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

Nikolay Shirokovskiy nshirokovskiy at virtuozzo.com
Thu Oct 26 14:22:11 UTC 2017

On 26.10.2017 17:15, Peter Krempa wrote:
> 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.

Great! Thanx!

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