ipv4: annotate data races arount inet->min_ttl

No report yet from KCSAN, yet worth documenting the races.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/ip_sockglue.c

@@ -1352,7 +1352,10 @@ static int do_ip_setsockopt(struct sock *sk, int level, int optname,
 			goto e_inval;
 		if (val < 0 || val > 255)
 			goto e_inval;
-		inet->min_ttl = val;
+		/* tcp_v4_err() and tcp_v4_rcv() might read min_ttl
+		 * while we are changint it.
+		 */
+		WRITE_ONCE(inet->min_ttl, val);
 		break;
 
 	default:

net/ipv4/tcp_ipv4.c

@@ -508,7 +508,8 @@ int tcp_v4_err(struct sk_buff *skb, u32 info)
 	if (sk->sk_state == TCP_CLOSE)
 		goto out;
 
-	if (unlikely(iph->ttl < inet_sk(sk)->min_ttl)) {
+	/* 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);
 		goto out;
 	}
@@ -2068,7 +2069,9 @@ int tcp_v4_rcv(struct sk_buff *skb)
 			return 0;
 		}
 	}
-	if (unlikely(iph->ttl < inet_sk(sk)->min_ttl)) {
+
+	/* 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);
 		goto discard_and_relse;
 	}