udp: Fix bogus UFO packet generation

It has been reported that the new UFO software fallback path
fails under certain conditions with NFS.  I tracked the problem
down to the generation of UFO packets that are smaller than the
MTU.  The software fallback path simply discards these packets.

This patch fixes the problem by not generating such packets on
the UFO path.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -873,8 +873,10 @@ int ip_append_data(struct sock *sk,
 	    !exthdrlen)
 		csummode = CHECKSUM_PARTIAL;
 
+	skb = skb_peek_tail(&sk->sk_write_queue);
+
 	inet->cork.length += length;
-	if (((length> mtu) || !skb_queue_empty(&sk->sk_write_queue)) &&
+	if (((length > mtu) || (skb && skb_is_gso(skb))) &&
 	    (sk->sk_protocol == IPPROTO_UDP) &&
 	    (rt->u.dst.dev->features & NETIF_F_UFO)) {
 		err = ip_ufo_append_data(sk, getfrag, from, length, hh_len,
@@ -892,7 +894,7 @@ int ip_append_data(struct sock *sk,
 	 * adding appropriate IP header.
 	 */
 
-	if ((skb = skb_peek_tail(&sk->sk_write_queue)) == NULL)
+	if (!skb)
 		goto alloc_new_skb;
 
 	while (length > 0) {
@@ -1121,7 +1123,8 @@ ssize_t	ip_append_page(struct sock *sk, struct page *page,
 		return -EINVAL;
 
 	inet->cork.length += size;
-	if ((sk->sk_protocol == IPPROTO_UDP) &&
+	if ((size + skb->len > mtu) &&
+	    (sk->sk_protocol == IPPROTO_UDP) &&
 	    (rt->u.dst.dev->features & NETIF_F_UFO)) {
 		skb_shinfo(skb)->gso_size = mtu - fragheaderlen;
 		skb_shinfo(skb)->gso_type = SKB_GSO_UDP;