Re: [RFC]Pid conversion between pid namespace

[Hu Tao]

Hi,

On Fri, Jul 25, 2014 at 05:34:43PM +0000, Serge Hallyn wrote:
> Quoting chenhanxiao@xxxxxxxxxxxxxx (chenhanxiao@xxxxxxxxxxxxxx):
> > Hi,
> > 
> > > -----Original Message-----
> > > From: Serge Hallyn [mailto:serge.hallyn@xxxxxxxxxx]
> > > Sent: Tuesday, July 15, 2014 12:16 PM
> > > To: Chen, Hanxiao/é æé
> > > 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:

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

Regards,
Hu

>
> 
> > In nested containers, does this syscall work as:
> > getnspid(9, 4) -> (6, 8, 9) 
> 
> No, assuming the querying task is in init_pid_ns,
> getnspid(9, 4) would return 6.
> 
> 4 is the observer pid given in the querier's own pidns, so
> it refers to t3.  9 is the pid being queried, in the oberver's
> pidns, so it revers to t4.  The result is, the pid in our own
> pidns.
> 
> Does that help clarify at all?  I'm not sure whether the problem is that
> I didn't explain well enough from the start, or whether this just shows
> that the API is one only its mother could love :)
> 
> -serge
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