Re: [libvirt] [RFC] Design executing commands from within domains

On Mon, Aug 08, 2016 at 05:00:38PM +0200, Michal Privoznik wrote: > Dear list, > > while wiring qemu-ga into libvirt I've noticed that it has ability to > spawn commands inside guest. I haven't paid much attention to it then as > implementing libvirt <-> qemu-ga communication was more important. But > lately couple of requests on the list showed up where ability to spawn > various commands inside guests would be much appreciated (e.g. when > fetching some stats that HV can't know or has no support for yet - > free/df/..). > I never really liked the qemu guest agent ability to run arbitrary commands. It is basically re-inventing the shell but with really awful features. eg the having to provide all the input upfront, not having any way to stream large stdout/stderr data back to the host. Further, from a security POV it is really bad practice to have this feature in QEMU guest agent, as it makes it impossible to provide any kind of sane security confinment for the GA. IIRC, default Fedora SELinux policy will not even permit the exec command to be to used. Most of the QEMU GA commands have very tight scope so are easily confined, but 'exec' by its very nature wants todo anything. From that POV, a general purpose exec facility is really better suited to a separate command.

Also from an API modelling POV, exposing the guest agent exec in libvirt is pretty much giving up on any sense of API design. It'll just discourage anyone from ever writing any further special case guest agent commands with formal APIs. IOW, I don't think we should ever expose the qemu guest agent exec command via libvirt APIs.