Re: [PATCH net 3/3] xen-netback: BUG_ON in xenvif_rx_action() not catching overflow

[Ian Campbell <Ian.Campbell@citrix.com>]

On Thu, 2014-03-27 at 12:23 +0000, Paul Durrant wrote:
> The BUG_ON to catch ring overflow in xenvif_rx_action() makes the assumption
> that meta_slots_used == ring slots used. This is not the case for GSO
> packets.

Can you explain why not here please.

>  This patch changes the test to actually check ring slots.
> 
> Signed-off-by: Paul 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>
> ---
>  drivers/net/xen-netback/netback.c |    8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
> index ac35489..ae7351f 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -481,6 +481,8 @@ 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
> @@ -520,8 +522,12 @@ 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);
>  	}