[RFC][PATCH] Disable CLONE_PARENT for init

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

Disable CLONE_PARENT for init

When global or container-init processes use CLONE_PARENT, they create a
multi-rooted process tree. Besides, if the siblings of init exit, the
SIGCHLD is not sent to init process resulting in the zombies sticking
around indefinitely. So disable CLONE_PARENT for init.

Lightly tested, RFC patch :-)

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-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
Acked-by: Roland McGrath <roland-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
---
 kernel/fork.c |   11 +++++++++++
 1 file changed, 11 insertions(+)

Index: linux-mmotm/kernel/fork.c
===================================================================
--- linux-mmotm.orig/kernel/fork.c	2009-06-30 23:01:06.000000000 -0700
+++ linux-mmotm/kernel/fork.c	2009-06-30 23:13:53.000000000 -0700
@@ -974,6 +974,17 @@ static struct task_struct *copy_process(
 	if ((clone_flags & CLONE_SIGHAND) && !(clone_flags & CLONE_VM))
 		return ERR_PTR(-EINVAL);
 
+	/*
+	 * Swapper process sets the handler for SIGCHLD to SIG_DFL. If init
+	 * creates a sibling and the sibling exits, the SIGCHLD is sent to
+	 * the swapper (since the swapper's handler for SIGCHLD is SIG_DFL).
+	 * But since the swapper does not reap its children, the zombie will
+	 * remain forever. So prevent init from using CLONE_PARENT.
+	 */
+	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;