlikely((NET_IP_ALIGN && ((unsigned long)skb->data & 3)) ||
+		     skb_headroom(skb) >= 0xFFFF)) {
+		struct sk_buff *nskb;
+
+		tcp_skb_tsorted_save(skb) {
+			nskb = __pskb_copy(skb, MAX_TCP_HEADER, GFP_ATOMIC);
+			if (nskb) {
+				nskb->dev = NULL;
+				err = tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC);
+			} else {
+				err = -ENOBUFS;
+			}
+		} tcp_skb_tsorted_restore(skb);
+
+		if (!err) {
+			tcp_update_skb_after_send(sk, skb, tp->tcp_wstamp_ns);
+			tcp_rate_skb_sent(sk, skb);
+		}
+	} else {
+		err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
+	}
+
+	/* To avoid taking spuriously low RTT samples based on a timestamp
+	 * for a transmit that never happened, always mark EVER_RETRANS
+	 */
+	TCP_SKB_CB(skb)->sacked |= TCPCB_EVER_RETRANS;
+
+	if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RETRANS_CB_FLAG))
+		tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RETRANS_CB,
+				  TCP_SKB_CB(skb)->seq, segs, err);
+
+	if (likely(!err)) {
+		trace_tcp_retransmit_skb(sk, skb);
+	} else if (err != -EBUSY) {
+		NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL, segs);
+	}
+	return err;
+}
+
+int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	int err = __tcp_retransmit_skb(sk, skb, segs);
+
+	if (err == 0) {
+#if FASTRETRANS_DEBUG > 0
+		if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
+			net_dbg_ratelimited("retrans_out leaked\n");
+		}
+#endif
+		TCP_SKB_CB(skb)->sacked |= TCPCB_RETRANS;
+		tp->retrans_out += tcp_skb_pcount(skb);
+	}
+
+	/* Save stamp of the first (attempted) retransmit. */
+	if (!tp->retrans_stamp)
+		tp->retrans_stamp = tcp_skb_timestamp(skb);
+
+	if (tp->undo_retrans < 0)
+		tp->undo_retrans = 0;
+	tp->undo_retrans += tcp_skb_pcount(skb);
+	return err;
+}
+
+/* This gets called after a retransmit timeout, and the initially
+ * retransmitted data is acknowledged.  It tries to continue
+ * resending the rest of the retransmit queue, until either
+ * we've sent it all or the congestion window limit is reached.
+ */
+void tcp_xmit_retransmit_queue(struct sock *sk)
+{
+	const struct inet_connection_sock *icsk = inet_csk(sk);
+	struct sk_buff *skb, *rtx_head, *hole = NULL;
+	struct tcp_sock *tp = tcp_sk(sk);
+	bool rearm_timer = false;
+	u32 max_segs;
+	int mib_idx;
+
+	if (!tp->packets_out)
+		return;
+
+	rtx_head = tcp_rtx_queue_head(sk);
+	skb = tp->retransmit_skb_hint ?: rtx_head;
+	max_segs = tcp_tso_segs(sk, tcp_current_mss(sk));
+	skb_rbtree_walk_from(skb) {
+		__u8 sacked;
+		int segs;
+
+		if (tcp_pacing_check(sk))
+			break;
+
+		/* we could do better than to assign each time */
+		if (!hole)
+			tp->retransmit_skb_hint = skb;
+
+		segs = tcp_snd_cwnd(tp) - tcp_packets_in_flight(tp);
+		if (segs <= 0)
+			break;
+		sacked = TCP_SKB_CB(skb)->sacked;
+		/* In case tcp_shift_skb_data() have aggregated large skbs,
+		 * we need to make sure not sending too bigs TSO packets
+		 */
+		segs = min_t(int, segs, max_segs);
+
+		if (tp->retrans_out >= tp->lost_out) {
+			break;
+		} else if (!(sacked & TCPCB_LOST)) {
+			if (!hole && !(sacked & (TCPCB_SACKED_RETRANS|TCPCB_SACKED_ACKED)))
+				hole = skb;
+			continue;
+
+		} else {
+			if (icsk->icsk_ca_state != TCP_CA_Loss)
+				mib_idx = LINUX_MIB_TCPFASTRETRANS;
+			else
+				mib_idx = LINUX_MIB_TCPSLOWSTARTRETRANS;
+		}
+
+		if (sacked & (TCPCB_SACKED_ACKED|TCPCB_SACKED_RETRANS))
+			continue;
+
+		if (tcp_small_queue_check(sk, skb, 1))
+			break;
+
+		if (tcp_retransmit_skb(sk, skb, segs))
+			break;
+
+		NET_ADD_STATS(sock_net(sk), mib_idx, tcp_skb_pcount(skb));
+
+		if (tcp_in_cwnd_reduction(sk))
+			tp->prr_out += tcp_skb_pcount(skb);
+
+		if (skb == rtx_head &&
+		    icsk->icsk_pending != ICSK_TIME_REO_TIMEOUT)
+			rearm_timer = true;
+
+	}
+	if (rearm_timer)
+		tcp_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
+				     inet_csk(sk)->icsk_rto,
+				     TCP_RTO_MAX);
+}
+
+/* We allow to exceed memory limits for FIN packets to expedite
+ * connection tear down and (memory) recovery.
+ * Otherwise tcp_send_fin() could be tempted to either delay FIN
+ * or even be forced to close flow without any FIN.
+ * In general, we want to allow one skb per socket to avoid hangs
+ * with edge trigger epoll()
+ */
+void sk_forced_mem_schedule(struct sock *sk, int size)
+{
+	int delta, amt;
+
+	delta = size - sk->sk_forward_alloc;
+	if (delta <= 0)
+		return;
+	amt = sk_mem_pages(delta);
+	sk_forward_alloc_add(sk, amt << PAGE_SHIFT);
+	sk_memory_allocated_add(sk, amt);
+
+	if (mem_cgroup_sockets_enabled && sk->sk_memcg)
+		mem_cgroup_charge_skmem(sk->sk_memcg, amt,
+					gfp_memcg_charge() | __GFP_NOFAIL);
+}
+
+/* Send a FIN. The caller locks the socket for us.
+ * We should try to send a FIN packet really hard, but eventually give up.
+ */
+void tcp_send_fin(struct sock *sk)
+{
+	struct sk_buff *skb, *tskb, *tail = tcp_write_queue_tail(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	/* Optimization, tack on the FIN if we have one skb in write queue and
+	 * this skb was not yet sent, or we are under memory pressure.
+	 * Note: in the latter case, FIN packet will be sent after a timeout,
+	 * as TCP stack thinks it has already been transmitted.
+	 */
+	tskb = tail;
+	if (!tskb && tcp_under_memory_pressure(sk))
+		tskb = skb_rb_last(&sk->tcp_rtx_queue);
+
+	if (tskb) {
+		TCP_SKB_CB(tskb)->tcp_flags |= TCPHDR_FIN;
+		TCP_SKB_CB(tskb)->end_seq++;
+		tp->write_seq++;
+		if (!tail) {
+			/* This means tskb was already sent.
+			 * Pretend we included the FIN on previous transmit.
+			 * We need to set tp->snd_nxt to the value it would have
+			 * if FIN had been sent. This is because retransmit path
+			 * does not change tp->snd_nxt.
+			 */
+			WRITE_ONCE(tp->snd_nxt, tp->snd_nxt + 1);
+			return;
+		}
+	} else {
+		skb = alloc_skb_fclone(MAX_TCP_HEADER, sk->sk_allocation);
+		if (unlikely(!skb))
+			return;
+
+		INIT_LIST_HEAD(&skb->tcp_tsorted_anchor);
+		skb_reserve(skb, MAX_TCP_HEADER);
+		sk_forced_mem_schedule(sk, skb->truesize);
+		/* FIN eats a sequence byte, write_seq advanced by tcp_queue_skb(). */
+		tcp_init_nondata_skb(skb, tp->write_seq,
+				     TCPHDR_ACK | TCPHDR_FIN);
+		tcp_queue_skb(sk, skb);
+	}
+	__tcp_push_pending_frames(sk, tcp_current_mss(sk), TCP_NAGLE_OFF);
+}
+
+/* We get here when a process closes a file descriptor (either due to
+ * an explicit close() or as a byproduct of exit()'ing) and there
+ * was unread data in the receive queue.  This behavior is recommended
+ * by RFC 2525, section 2.17.  -DaveM
+ */
+void tcp_send_active_reset(struct sock *sk, gfp_t priority)
+{
+	struct sk_buff *skb;
+
+	TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTRSTS);
+
+	/* NOTE: No TCP options attached and we never retransmit this. */
+	skb = alloc_skb(MAX_TCP_HEADER, priority);
+	if (!skb) {
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
+		return;
+	}
+
+	/* Reserve space for headers and prepare control bits. */
+	skb_reserve(skb, MAX_TCP_HEADER);
+	tcp_init_nondata_skb(skb, tcp_acceptable_seq(sk),
+			     TCPHDR_ACK | TCPHDR_RST);
+	tcp_mstamp_refresh(tcp_sk(sk));
+	/* Send it off. */
+	if (tcp_transmit_skb(sk, skb, 0, priority))
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
+
+	/* skb of trace_tcp_send_reset() keeps the skb that caused RST,
+	 * skb here is different to the troublesome skb, so use NULL
+	 */
+	trace_tcp_send_reset(sk, NULL);
+}
+
+/* Send a crossed SYN-ACK during socket establishment.
+ * WARNING: This routine must only be called when we have already sent
+ * a SYN packet that crossed the incoming SYN that caused this routine
+ * to get called. If this assumption fails then the initial rcv_wnd
+ * and rcv_wscale values will not be correct.
+ */
+int tcp_send_synack(struct sock *sk)
+{
+	struct sk_buff *skb;
+
+	skb = tcp_rtx_queue_head(sk);
+	if (!skb || !(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)) {
+		pr_err("%s: wrong queue state\n", __func__);
+		return -EFAULT;
+	}
+	if (!(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_ACK)) {
+		if (skb_cloned(skb)) {
+			struct sk_buff *nskb;
+
+			tcp_skb_tsorted_save(skb) {
+				nskb = skb_copy(skb, GFP_ATOMIC);
+			} tcp_skb_tsorted_restore(skb);
+			if (!nskb)
+				return -ENOMEM;
+			INIT_LIST_HEAD(&nskb->tcp_tsorted_anchor);
+			tcp_highest_sack_replace(sk, skb, nskb);
+			tcp_rtx_queue_unlink_and_free(skb, sk);
+			__skb_header_release(nskb);
+			tcp_rbtree_insert(&sk->tcp_rtx_queue, nskb);
+			sk_wmem_queued_add(sk, nskb->truesize);
+			sk_mem_charge(sk, nskb->truesize);
+			skb = nskb;
+		}
+
+		TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ACK;
+		tcp_ecn_send_synack(sk, skb);
+	}
+	return tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
+}
+
+/**
+ * tcp_make_synack - Allocate one skb and build a SYNACK packet.
+ * @sk: listener socket
+ * @dst: dst entry attached to the SYNACK. It is consumed and caller
+ *       should not use it again.
+ * @req: request_sock pointer
+ * @foc: cookie for tcp fast open
+ * @synack_type: Type of synack to prepare
+ * @syn_skb: SYN packet just received.  It could be NULL for rtx case.
+ */
+struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
+				struct request_sock *req,
+				struct tcp_fastopen_cookie *foc,
+				enum tcp_synack_type synack_type,
+				struct sk_buff *syn_skb)
+{
+	struct inet_request_sock *ireq = inet_rsk(req);
+	const struct tcp_sock *tp = tcp_sk(sk);
+	struct tcp_md5sig_key *md5 = NULL;
+	struct tcp_out_options opts;
+	struct sk_buff *skb;
+	int tcp_header_size;
+	struct tcphdr *th;
+	int mss;
+	u64 now;
+
+	skb = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
+	if (unlikely(!skb)) {
+		dst_release(dst);
+		return NULL;
+	}
+	/* Reserve space for headers. */
+	skb_reserve(skb, MAX_TCP_HEADER);
+
+	switch (synack_type) {
+	case TCP_SYNACK_NORMAL:
+		skb_set_owner_w(skb, req_to_sk(req));
+		break;
+	case TCP_SYNACK_COOKIE:
+		/* Under synflood, we do not attach skb to a socket,
+		 * to avoid false sharing.
+		 */
+		break;
+	case TCP_SYNACK_FASTOPEN:
+		/* sk is a const pointer, because we want to express multiple
+		 * cpu might call us concurrently.
+		 * sk->sk_wmem_alloc in an atomic, we can promote to rw.
+		 */
+		skb_set_owner_w(skb, (struct sock *)sk);
+		break;
+	}
+	skb_dst_set(skb, dst);
+
+	mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
+
+	memset(&opts, 0, sizeof(opts));
+	now = tcp_clock_ns();
+#ifdef CONFIG_SYN_COOKIES
+	if (unlikely(synack_type == TCP_SYNACK_COOKIE && ireq->tstamp_ok))
+		skb_set_delivery_time(skb, cookie_init_timestamp(req, now),
+				      true);
+	else
+#endif
+	{
+		skb_set_delivery_time(skb, now, true);
+		if (!tcp_rsk(req)->snt_synack) /* Timestamp first SYNACK */
+			tcp_rsk(req)->snt_synack = tcp_skb_timestamp_us(skb);
+	}
+
+#ifdef CONFIG_TCP_MD5SIG
+	rcu_read_lock();
+	md5 = tcp_rsk(req)->af_specific->req_md5_lookup(sk, req_to_sk(req));
+#endif
+	skb_set_hash(skb, READ_ONCE(tcp_rsk(req)->txhash), PKT_HASH_TYPE_L4);
+	/* bpf program will be interested in the tcp_flags */
+	TCP_SKB_CB(skb)->tcp_flags = TCPHDR_SYN | TCPHDR_ACK;
+	tcp_header_size = tcp_synack_options(sk, req, mss, skb, &opts, md5,
+					     foc, synack_type,
+					     syn_skb) + sizeof(*th);
+
+	skb_push(skb, tcp_header_size);
+	skb_reset_transport_header(skb);
+
+	th = (struct tcphdr *)skb->data;
+	memset(th, 0, sizeof(struct tcphdr));
+	th->syn = 1;
+	th->ack = 1;
+	tcp_ecn_make_synack(req, th);
+	th->source = htons(ireq->ir_num);
+	th->dest = ireq->ir_rmt_port;
+	skb->mark = ireq->ir_mark;
+	skb->ip_summed = CHECKSUM_PARTIAL;
+	th->seq = htonl(tcp_rsk(req)->snt_isn);
+	/* XXX data is queued and acked as is. No buffer/window check */
+	th->ack_seq = htonl(tcp_rsk(req)->rcv_nxt);
+
+	/* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
+	th->window = htons(min(req->rsk_rcv_wnd, 65535U));
+	tcp_options_write(th, NULL, &opts);
+	th->doff = (tcp_header_size >> 2);
+	TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTSEGS);
+
+#ifdef CONFIG_TCP_MD5SIG
+	/* Okay, we have all we need - do the md5 hash if needed */
+	if (md5)
+		tcp_rsk(req)->af_specific->calc_md5_hash(opts.hash_location,
+					       md5, req_to_sk(req), skb);
+	rcu_read_unlock();
+#endif
+
+	bpf_skops_write_hdr_opt((struct sock *)sk, skb, req, syn_skb,
+				synack_type, &opts);
+
+	skb_set_delivery_time(skb, now, true);
+	tcp_add_tx_delay(skb, tp);
+
+	return skb;
+}
+EXPORT_SYMBOL(tcp_make_synack);
+
+static void tcp_ca_dst_init(struct sock *sk, const struct dst_entry *dst)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	const struct tcp_congestion_ops *ca;
+	u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
+
+	if (ca_key == TCP_CA_UNSPEC)
+		return;
+
+	rcu_read_lock();
+	ca = tcp_ca_find_key(ca_key);
+	if (likely(ca && bpf_try_module_get(ca, ca->owner))) {
+		bpf_module_put(icsk->icsk_ca_ops, icsk->icsk_ca_ops->owner);
+		icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
+		icsk->icsk_ca_ops = ca;
+	}
+	rcu_read_unlock();
+}
+
+/* Do all connect socket setups that can be done AF independent. */
+static void tcp_connect_init(struct sock *sk)
+{
+	const struct dst_entry *dst = __sk_dst_get(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	__u8 rcv_wscale;
+	u32 rcv_wnd;
+
+	/* We'll fix this up when we get a response from the other end.
+	 * See tcp_input.c:tcp_rcv_state_process case TCP_SYN_SENT.
+	 */
+	tp->tcp_header_len = sizeof(struct tcphdr);
+	if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_timestamps))
+		tp->tcp_header_len += TCPOLEN_TSTAMP_ALIGNED;
+
+	/* If user gave his TCP_MAXSEG, record it to clamp */
+	if (tp->rx_opt.user_mss)
+		tp->rx_opt.mss_clamp = tp->rx_opt.user_mss;
+	tp->max_window = 0;
+	tcp_mtup_init(sk);
+	tcp_sync_mss(sk, dst_mtu(dst));
+
+	tcp_ca_dst_init(sk, dst);
+
+	if (!tp->window_clamp)
+		tp->window_clamp = dst_metric(dst, RTAX_WINDOW);
+	tp->advmss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
+
+	tcp_initialize_rcv_mss(sk);
+
+	/* limit the window selection if the user enforce a smaller rx buffer */
+	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
+	    (tp->window_clamp > tcp_full_space(sk) || tp->window_clamp == 0))
+		tp->window_clamp = tcp_full_space(sk);
+
+	rcv_wnd = tcp_rwnd_init_bpf(sk);
+	if (rcv_wnd == 0)
+		rcv_wnd = dst_metric(dst, RTAX_INITRWND);
+
+	tcp_select_initial_window(sk, tcp_full_space(sk),
+				  tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0),
+				  &tp->rcv_wnd,
+				  &tp->window_clamp,
+				  READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_window_scaling),
+				  &rcv_wscale,
+				  rcv_wnd);
+
+	tp->rx_opt.rcv_wscale = rcv_wscale;
+	tp->rcv_ssthresh = tp->rcv_wnd;
+
+	WRITE_ONCE(sk->sk_err, 0);
+	sock_reset_flag(sk, SOCK_DONE);
+	tp->snd_wnd = 0;
+	tcp_init_wl(tp, 0);
+	tcp_write_queue_purge(sk);
+	tp->snd_una = tp->write_seq;
+	tp->snd_sml = tp->write_seq;
+	tp->snd_up = tp->write_seq;
+	WRITE_ONCE(tp->snd_nxt, tp->write_seq);
+
+	if (likely(!tp->repair))
+		tp->rcv_nxt = 0;
+	else
+		tp->rcv_tstamp = tcp_jiffies32;
+	tp->rcv_wup = tp->rcv_nxt;
+	WRITE_ONCE(tp->copied_seq, tp->rcv_nxt);
+
+	inet_csk(sk)->icsk_rto = tcp_timeout_init(sk);
+	inet_csk(sk)->icsk_retransmits = 0;
+	tcp_clear_retrans(tp);
+}
+
+static void tcp_connect_queue_skb(struct sock *sk, struct sk_buff *skb)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);
+
+	tcb->end_seq += skb->len;
+	__skb_header_release(skb);
+	sk_wmem_queued_add(sk, skb->truesize);
+	sk_mem_charge(sk, skb->truesize);
+	WRITE_ONCE(tp->write_seq, tcb->end_seq);
+	tp->packets_out += tcp_skb_pcount(skb);
+}
+
+/* Build and send a SYN with data and (cached) Fast Open cookie. However,
+ * queue a data-only packet after the regular SYN, such that regular SYNs
+ * are retransmitted on timeouts. Also if the remote SYN-ACK acknowledges
+ * only the SYN sequence, the data are retransmitted in the first ACK.
+ * If cookie is not cached or other error occurs, falls back to send a
+ * regular SYN with Fast Open cookie request option.
+ */
+static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct tcp_fastopen_request *fo = tp->fastopen_req;
+	struct page_frag *pfrag = sk_page_frag(sk);
+	struct sk_buff *syn_data;
+	int space, err = 0;
+
+	tp->rx_opt.mss_clamp = tp->advmss;  /* If MSS is not cached */
+	if (!tcp_fastopen_cookie_check(sk, &tp->rx_opt.mss_clamp, &fo->cookie))
+		goto fallback;
+
+	/* MSS for SYN-data is based on cached MSS and bounded by PMTU and
+	 * user-MSS. Reserve maximum option space for middleboxes that add
+	 * private TCP options. The cost is reduced data space in SYN :(
+	 */
+	tp->rx_opt.mss_clamp = tcp_mss_clamp(tp, tp->rx_opt.mss_clamp);
+	/* Sync mss_cache after updating the mss_clamp */
+	tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
+
+	space = __tcp_mtu_to_mss(sk, icsk->icsk_pmtu_cookie) -
+		MAX_TCP_OPTION_SPACE;
+
+	space = min_t(size_t, space, fo->size);
+
+	if (space &&
+	    !skb_page_frag_refill(min_t(size_t, space, PAGE_SIZE),
+				  pfrag, sk->sk_allocation))
+		goto fallback;
+	syn_data = tcp_stream_alloc_skb(sk, sk->sk_allocation, false);
+	if (!syn_data)
+		goto fallback;
+	memcpy(syn_data->cb, syn->cb, sizeof(syn->cb));
+	if (space) {
+		space = min_t(size_t, space, pfrag->size - pfrag->offset);
+		space = tcp_wmem_schedule(sk, space);
+	}
+	if (space) {
+		space = copy_page_from_iter(pfrag->page, pfrag->offset,
+					    space, &fo->data->msg_iter);
+		if (unlikely(!space)) {
+			tcp_skb_tsorted_anchor_cleanup(syn_data);
+			kfree_skb(syn_data);
+			goto fallback;
+		}
+		skb_fill_page_desc(syn_data, 0, pfrag->page,
+				   pfrag->offset, space);
+		page_ref_inc(pfrag->page);
+		pfrag->offset += space;
+		skb_len_add(syn_data, space);
+		skb_zcopy_set(syn_data, fo->uarg, NULL);
+	}
+	/* No more data pending in inet_wait_for_connect() */
+	if (space == fo->size)
+		fo->data = NULL;
+	fo->copied = space;
+
+	tcp_connect_queue_skb(sk, syn_data);
+	if (syn_data->len)
+		tcp_chrono_start(sk, TCP_CHRONO_BUSY);
+
+	err = tcp_transmit_skb(sk, syn_data, 1, sk->sk_allocation);
+
+	skb_set_delivery_time(syn, syn_data->skb_mstamp_ns, true);
+
+	/* Now full SYN+DATA was cloned and sent (or not),
+	 * remove the SYN from the original skb (syn_data)
+	 * we keep in write queue in case of a retransmit, as we
+	 * also have the SYN packet (with no data) in the same queue.
+	 */
+	TCP_SKB_CB(syn_data)->seq++;
+	TCP_SKB_CB(syn_data)->tcp_flags = TCPHDR_ACK | TCPHDR_PSH;
+	if (!err) {
+		tp->syn_data = (fo->copied > 0);
+		tcp_rbtree_insert(&sk->tcp_rtx_queue, syn_data);
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPORIGDATASENT);
+		goto done;
+	}
+
+	/* data was not sent, put it in write_queue */
+	__skb_queue_tail(&sk->sk_write_queue, syn_data);
+	tp->packets_out -= tcp_skb_pcount(syn_data);
+
+fallback:
+	/* Send a regular SYN with Fast Open cookie request option */
+	if (fo->cookie.len > 0)
+		fo->cookie.len = 0;
+	err = tcp_transmit_skb(sk, syn, 1, sk->sk_allocation);
+	if (err)
+		tp->syn_fastopen = 0;
+done:
+	fo->cookie.len = -1;  /* Exclude Fast Open option for SYN retries */
+	return err;
+}
+
+/* Build a SYN and send it off. */
+int tcp_connect(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *buff;
+	int err;
+
+	tcp_call_bpf(sk, BPF_SOCK_OPS_TCP_CONNECT_CB, 0, NULL);
+
+	if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
+		return -EHOSTUNREACH; /* Routing failure or similar. */
+
+	tcp_connect_init(sk);
+
+	if (unlikely(tp->repair)) {
+		tcp_finish_connect(sk, NULL);
+		return 0;
+	}
+
+	buff = tcp_stream_alloc_skb(sk, sk->sk_allocation, true);
+	if (unlikely(!buff))
+		return -ENOBUFS;
+
+	tcp_init_nondata_skb(buff, tp->write_seq++, TCPHDR_SYN);
+	tcp_mstamp_refresh(tp);
+	tp->retrans_stamp = tcp_time_stamp(tp);
+	tcp_connect_queue_skb(sk, buff);
+	tcp_ecn_send_syn(sk, buff);
+	tcp_rbtree_insert(&sk->tcp_rtx_queue, buff);
+
+	/* Send off SYN; include data in Fast Open. */
+	err = tp->fastopen_req ? tcp_send_syn_data(sk, buff) :
+	      tcp_transmit_skb(sk, buff, 1, sk->sk_allocation);
+	if (err == -ECONNREFUSED)
+		return err;
+
+	/* We change tp->snd_nxt after the tcp_transmit_skb() call
+	 * in order to make this packet get counted in tcpOutSegs.
+	 */
+	WRITE_ONCE(tp->snd_nxt, tp->write_seq);
+	tp->pushed_seq = tp->write_seq;
+	buff = tcp_send_head(sk);
+	if (unlikely(buff)) {
+		WRITE_ONCE(tp->snd_nxt, TCP_SKB_CB(buff)->seq);
+		tp->pushed_seq	= TCP_SKB_CB(buff)->seq;
+	}
+	TCP_INC_STATS(sock_net(sk), TCP_MIB_ACTIVEOPENS);
+
+	/* Timer for repeating the SYN until an answer. */
+	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
+				  inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
+	return 0;
+}
+EXPORT_SYMBOL(tcp_connect);
+
+u32 tcp_delack_max(const struct sock *sk)
+{
+	const struct dst_entry *dst = __sk_dst_get(sk);
+	u32 delack_max = inet_csk(sk)->icsk_delack_max;
+
+	if (dst && dst_metric_locked(dst, RTAX_RTO_MIN)) {
+		u32 rto_min = dst_metric_rtt(dst, RTAX_RTO_MIN);
+		u32 delack_from_rto_min = max_t(int, 1, rto_min - 1);
+
+		delack_max = min_t(u32, delack_max, delack_from_rto_min);
+	}
+	return delack_max;
+}
+
+/* Send out a delayed ack, the caller does the policy checking
+ * to see if we should even be here.  See tcp_input.c:tcp_ack_snd_check()
+ * for details.
+ */
+void tcp_send_delayed_ack(struct sock *sk)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	int ato = icsk->icsk_ack.ato;
+	unsigned long timeout;
+
+	if (ato > TCP_DELACK_MIN) {
+		const struct tcp_sock *tp = tcp_sk(sk);
+		int max_ato = HZ / 2;
+
+		if (inet_csk_in_pingpong_mode(sk) ||
+		    (icsk->icsk_ack.pending & ICSK_ACK_PUSHED))
+			max_ato = TCP_DELACK_MAX;
+
+		/* Slow path, intersegment interval is "high". */
+
+		/* If some rtt estimate is known, use it to bound delayed ack.
+		 * Do not use inet_csk(sk)->icsk_rto here, use results of rtt measurements
+		 * directly.
+		 */
+		if (tp->srtt_us) {
+			int rtt = max_t(int, usecs_to_jiffies(tp->srtt_us >> 3),
+					TCP_DELACK_MIN);
+
+			if (rtt < max_ato)
+				max_ato = rtt;
+		}
+
+		ato = min(ato, max_ato);
+	}
+
+	ato = min_t(u32, ato, tcp_delack_max(sk));
+
+	/* Stay within the limit we were given */
+	timeout = jiffies + ato;
+
+	/* Use new timeout only if there wasn't a older one earlier. */
+	if (icsk->icsk_ack.pending & ICSK_ACK_TIMER) {
+		/* If delack timer is about to expire, send ACK now. */
+		if (time_before_eq(icsk->icsk_ack.timeout, jiffies + (ato >> 2))) {
+			tcp_send_ack(sk);
+			return;
+		}
+
+		if (!time_before(timeout, icsk->icsk_ack.timeout))
+			timeout = icsk->icsk_ack.timeout;
+	}
+	icsk->icsk_ack.pending |= ICSK_ACK_SCHED | ICSK_ACK_TIMER;
+	icsk->icsk_ack.timeout = timeout;
+	sk_reset_timer(sk, &icsk->icsk_delack_timer, timeout);
+}
+
+/* This routine sends an ack and also updates the window. */
+void __tcp_send_ack(struct sock *sk, u32 rcv_nxt)
+{
+	struct sk_buff *buff;
+
+	/* If we have been reset, we may not send again. */
+	if (sk->sk_state == TCP_CLOSE)
+		return;
+
+	/* We are not putting this on the write queue, so
+	 * tcp_transmit_skb() will set the ownership to this
+	 * sock.
+	 */
+	buff = alloc_skb(MAX_TCP_HEADER,
+			 sk_gfp_mask(sk, GFP_ATOMIC | __GFP_NOWARN));
+	if (unlikely(!buff)) {
+		struct inet_connection_sock *icsk = inet_csk(sk);
+		unsigned long delay;
+
+		delay = TCP_DELACK_MAX << icsk->icsk_ack.retry;
+		if (delay < TCP_RTO_MAX)
+			icsk->icsk_ack.retry++;
+		inet_csk_schedule_ack(sk);
+		icsk->icsk_ack.ato = TCP_ATO_MIN;
+		inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK, delay, TCP_RTO_MAX);
+		return;
+	}
+
+	/* Reserve space for headers and prepare control bits. */
+	skb_reserve(buff, MAX_TCP_HEADER);
+	tcp_init_nondata_skb(buff, tcp_acceptable_seq(sk), TCPHDR_ACK);
+
+	/* We do not want pure acks influencing TCP Small Queues or fq/pacing
+	 * too much.
+	 * SKB_TRUESIZE(max(1 .. 66, MAX_TCP_HEADER)) is unfortunately ~784
+	 */
+	skb_set_tcp_pure_ack(buff);
+
+	/* Send it off, this clears delayed acks for us. */
+	__tcp_transmit_skb(sk, buff, 0, (__force gfp_t)0, rcv_nxt);
+}
+EXPORT_SYMBOL_GPL(__tcp_send_ack);
+
+void tcp_send_ack(struct sock *sk)
+{
+	__tcp_send_ack(sk, tcp_sk(sk)->rcv_nxt);
+}
+
+/* This routine sends a packet with an out of date sequence
+ * number. It assumes the other end will try to ack it.
+ *
+ * Question: what should we make while urgent mode?
+ * 4.4BSD forces sending single byte of data. We cannot send
+ * out of window data, because we have SND.NXT==SND.MAX...
+ *
+ * Current solution: to send TWO zero-length segments in urgent mode:
+ * one is with SEG.SEQ=SND.UNA to deliver urgent pointer, another is
+ * out-of-date with SND.UNA-1 to probe window.
+ */
+static int tcp_xmit_probe_skb(struct sock *sk, int urgent, int mib)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *skb;
+
+	/* We don't queue it, tcp_transmit_skb() sets ownership. */
+	skb = alloc_skb(MAX_TCP_HEADER,
+			sk_gfp_mask(sk, GFP_ATOMIC | __GFP_NOWARN));
+	if (!skb)
+		return -1;
+
+	/* Reserve space for headers and set control bits. */
+	skb_reserve(skb, MAX_TCP_HEADER);
+	/* Use a previous sequence.  This should cause the other
+	 * end to send an ack.  Don't queue or clone SKB, just
+	 * send it.
+	 */
+	tcp_init_nondata_skb(skb, tp->snd_una - !urgent, TCPHDR_ACK);
+	NET_INC_STATS(sock_net(sk), mib);
+	return tcp_transmit_skb(sk, skb, 0, (__force gfp_t)0);
+}
+
+/* Called from setsockopt( ... TCP_REPAIR ) */
+void tcp_send_window_probe(struct sock *sk)
+{
+	if (sk->sk_state == TCP_ESTABLISHED) {
+		tcp_sk(sk)->snd_wl1 = tcp_sk(sk)->rcv_nxt - 1;
+		tcp_mstamp_refresh(tcp_sk(sk));
+		tcp_xmit_probe_skb(sk, 0, LINUX_MIB_TCPWINPROBE);
+	}
+}
+
+/* Initiate keepalive or window probe from timer. */
+int tcp_write_wakeup(struct sock *sk, int mib)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *skb;
+
+	if (sk->sk_state == TCP_CLOSE)
+		return -1;
+
+	skb = tcp_send_head(sk);
+	if (skb && before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp))) {
+		int err;
+		unsigned int mss = tcp_current_mss(sk);
+		unsigned int seg_size = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
+
+		if (before(tp->pushed_seq, TCP_SKB_CB(skb)->end_seq))
+			tp->pushed_seq = TCP_SKB_CB(skb)->end_seq;
+
+		/* We are probing the opening of a window
+		 * but the window size is != 0
+		 * must have been a result SWS avoidance ( sender )
+		 */
+		if (seg_size < TCP_SKB_CB(skb)->end_seq - TCP_SKB_CB(skb)->seq ||
+		    skb->len > mss) {
+			seg_size = min(seg_size, mss);
+			TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
+			if (tcp_fragment(sk, TCP_FRAG_IN_WRITE_QUEUE,
+					 skb, seg_size, mss, GFP_ATOMIC))
+				return -1;
+		} else if (!tcp_skb_pcount(skb))
+			tcp_set_skb_tso_segs(skb, mss);
+
+		TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
+		err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
+		if (!err)
+			tcp_event_new_data_sent(sk, skb);
+		return err;
+	} else {
+		if (between(tp->snd_up, tp->snd_una + 1, tp->snd_una + 0xFFFF))
+			tcp_xmit_probe_skb(sk, 1, mib);
+		return tcp_xmit_probe_skb(sk, 0, mib);
+	}
+}
+
+/* A window probe timeout has occurred.  If window is not closed send
+ * a partial packet else a zero probe.
+ */
+void tcp_send_probe0(struct sock *sk)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct net *net = sock_net(sk);
+	unsigned long timeout;
+	int err;
+
+	err = tcp_write_wakeup(sk, LINUX_MIB_TCPWINPROBE);
+
+	if (tp->packets_out || tcp_write_queue_empty(sk)) {
+		/* Cancel probe timer, if it is not required. */
+		icsk->icsk_probes_out = 0;
+		icsk->icsk_backoff = 0;
+		icsk->icsk_probes_tstamp = 0;
+		return;
+	}
+
+	icsk->icsk_probes_out++;
+	if (err <= 0) {
+		if (icsk->icsk_backoff < READ_ONCE(net->ipv4.sysctl_tcp_retries2))
+			icsk->icsk_backoff++;
+		timeout = tcp_probe0_when(sk, TCP_RTO_MAX);
+	} else {
+		/* If packet was not sent due to local congestion,
+		 * Let senders fight for local resources conservatively.
+		 */
+		timeout = TCP_RESOURCE_PROBE_INTERVAL;
+	}
+
+	timeout = tcp_clamp_probe0_to_user_timeout(sk, timeout);
+	tcp_reset_xmit_timer(sk, ICSK_TIME_PROBE0, timeout, TCP_RTO_MAX);
+}
+
+int tcp_rtx_synack(const struct sock *sk, struct request_sock *req)
+{
+	const struct tcp_request_sock_ops *af_ops = tcp_rsk(req)->af_specific;
+	struct flowi fl;
+	int res;
+
+	/* Paired with WRITE_ONCE() in sock_setsockopt() */
+	if (READ_ONCE(sk->sk_txrehash) == SOCK_TXREHASH_ENABLED)
+		WRITE_ONCE(tcp_rsk(req)->txhash, net_tx_rndhash());
+	res = af_ops->send_synack(sk, NULL, &fl, req, NULL, TCP_SYNACK_NORMAL,
+				  NULL);
+	if (!res) {
+		TCP_INC_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS);
+		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
+		if (unlikely(tcp_passive_fastopen(sk))) {
+			/* sk has const attribute because listeners are lockless.
+			 * However in this case, we are dealing with a passive fastopen
+			 * socket thus we can change total_retrans value.
+			 */
+			tcp_sk_rw(sk)->total_retrans++;
+		}
+		trace_tcp_retransmit_synack(sk, req);
+	}
+	return res;
+}
+EXPORT_SYMBOL(tcp_rtx_synack);