[CRIU] How can I get "real pid" in pid namespace?

[Chanho Park]

> I think your need is "Other processes can identify the process from pid",
> and "convert vpid to pid" is one solution you figured out, right?
> 
> So here I offer another solution to satisfy the needs, since we have
> this function in kernel:

I know we can get vpid and pid and pidns and any other things in the kernel
level.
I mean, "Does it possible in the user level?"
To export vpid and pidns, we should implement more things in the kernel
layer.

> 
> struct task_struct *find_task_by_vpid(pid_t vnr) {
> 	return find_task_by_pid_ns(vnr, current->nsproxy->pid_ns); }
> 
> struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace
> *ns) {
> 	rcu_lockdep_assert(rcu_read_lock_held(),
> 			   "find_task_by_pid_ns() needs rcu_read_lock()"
> 			   " protection");
> 	return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID); }
> 
> In your condition, you can't get pid_ns through current, so I suggest
> you export pid_ns *and* vpid, so you can get that task_struct.
> 
> >
> >>
> >>>
> >>> I know the behavior is not suitable for the purpose of container.
> >>> I think if the virtual pid is exported to out of namespace, it
> >>> should be converted to the real pid.
> >>>
> >>> How can the process get "real pid" in the pid namespace?
> >>
> >> It can't, the process in the pid namespace can't know they are in a
> >> sub pidns or not, the mechanism decide this.
> >
> > Do we need new API like export_pid() to get the real pid in the pid
> > namespace?
> 
> I don't think so, like I said, get the real pid in the pid namespace
> will break the design.

I think it looks like blue pill and red pill in the Matrix movie.
To exit from the container, we should eat the red pill :)

Return to CRIU, if we don't use pid namespace, dumped pid will be not
changed.
If a machine already use the pid, we can't restore the state.
To avoid this problem, we should use the namespace features.
However, if we use the pid namespace, we can't support the behavior which
communicates pid information out of the container.

Best Regards,
Chanho Park