Re: [PATCH v2] ns: introduce getnspid syscall

Quoting chenhanxiao@cn.fujitsu.com (chenhanxiao@cn.fujitsu.com):

Hi

quoted

-----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:

quoted

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?

quoted

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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