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Re: [PATCH v5] qcow2: Forbid discard in qcow2 v2 images with backing fil

From: Eric Blake
Subject: Re: [PATCH v5] qcow2: Forbid discard in qcow2 v2 images with backing files
Date: Tue, 31 Mar 2020 10:12:57 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0

On 3/31/20 6:43 AM, 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

Signed-off-by: Alberto Garcia <address@hidden>
- Fix iotests 046 and 177 with compat=0.10 [Max]

Reviewed-by: Eric Blake <address@hidden>

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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