Re: [PATCH v2 1/3] iotests: Specify explicit backing format where sensible

[Peter Krempa]

On Wed, Feb 26, 2020 at 20:39:26 -0600, Eric Blake wrote:
> 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, convert, and amend 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, convert
> has -B but no -F, and amend has no shortcuts.
> 
> Signed-off-by: Eric Blake <address@hidden>
> ---

[...]

>  113 files changed, 414 insertions(+), 338 deletions(-)
> 
> diff --git a/tests/qemu-iotests/017 b/tests/qemu-iotests/017
> index 0a4b854e6520..585512bb296b 100755
> --- a/tests/qemu-iotests/017
> +++ b/tests/qemu-iotests/017
> @@ -66,7 +66,7 @@ echo "Creating test image with backing file"
>  echo
> 
>  TEST_IMG=$TEST_IMG_SAVE
> -_make_test_img -b "$TEST_IMG.base" 6G
> +_make_test_img -b "$TEST_IMG.base" -F $IMGFMT 6G
> 

My understanding of the intricacies of the qemu-iotest suite is not good
enoug to be able to review this patch. Specifically $IMGFMT in this
instance is also used in the '-f' switch of qemu-img in _make_test_img
and I don't know if it's expected for the backing file to share the
format.