Re: [PATCH v3 1/4] block: Add trivial backing_fmt support to qcow, sheepdog, vmdk

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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@xxxxxxxxxx>

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





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