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Re: [libvirt] [Qemu-devel] live snapshot wiki updated

On 07/19/2011 09:30 AM, Jes Sorensen wrote:
On 07/19/11 16:24, Eric Blake wrote:
[adding the libvir-list]
On 07/19/2011 08:09 AM, Jes Sorensen wrote:
Urgh, libvirt parsing image files is really unfortunate, it really
doesn't give me warm fuzzy feelings :( libvirt really should not know
about internals of image formats.

But even if you add new features to qemu to avoid needing this in the
future, it doesn't change the past - libvirt will always have to know
how to parse image files understood by older qemu, and so as long as
libvirt already knows how to do that parsing, we might as well take
advantage of it.

What has been done here in the past is plain wrong. Continuing to do it
isn't the right thing to do here.

Besides, I feel that having a well-documented file format, so that
independent applications can both parse the same file with the same
semantics by obeying the file format specification, is a good design goal.

We all know that documentation is rarely uptodate, new features may not
get added and libvirt will never be able to keep up. The driver for a
file format belongs in QEMU and nowhere else.

It would be nice if libvirt had a way to pass fds for every disk and
backing file up front; then, SELinux can work around the lack of NFS
per-file labelling by blocking open() in qemu.  In fact, this has
already been proposed:

A cleaner solution seems to have libvirt provide a call-back allowing
QEMU to call out and have libvirt open a file descriptor instead. This
way libvirt can validate it and open it for QEMU and pass it back.

Yes, that could probably be made to work with libvirt.

I am a little frustrated this approach wasn't taken up front instead of
the evil hack of having libvirt attempt to parse image files.

If we cannot do something like this, I would prefer to have backing
files on NFS should simply not be supported when running in an selinux

As nice as that sentiment is, it will never fly, because it would be a
regression in current behavior.  The whole reason that the virt_use_nfs
SELinux bool exists is that some people are willing to make the partial
security tradeoff.  Besides, the use of sVirt via SELinux is more than
just open() protection - while the current virt_use_nfs bool makes NFS
less secure than otherwise possible, it still gives some nice guarantees
to the rest of the qemu process such as passthrough accesses to local
pci devices.

Well leaving things at status quo is not making it worse, it just leaves
an evil in place.

NFS and SELinux is a fundamental problem with SELinux and NFS. We can piss and moan as much as we want about it but it's reality. SELinux fundamentally requires extended attributes. By the time NFS adds extended attribute support, we'll all be flying around in hover cars.

As terrible as NFS is, people use it all of the time.

It would be nice if libvirt had the ability to make better use of DAC to support isolation. The fact that MAC is the only way you can do isolation between guests is pretty unfortunate. If I could assign specific UIDs to a guest and use that to enforce isolation, it would go a long ways to solving this problem.


Anthony Liguori

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