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SubjectRe: [RFC]Pid conversion between pid namespace
Quoting Hu Tao (hutao@cn.fujitsu.com):
> Hi,
>
> On Fri, Jul 25, 2014 at 05:34:43PM +0000, Serge Hallyn wrote:
> > Quoting chenhanxiao@cn.fujitsu.com (chenhanxiao@cn.fujitsu.com):
> > > Hi,
> > >
> > > > -----Original Message-----
> > > > From: Serge Hallyn [mailto:serge.hallyn@ubuntu.com]
> > > > Sent: Tuesday, July 15, 2014 12:16 PM
> > > > To: Chen, Hanxiao/陈 晗霄
> > > > Subject: Re: [RFC]Pid conversion between pid namespace
> > > > > A-2) syscall pid_t getnspid(pid_t query_pid, pid_t observer_pid)
> > > > > pros:
> > > > > - ns procfs free, easy to use.
> > > > > We could get rid of mounted ns procfs.
> > > > >
> > > > > cons:
> > > > > - may find multiple results in nested ns.
> > > > > We wished the new API could tell us the exact answer.
> > > > > But if getnspid return more than one results will bring trouble to admins,
> > > >
> > > > (See below for more, but) the question being posed to getnspid has precisely
> > > > one answer.
> > > >
> > > > > they had to make another decision.
> > > > > Or we marked the deepest level for translation as prerequisite.
> > > > >
> > > > > -based on current pidns, no reference ns.
> > > >
> > > > Hm, no. The intent here was that
> > > >
> > > > observer_pid would be in current ns
> > > > query_pid would be in observer_pid's ns.
> > > >
> > > > So this would be ideal for "I got a pid in a logfile created by rsyslog in
> > > > a nested contaner, what is the logged pid in my pidns."
> > > >
> > > > Taking a set of tasks (like a container with nesting) and bulding a tree
> > > > of all pids shouldn't be too difficult either. Start with the init pid,
> > > > call getnspid($pid, $init_pid) for every $pid in the container; to figure
> > > > out whether any $pid is itself a nested init_pid, we can compare the
> > > > /proc/$$/ns/pid, as well as look at getnspid($pid, $pid).
> > > I'm a little confused in this section:
> > >
> > > Ex:
> > > init_pid_ns ns1 ns2
> > > t1 2
> > > t2 `- 3 1
> > > t3 `- 4 `- 5 1
> > > t4 `-6 `-8 `-9
> > > t5 `-10 `-9 `-10
> > >
> > > For getnspid($pid, $init_pid),
> > > Does init_pid means container's init_pid such as 3 for t2?
> >
> > Right, if you're in init_pid_ns and making the query, then
> > you'd pass 3.
>
> Sorry for jumping in, but I'm not quite understanding the purpose of
> $init_pid here, does it identify the ns which the process to be
> queried is in? Also see my questions below:

I was passing in initpid for a particular reason before, the second
argument is NOT meant to be an "initpid", it's meant to be the pid
(in caller's ns) of the observer pid - the pid in whose namespace we
are querying.

> 1. Given the example above, what's the return of getnspid(9, 3)?
> Is it 6(task t4) or 10(task t5)?

Assuming the caller is in init_pid_ns, then the return value is t5.

>
> 2. if there is a process in ns1 which is a child of process 1 has pid
> 10, but not in ns2, like below:
>
> init_pid_ns ns1 ns2
> t1 2
> t2 `- 3 1
> t3 `- 4 +- 5 1
> t4 `-6 | `-8 `-9
> t5 `-10 | `-9 `-10
> t6 `-11 `-10
>
> then what is the return of getnspid(10, 3)?

Assuming the caller is in init_pid_ns, the answer is t6. The question
was "In the pid_ns belonging to t2 (pid 3), what task does the pid
10 refer to".
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