Gianfar: RX Recycle skb->len error

["Ben Menchaca (ben@bigfootnetworks.com)" <ben@bigfootnetworks.com>]

We are seeing some random skb data length errors on RX after long-running, full-gigabit traffic.  First, my debugging and solution are based on the following invariant assumption:
(skb->tail - skb->data) == skb->len

If this is wrong, please educate.

After some tracing, here is where the error packets seem to originate:
1.  We are cleaning rx, in gfar_clean_rx_ring;
2.  A new RX skb is drawn from the rx_recycle queue, and obey the above invariant (so, in gfar_new_skb(), __skb_dequeue returns an skb);
3.  At this point skb_reserve is called, which moves data and tail by the same calculated alignamount;
4.  So, newskb is not NULL.  However, !(bdp->status & RXBD_LAST) || (bdp->status & RXBD_ERR)) is evaluates to true;
5.  Since newskb is not NULL, we arrive at the else if (skb), which is true;
6.  skb->data = skb->head + NET_SKB_PAD is applied, and then the skb is requeued for recycling.

At this point, skb->data != skb->tail, but skb->len == 0.  When this skb is used for the next RX, it is causing issues later when we skb_put trailers, and then trust skb->len.

I would propose something like:

--- a/drivers/net/gianfar.c
+++ b/drivers/net/gianfar.c
@@ -2540,6 +2540,7 @@ 
				 * recycle list.
 				 */
 				skb->data = skb->head + NET_SKB_PAD;
+				skb_reset_tail_pointer(skb);
				__skb_queue_head(&priv->rx_recycle, skb);
			}
		} else {

Ben Menchaca
Bigfoot Networks