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

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH] qcow2: fix image corruption after committing qcow2 image into base
Date: Fri, 3 Nov 2017 13:00:22 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

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.

> +++ b/block/qcow2.c
> @@ -3594,7 +3594,8 @@ static int qcow2_make_empty(BlockDriverState *bs)
>      l1_clusters = DIV_ROUND_UP(s->l1_size, s->cluster_size / 
> sizeof(uint64_t));
>      if (s->qcow_version >= 3 && !s->snapshots &&
> -        3 + l1_clusters <= s->refcount_block_size) {
> +        3 + l1_clusters <= s->refcount_block_size &&
> +        s->crypt_method_header != QCOW_CRYPT_LUKS) {
>          /* The following function only works for qcow2 v3 images (it requires
>           * the dirty flag) and only as long as there are no snapshots 
> (because
>           * it completely empties the image). Furthermore, the L1 table and 
> three

Worth updating the comment to explain why we can't use the fast path
with LUKS encryption?

But that's minor enough that with or without a comment tweak, I'm fine with:

Reviewed-by: Eric Blake <address@hidden>

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

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