Re: [Qemu-devel] [RFC][PATCH v6 00/23] virtagent: host/guest RPC communication agent

[Michael Roth]

On 02/17/2011 02:26 AM, Jes Sorensen wrote:
> On 02/16/11 18:22, Michael Roth wrote:
> > We've seen similar behavior. I think it comes down to qemu-va being
> > linked against shared objects in the host that don't necessarily
> > coincide with what's in the guest. It's somewhat misleading that we
> > currently build qemu-va along with the binary, since qemu-va is not
> > meant to be used on the host, and the version built on the host is not
> > meant to be used in the guest.
> >
> > Really the guest binary, qemu-va, should be built in a proper build
> > environment for that particular guest. Long term it may make sense to
> > have a "guest-utils" target that isn't part of the normal host-build
> > process to reflect binaries with these kinds of requirements. For now I
> > think we'll may just end up removing qemu-va from the default make
> > target, and only build it explicitly with "make qemu-va".
>
> Hi Michael,
>
> I am not sure I was totally clear in my mail, but the crashes I saw were
> QEMU on the host that went down.  Not qemu-va running in the guest. My

Sorry, selective reading on my part. Had recently been tracking down 
issues on the guest side.

> worry is that we are adding a lot of complexity into QEMU on the host
> side which is going to be difficult to audit, especially with things
> like the HTML and XML processing. If we separated host side processing
> into a separate command, we could better protect ourselves against a
> situation where a rogue guest could kill QEMU and possibly exploit it on
> the host side. I think we should seriously look at moving the agent
> processing code out of main QEMU and into a standalone command, maybe
> qemu-va-host or something like that.

I don't think the problem is really so fundamental...if you saw a 
host-side crash it's most likely a bug/sloppy error-handling in 
virtagent. Malformed xml (from version mismatches, transports errors, 
etc) shouldn't crash xmlrpc-c... it's using a libxml parser that just 
returns an error on unexpected xml...we just need to make sure we handle 
errors appropriately.

Can you provide some details on what you ran and what the error message was?

> There has been talk about doing the same thing with the monitor in the
> past, and have it talk to the main QEMU process over QMP. This pretty
> much goes along the same lines, except that I think we need the XML
> handling moved out with it, so we couldn't just layer it directly on top
> of QMP.

I've reworked the code quite a bit so that we could potentially swap out 
the xmlrpc-c layer transparently, while retaining the same HMP/QMP 
commands/formats. A very realistic thing we can consider in the future 
is using QMP for data encapsulation in place of xmlrpc, so I'd prefer 
not to drastically change the design to work around dependencies for the 
current data encapsulation scheme. Especially considering that 
terminating the protocol within QEMU, at the HMP/QMP layer, is one of 
the key benefits that differentiates virtagent from other guest agent 
proposals.

But I think this is all a bit of a tangent if what we have here is just 
a virtagent bug.

> > P.S. Hoping to have the execute-RPCs-in-seperate-threads work done soon
> > so we can get back to integrating your patches.
>
> Sounds good!
>
> Cheers,
> Jes