Re: [PATCH v4] qcow2: Forbid discard in qcow2 v2 images with backing files

[Eric Blake <eblake@redhat.com>]

On 3/27/20 1:59 PM, Alberto Garcia wrote:
> A discard request deallocates the selected clusters so they read back
> as zeroes. This is done by clearing the cluster offset field and
> setting QCOW_OFLAG_ZERO in the L2 entry.
> 
> This flag is however only supported when qcow_version >= 3. In older
> images the cluster is simply deallocated, exposing any possible stale
> data from the backing file.
> 
> Since discard is an advisory operation it's safer to simply forbid it
> in this scenario.
> 
> Note that we are adding this check to qcow2_co_pdiscard() and not to
> qcow2_cluster_discard() or discard_in_l2_slice() because the last
> two are also used by qcow2_snapshot_create() to discard the clusters
> used by the VM state. In this case there's no risk of exposing stale
> data to the guest and we really want that the clusters are always
> discarded.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
> v4:
> - Show output of qemu-img map when there's no backing file [Eric]

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
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