tcp: add TCP_MINTTL drop reason

In the unlikely case incoming packets are dropped because
of IP_MINTTL / IPV6_MINHOPCOUNT constraints...

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230201174345.2708943-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

include/net/dropreason.h

@@ -71,6 +71,7 @@
 	FN(DUP_FRAG)			\
 	FN(FRAG_REASM_TIMEOUT)		\
 	FN(FRAG_TOO_FAR)		\
+	FN(TCP_MINTTL)			\
 	FNe(MAX)
 
 /**
@@ -312,6 +313,11 @@ enum skb_drop_reason {
 	 * (/proc/sys/net/ipv4/ipfrag_max_dist)
 	 */
 	SKB_DROP_REASON_FRAG_TOO_FAR,
+	/**
+	 * @SKB_DROP_REASON_TCP_MINTTL: ipv4 ttl or ipv6 hoplimit below
+	 * the threshold (IP_MINTTL or IPV6_MINHOPCOUNT).
+	 */
+	SKB_DROP_REASON_TCP_MINTTL,
 	/**
 	 * @SKB_DROP_REASON_MAX: the maximum of drop reason, which shouldn't be
 	 * used as a real 'reason'

net/ipv4/tcp_ipv4.c

@@ -2102,6 +2102,7 @@ int tcp_v4_rcv(struct sk_buff *skb)
 		/* min_ttl can be changed concurrently from do_ip_setsockopt() */
 		if (unlikely(iph->ttl < READ_ONCE(inet_sk(sk)->min_ttl))) {
 			__NET_INC_STATS(net, LINUX_MIB_TCPMINTTLDROP);
+			drop_reason = SKB_DROP_REASON_TCP_MINTTL;
 			goto discard_and_relse;
 		}
 	}

net/ipv6/tcp_ipv6.c

@@ -1708,8 +1708,9 @@ INDIRECT_CALLABLE_SCOPE int tcp_v6_rcv(struct sk_buff *skb)
 
 	if (static_branch_unlikely(&ip6_min_hopcount)) {
 		/* min_hopcount can be changed concurrently from do_ipv6_setsockopt() */
-		if (hdr->hop_limit < READ_ONCE(tcp_inet6_sk(sk)->min_hopcount)) {
+		if (unlikely(hdr->hop_limit < READ_ONCE(tcp_inet6_sk(sk)->min_hopcount))) {
 			__NET_INC_STATS(net, LINUX_MIB_TCPMINTTLDROP);
+			drop_reason = SKB_DROP_REASON_TCP_MINTTL;
 			goto discard_and_relse;
 		}
 	}