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

On a Wednesday in 2020, 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 <eblake(a)redhat.com&gt; > --- [...] Test #225 still uses -b without a format: ./check -vmdk 225