[libvirt] RFC: how to help to detect backing chain when no backing format info in images

[Peter Krempa]

On Thu, Oct 26, 2017 at 17:05:41 +0300, Nikolay Shirokovskiy wrote:
> I create transient domain with disk based on qcow2 image with 2 backing images.
> I specify qcow2 format explicitly for the top image and omit the backing
> chain in xml for create becase libvirt does not utilize this data anyway.
> Top image and its backing file don't have backing file format information 
> and format autodetection is off by default thus I get wrong backing chain
> for the disk. Second image have format raw and there is no third backing
> image obviously.

You would have to enable format detection, but that's not safe.

> 
> How can I get correct backing chain in domain xml in this case? I don't
> want to set backing file format on this images because they are backup
> images and I don't want to touch them in any way. Can we add option
> to create API not to drop backing chain info that I can specify
> on start? (Drop is forced on start in qemuDomainDetermineDiskChain)

You'll have to wait until I send the patches to allow specifying the
backing chain in the XML (some prequels were already posted with
'blockdev-add saga' in the subject)

With that you'll be able to declare the backing chain in the XML along
with the format so libvirt will not have to detect the chain (thus no
security hole and also it provides workaround).

I don't want to add any other workarounds for those since it would
likely introduce security risks.
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