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Re: [Qemu-devel] [PATCH for-2.11 5/5] qcow2: Refuse to get unaligned off

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH for-2.11 5/5] qcow2: Refuse to get unaligned offsets from cache
Date: Fri, 10 Nov 2017 23:16:14 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 2017-11-10 23:15, Eric Blake wrote:
> On 11/10/2017 04:00 PM, Max Reitz wrote:
>>> Trying to understand this: we have a double corruption, because we
>>> encountered a refblock that points outside of the image, but fixing the
>>> refblock in turn encounters a second refblock that points within the
>>> image but to an unaligned area.
>> No, it's the very same.  As far as I've seen it, the repair function
>> tries to fix the "refblock is outside image" error by resizing the image
>> so the refblock is inside the image.  However, the subsequent
>> bdrv_truncate() detects the alignment corruption, too, and thus marks
>> the image corrupt.
> Is resizing the image to be larger always a wise thing compared to just
> rebuilding the refcount?  If I stick a large enough out-of-image value
> in the table, can I cause a denial-of-service by making qemu try to
> allocate petabytes of storage just to bring it into range?

But it's just a qcow2 resize (with no preallocation), so nothing will be


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