tcp/dccp: drop SYN packets if accept queue is full

Eric Dumazet · davem330 · commit 5ea8ea2cb7f1 · 2016-10-29T15:09:21.000-04:00
Per listen(fd, backlog) rules, there is really no point accepting a SYN,
sending a SYNACK, and dropping the following ACK packet if accept queue
is full, because application is not draining accept queue fast enough.

This behavior is fooling TCP clients that believe they established a
flow, while there is nothing at server side. They might then send about
10 MSS (if using IW10) that will be dropped anyway while server is under
stress.

Signed-off-by: Eric Dumazet &lt;edumazet@google.com&gt;
Acked-by: Neal Cardwell &lt;ncardwell@google.com&gt;
Acked-by: Yuchung Cheng &lt;ycheng@google.com&gt;
Signed-off-by: David S. Miller &lt;davem@davemloft.net&gt;
diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
@@ -289,11 +289,6 @@ static inline int inet_csk_reqsk_queue_len(const struct sock *sk)
 	return reqsk_queue_len(&inet_csk(sk)->icsk_accept_queue);
 }
 
-static inline int inet_csk_reqsk_queue_young(const struct sock *sk)
-{
-	return reqsk_queue_len_young(&inet_csk(sk)->icsk_accept_queue);
-}
-
 static inline int inet_csk_reqsk_queue_is_full(const struct sock *sk)
 {
 	return inet_csk_reqsk_queue_len(sk) >= sk->sk_max_ack_backlog;
diff --git a/net/dccp/ipv4.c b/net/dccp/ipv4.c
@@ -588,13 +588,7 @@ int dccp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
 	if (inet_csk_reqsk_queue_is_full(sk))
 		goto drop;
 
-	/*
-	 * Accept backlog is full. If we have already queued enough
-	 * of warm entries in syn queue, drop request. It is better than
-	 * clogging syn queue with openreqs with exponentially increasing
-	 * timeout.
-	 */
-	if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1)
+	if (sk_acceptq_is_full(sk))
 		goto drop;
 
 	req = inet_reqsk_alloc(&dccp_request_sock_ops, sk, true);
diff --git a/net/dccp/ipv6.c b/net/dccp/ipv6.c
@@ -325,7 +325,7 @@ static int dccp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
 	if (inet_csk_reqsk_queue_is_full(sk))
 		goto drop;
 
-	if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1)
+	if (sk_acceptq_is_full(sk))
 		goto drop;
 
 	req = inet_reqsk_alloc(&dccp6_request_sock_ops, sk, true);
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
@@ -6298,13 +6298,7 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
 			goto drop;
 	}
 
-
-	/* Accept backlog is full. If we have already queued enough
-	 * of warm entries in syn queue, drop request. It is better than
-	 * clogging syn queue with openreqs with exponentially increasing
-	 * timeout.
-	 */
-	if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1) {
+	if (sk_acceptq_is_full(sk)) {
 		NET_INC_STATS(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
 		goto drop;
 	}

Original file line number	Diff line number	Diff line change
`@@ -289,11 +289,6 @@ static inline int inet_csk_reqsk_queue_len(const struct sock *sk)`
`289`	`289`	`return reqsk_queue_len(&inet_csk(sk)->icsk_accept_queue);`
`290`	`290`	`}`
`291`	`291`
`292`		`-static inline int inet_csk_reqsk_queue_young(const struct sock *sk)`
`293`		`-{`
`294`		`- return reqsk_queue_len_young(&inet_csk(sk)->icsk_accept_queue);`
`295`		`-}`
`296`		`-`
`297`	`292`	`static inline int inet_csk_reqsk_queue_is_full(const struct sock *sk)`
`298`	`293`	`{`
`299`	`294`	`return inet_csk_reqsk_queue_len(sk) >= sk->sk_max_ack_backlog;`
Original file line number	Diff line number	Diff line change
`@@ -6298,13 +6298,7 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,`
`6298`	`6298`	`goto drop;`
`6299`	`6299`	`}`
`6300`	`6300`
`6301`		`-`
`6302`		`- /* Accept backlog is full. If we have already queued enough`
`6303`		`- * of warm entries in syn queue, drop request. It is better than`
`6304`		`- * clogging syn queue with openreqs with exponentially increasing`
`6305`		`- * timeout.`
`6306`		`- */`
`6307`		`- if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1) {`
	`6301`	`+ if (sk_acceptq_is_full(sk)) {`
`6308`	`6302`	`NET_INC_STATS(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);`
`6309`	`6303`	`goto drop;`
`6310`	`6304`	`}`