xen-netback: correctly return errors from netbk_count_requests()

netbk_count_requests() could detect an error, call
netbk_fatal_tx_error() but return 0.  The vif may then be used
afterwards (e.g., in a call to netbk_tx_error().

Since netbk_fatal_tx_error() could set vif->refcnt to 1, the vif may
be freed immediately after the call to netbk_fatal_tx_error() (e.g.,
if the vif is also removed).

Netback thread              Xenwatch thread
-------------------------------------------
netbk_fatal_tx_err()        netback_remove()
                              xenvif_disconnect()
                                ...
                                free_netdev()
netbk_tx_err() Oops!
Signed-off-by: NWei Liu <wei.liu2@citrix.com>
Signed-off-by: NJan Beulich <JBeulich@suse.com>
Signed-off-by: NDavid Vrabel <david.vrabel@citrix.com>
Reported-by: NChristopher S. Aker <caker@theshore.net>
Acked-by: NIan Campbell <ian.campbell@citrix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netback/netback.c

@@ -911,13 +911,13 @@ static int netbk_count_requests(struct xenvif *vif,
 		if (frags >= work_to_do) {
 			netdev_err(vif->dev, "Need more frags\n");
 			netbk_fatal_tx_err(vif);
-			return -frags;
+			return -ENODATA;
 		}
 
 		if (unlikely(frags >= MAX_SKB_FRAGS)) {
 			netdev_err(vif->dev, "Too many frags\n");
 			netbk_fatal_tx_err(vif);
-			return -frags;
+			return -E2BIG;
 		}
 
 		memcpy(txp, RING_GET_REQUEST(&vif->tx, cons + frags),
@@ -925,7 +925,7 @@ static int netbk_count_requests(struct xenvif *vif,
 		if (txp->size > first->size) {
 			netdev_err(vif->dev, "Frag is bigger than frame.\n");
 			netbk_fatal_tx_err(vif);
-			return -frags;
+			return -EIO;
 		}
 
 		first->size -= txp->size;
@@ -935,7 +935,7 @@ static int netbk_count_requests(struct xenvif *vif,
 			netdev_err(vif->dev, "txp->offset: %x, size: %u\n",
 				 txp->offset, txp->size);
 			netbk_fatal_tx_err(vif);
-			return -frags;
+			return -EINVAL;
 		}
 	} while ((txp++)->flags & XEN_NETTXF_more_data);
 	return frags;