tcp: fix MSG_PEEK race check

Commit 518a09ef (tcp: Fix recvmsg MSG_PEEK influence of
blocking behavior) lets the loop run longer than the race check
did previously expect, so we need to be more careful with this
check and consider the work we have been doing.

I tried my best to deal with urg hole madness too which happens
here:
	if (!sock_flag(sk, SOCK_URGINLINE)) {
		++*seq;
		...
by using additional offset by one but I certainly have very
little interest in testing that part.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Tested-by: Frans Pop <elendil@planet.nl>
Tested-by: Ian Zimmermann <itz@buug.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -1321,6 +1321,7 @@ int tcp_recvmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 	struct task_struct *user_recv = NULL;
 	int copied_early = 0;
 	struct sk_buff *skb;
+	u32 urg_hole = 0;
 
 	lock_sock(sk);
 
@@ -1532,7 +1533,8 @@ do_prequeue:
 				}
 			}
 		}
-		if ((flags & MSG_PEEK) && peek_seq != tp->copied_seq) {
+		if ((flags & MSG_PEEK) &&
+		    (peek_seq - copied - urg_hole != tp->copied_seq)) {
 			if (net_ratelimit())
 				printk(KERN_DEBUG "TCP(%s:%d): Application bug, race in MSG_PEEK.\n",
 				       current->comm, task_pid_nr(current));
@@ -1553,6 +1555,7 @@ do_prequeue:
 				if (!urg_offset) {
 					if (!sock_flag(sk, SOCK_URGINLINE)) {
 						++*seq;
+						urg_hole++;
 						offset++;
 						used--;
 						if (!used)