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Re: [Qemu-block] [PATCH v3 4/4] qcow2: Avoid memory over-allocation on c

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH v3 4/4] qcow2: Avoid memory over-allocation on compressed images
Date: Thu, 22 Feb 2018 13:02:44 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

On 02/22/2018 10:23 AM, Alberto Garcia wrote:
On Thu 22 Feb 2018 04:59:22 PM CET, Eric Blake wrote:
          sector_offset = coffset & 511;
          csize = nb_csectors * 512 - sector_offset;
+        assert(csize < 2 * s->cluster_size);

I think it should be <=

If sector_offset is 0 and nb_csector is the maximum allowed value then
csize is exactly 2 * s->cluster_size bytes.

Sigh, yes you're right. I was thinking that "qemu sets csize to a maximum of s->cluster_size, but only when sector_offset is not 0" - but as long as we're dealing with externally-produced images, sector_offset can be 0 at the same time as providing all 1s to the field. So I did indeed have an off-by-one.

Perhaps the maintainer can fix it up, instead of me spinning a v4?

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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