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Re: [PATCH v4] qcow2: Forbid discard in qcow2 v2 images with backing fil
From: |
Max Reitz |
Subject: |
Re: [PATCH v4] qcow2: Forbid discard in qcow2 v2 images with backing files |
Date: |
Mon, 30 Mar 2020 12:05:03 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 |
On 27.03.20 19:59, 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>
> ---
> v4:
> - Show output of qemu-img map when there's no backing file [Eric]
>
> v3:
> - Rebase and change iotest number
> - Show output of qemu-img map in iotest 290 [Kevin]
> - Use the l2_offset and rb_offset variables in iotest 060
>
> 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 | 12 ++---
> tests/qemu-iotests/060.out | 2 -
> tests/qemu-iotests/290 | 97 ++++++++++++++++++++++++++++++++++++++
> tests/qemu-iotests/290.out | 61 ++++++++++++++++++++++++
> tests/qemu-iotests/group | 1 +
> 6 files changed, 170 insertions(+), 9 deletions(-)
> create mode 100755 tests/qemu-iotests/290
> create mode 100644 tests/qemu-iotests/290.out
Thanks, applied to my block branch:
https://git.xanclic.moe/XanClic/qemu/commits/branch/block
Max
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