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

[Max Reitz]

On 30.03.20 12:05, Max Reitz wrote:
> 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

I’ll have to dequeue it again, because it breaks iotests 046 and 177
(both of which already have special handling for v2-specific discard;
but it needs to be adjusted now that the discard operation no longer
reveals the backing file contents).

Max

signature.asc
Description: OpenPGP digital signature