[PATCH v3] tcp: Add listening address to SYN flood message

[Jamie Bainbridge]

The SYN flood message prints the listening port number, but 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.

For IPv6 use "[IP]:port" as per RFC-5952 and to provide ease of
copy-paste to "ss" filters. For IPv4 use "IP:port" to match.

Each protcol's "any" address and a host address now look like:

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

Signed-off-by: Jamie Bainbridge <jamie.bainbridge@xxxxxxxxx>
---
v2: Place IS_ENABLED() inside if condition c/o Andrew Lunn.
    Change port printf to unsigned c/o Stephen Hemminger.
    Remove long and unhelpful "Check SNMP counters" c/o Stephen H.
v3: Use "IP:port" format c/o Eric Duamzet and Stephen H.
---
 net/ipv4/tcp_input.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 0640453fce54b6daae0861d948f3db075830daf6..6e51d8eefe19075721ec6d31036ecae9b6e0d698 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6831,9 +6831,17 @@ 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) && sk->sk_family == AF_INET6) {
+			net_info_ratelimited("%s: Possible SYN flooding on port [%pI6c]:%u. %s.\n",
+					proto, &sk->sk_v6_rcv_saddr,
+					sk->sk_num, msg);
+		} else {
+			net_info_ratelimited("%s: Possible SYN flooding on port %pI4:%u. %s.\n",
+					proto, &sk->sk_rcv_saddr,
+					sk->sk_num, msg);
+		}
+	}
 
 	return want_cookie;
 }
-- 
2.38.1