[IPv4] UFO: prevent generation of chained skb destined to UFO device

Problem: ip_append_data() could wrongly generate a chained skb for
devices which support UFO.  When sk_write_queue is not empty
(e.g. MSG_MORE), __instead__ of appending data into the next nr_frag
of the queued skb, a new chained skb is created.

I would normally assume UFO device should get data in nr_frags and not
in frag_list.  Later the udp4_hwcsum_outgoing() resets csum to NONE
and skb_gso_segment() has oops.

Proposal:
1. Even length is less than mtu, employ ip_ufo_append_data()
and append data to the __existed__ skb in the sk_write_queue.

2. ip_ufo_append_data() is fixed due to a wrong manipulation of
peek-ing and later enqueue-ing of the same skb.  Now, enqueuing is
always performed, because on error the further
ip_flush_pending_frames() would release the queued skb.
Signed-off-by: Kostya B <bkostya@hotmail.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -753,23 +753,15 @@ static inline int ip_ufo_append_data(struct sock *sk,
 		skb->ip_summed = CHECKSUM_PARTIAL;
 		skb->csum = 0;
 		sk->sk_sndmsg_off = 0;
-	}
 
-	err = skb_append_datato_frags(sk,skb, getfrag, from,
-			       (length - transhdrlen));
-	if (!err) {
-		/* specify the length of each IP datagram fragment*/
+		/* specify the length of each IP datagram fragment */
 		skb_shinfo(skb)->gso_size = mtu - fragheaderlen;
 		skb_shinfo(skb)->gso_type = SKB_GSO_UDP;
 		__skb_queue_tail(&sk->sk_write_queue, skb);
-
-		return 0;
 	}
-	/* There is not enough support do UFO ,
-	 * so follow normal path
-	 */
-	kfree_skb(skb);
-	return err;
+
+	return skb_append_datato_frags(sk, skb, getfrag, from,
+				       (length - transhdrlen));
 }
 
 /*
@@ -863,9 +855,9 @@ int ip_append_data(struct sock *sk,
 		csummode = CHECKSUM_PARTIAL;
 
 	inet->cork.length += length;
-	if (((length > mtu) && (sk->sk_protocol == IPPROTO_UDP)) &&
+	if (((length> mtu) || !skb_queue_empty(&sk->sk_write_queue)) &&
+	    (sk->sk_protocol == IPPROTO_UDP) &&
 	    (rt->u.dst.dev->features & NETIF_F_UFO)) {
-
 		err = ip_ufo_append_data(sk, getfrag, from, length, hh_len,
 					 fragheaderlen, transhdrlen, mtu,
 					 flags);