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

From: Alberto Garcia
Subject: Re: [Qemu-block] [PATCH v3 4/4] qcow2: Avoid memory over-allocation on compressed images
Date: Mon, 26 Feb 2018 17:35:02 +0100
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

On Thu 22 Feb 2018 08:02:44 PM CET, Eric Blake wrote:
> 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?

That would work for me, but note that this part of the commit message
also needs to be updated:

   In fact, the qcow2 spec permits an all-ones sector count, plus 511
   bytes from the sector containing the initial offset, for a maximum
   read of nearly 2 full clusters;

With those two things corrected,

Reviewed-by: Alberto Garcia <address@hidden>


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