xen/netfront: tolerate frags with no data

[ Upstream commit d81c505 ]

At least old Xen net backends seem to send frags with no real data
sometimes. In case such a fragment happens to occur with the frag limit
already reached the frontend will BUG currently even if this situation
is easily recoverable.

Modify the BUG_ON() condition accordingly.

Tested-by: Dietmar Hahn <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

(cherry picked from commit f033fba)
Orabug: 29632145
Signed-off-by: Calum Mackay <[email protected]>
Tested-by: Saar Maoz <[email protected]>
Reviewed-by: John Haxby <[email protected]>

Author: jgross1

drivers/net/xen-netfront.c

@@ -1141,7 +1141,7 @@ static RING_IDX xennet_fill_frags(struct netfront_queue *queue,
 		if (skb_shinfo(skb)->nr_frags == MAX_SKB_FRAGS) {
 			unsigned int pull_to = NETFRONT_SKB_CB(skb)->pull_to;
 
-			BUG_ON(pull_to <= skb_headlen(skb));
+			BUG_ON(pull_to < skb_headlen(skb));
 			__pskb_pull_tail(skb, pull_to - skb_headlen(skb));
 		}
 		if (unlikely(skb_shinfo(skb)->nr_frags >= MAX_SKB_FRAGS)) {