net: datagram: fix unbounded loop in __skb_try_recv_datagram()

Christoph reported a stall while peeking datagram with an offset when
busy polling is enabled. __skb_try_recv_datagram() uses as the loop
termination condition 'queue empty'. When peeking, the socket
queue can be not empty, even when no additional packets are received.

Address the issue explicitly checking for receive queue changes,
as currently done by __skb_wait_for_more_packets().

Fixes: 2b5cd0df ("net: Change return type of sk_busy_loop from bool to void")
Reported-and-tested-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/datagram.c

@@ -279,7 +279,7 @@ struct sk_buff *__skb_try_recv_datagram(struct sock *sk, unsigned int flags,
 			break;
 
 		sk_busy_loop(sk, flags & MSG_DONTWAIT);
-	} while (!skb_queue_empty(&sk->sk_receive_queue));
+	} while (sk->sk_receive_queue.prev != *last);
 
 	error = -EAGAIN;