AW: [vserver] Problem with host process ending up to be child of guest process

From: Fiedler Roman <>
Date: Mon 13 Oct 2014 - 12:47:48 BST
Message-ID: <2ECE9D9EEF1F524185270138AE23265947D15D2A@S0MSMAIL111.arc.local>


Thanks for your reply,

> Von: Herbert Poetzl []
> On Mon, Oct 13, 2014 at 08:52:36AM +0000, Fiedler Roman wrote:
> > Hello List,
> > Analyzing a problem today, It seems the the root cause is, that
> > a host process ended up being the child of a guest process
> > after using "vserver exec".
> Not sure what you expect from "vserver exec" but it is intended
> to execute processes inside the guest after all.
> If the guest has no reaper/init running, every top level guest
> process will become a child of a host process.

Yes, but how can a host process become a child of a guest process in that
way? Could some strange corner case/bug in setpgid cause that?

> > Since the guest process is in the guest's PID namespace, tools
> > on the host fail to react accordingly, e.g. bash job-control
> > gets stuck or start-stop-daemon reacts unexpected.
> Maybe details about kernel/patch/util-vserver version as well
> as the problematic command sequence which causes the "unexpected"
> behaviour plus a description what the "expected" behaviour would
> be would help a lot.

Version should be the latest available for download:

Linux version 3.14.17-vs2.3.6.13 (root@localhost) (gcc version 4.8.2) #1 SMP
Tue Sep 16 09:13:43 UTC 2014

Command sequence is

start-stop-daemon --start --background --make-pidfile --pidfile
"/var/run/" --exec /usr/bin/socat EXEC:host-stdin-bridge
"EXEC:vserver somename exec guest-stdout-bridge,nofork=1"

When running the command, the host-stdin-bridge process running on the host
will become the child of the guest-stdout-bridge command running in the
guest, thus host process returning a guest-PID via getppid

I would expect the host process to have start-stop-daemon as parent at
first, after termination if start-stop-daemon, I would guess getppid()=1
should be it for the host process.

> > Some questions to that:
> > * Can someone give a hint, how this could happen, thus being
> > able to attempt to fix the problem for now using a workaround?
> > * Is this caused by known behavior of vserver exec and misuse
> > from my side?
> > * No matter if misuse or bug, should this case be prevented
> > since it should also cause security risks?
> > A host process uses the parent pid of itself or another host
> > process, but parent pid is from guest namespace. So from my
> > understanding it could collide with PIDs from host namespace,
> > thus host process might operate on arbitrary other host
> > process, e.g. signalling or modifying it or just like with
> > bash, getting stuck running wait(-1) in endless loop?
> Let's get some details first, thanks,


Received on Mon Oct 13 12:48:29 2014
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