void tcp_rcv_established(struct sock *sk, struct sk_buff *skb,
			 const struct tcphdr *th, unsigned int len)
{
	struct tcp_sock *tp = tcp_sk(sk);

	if (unlikely(!sk->sk_rx_dst))
		inet_csk(sk)->icsk_af_ops->sk_rx_dst_set(sk, skb);
	/*
	 *	Header prediction.
	 *	The code loosely follows the one in the famous
	 *	"30 instruction TCP receive" Van Jacobson mail.
	 *
	 *	Van's trick is to deposit buffers into socket queue
	 *	on a device interrupt, to call tcp_recv function
	 *	on the receive process context and checksum and copy
	 *	the buffer to user space. smart...
	 *
	 *	Our current scheme is not silly either but we take the
	 *	extra cost of the net_bh soft interrupt processing...
	 *	We do checksum and copy also but from device to kernel.
	 */

	tp->rx_opt.saw_tstamp = 0;

	/*	pred_flags is 0xS?10 << 16 + snd_wnd
	 *	if header_prediction is to be made
	 *	'S' will always be tp->tcp_header_len >> 2
	 *	'?' will be 0 for the fast path, otherwise pred_flags is 0 to
	 *  turn it off	(when there are holes in the receive
	 *	 space for instance)
	 *	PSH flag is ignored.
	 */

	if ((tcp_flag_word(th) & TCP_HP_BITS) == tp->pred_flags &&
	    TCP_SKB_CB(skb)->seq == tp->rcv_nxt &&
	    !after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
		int tcp_header_len = tp->tcp_header_len;

		/* Timestamp header prediction: tcp_header_len
		 * is automatically equal to th->doff*4 due to pred_flags
		 * match.
		 */

		/* Check timestamp */
		if (tcp_header_len == sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) {
			/* No? Slow path! */
			if (!tcp_parse_aligned_timestamp(tp, th))
				goto slow_path;

			/* If PAWS failed, check it more carefully in slow path */
			if ((s32)(tp->rx_opt.rcv_tsval - tp->rx_opt.ts_recent) < 0)
				goto slow_path;

			/* DO NOT update ts_recent here, if checksum fails
			 * and timestamp was corrupted part, it will result
			 * in a hung connection since we will drop all
			 * future packets due to the PAWS test.
			 */
		}

		if (len <= tcp_header_len) {
			/* Bulk data transfer: sender */
			if (len == tcp_header_len) {
				/* Predicted packet is in window by definition.
				 * seq == rcv_nxt and rcv_wup <= rcv_nxt.
				 * Hence, check seq<=rcv_wup reduces to:
				 */
				if (tcp_header_len ==
				    (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
				    tp->rcv_nxt == tp->rcv_wup)
					tcp_store_ts_recent(tp);

				/* We know that such packets are checksummed
				 * on entry.
				 */
				tcp_ack(sk, skb, 0);
				__kfree_skb(skb);
				tcp_data_snd_check(sk);
				return;
			} else { /* Header too small */
				TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
				goto discard;
			}
		} else {
			int eaten = 0;
			bool fragstolen = false;

			if (tp->ucopy.task == current &&
			    tp->copied_seq == tp->rcv_nxt &&
			    len - tcp_header_len <= tp->ucopy.len &&
			    sock_owned_by_user(sk)) {
				__set_current_state(TASK_RUNNING);

				if (!tcp_copy_to_iovec(sk, skb, tcp_header_len)) {
					/* Predicted packet is in window by definition.
					 * seq == rcv_nxt and rcv_wup <= rcv_nxt.
					 * Hence, check seq<=rcv_wup reduces to:
					 */
					if (tcp_header_len ==
					    (sizeof(struct tcphdr) +
					     TCPOLEN_TSTAMP_ALIGNED) &&
					    tp->rcv_nxt == tp->rcv_wup)
						tcp_store_ts_recent(tp);

					tcp_rcv_rtt_measure_ts(sk, skb);

					__skb_pull(skb, tcp_header_len);
					tcp_rcv_nxt_update(tp, TCP_SKB_CB(skb)->end_seq);
					NET_INC_STATS(sock_net(sk),
							LINUX_MIB_TCPHPHITSTOUSER);
					eaten = 1;
				}
			}
			if (!eaten) {
				if (tcp_checksum_complete(skb))
					goto csum_error;

				if ((int)skb->truesize > sk->sk_forward_alloc)
					goto step5;

				/* Predicted packet is in window by definition.
				 * seq == rcv_nxt and rcv_wup <= rcv_nxt.
				 * Hence, check seq<=rcv_wup reduces to:
				 */
				if (tcp_header_len ==
				    (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
				    tp->rcv_nxt == tp->rcv_wup)
					tcp_store_ts_recent(tp);

				tcp_rcv_rtt_measure_ts(sk, skb);

				NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPHPHITS);

				/* Bulk data transfer: receiver */
				eaten = tcp_queue_rcv(sk, skb, tcp_header_len,
						      &fragstolen);
			}

			tcp_event_data_recv(sk, skb);

			if (TCP_SKB_CB(skb)->ack_seq != tp->snd_una) {
				/* Well, only one small jumplet in fast path... */
				tcp_ack(sk, skb, FLAG_DATA);
				tcp_data_snd_check(sk);
				if (!inet_csk_ack_scheduled(sk))
					goto no_ack;
			}

			__tcp_ack_snd_check(sk, 0);
no_ack:
			if (eaten)
				kfree_skb_partial(skb, fragstolen);
			sk->sk_data_ready(sk);
			return;
		}
	}

slow_path:
	if (len < (th->doff << 2) || tcp_checksum_complete(skb))
		goto csum_error;

	if (!th->ack && !th->rst && !th->syn)
		goto discard;

	/*
	 *	Standard slow path.
	 */

	if (!tcp_validate_incoming(sk, skb, th, 1))
		return;

step5:
	if (tcp_ack(sk, skb, FLAG_SLOWPATH | FLAG_UPDATE_TS_RECENT) < 0)
		goto discard;

	tcp_rcv_rtt_measure_ts(sk, skb);

	/* Process urgent data. */
	tcp_urg(sk, skb, th);

	/* step 7: process the segment text */
	tcp_data_queue(sk, skb);

	tcp_data_snd_check(sk);
	tcp_ack_snd_check(sk);
	return;

csum_error:
	TCP_INC_STATS(sock_net(sk), TCP_MIB_CSUMERRORS);
	TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);

discard:
	tcp_drop(sk, skb);
}
Unknown