Re: [Qemu-devel] [qemu-devel]question on virtqueue_get_avail_bytes

[Amit Shah]

On (Mon) 19 Aug 2013 [16:30:54], Stefan Hajnoczi wrote:
> On Mon, Aug 19, 2013 at 05:28:44PM +0800, yinyin wrote:
> > Hi,all:
> >     in func virtqueue_get_avail_bytes， when found a indirect desc, we need 
> > loop over it.
> >             /* loop over the indirect descriptor table */
> >             indirect = 1;
> >             max = vring_desc_len(desc_pa, i) / sizeof(VRingDesc);
> >             num_bufs = i = 0;
> >             desc_pa = vring_desc_addr(desc_pa, i);
> >     But, It init i to 0, then use i to update desc_pa. so we will always 
> > get  :
> >     desc_pa = vring_desc_addr(desc_pa, 0);
> >     is it right？or should we update desc_pa first, then init i to 0?
> 
> Is there a way to trigger a crash or erorr from a normal running guest?
> 
> Affected devices: serial, rng, and net - they call
> virtqueue_get_avail_bytes() directly or indirectly.
> 
> > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > index 09f62c6..554ae6f 100644
> > --- a/hw/virtio/virtio.c
> > +++ b/hw/virtio/virtio.c
> > @@ -377,8 +377,8 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned 
> > int *in_bytes,
> >              /* loop over the indirect descriptor table */
> >              indirect = 1;
> >              max = vring_desc_len(desc_pa, i) / sizeof(VRingDesc);
> > -            num_bufs = i = 0;
> >              desc_pa = vring_desc_addr(desc_pa, i);
> > +           num_bufs = i = 0;
> 
> I agree, this looks wrong.  git-blame(1) doesn't reveal anything
> interesting.  Looks like this bug has been around since 2009!

Hm, why hasn't this bitten anyone yet?

                Amit