Re: [Xen-devel] [PATCH v2 net-next] xen-netback: add support for multicast control

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Ian Campbell [mailto:ian.campbell@xxxxxxxxxx]
> Sent: 03 September 2015 10:31
> To: Paul Durrant; Jan Beulich
> Cc: Wei Liu; xen-devel@xxxxxxxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] [PATCH v2 net-next] xen-netback: add support for
> multicast control
> 
> On Thu, 2015-09-03 at 10:00 +0100, Paul Durrant wrote:
> > >
> > > -----Original Message-----
> > > From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> > > Sent: 03 September 2015 09:57
> > > To: Paul Durrant
> > > Cc: Ian Campbell; Wei Liu; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> > > netdev@xxxxxxxxxxxxxxx
> > > Subject: Re: [Xen-devel] [PATCH v2 net-next] xen-netback: add support
> > > for
> > > multicast control
> > >
> > > > > > On 02.09.15 at 18:58, <paul.durrant@xxxxxxxxxx> wrote:
> > > > @@ -1215,6 +1289,31 @@ static void xenvif_tx_build_gops(struct
> > > xenvif_queue *queue,
> > > >                                 break;
> > > >                 }
> > > >
> > > > +               if (extras[XEN_NETIF_EXTRA_TYPE_MCAST_ADD - 1].type)
> > > > {
> > > > +                       struct xen_netif_extra_info *extra;
> > > > +
> > > > +                       extra =
> > > &extras[XEN_NETIF_EXTRA_TYPE_MCAST_ADD - 1];
> > > > +                       ret = xenvif_mcast_add(queue->vif, extra-
> > > > u.mcast.addr);
> > >
> > > What's the reason this call isn't gated on vif->multicast_control?
> > >
> >
> > No particular reason. I guess it eats a small amount of memory for no
> > gain but a well behaved frontend wouldn't send such a request and a
> > malicious one can only send 64 of them before netback starts to reject
> > them.
> 
> Perhaps a confused guest might submit them thinking they would work
> when
> actually the feature hasn't been properly negotiated and since it would
> succeed it wouldn't generate an error on the guest side?

It would, but that's essentially harmless to functionality. If the feature had 
not been negotiated properly then multicast flooding would still be in 
operation so the guest would not lose any multicasts. I can tighten things up 
if you like but as you say below it is a bit of a corner case.

  Paul

> 
> (A bit of a niche corner case I confess...)
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