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Re: [Qemu-devel] [PATCH] qcow2: fix image corruption after committing qc

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH] qcow2: fix image corruption after committing qcow2 image into base
Date: Tue, 14 Nov 2017 14:52:27 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 2017-11-10 18:22, Daniel P. Berrange wrote:
> On Fri, Nov 10, 2017 at 10:34:59AM -0600, Eric Blake wrote:
>> On 11/03/2017 09:41 AM, Daniel P. Berrange wrote:
>>> After committing the qcow2 image contents into the base image, qemu-img
>>> will call bdrv_make_empty to drop the payload in the layered image.
>>> When this is done for qcow2 images, it blows away the LUKS encryption
>>> header, making the resulting image unusable. There are two codepaths
>>> for emptying a qcow2 image, and the second (slower) codepaths leaves
>>> the LUKS header intact, so force use of that codepath.
>>> Signed-off-by: Daniel P. Berrange <address@hidden>
>>> ---
>>> NB, ideally we would fix the faster codepath in make_completely_empty, but
>>> having looked at the code, I've really no idea how to even start on fixing 
>>> that
>>> to not kill the LUKS header clusters.
>> Hmm - I wonder if persistent bitmaps are also corrupted in the fast path.
> I also wonder if there's anything better we can do to make us safer by
> default, so we default to the slow & safe path, unless we can provide
> we *only* have the subset of features that are safe for the fast path ?

I have wondered the same but I can't think of any.  The only thing that
comes close would be to check for which header extensions there are; but
at the same time, we could just add a comment to qcow2_read_extensions()
("If you add a new feature to qcow2, note that you may want to adjust
the qcow2_make_empty() fastpath conditions").


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