[RFC][PATCH] Disable CLONE_PARENT for init

[Sukadev Bhattiprolu]

Roland McGrath [roland at redhat.com] wrote:
| Yeah, that's fine.  Since Oleg's pointed out that there is indeed no way to
| avoid the leak in some global-init uses, it is fine to have a comment that
| says that global init problems are part of the reason to outlaw this usage.
| I just objected to a change that was really for container init but said it
| was only to fix something different.

Ok. How about this comment:

---

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 at us.ibm.com>
Acked-by: "Eric W. Biederman" <ebiederm at xmission.com>
---
 kernel/fork.c |   10 ++++++++++
 1 file changed, 10 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	2009-07-01 17:29:09.000000000 -0700
@@ -974,6 +974,16 @@ static struct task_struct *copy_process(
 	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;