Re: [PATCH] qcow2: Avoid integer wraparound in qcow2_co_truncate()

[Eric Blake]

On 5/4/20 8:47 AM, Alberto Garcia wrote:

> > Drop this hunk (leave zero_start unchanged), and instead...
> >
> > > So, using your numbers, pre-patch, we have zero_start = 0x90000 (0x82000
> > > rounded up to 0x10000 alignment).  post-patch, the new MIN() lowers it
> > > back to 0x8dc00 (the new size), which is unaligned.
> > >
> > > >            /*
> > > >             * Use zero clusters as much as we can. qcow2_cluster_zeroize()
> > > >             * requires a cluster-aligned start. The end may be
> > > > unaligned if it is
> > >            * at the end of the image (which it is here).
> > >            */
> > >           ret = qcow2_cluster_zeroize(bs, zero_start, offset -
> > > zero_start, 0);
> >
> > ...patch _this_ call to compute 'QEMU_ALIGN_UP(offset, s->cluster_size)
> > - zero_start' for the length.
>
> That would work, but then we would be writing zeroes beyond the end of
> the image (but still within the last cluster).
>
> The other solution is to keep my hunk and call qcow2_cluster_zeroize()
> only when offset > zero_start.

Yes, that would work, and probably less complicated.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org