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Re: [libvirt] [PATCH 5/5] utils: storage: Fall back to "raw" disk on backing store parse failure

On Tue, Sep 09, 2014 at 11:04:18AM +0200, Peter Krempa wrote:
> On 09/09/14 11:01, Daniel P. Berrange wrote:
> > On Tue, Sep 09, 2014 at 10:45:48AM +0200, Peter Krempa wrote:
> >> When libvirt can't parse the backing store format for some reasons we
> >> should fall back to something safe rather than marking the backing chain
> >> as broken.
> > 
> > I'm not really convinced that falling back to raw is the "safe" option
> > vs reporting an error for the backing chain.
> > 
> > There are a few reasons why we probe backing files
> > 
> >  - To report the backing file in the storage vol XML
> >  - To relabel disks to grant QEMU access (SElinux, DAC, CGroups)
> >  - To support APIs like block rebase that affect backing file
> > 
> > I don't think that falling back to raw is going to make any of those
> > scenarios "do the right thing" when they find an unknown backing
> > store format. Rather things will simply fail in different, more
> > obscure ways due to libvirt treating the backing file differently
> > than QEMU. I think it is better than we report an explicit error
> > upfront when we can't interpret backing store, as this will make it
> > clear that libvirt can't work and likely mean that we find out about
> > new features we need to support sooner.
> > 
> Well we do exactly that right now. Although this disallows to start a VM
> that uses an obscure (read as: unknown to libvirt) backing path
> specification which causes grief.

Not being able to start a VM because we can't parse the backing store
is a desirable feature, because that means we will not have correctly
configured SELinux / DAC / cgroups for the backing store, nor correctly
reported auditing info for the disks.

> Should we then add a flag for the vm starting API that will bypass the
> check for backing chain integrity?

I don't think we need that. If we don't support the new disk format
then reporting errors is the right thing to do.

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