RE: [PATCH v2] ns: introduce getnspid syscall

Hi,

-----Original Message-----
From: Serge E. Hallyn [mailto:serge@hallyn.com]
Sent: Monday, June 23, 2014 9:33 PM
To: Chen, Hanxiao/陈 晗霄
Cc: Richard Weinberger; containers@lists.linux-foundation.org;
linux-kernel@vger.kernel.org; Pavel Emelyanov; linux-api@vger.kernel.org;
Serge Hallyn; Oleg Nesterov; David Howells; Eric W. Biederman; Al Viro
Subject: Re: [PATCH v2] ns: introduce getnspid syscall

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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

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numbers.

Could you please give some hints on how to expand this interface?

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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?

But this could be confused in nested ns.

Ex:
(thanks for Pavel's figure)
    init_pid_ns    ns1         ns2
t1  2
t2   `- 3          1 
t3       `- 4      `- 5        1
t4  5

a) getnspid(1, 1):
We expected it could return t2's pid(2nd 1 as pid
such as systemd in init_pid_ns),
but t3'pid is also an appropriate result.
We may get more than one returns.

b) getnspid(5, 1):
(1st 5 was expected as  pid in ns1)
t3'pid and t4's pid could both be the answer.
We could not determine which one is what we want.

So something unique like fds of ns should be
a better reference. 

Thanks,
- Chen

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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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