Quoting sukadev@us.ibm.com (sukadev@us.ibm.com):
> 
> Subject: Enable signaling child reaper from parent ns.
> 
> From: Sukadev Bhattiprolu <sukadev@us.ibm.com>
> 
> The reaper of a child namespace must receive signals from its parent pid
> namespace but not receive any signals from its own namespace. 
> 
> This is a very early draft :-) and following tests seem to pass 
> 
> 	- Successfully kill child reaper from parent namespace (init_pid_ns)
> 
> 	- Fail to kill child reaper from within its namespace (non init_pid_ns)
> 
> 	- kill -1 1 from init_pid_ns seemed to work (rescanned inittab)
> 
> TODO:
> 	- Test async io and SIGIO delivery.
> 
> 	- Allow any legitimate signals that the child reaper can receive
> 	  from within its namespace? (we block all signals now)
> 
>         - Sending SIGKILL to the child reaper of a namespace terminates the
>           namespace But if the namespace remounted /proc from user space,
> 	  /proc would remain mounted even after reaper and other process in
> 	  the namespace go away.
> 
> Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
> ---
>  kernel/signal.c |   22 +++++++++++++++++++++-
>  1 file changed, 21 insertions(+), 1 deletion(-)
> 
> Index: lx26-21-mm2/kernel/signal.c
> ===================================================================
> --- lx26-21-mm2.orig/kernel/signal.c	2007-05-22 16:59:42.000000000 -0700
> +++ lx26-21-mm2/kernel/signal.c	2007-05-22 16:59:57.000000000 -0700
> @@ -507,6 +507,20 @@ static int check_kill_permission(int sig
>  	    && !capable(CAP_KILL))
>  		return error;
> 
> +	/*
> +	 * If t is the reaper of its namespace and someone from that
> +	 * namespace is trying to send a signal.
> +	 *
> +	 * Note: If some one from parent namespace is sending a signal,
> +	 *       task_child_reaper() != t and we allow the signal.
> +	 *
> +	 * In the child namespace, does this block even legitimate signals
> +	 * like the ones telinit sends to /sbin/init ?
> +	 *
> +	 */
> +	if ((!is_global_init(t)) && (t == task_child_reaper(t)))

Couldn't you more clearly achieve what you want by doing:
	if ((!is_global_init(t)) && (t == task_child_reaper(current)))

Still like you say I think you need to study more how current code does
the right thing for the global init.  Reproduce exactly that if t ==
task_child_reaper(current), else treat like any other task.  And though
I said "reproduce", I should think you could do it without separate
checks as you have here.

-serge

> +		return -EPERM;
> +
>  	error = security_task_kill(t, info, sig, 0);
>  	if (!error)
>  		audit_signal_info(sig, t); /* Let audit system see the signal */
> @@ -1910,7 +1924,13 @@ relock:
>  		/*
>  		 * Init of a pid space gets no signals it doesn't want from
>  		 * within that pid space. It can of course get signals from
> -		 * its parent pid space.
> +		 * its parent pid space. But we have no way of knowing the
> +		 * namespace from which the signal was sent. For now check
> +		 * if we are global init here and add additional checks in
> +		 * sys_kill() and friends.
> +		 *
> +		 * Note that t == task_child_reaper(t) implies t is the global
> +		 * init (and we are in init_pid_ns).
>  		 */
>  		if (current == task_child_reaper(current))
>  			continue;
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