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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Ian Campbell
> Sent: 27 March 2014 12:28
> To: Paul Durrant
> Cc: xen-devel@xxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; Wei Liu; Sander
> Eikelenboom
> Subject: Re: [PATCH net 3/3] xen-netback: BUG_ON in xenvif_rx_action()
> not catching overflow
> 
> 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.
>

Sure. I'll add an explanation.

  Paul
 
> >  This patch changes the test to actually check ring slots.
> >
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> > Cc: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
> > ---
> >  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);
> >     }
> 

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