[PATCH] Disable CLONE_PARENT for init

[Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>]

Disable CLONE_PARENT for init.

When global or container-init processes use CLONE_PARENT, they create a
multi-rooted process tree. Besides siblings of global init remain as
zombies on exit since they are not reaped by their parent (swapper). So
prevent global and container-inits from creating siblings.

Changelog[v3]:
	- [Roland, Oleg] Simplify comment describing the change
Changelog[v2]:
	- Simplify patch description based on comments from Eric Biederman
	  and Oleg Nesterov.
	- [Oleg Nesterov] Use SIGNAL_UNKILLABLE instead of is_global_init()

Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Acked-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Roland McGrath <roland@redhat.com>
---
 kernel/fork.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

Index: linux-mmotm/kernel/fork.c
===================================================================
--- linux-mmotm.orig/kernel/fork.c	2009-07-16 10:03:01.000000000 -0700
+++ linux-mmotm/kernel/fork.c	2009-07-16 11:36:28.000000000 -0700
@@ -992,6 +992,16 @@
 	if ((clone_flags & CLONE_SIGHAND) && !(clone_flags & CLONE_VM))
 		return ERR_PTR(-EINVAL);
 
+	/*
+	 * Siblings of global init remain as zombies on exit since they are
+	 * not reaped by their parent (swapper). To solve this and to avoid
+	 * multi-rooted process trees, prevent global and container-inits
+	 * from creating siblings.
+	 */
+	if ((clone_flags & CLONE_PARENT) &&
+				current->signal->flags & SIGNAL_UNKILLABLE)
+		return ERR_PTR(-EINVAL);
+
 	retval = security_task_create(clone_flags);
 	if (retval)
 		goto fork_out;