Re: [PATCH] Introduce ActivePid: in /proc/self/status (v2, was Vpid:)

From: Eric W. Biederman
Date: Thu Jun 16 2011 - 11:22:32 EST

Cedric Le Goater <clg@xxxxxxxxxx> writes:

> On 06/16/2011 03:06 PM, Oleg Nesterov wrote:
>> On 06/16, Cedric Le Goater wrote:
>>> We have a case where a task in a parent pid namespace needs to kill
>>> another task in a sub pid namespace only knowing its internal pid.
>>> the latter has been communicated to the parent task through a file or
>>> a unix socket.
>> OK, thanks, this partly answers my question... But if they communicate
>> anyway, it is not clear why the signal is needed.
> Well, user space always finds ways to challenge the kernel.
> Our case is related to HPC. The batch manager runs jobs inside lxc
> containers (using namespaces) and signals are sent to the application
> for different reasons. First, to cleanly exit but also for other more
> specific actions related to the cluster interconnects.

In that case I really recommend unix domain sockets. You likely
won't need a kernel upgrade to make use of those and their pid
translation ability.

>>> a new kill syscall could be the solution:
>>> int pidns_kill(pid_t init_pid, pid_t some_pid);
>>> where 'init_pid' identifies the namespace and 'some_pid' identifies
>>> a task in this namespace. this is very specific but why not.
>> Yes, I also thought about this. Should be trivial.
>> Or int sys_tell_me_its_pid(pid_t init_pid, pid_t some_pid).
> why not. it's even better because more general.

If we get as far as a new system call (and I don't think any of this
needs a new system call) we really should use a namespace file
descriptor to identify the pid namespace not a pid.

>> Just in case.... This is hack, yes, but in fact you do not need the
>> kernel changes to send a signal inside the namespace. You could
>> ptrace sub_init, and execute the necessary code "inside" the namespace.
> hmm, I look at that.

Looking at the ptrace interactions are definitely worthwhile.

I remember there were a few very weird things with pids when ptracing
a process in another pid namespace. It may be that ActivePid is enough
to allow the tracer to figure out the confusing information it is

I would be surprised if using ptrace to send signals is how you
want to do things. It works, and it is a great argument from
a security perspective on allowing things that we already allow.
Using ptrace to run system calls was cumbersome and not easily
portable across architectures last time I looked.

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