[PATCH v2] ns: introduce getnspid syscall

[Serge Hallyn]

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

Clearly the passed-in pids should be interpreted as relative
to current's pidns.  There can be no ambiguity at that point,
unless I'm overlooking something.

> 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
> 
> > 
> > > 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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> 
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