tcp: Try to catch MSG_PEEK bug

This patch tries to print out more information when we hit the
MSG_PEEK bug in tcp_recvmsg.  It's been around since at least
2005 and it's about time that we finally fix it.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -1442,7 +1442,9 @@ int tcp_recvmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 				goto found_ok_skb;
 			if (tcp_hdr(skb)->fin)
 				goto found_fin_ok;
-			WARN_ON(!(flags & MSG_PEEK));
+			if (WARN_ON(!(flags & MSG_PEEK)))
+				printk(KERN_INFO "recvmsg bug 2: copied %X "
+				       "seq %X\n", *seq, TCP_SKB_CB(skb)->seq);
 		}
 
 		/* Well, if we have backlog, try to process it now yet. */