// 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 <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));

  }

  /* 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 = tp->snd_cwnd;

  	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;
  	tp->snd_cwnd = 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);

  	/* If this is the first data packet sent in response to the
  	 * previous received data,
  	 * and it is a reply for ato after last received packet,
  	 * increase pingpong count.

  	 */

  	if (before(tp->lsndtime, icsk->icsk_ack.lrcvtime) &&
  	    (u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
  		inet_csk_inc_pingpong_cnt(sk);
  
  	tp->lsndtime = now;

  }

  /* Account for an ACK we sent. */

  static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts,
  				      u32 rcv_nxt)

  {

  	struct tcp_sock *tp = tcp_sk(sk);

  	if (unlikely(tp->compressed_ack > TCP_FASTRETRANS_THRESH)) {

  		NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,

  			      tp->compressed_ack - TCP_FASTRETRANS_THRESH);
  		tp->compressed_ack = TCP_FASTRETRANS_THRESH;

  		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, pkts);
  	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 (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, sock_net(sk)->ipv4.sysctl_tcp_rmem[2]);

  		space = max_t(u32, space, 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);

  	u32 old_win = tp->rcv_wnd;

  	u32 cur_win = tcp_receive_window(tp);
  	u32 new_win = __tcp_select_window(sk);
  
  	/* Never shrink the offered window */

  	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 (new_win == 0)
  			NET_INC_STATS(sock_net(sk),
  				      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 &&
  	    sock_net(sk)->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(sock_net(sk),
  				      LINUX_MIB_TCPTOZEROWINDOWADV);
  	} else if (old_win == 0) {
  		NET_INC_STATS(sock_net(sk), 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 = 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 (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_CB(skb)->sacked = 0;

  	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	(1 << 0)
  #define OPTION_TS		(1 << 1)
  #define OPTION_MD5		(1 << 2)

  #define OPTION_WSCALE		(1 << 3)

  #define OPTION_FAST_OPEN_COOKIE	(1 << 8)

  #define OPTION_SMC		(1 << 9)
  
  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 *hash_location;	/* temporary pointer, overloaded */

  	__u32 tsval, tsecr;	/* need to include OPTION_TS */
  	struct tcp_fastopen_cookie *fastopen_cookie;	/* Fast open cookie */

  };

  /* 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(__be32 *ptr, struct tcp_sock *tp,

  			      struct tcp_out_options *opts)
  {

  	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);
  }
  
  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

  }

  /* 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) &&

  	    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(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(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(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);

  	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)

  {

  	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.
  		 */

  		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 = 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;
  		}
  	}

  	smc_set_option_cond(tcp_sk(sk), ireq, 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) &&

  	    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;
  	}

  	eff_sacks = tp->rx_opt.num_sacks + tp->rx_opt.dsack;
  	if (unlikely(eff_sacks)) {

  		const unsigned int remaining = MAX_TCP_OPTION_SPACE - size;

  		opts->num_sack_blocks =

  			min_t(unsigned int, eff_sacks,

  			      (remaining - TCPOLEN_SACK_BASE_ALIGNED) /
  			      TCPOLEN_SACK_PERBLOCK);

  		if (likely(opts->num_sack_blocks))
  			size += TCPOLEN_SACK_BASE_ALIGNED +
  				opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;

  	}
  
  	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 &&

  		    tp->snd_cwnd > 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(unsigned long data)
  {
  	struct tsq_tasklet *tsq = (struct tsq_tasklet *)data;
  	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, nflags;

  	/* perform an atomic operation only if at least one flag is set */
  	do {

  		flags = sk->sk_tsq_flags;

  		if (!(flags & TCP_DEFERRED_ALL))
  			return;
  		nflags = flags & ~TCP_DEFERRED_ALL;

  	} while (cmpxchg(&sk->sk_tsq_flags, flags, nflags) != flags);

  	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_init(&tsq->tasklet,
  			     tcp_tasklet_func,
  			     (unsigned long)tsq);
  	}
  }
  
  /*
   * 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;

  
  	/* 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;

  	for (oval = READ_ONCE(sk->sk_tsq_flags);; oval = nval) {

  		struct tsq_tasklet *tsq;

  		bool empty;

  		if (!(oval & TSQF_THROTTLED) || (oval & TSQF_QUEUED))
  			goto out;

  		nval = (oval & ~TSQF_THROTTLED) | TSQF_QUEUED;

  		nval = cmpxchg(&sk->sk_tsq_flags, oval, nval);

  		if (nval != oval)
  			continue;

  		/* 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);
  }

  /* 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->skb_mstamp_ns = tp->tcp_wstamp_ns;

  	if (clone_it) {

  		TCP_SKB_CB(skb)->tx.in_flight = TCP_SKB_CB(skb)->end_seq
  			- tp->snd_una;

  		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;
  	}

  	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);

  	/* if no packet is in qdisc/device queue, then allow XPS to select

  	 * another queue. We can be called from tcp_tsq_handler()

  	 * which holds one reference to sk.

  	 *
  	 * TODO: Ideally, in-flight pure ACK packets should not matter here.
  	 * One way to get this would be to set skb->truesize = 2 on them.

  	 */

  	skb->ooo_okay = sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1);

  	/* 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;

  	skb_set_hash_from_sk(skb, sk);

  	refcount_add(skb->truesize, &sk->sk_wmem_alloc);

  	skb_set_dst_pending_confirm(skb, 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;
  		}

  	}

  	tcp_options_write((__be32 *)(th + 1), tp, &opts);

  	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));
  	}

  #ifdef CONFIG_TCP_MD5SIG
  	/* Calculate the MD5 hash, as we have all we need now */
  	if (md5) {

  		sk_nocaps_add(sk, NETIF_F_GSO_MASK);

  		tp->af_specific->calc_md5_hash(opts.hash_location,

  					       md5, sk, skb);

  	}
  #endif

  	icsk->icsk_af_ops->send_check(sk, skb);

  	if (likely(tcb->tcp_flags & TCPHDR_ACK))

  		tcp_event_ack_sent(sk, tcp_skb_pcount(skb), 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);

  	/* 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 = icsk->icsk_af_ops->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 nsize, old_factor;

  	long limit;

  	int nlen;

  	u8 flags;

  	if (WARN_ON(len > skb->len))
  		return -EINVAL;

  	nsize = skb_headlen(skb) - len;
  	if (nsize < 0)
  		nsize = 0;

  	/* 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_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(skb, gfp))

  		return -ENOMEM;
  
  	/* Get a new skb... force flag on. */

  	buff = sk_stream_alloc_skb(sk, nsize, gfp, true);

  	if (!buff)

  		return -ENOMEM; /* We'll just try again later. */

  	skb_copy_decrypted(buff, skb);

  	sk_wmem_queued_add(sk, buff->truesize);

  	sk_mem_charge(sk, buff->truesize);

  	nlen = skb->len - len - nsize;
  	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);

  	buff->ip_summed = CHECKSUM_PARTIAL;

  	buff->tstamp = skb->tstamp;

  	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;

  	eat = min_t(int, len, skb_headlen(skb));
  	if (eat) {
  		__skb_pull(skb, eat);
  		len -= eat;
  		if (!len)

  			return 0;

  	}

  	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(skb, GFP_ATOMIC))

  		return -ENOMEM;

  	delta_truesize = __pskb_trim_head(skb, len);

  
  	TCP_SKB_CB(skb)->seq += len;

  	skb->ip_summed = CHECKSUM_PARTIAL;

  	if (delta_truesize) {
  		skb->truesize	   -= delta_truesize;

  		sk_wmem_queued_add(sk, -delta_truesize);

  		sk_mem_uncharge(sk, delta_truesize);
  		sock_set_flag(sk, SOCK_QUEUE_SHRUNK);
  	}

  	/* 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, 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));
  }