[libvirt] [Libguestfs] Quantifying libvirt errors in launching the libguestfs appliance

[Daniel P. Berrange]

On Thu, Jan 14, 2016 at 10:51:47AM +0100, Jiri Denemark wrote:
> On Wed, Jan 13, 2016 at 16:25:14 +0100, Martin Kletzander wrote:
> > On Wed, Jan 13, 2016 at 10:18:42AM +0000, Richard W.M. Jones wrote:
> > >As people may know, we frequently encounter errors caused by libvirt
> > >when running the libguestfs appliance.
> > >
> > >I wanted to find out exactly how frequently these happen and classify
> > >the errors, so I ran the 'virt-df' tool overnight 1700 times.  This
> > >tool runs several parallel qemu:///session libvirt connections both
> > >creating a short-lived appliance guest.
> > >
> > >Note that I have added Cole's patch to fix https://bugzilla.redhat.com/1271183
> > >"XML-RPC error : Cannot write data: Transport endpoint is not connected"
> > >
> > >Results:
> > >
> > >The test failed 538 times (32% of the time), which is pretty dismal.
> > >To be fair, virt-df is aggressive about how it launches parallel
> > >libvirt connections.  Most other virt-* tools use only a single
> > >libvirt connection and are consequently more reliable.
> > >
> > >Of the failures, 518 (96%) were of the form:
> > >
> > >  process exited while connecting to monitor: qemu: could not load kernel '/home/rjones/d/libguestfs/tmp/.guestfs-1000/appliance.d/kernel': Permission denied
> > >
> > >which is https://bugzilla.redhat.com/921135 or maybe
> > >https://bugzilla.redhat.com/1269975.  It's not clear to me if these
> > >bugs have different causes, but if they do then potentially we're
> > >seeing a mix of both since my test has no way to distinguish them.
> > >
> > 
> > It looks to me as the same problem.  And as the same problem we were
> > talking about bunch of time and, apparently, didn't get to a conclusion.
> > 
> > For each of the kernels, libvirt labels them (with both DAC and selinux
> > labels), then proceeds to launching qemu.  If this is done parallel, the
> > race is pretty obvious.  Could you remind me why you couldn't use
> > <seclabel model='none'/> or <seclabel relabel='no'/> or something that
> > would mitigate this?  If we cannot use this, then we need to implement
> > the <seclabel/> element for kernel and initrd.
> 
> Hmm, can't we just label kernel and initrd files the same way we label
> <shareable/> disk images, i.e., non-exclusive label so that all QEMU
> process can access them and avoid removing the label once a domain
> disappears?

We actually should treat it in the same way as <readonly/> disks,
and give it a shared read-only label. And indeed we *do* that.

The difference comes in the restore step - where we blow away the
readonly label and put it back to the original. For disks we never
restore readonly/shared labels, but for kernels we do. If we just
kill the restore step for kernels too, we should be fine AFAICT.

Regards,
Daniel
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