[PATCH] tcp: Add listening address to SYN flood message

[Jamie Bainbridge]

The SYN flood message prints the listening port number, but on a system
with many processes bound to the same port on different IPs, it's
impossible to tell which socket is the problem.

Add the listen IP address to the SYN flood message. It might have been
nicer to print the address first, but decades of monitoring tools are
watching for the string "SYN flooding on port" so don't break that.

Tested with each protcol's "any" address and a host address:

 Possible SYN flooding on port 9001. IP 0.0.0.0.
 Possible SYN flooding on port 9001. IP 127.0.0.1.
 Possible SYN flooding on port 9001. IP ::.
 Possible SYN flooding on port 9001. IP fc00::1.

Signed-off-by: Jamie Bainbridge <jamie.bainbridge@xxxxxxxxx>
---
 net/ipv4/tcp_input.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 0640453fce54b6daae0861d948f3db075830daf6..fb86056732266fedc8ad574bbf799dbdd7a425a3 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6831,9 +6831,19 @@ static bool tcp_syn_flood_action(const struct sock *sk, const char *proto)
 		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPREQQFULLDROP);
 
 	if (!queue->synflood_warned && syncookies != 2 &&
-	    xchg(&queue->synflood_warned, 1) == 0)
-		net_info_ratelimited("%s: Possible SYN flooding on port %d. %s.  Check SNMP counters.\n",
-				     proto, sk->sk_num, msg);
+	    xchg(&queue->synflood_warned, 1) == 0) {
+#if IS_ENABLED(CONFIG_IPV6)
+		if (sk->sk_family == AF_INET6) {
+			net_info_ratelimited("%s: Possible SYN flooding on port %d. IP %pI6c. %s.  Check SNMP counters.\n",
+					proto, sk->sk_num,
+					&sk->sk_v6_rcv_saddr, msg);
+		} else
+#endif
+		{
+			net_info_ratelimited("%s: Possible SYN flooding on port %d. IP %pI4. %s.  Check SNMP counters.\n",
+					proto, sk->sk_num, &sk->sk_rcv_saddr, msg);
+		}
+	}
 
 	return want_cookie;
 }
-- 
2.38.1