xen-netback: BUG_ON in xenvif_rx_action() not catching overflow

The BUG_ON to catch ring overflow in xenvif_rx_action() makes the assumption
that meta_slots_used == ring slots used. This is not necessarily the case
for GSO packets, because the non-prefix GSO protocol consumes one more ring
slot than meta-slot for the 'extra_info'. This patch changes the test to
actually check ring slots.
Signed-off-by: NPaul Durrant <paul.durrant@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netback/netback.c

@@ -482,6 +482,8 @@ static void xenvif_rx_action(struct xenvif *vif)
 
 	while ((skb = skb_dequeue(&vif->rx_queue)) != NULL) {
 		RING_IDX max_slots_needed;
+		RING_IDX old_req_cons;
+		RING_IDX ring_slots_used;
 		int i;
 
 		/* We need a cheap worse case estimate for the number of
@@ -530,8 +532,12 @@ static void xenvif_rx_action(struct xenvif *vif)
 			vif->rx_last_skb_slots = 0;
 
 		sco = (struct skb_cb_overlay *)skb->cb;
+
+		old_req_cons = vif->rx.req_cons;
 		sco->meta_slots_used = xenvif_gop_skb(skb, &npo);
-		BUG_ON(sco->meta_slots_used > max_slots_needed);
+		ring_slots_used = vif->rx.req_cons - old_req_cons;
+
+		BUG_ON(ring_slots_used > max_slots_needed);
 
 		__skb_queue_tail(&rxq, skb);
 	}