tcp: Do not use TSO/GSO when there is urgent data

This patch fixes http://bugzilla.kernel.org/show_bug.cgi?id=12014

Since most (if not all) implementations of TSO and even the in-kernel
software GSO do not update the urgent pointer when splitting a large
segment, it is necessary to turn off TSO/GSO for all outgoing traffic
with the URG pointer set.

Looking at tcp_current_mss (and the preceding comment) I even think
this was the original intention. However, this approach is insufficient,
because TSO/GSO is turned off only for newly created frames, not for
frames which were already pending at the arrival of a message with
MSG_OOB set. These frames were created when TSO/GSO was enabled,
so they may be large, and they will have the urgent pointer set
in tcp_transmit_skb().

With this patch, such large packets will be fragmented again before
going to the transmit routine.

As a side note, at least the following NICs are known to screw up
the urgent pointer in the TCP header when doing TSO:

	Intel 82566MM (PCI ID 8086:1049)
	Intel 82566DC (PCI ID 8086:104b)
	Intel 82541GI (PCI ID 8086:1076)
	Broadcom NetXtreme II BCM5708 (PCI ID 14e4:164c)
Signed-off-by: NPetr Tesarik <ptesarik@suse.cz>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -722,7 +722,8 @@ static void tcp_queue_skb(struct sock *sk, struct sk_buff *skb)
 static void tcp_set_skb_tso_segs(struct sock *sk, struct sk_buff *skb,
 				 unsigned int mss_now)
 {
-	if (skb->len <= mss_now || !sk_can_gso(sk)) {
+	if (skb->len <= mss_now || !sk_can_gso(sk) ||
+	    tcp_urg_mode(tcp_sk(sk))) {
 		/* Avoid the costly divide in the normal
 		 * non-TSO case.
 		 */
@@ -1163,7 +1164,9 @@ static int tcp_init_tso_segs(struct sock *sk, struct sk_buff *skb,
 {
 	int tso_segs = tcp_skb_pcount(skb);
 
-	if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now)) {
+	if (!tso_segs ||
+	    (tso_segs > 1 && (tcp_skb_mss(skb) != mss_now ||
+			      tcp_urg_mode(tcp_sk(sk))))) {
 		tcp_set_skb_tso_segs(sk, skb, mss_now);
 		tso_segs = tcp_skb_pcount(skb);
 	}