Re: Mapping PIDs from parent->child namespaces

[Daniel Lezcano <daniel.lezcano-GANU6spQydw@public.gmane.org>]

On 01/04/2011 08:57 PM, Mike Heffner wrote:
> On 01/04/2011 11:04 AM, Daniel Lezcano wrote:
>> On 01/04/2011 12:02 AM, Mike Heffner wrote:
>>> Hi,
>>>
>>> Is it possible for a process running in a parent PID namespace to map
>>> the PID of a process running in a child's namespace from the
>>> parent->child's namespace? For example, if I span the process "myproc"
>>> with CLONE_NEWPID then a call to getpid() inside myproc will return "1"
>>> whereas in the parent's namespace that process could actually be PID
>>> "23495". I'd like to be able to know that 23495 maps to 1 in the new 
>>> NS.
>>> Obviously, just mapping the first PID is straightforward since I can
>>> just look at the result of clone(). However, mapping the PIDs of
>>> processes subsequently forked from "myproc" -- in this example -- I
>>> haven't been able to figure out.
>>
>> AFAIK, it is not possible.
>>
>> That would be very nice to show the pid<->  vpid association.
>>
>> The procfs is a good candidate to show these informations.
>>
>> That would makes sense to show the content of /proc/<pid>/status with
>> the pid relatively to the namespace.
>>
>> Let me give an example:
>>
>> Assuming the process '1234' creates a new pid namespace, and the child
>> which is '1' in the new namespace has the real pid '4321'. This one
>> mounts its /proc.
>>
>> If the process '1234' looks at /proc/4321/root/proc/1/status, it sees:
>>
>> ...
>> Tgid:    1
>> Pid:    1
>> PPid:    0
>> ...
>>
>>
>> It could be:
>>
>> ...
>> Tgid:    4321
>> Pid:    4321
>> PPid:    1234
>> ...
>>
>> as the file is inspected from the parent namespace. Of course, if the
>> file is looked from the child namespace context, we will see '1', '1'
>> and '0'.
>>
>> I suppose the patch in the kernel should very small also.
>>
>> Thoughts ?
>
> Would that mean that finding the pid->vpid association for a real PID 
> X requires checking all files /proc/<X>/root/proc/<Y>/status where Y 
> is all vpids until you find the one where Pid == X? It would be nice 
> to have a have a way to check a single file for the association where 
> vpid is not known beforehand -- unless I'm misunderstanding your 
> solution.

Hmm, right. But how do you know a pid is belonging to a specific pid 
namespace ? I mean you can have a single process creating several pid 
namespaces. So while looking at the /proc/<pid>/status, you can see 
several times the same vpid, no ?
I am not sure the kind of informations you want to collect but it is not 
really a problem to build an association table from the userspace by 
browsing the /proc/<pid>/root/proc/<vpids> and their corresponding pid 
from the 'status' file information.

Do you have an example for a pid -> vpid association without looking at 
more informations from /proc ?