10:49 a.m.
On Thu, Jan 14, 2021 at 03:22:41PM +0000, Daniel P. Berrangé wrote:
On Thu, Jan 14, 2021 at 04:02:56PM +0100, Kevin Wolf wrote:
> Am 14.01.2021 um 14:59 hat Daniel P. Berrangé geschrieben:
> > On Thu, Jan 14, 2021 at 01:52:34PM +0000, Stefan Hajnoczi wrote:
> > > On Wed, Jan 13, 2021 at 01:59:43PM -0500, John Snow wrote:
> > > > On 1/13/21 3:53 AM, Stefan Hajnoczi wrote:
> > > > > On Tue, Jan 12, 2021 at 9:10 PM John Snow
<jsnow(a)redhat.com> wrote:
> > > > > 2. Ability to watch QMP activity on a running QEMU process, e.g.
even
> > > > > when libvirt is directly connected to the monitor.
> > > > >
> > > >
> > > > That *WOULD* be extremely cool, and moves a lot closer to how
mitmproxy
> > > > works.
> > > >
> > > > (Actually, mitmproxy could theoretically be taught how to read and
> > > > understand QMP traffic, but that's not something I know how to do
or would
> > > > be prepared to mentor.)
> > > >
> > > > Is this possible to do in a post-hoc fashion? Let's say you are
using
> > > > production environment QEMU, how do we attach the QMP listener to it?
Or
> > > > does this idea require that we start QEMU in a specific fashion with
a
> > > > second debug socket that qmp-shell can connect to in order to
listen?
> > > >
> > > > ... Or do we engineer qmp-shell to open its own socket that libvirt
connects
> > > > to ...?
> > >
> > > Here is the QEMU command-line that libvirt uses on my F33 system:
> > >
> > > -chardev socket,id=charmonitor,fd=36,server,nowait
> > > -mon chardev=charmonitor,id=monitor,mode=control
> > >
> > > Goals for this feature:
> > >
> > > 1. No manual steps required for setup.
> > > 2. Ability to start/stop monitoring traffic at runtime without
> > > restarting QEMU.
> > > 3. Available to unprivileged users.
> > >
> > > I think the easiest way to achieve this is through a new QEMU monitor
> > > command. Approaches that come to mind:
> > >
> > > 1. Add a -mon debug-chardev property and a QMP command to set it at
> > > runtime. The debug-chardev receives both monitor input (commands) and
> > > output (responses and events). This does not allow MITM, rather it
> > > mirrors traffic.
> > >
> > > 2. Add a chardev-get-fd command that fetches the fd from a chardev and
> > > then use the existing chardev-change command to replace the monitor
> > > chardev with a chardev connected to qmp-shell. This inserts qmp-shell
> > > as a proxy between the QMP client and server. qmp-shell can remove
> > > itself again with another chardev-change command. This approach
> > > allows MITM. The downside is it assumes the QMP chardev is a file
> > > descriptor, so it won't work with all types of chardev.
> > >
> > > 3. Add a new chardev-proxy type that aggregates 3 chardevs: 1. an origin
> > > source chardev, 2. a monitoring sink chardev, and 3. a monitoring
> > > source chardev. The data flow is origin <-> monitoring sink
<->
> > > monitoring source <-> QMP monitor. qmp-shell creates the
monitoring
> > > sink (for receiving incoming QMP commands) and monitoring source
> > > chardev (for forwarding QMP commands or MITM commands), and then it
> > > uses change-chardev to instantiate a chardev-proxy that directs the
> > > original libvirt chardev through the monitoring sink and source.
> > >
> > > This is the most complex but also completely contained within the
> > > QEMU chardev layer.
>
> I have an idea for the QMP command name: chardev-snapshot-sync!
>
> Finally we get backing file chains for chardevs! :-)
>
> > > In all these approaches qmp-shell uses virsh qemu-monitor-command or an
> > > equivalent API to start/stop monitoring a running VM without manual
> > > setup steps.
> >
> > Why go to the trouble of adding more chardevs to a running QEMU that
> > libvirt has. qmp-shell can just directly use the libvirt Python API
> > to invoke virDomainQemuMonitorCommand to invoke QMP commands, and
> > the othe API for receiving QMP events.
> >
> > Essentially it just needs to be split into two layers. The upper
> > layer works in terms of individual QMP command/replies, and QMP
> > events. The lower layer provides a transport that is either a
> > UNIX socket, or is the libvirt QMP passthrough API.
> >
> > Or alternatively, provide a virt-qmp-shim command that listens on
> > a UNIX socket, accepts QMP commands and turns them into calls to
> > virDomainQemuMonitorCommand, and funnells back the response.
>
> I think the idea was to show the QMP traffic that libvirt produces for
> other management applications, not for the QMP shell. These APIs
> probably don't allow this?
FWIW if you want to monitor what libvirt is sending/receiving we have
a script for that that uses our systemtap probe points:
https://gitlab.com/libvirt/libvirt/-/blob/master/examples/systemtap/qemu-...
Does that require root?
Stefan