[Lxc-users] mapping host PID <-> container PID

[Serge Hallyn]

Quoting Daniel Lezcano (daniel.lezcano at free.fr):
> On 05/03/2011 05:36 PM, Greg Kurz wrote:
> >On Thu, 2011-04-28 at 09:41 -0500, Serge Hallyn wrote:
> >>Quoting Ulli Horlacher (framstag at rus.uni-stuttgart.de):
> >>>Is there a way to get the corresponding host PID for a container PID?
> >>>
> >>>For example: inside the the container the process "init" has always PID 1.
> >>>But what PID has this process in the host process table?
> >>>
> >>>ps aux | grep ... is not what I am looking for, I want more robust solution.
> >>There is nothing that gives you a 100% guaranteed correct race-free
> >>correspondence right now.  You can look under /proc/<pid>/root/proc/ to
> >>see the pids valid in the container, and you can relate output of
> >>lxc-ps --forest to ps --forest output.  But nothing under /proc that I
> >>know of tells you "this task is the same as that task".  You can't
> >>even look at /proc/<pid>  inode numbers since they are different
> >>filesystems for each proc mount.
> >>
> >>It's tempting to say that we should put a per-task unique id under
> >>/proc/<pid>  for each task.  However that would likely be nacked because
> >>it introduces a new namespace of its own.
> >>
> >An alternative could be to expose the container pid
> >in /proc/<pid>/status. Could such a patch make it to mainline ?
> >
> >--- a/fs/proc/array.c
> >+++ b/fs/proc/array.c
> >@@ -337,6 +337,12 @@ static void task_cpus_allowed(struct seq_file *m,
> >struct task_struct *task)
> >  	seq_putc(m, '\n');
> >  }
> >
> >+static void task_vpid(struct seq_file *m, struct task_struct *task)
> >+{
> >+	struct pid_namespace *ns = task_active_pid_ns(task);
> >+	seq_printf(m, "Vpid:\t%d\n", ns ? task_pid_nr_ns(task, ns) : 0);
> >+}
> >+
> >  int proc_pid_status(struct seq_file *m, struct pid_namespace *ns,
> >  			struct pid *pid, struct task_struct *task)
> >  {
> >@@ -354,6 +360,7 @@ int proc_pid_status(struct seq_file *m, struct
> >pid_namespace *ns,
> >  	task_cpus_allowed(m, task);
> >  	cpuset_task_status_allowed(m, task);
> >  	task_context_switch_counts(m, task);
> >+	task_vpid(m, task);
> >  	return 0;
> >  }
> >
> >Signed-off-by: Greg Kurz<gkurz at fr.ibm.com>
> 
> I think we should propose this patch for mainline inclusion. The
> vpid does not give, by its own, enough information for the pid
> namespace. How can we rebuild a pid ns tree ? I guess we can look
> for the vpid 1 as the root node of the process tree no ?

You mean find pid 1 for the task's container, and print out it's
pid in current's pid_ns, i.e.

Container_init: <pid>

That'd be very useful, and, again, does not AFAICS risk introducing
a new namespace.

> Otherwise:
> Acked-by: Daniel Lezcano <daniel.lezcano at free.fr>