Re: [Xen-devel] [PATCH V5 6/7] xen-netback: coalesce slots in TX path and fix regressions

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Sun, Apr 21, 2013 at 11:06:10PM +0100, annie li wrote:
[...]
> > -           if (unlikely(frags >= MAX_SKB_FRAGS)) {
> > -                   netdev_err(vif->dev, "Too many frags\n");
> > +           /* This guest is really using too many slots and
> > +            * considered malicious.
> > +            */
> > +           if (unlikely(slots >= max_skb_slots)) {
> > +                   netdev_err(vif->dev,
> > +                              "Malicious frontend using %d slots, 
> > threshold %u\n",
> > +                              slots, max_skb_slots);
> >                     netbk_fatal_tx_err(vif);
> >                     return -E2BIG;
> 
> It is possible that vif is freed when packet size is less than 64K here 
> but slots required >= max_skb_slots. Alough max_skb_slots can be 
> configured, this kind of packets would be dropped in following if 
> (!drop_err && slots >= XEN_NETIF_NR_SLOTS_MIN).
> 

Yes, because slots required >= max_skb_slots is considered malicious
behavior. The size of the packet doesn't matter here.

> >             }
> >   
> > -           memcpy(txp, RING_GET_REQUEST(&vif->tx, cons + frags),
> > +           /* Xen network protocol had implicit dependency on
> > +            * MAX_SKB_FRAGS. XEN_NETIF_NR_SLOTS_MIN is set to the
> > +            * historical MAX_SKB_FRAGS value 18 to honor the same
> > +            * behavior as before. Any packet using more than 18
> > +            * slots but less than max_skb_slots slots is dropped
> > +            */
> > +           if (!drop_err && slots >= XEN_NETIF_NR_SLOTS_MIN) {
> > +                   if (net_ratelimit())
> > +                           netdev_dbg(vif->dev,
> > +                                      "Too many slots (%d) exceeding limit 
> > (%d), dropping packet\n",
> > +                                      slots, XEN_NETIF_NR_SLOTS_MIN);
> > +                   drop_err = -E2BIG;
> 
> It is possible to drop packets like above(size < 64K && slot >= 
> XEN_NETIF_NR_SLOTS_MIN). I do not know how frequently this kind of 
> packets appear, maybe some SKBs with compound page(size < 64K && slot >= 
> XEN_NETIF_NR_SLOTS_MIN) are dropped here?
> 

I don't know either, but this is the default behavior of netback so here
we retain the same behavior here to avoid further regression.


Wei.

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