[PATCH 3.16 20/63] tcp/dccp: drop SYN packets if accept queue is full

[Ben Hutchings <ben@decadent.org.uk>]

3.16.81-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet@google.com>

commit 5ea8ea2cb7f1d0db15762c9b0bb9e7330425a071 upstream.

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 <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[bwh: Backported to 3.16: Apply TCP changes in both tcp_ipv4.c and tcp_ipv6.c]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/include/net/inet_connection_sock.h
+++ b/include/net/inet_connection_sock.h
@@ -287,11 +287,6 @@ static inline int inet_csk_reqsk_queue_l
 	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 reqsk_queue_is_full(&inet_csk(sk)->icsk_accept_queue);
--- a/net/dccp/ipv4.c
+++ b/net/dccp/ipv4.c
@@ -621,13 +621,7 @@ int dccp_v4_conn_request(struct sock *sk
 	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);
--- a/net/dccp/ipv6.c
+++ b/net/dccp/ipv6.c
@@ -391,7 +391,7 @@ static int dccp_v6_conn_request(struct s
 	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 = inet6_reqsk_alloc(&dccp6_request_sock_ops);
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1292,12 +1292,7 @@ int tcp_v4_conn_request(struct sock *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)) {
 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
 		goto drop;
 	}
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1009,7 +1009,7 @@ static int tcp_v6_conn_request(struct so
 			goto drop;
 	}
 
-	if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1) {
+	if (sk_acceptq_is_full(sk)) {
 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
 		goto drop;
 	}