Re: [PATCH v3 1/4] block: Add trivial backing_fmt support to qcow, sheepdog, vmdk
On 3/9/20 10:36 AM, Daniel P. Berrangé wrote:
On Mon, Mar 09, 2020 at 04:21:12PM +0100, Kevin Wolf wrote:
> Am 06.03.2020 um 23:51 hat Eric Blake geschrieben:
>> For qcow2 and qed, we want to encourage the use of -F always, as these
>> formats can suffer from data corruption or security holes if backing
>> format is probed. But for other formats, the backing format cannot be
>> recorded. Making the user decide on a per-format basis whether to
>> supply a backing format string is awkward, better is to just blindly
>> accept a backing format argument even if it is ignored by the
>> contraints of the format at hand.
>>
>> Signed-off-by: Eric Blake <eblake(a)redhat.com>
>
> I'm not sure if I agree with this reasoning. Accepting and silently
> ignoring -F could give users a false sense of security. If I specify a
> -F raw and QEMU later probes qcow2, that would be very surprising.
And if the user specifies "-F raw" and we probe qcow2, and the user
does not realize this, they can become silently reliant on always
probing qcow2. If we then honour the "-F raw" option in a later
QEMU release, we'll break the behaviour they've relied on.
IMHO, we must not accept "-F fmt" unless we're in a position to
honour it.
So I'm thinking:
qemu-img create -f qcow -b backing.qcow -F qcow img.qcow => okay
qemu-img create -f qcow -b backing.raw -F raw img.qcow => okay,
slightly risky (if backing.raw is ever changed to be non-raw), but then
again, backing files tend to be read-only (do we even support commit on
qcow images, or do we limit that to qcow2?)
qemu-img create -f qcow -b backing.qcow -F raw img.qcow => fails, due
to mismatch
qemu-img create -u -f qcow -b anything -F anything img.qcow $size =>
fails: we can't write -F into the image, nor can we open anything to
probe its type to check that -F was correct
qemu-img create -f qcow -b backing.qcow img.qcow => warns, but okay
(we did not get -F, but the probe works out)
qemu-img create -f qcow -b backing.raw img.qcow => likewise warns
qemu-img create -f qcow -b backing.qcow2 img.qcow => error; new qcow
images (which you should avoid where possible anyways) must be backed by
only raw or qcow, going forward
Other scenarios? Do the above ideas look reasonable?
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org