tcp: handle pure FIN case correctly

When skb->len==0, the recv_actor() returns 0 too, but we also use 0
for error conditions. This patch amends this by propagating the errors
to tcp_read_skb() so that we can distinguish skb->len==0 case from
error cases.

Fixes: 04919be ("tcp: Introduce tcp_read_skb()")
Reported-by: Eric Dumazet <edumazet@google.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/skmsg.c

@@ -1194,8 +1194,9 @@ static int sk_psock_verdict_recv(struct sock *sk, struct sk_buff *skb)
 		ret = bpf_prog_run_pin_on_cpu(prog, skb);
 		ret = sk_psock_map_verd(ret, skb_bpf_redirect_fetch(skb));
 	}
-	if (sk_psock_verdict_apply(psock, skb, ret) < 0)
-		len = 0;
+	ret = sk_psock_verdict_apply(psock, skb, ret);
+	if (ret < 0)
+		len = ret;
 out:
 	rcu_read_unlock();
 	return len;

net/ipv4/tcp.c

@@ -1768,7 +1768,7 @@ int tcp_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
 	__skb_unlink(skb, &sk->sk_receive_queue);
 	WARN_ON(!skb_set_owner_sk_safe(skb, sk));
 	copied = recv_actor(sk, skb);
-	if (copied > 0) {
+	if (copied >= 0) {
 		seq += copied;
 		if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
 			++seq;