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Subject[RFC][PATCH] Disable CLONE_PARENT for init

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@us.ibm.com>
Acked-by: Roland McGrath <roland@redhat.com>
---
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;

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