tcp_recvmsg: Fix application bug induced races with MSG_PEEK and copied_seq.

net/ipv4/tcp.c

@@ -1674,6 +1674,12 @@ int tcp_recvmsg(struct sock *sk, struct msghdr *msg,
 				}
 			}
 		}
+		if ((flags & MSG_PEEK) && peek_seq != tp->copied_seq) {
+			if (net_ratelimit())
+				printk(KERN_DEBUG "TCP(%s:%d): Application bug, race in MSG_PEEK.\n",
+				       current->comm, current->pid);
+			peek_seq = tp->copied_seq;
+		}
 		continue;
 
 	found_ok_skb: