Re: [PATCH v2] ns: introduce getnspid syscall

["Serge E. Hallyn" <serge-A9i7LUbDfNHQT0dZR+AlfA@public.gmane.org>]

Quoting chenhanxiao@cn.fujitsu.com (chenhanxiao@cn.fujitsu.com):
> Hi
> 
> > -----Original Message-----
> > From: Richard Weinberger [mailto:richard@nod.at]
> > Sent: Friday, June 20, 2014 7:02 PM
> > To: Chen, Hanxiao/陈 晗霄; containers@lists.linux-foundation.org;
> > linux-kernel@vger.kernel.org
> > Cc: Eric W. Biederman; Serge Hallyn; Daniel P. Berrange; Oleg Nesterov; Al Viro;
> > David Howells; Pavel Emelyanov; Vasiliy Kulikov; Gotou, Yasunori/五島 康文;
> > linux-api@vger.kernel.org
> > Subject: Re: [PATCH v2] ns: introduce getnspid syscall
> > 
> > Am 20.06.2014 12:18, schrieb Chen Hanxiao:
> > > We need a direct method of getting the pid inside containers.
> > > If some issues occurred inside container guest, host user
> > > could not know which process is in trouble just by guest pid:
> > > the users of container guest only knew the pid inside containers.
> > > This will bring obstacle for trouble shooting.
> > >
> > > int getnspid(pid_t pid, int fd1, int fd2);
> > >
> > > pid: the pid number need to be translated.
> > >
> > > fd: a file descriptor referring to one of
> > >     the namespace entries in a /proc/[pid]/ns/pid.
> > >     fd1 for destination ns(ns1), where the pid came from.
> > >     fd2 for reference ns(ns2), while fd2 = -2 means for current ns.
> > >
> > > return value:
> > >     >0 : translated pid in ns1(fd1) seen from ns2(fd2).
> > >     <=0: on failure.
> > >
> > 
> > I don't think that adding a new system call for this is a good solution.
> > We need a more generic way. I bet people are interested in more than just PID
> > numbers.
> 
> Could you please give some hints on how to expand this interface?
> 
> > 
> > I agree with Eric that a procfs solution is more appropriate.
> > 
> 
> Procfs is a good solution, but syscall is not bad though.

I might be inclined to agree, except that in this case you are still
needing mounted procfs anyway to get the proc/$pid/ns/pid fds.

I'm sorry, I've not been watching this thread, so this probably has been
considered and decided against, but I'm going to ask anyway.  Keeping
in mind both checkpoint-restart and and introspection for use in a
setns'd commend, why not make it

pid_t getnspid(pid_t query_pid, pid_t observer_pid)

which returns the process id of query_pid as seen from observer_pid's
pidns?


> Procfs works for me, but that seems could not fit
> Pavel's requirement.
> His opinion is that a syscall is a more generic interface
> than proc files, and  also very helpful.
> And syscall could tell whether a pid lives in a specific pid namespace,
> much convenient than procfs.
> 
> Thanks,
> - Chen

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