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

[Eric Blake <eblake@redhat.com>]

On 3/9/20 10:21 AM, 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@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.

Do we know what formats qcow, sheepdog, and vmdk expect to probe?  I'm 
wondering if we can compromise by checking that the requested backing 
image has the specified format, and error if it is not, rather than 
completely ignoring it - but at the same time, the image formats have no 
where to record a backing format.

I'm guessing that qcow works with either raw or qcow as backing format 
(and anything else is odd - a qcow2 backing to a qcow is unusual, and 
would be better to reject).  I'm not sure if sheepdog can be backed by 
anything but another sheepdog, similarly, I'm not sure if a vmdk can be 
backed by anything but another vmdk.  If so, it should be simple enough 
to do a v4 of this patch which requires -F to be a known-acceptable 
probe type for these images.

Still, the point of this patch is that I want to add -F into all the 
iotests, and without something along the lines of this patch, all of 
those iotests are broken for these image formats.  Patch 2 is a lot 
harder to write if we have to make our use of -F conditional on the 
image format in question.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org