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

[Daniel P. Berrangé]

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 at 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.

Regards,
Daniel
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