[RFC][PATCH 2/2] Prevent container-inits from using CLONE_PARENT

[Sukadev Bhattiprolu <sukadev-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>]

Prevent container-inits from using CLONE_PARENT

If a container-init creates a sibling (using CLONE_PARENT), pid namespace
semantics become complicated:

	- the "active pid namespace" of the sibling will be the descendant
	  container, but its not obvious if that is correct.

	- if container-init exits, it will terminate the sibling, but again
	  its not clear if that is the correct behavior.

	- the sibling exists in both parent and child containers while current
	  pid namespace semantics assume that only container-init can exist
	  in both parent/child containers.

	- the parent of the sibling is not a descendant of container-init
	  (while pid namespaces assume that all processes in the container
	  are descendants of the container-init)

	- When the sibling dies, the SIGCHLD is sent to its parent (if
	  alive), i.e the signal escapes the container to a parent container.
	  (if the parent of the sibling exits, the container-init then becomes
	  the reaper of the sibling).

To keep pid namespace semantics simple, prevent container-inits from using
CLONE_PARENT at least until we have a better understanding of CLONE_PARENT
and pid-namespace interactions.

Untested, RFC patch :-)

Signed-off-by: Sukadev Bhattiprolu <sukadev-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
---
 kernel/fork.c |    8 ++++++++
 1 file changed, 8 insertions(+)

Index: linux-mmotm/kernel/fork.c
===================================================================
--- linux-mmotm.orig/kernel/fork.c	2009-06-17 18:23:23.000000000 -0700
+++ linux-mmotm/kernel/fork.c	2009-06-17 19:17:54.000000000 -0700
@@ -974,6 +974,14 @@ static struct task_struct *copy_process(
 	if ((clone_flags & CLONE_SIGHAND) && !(clone_flags & CLONE_VM))
 		return ERR_PTR(-EINVAL);
 
+	/*
+	 * To keep pid namespace semantics simple, prevent container-inits
+	 * from creating siblings.
+	 */
+	if ((clone_flags & CLONE_PARENT) &&
+			is_container_init(current) && !is_global_init(current))
+		return ERR_PTR(-EINVAL);
+
 	retval = security_task_create(clone_flags);
 	if (retval)
 		goto fork_out;