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

[Max Reitz]

On 24.03.20 13:16, 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 <address@hidden>
> ---
> v2:
> 
> - Don't create the image with compat=0.10 in iotest 060 [Max]
> - Use $TEST_IMG.base for the backing image name in iotest 289 [Max]
> - Add list of unsupported options to iotest 289 [Max]
> 
>  block/qcow2.c              |  6 +++
>  tests/qemu-iotests/060     | 10 ++---
>  tests/qemu-iotests/060.out |  2 -
>  tests/qemu-iotests/289     | 91 ++++++++++++++++++++++++++++++++++++++
>  tests/qemu-iotests/289.out | 52 ++++++++++++++++++++++
>  tests/qemu-iotests/group   |  1 +
>  6 files changed, 154 insertions(+), 8 deletions(-)
>  create mode 100755 tests/qemu-iotests/289
>  create mode 100644 tests/qemu-iotests/289.out

Reviewed-by: Max Reitz <address@hidden>

(Maybe someone else wants to give a comment still, though)

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