udp: fix poll() issue with zero sized packets

Laura tracked poll() [and friends] regression caused by commit
e6afc8ac ("udp: remove headers from UDP packets before queueing")

udp_poll() needs to know if there is a valid packet in receive queue,
even if its payload length is 0.

Change first_packet_length() to return an signed int, and use -1
as the indication of an empty queue.

Fixes: e6afc8ac ("udp: remove headers from UDP packets before queueing")
Reported-by: NLaura Abbott <labbott@redhat.com>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Tested-by: NLaura Abbott <labbott@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -1182,13 +1182,13 @@ int udp_sendpage(struct sock *sk, struct page *page, int offset,
  *	@sk: socket
  *
  *	Drops all bad checksum frames, until a valid one is found.
- *	Returns the length of found skb, or 0 if none is found.
+ *	Returns the length of found skb, or -1 if none is found.
  */
-static unsigned int first_packet_length(struct sock *sk)
+static int first_packet_length(struct sock *sk)
 {
 	struct sk_buff_head list_kill, *rcvq = &sk->sk_receive_queue;
 	struct sk_buff *skb;
-	unsigned int res;
+	int res;
 
 	__skb_queue_head_init(&list_kill);
 
@@ -1203,7 +1203,7 @@ static unsigned int first_packet_length(struct sock *sk)
 		__skb_unlink(skb, rcvq);
 		__skb_queue_tail(&list_kill, skb);
 	}
-	res = skb ? skb->len : 0;
+	res = skb ? skb->len : -1;
 	spin_unlock_bh(&rcvq->lock);
 
 	if (!skb_queue_empty(&list_kill)) {
@@ -1232,7 +1232,7 @@ int udp_ioctl(struct sock *sk, int cmd, unsigned long arg)
 
 	case SIOCINQ:
 	{
-		unsigned int amount = first_packet_length(sk);
+		int amount = max_t(int, 0, first_packet_length(sk));
 
 		return put_user(amount, (int __user *)arg);
 	}
@@ -2184,7 +2184,7 @@ unsigned int udp_poll(struct file *file, struct socket *sock, poll_table *wait)
 
 	/* Check for false positives due to checksum errors */
 	if ((mask & POLLRDNORM) && !(file->f_flags & O_NONBLOCK) &&
-	    !(sk->sk_shutdown & RCV_SHUTDOWN) && !first_packet_length(sk))
+	    !(sk->sk_shutdown & RCV_SHUTDOWN) && first_packet_length(sk) == -1)
 		mask &= ~(POLLIN | POLLRDNORM);
 
 	return mask;