On 06/04/2012 03:33 PM, Glauber Costa wrote:

Currently, it is possible for a process  to join existing
net, uts and ipc namespaces. This patch allows a process to join an
existing pid namespace as well.

For that to remain sane, some restrictions are made in the calling process:

* It needs to be in the parent namespace of the namespace it wants to jump to
* It needs to sit in its own session and group as a leader.

The rationale for that, is that people want to trigger actions in a Container
from the outside. For instance, mainstream linux recently gained the ability
to safely reboot a container. It would be desirable, however, that this
action is triggered from an admin in the outside world, very much like a
power switch in a physical box.

This would also allow us to connect a console to the container, provide a
repair mode for setups without networking (or with a broken one), etc.

Hi Glauber,

I am in favor of this patch but I think the pidns support won't be
complete and some corner-cases are not handled.

May be you can look at Eric's patchset [1] where, IMO, everything is
taken into account. Some of the patches may be already upstream.

Thanks
  -- Daniel

[1]
http://git.kernel.org/?p=linux/kernel/git/ebiederm/linux-namespace-control-devel.git;a=summary