tcp: TCP Fast Open Server - take SYNACK RTT after completing 3WHS

When taking SYNACK RTT samples for servers using TCP Fast Open, fix
the code to ensure that we only call tcp_valid_rtt_meas() after we
receive the ACK that completes the 3-way handshake.

Previously we were always taking an RTT sample in
tcp_v4_syn_recv_sock(). However, for TCP Fast Open connections
tcp_v4_conn_req_fastopen() calls tcp_v4_syn_recv_sock() at the time we
receive the SYN. So for TFO we must wait until tcp_rcv_state_process()
to take the RTT sample.

To fix this, we wait until after TFO calls tcp_v4_syn_recv_sock()
before we set the snt_synack timestamp, since tcp_synack_rtt_meas()
already ensures that we only take a SYNACK RTT sample if snt_synack is
non-zero. To be careful, we only take a snt_synack timestamp when
a SYNACK transmit or retransmit succeeds.

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1125,6 +1125,7 @@ static inline void tcp_openreq_init(struct request_sock *req,
 	req->cookie_ts = 0;
 	tcp_rsk(req)->rcv_isn = TCP_SKB_CB(skb)->seq;
 	tcp_rsk(req)->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
+	tcp_rsk(req)->snt_synack = 0;
 	req->mss = rx_opt->mss_clamp;
 	req->ts_recent = rx_opt->saw_tstamp ? rx_opt->rcv_tsval : 0;
 	ireq->tstamp_ok = rx_opt->tstamp_ok;

net/ipv4/tcp_input.c

@@ -5983,6 +5983,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
 				 * need req so release it.
 				 */
 				if (req) {
+					tcp_synack_rtt_meas(sk, req);
 					reqsk_fastopen_remove(sk, req, false);
 				} else {
 					/* Make sure socket is routed, for

net/ipv4/tcp_ipv4.c

@@ -868,6 +868,8 @@ static int tcp_v4_send_synack(struct sock *sk, struct dst_entry *dst,
 					    ireq->rmt_addr,
 					    ireq->opt);
 		err = net_xmit_eval(err);
+		if (!tcp_rsk(req)->snt_synack && !err)
+			tcp_rsk(req)->snt_synack = tcp_time_stamp;
 	}
 
 	return err;
@@ -1382,6 +1384,7 @@ static int tcp_v4_conn_req_fastopen(struct sock *sk,
 	struct request_sock_queue *queue = &inet_csk(sk)->icsk_accept_queue;
 	const struct inet_request_sock *ireq = inet_rsk(req);
 	struct sock *child;
+	int err;
 
 	req->retrans = 0;
 	req->sk = NULL;
@@ -1393,8 +1396,11 @@ static int tcp_v4_conn_req_fastopen(struct sock *sk,
 		kfree_skb(skb_synack);
 		return -1;
 	}
-	ip_build_and_send_pkt(skb_synack, sk, ireq->loc_addr,
+	err = ip_build_and_send_pkt(skb_synack, sk, ireq->loc_addr,
 				    ireq->rmt_addr, ireq->opt);
+	err = net_xmit_eval(err);
+	if (!err)
+		tcp_rsk(req)->snt_synack = tcp_time_stamp;
 	/* XXX (TFO) - is it ok to ignore error and continue? */
 
 	spin_lock(&queue->fastopenq->lock);
@@ -1612,7 +1618,6 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
 		isn = tcp_v4_init_sequence(skb);
 	}
 	tcp_rsk(req)->snt_isn = isn;
-	tcp_rsk(req)->snt_synack = tcp_time_stamp;
 
 	if (dst == NULL) {
 		dst = inet_csk_route_req(sk, &fl4, req);
@@ -1650,6 +1655,7 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
 		if (err || want_cookie)
 			goto drop_and_free;
 
+		tcp_rsk(req)->snt_synack = tcp_time_stamp;
 		tcp_rsk(req)->listener = NULL;
 		/* Add the request_sock to the SYN table */
 		inet_csk_reqsk_queue_hash_add(sk, req, TCP_TIMEOUT_INIT);

net/ipv6/tcp_ipv6.c

@@ -1169,7 +1169,6 @@ static int tcp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
 	}
 have_isn:
 	tcp_rsk(req)->snt_isn = isn;
-	tcp_rsk(req)->snt_synack = tcp_time_stamp;
 
 	if (security_inet_conn_request(sk, skb, req))
 		goto drop_and_release;
@@ -1180,6 +1179,7 @@ have_isn:
 	    want_cookie)
 		goto drop_and_free;
 
+	tcp_rsk(req)->snt_synack = tcp_time_stamp;
 	tcp_rsk(req)->listener = NULL;
 	inet6_csk_reqsk_queue_hash_add(sk, req, TCP_TIMEOUT_INIT);
 	return 0;