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Re: [PATCH v6 08/10] iotests: Specify explicit backing format where sens

From: Eric Blake
Subject: Re: [PATCH v6 08/10] iotests: Specify explicit backing format where sensible
Date: Tue, 7 Jul 2020 14:57:04 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0

On 7/7/20 11:07 AM, Kevin Wolf wrote:
Am 06.07.2020 um 22:39 hat Eric Blake geschrieben:
There are many existing qcow2 images that specify a backing file but
no format.  This has been the source of CVEs in the past, but has
become more prominent of a problem now that libvirt has switched to
-blockdev.  With older -drive, at least the probing was always done by
qemu (so the only risk of a changed format between successive boots of
a guest was if qemu was upgraded and probed differently).  But with
newer -blockdev, libvirt must specify a format; if libvirt guesses raw
where the image was formatted, this results in data corruption visible
to the guest; conversely, if libvirt guesses qcow2 where qemu was
using raw, this can result in potential security holes, so modern
libvirt instead refuses to use images without explicit backing format.

The change in libvirt to reject images without explicit backing format
has pointed out that a number of tools have been far too reliant on
probing in the past.  It's time to set a better example in our own
iotests of properly setting this parameter.

iotest calls to create, rebase, and convert are all impacted to some
degree.  It's a bit annoying that we are inconsistent on command line
- while all of those accept -o backing_file=...,backing_fmt=..., the
shortcuts are different: create and rebase have -b and -F, while
convert has -B but no -F.  (amend has no shortcuts, but the previous
patch just deprecated the use of amend to change backing chains).

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

This breaks at least 024 and 043 for qed because qemu-img info can't
print the backing file format there (qed only saves a flag whether it's
raw or non-raw).

Shoot. I tend to avoid tests of qed and qcow (because they take so long), so while I may have tested them around v1 or v2 of the series, all the rebasing has changed the results by now. Such is life.

We can fix the output filtering during the freeze, though.

Yes indeed, and I'll post such patches soon, now that you've pointed it out.

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

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