Re: Mapping between host & container PIDs ?

["Daniel P. Berrange" <berrange-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Tue, Nov 27, 2012 at 07:36:09AM -0600, Serge Hallyn wrote:
> Quoting Daniel P. Berrange (berrange-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org):
> > I'm trying to find out if there is a way to map between host and container
> > PIDs, at minimum in the host -> container direction. My use case is to be
> > able to kill processes associated with a container, based on the host PID,
> > in a race free manner.
> > 
> > Given a host PID, I can read the 'tasks' file for the container's cgroup
> > to verify that the PID is associated with the container in question. Then
> > I can kill the PID with a signal. There is a small race condition in there,
> > where the PID could die & a new process could be born using the original
> > PID. Now this might not be very likely but I was thinking that if it is
> > possible to map from a host PID to a container PID, you can do it more
> > safely. eg Lookup the container PID associted with the host PID, then
> > setns() into the container and kill the container PID. Now although there
> > is still a race condition, you are guaranteed that if the race hits you'll
> > only kill a process within the same container, not the host at large,
> > which is good when the user invoking the API is unprivileged.
> 
> I'm afraid I don't know of any way to do that.  At some point a new
> /proc/self/pids or somesuch file was suggested to get that info.
> 
> However, for your use case, what about freezing the container, checking
> again that the task exists and is in the container, killing it, then
> unfreezing it?

Yep, that's the bulletproof way, but it feels like rather a big hammer
to use

> (You also should be able to look at /proc/$pid/cgroups as a perhaps
> faster way to verify its container, as opposed to searching
> /sys/fs/cgroups/freezer/libvirt/lxc/$container/tasks;  then again it's
> more complicated to parse which might offset the searching time in most
> cases...)

Thinking about it more generally, this isn't really a container specific
problem, but rather an issue with the kill() syscall. It is the same
general class of problem as you see checking file permissions for example,
which is why you would use fstat() instead of stat() in many cases. It
might call for a way to get a FD associated with a pid (eg the /proc/$pid
dir handle) and then be able to kill() via that FD. eg something like

  dirfd = open("/proc/$pid", O_RDONLY);

  exefd = openat(dirfd, "exe", O_RDONLY);
  ...check it is the exe you think it is...

  cgroupfd = openat(dirfd, "cgroups", O_RDONLY);
  ...check the process is where you expect it to be...

  fkill(dirfd, SIG_KILL)

that's probably a whole can of worms though, so I think i'll just
restrict myself to killing processes based on the container's view
of the PID for now.

Daniel
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