net: annotate sk->sk_rcvlowat lockless reads

sock_rcvlowat() or int_sk_rcvlowat() might be called without the socket
lock for example from tcp_poll().

Use READ_ONCE() to document the fact that other cpus might change
sk->sk_rcvlowat under us and avoid KCSAN splats.

Use WRITE_ONCE() on write sides too.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>

include/net/sock.h

@@ -2271,7 +2271,9 @@ static inline long sock_sndtimeo(const struct sock *sk, bool noblock)
 
 static inline int sock_rcvlowat(const struct sock *sk, int waitall, int len)
 {
-	return (waitall ? len : min_t(int, sk->sk_rcvlowat, len)) ? : 1;
+	int v = waitall ? len : min_t(int, READ_ONCE(sk->sk_rcvlowat), len);
+
+	return v ?: 1;
 }
 
 /* Alas, with timeout socket operations are not restartable.

net/core/filter.c

@@ -4274,7 +4274,7 @@ BPF_CALL_5(bpf_setsockopt, struct bpf_sock_ops_kern *, bpf_sock,
 		case SO_RCVLOWAT:
 			if (val < 0)
 				val = INT_MAX;
-			sk->sk_rcvlowat = val ? : 1;
+			WRITE_ONCE(sk->sk_rcvlowat, val ? : 1);
 			break;
 		case SO_MARK:
 			if (sk->sk_mark != val) {

net/core/sock.c

@@ -974,7 +974,7 @@ int sock_setsockopt(struct socket *sock, int level, int optname,
 		if (sock->ops->set_rcvlowat)
 			ret = sock->ops->set_rcvlowat(sk, val);
 		else
-			sk->sk_rcvlowat = val ? : 1;
+			WRITE_ONCE(sk->sk_rcvlowat, val ? : 1);
 		break;
 
 	case SO_RCVTIMEO_OLD:

net/ipv4/tcp.c

@@ -1699,7 +1699,7 @@ int tcp_set_rcvlowat(struct sock *sk, int val)
 	else
 		cap = sock_net(sk)->ipv4.sysctl_tcp_rmem[2] >> 1;
 	val = min(val, cap);
-	sk->sk_rcvlowat = val ? : 1;
+	WRITE_ONCE(sk->sk_rcvlowat, val ? : 1);
 
 	/* Check if we need to signal EPOLLIN right now */
 	tcp_data_ready(sk);

net/sched/em_meta.c

@@ -554,7 +554,7 @@ META_COLLECTOR(int_sk_rcvlowat)
 		*err = -1;
 		return;
 	}
-	dst->value = sk->sk_rcvlowat;
+	dst->value = READ_ONCE(sk->sk_rcvlowat);
 }
 
 META_COLLECTOR(int_sk_rcvtimeo)