[PATCH] Allow signalling container-init

[Oleg Nesterov]

On 08/09, sukadev at us.ibm.com wrote:
>
> Oleg Nesterov [oleg at tv-sign.ru] wrote:
> | On 08/08, sukadev at us.ibm.com wrote:
> | > 
> | > From: Sukadev Bhattiprolu <sukadev at us.ibm.com>
> | > Subject: [PATCH] Allow signalling container-init
> | > 
> | > Only the global-init process must be special - any other container-init
> | > process must be killable to prevent run-away processes in the system.
> | 
> | I think you are right, but....
> | 
> | > --- lx26-23-rc1-mm1.orig/kernel/signal.c	2007-08-07 13:52:12.000000000 -0700
> | > +++ lx26-23-rc1-mm1/kernel/signal.c	2007-08-08 15:09:27.000000000 -0700
> | > @@ -1861,11 +1861,9 @@ relock:
> | >  			continue;
> | >  
> | >  		/*
> | > -		 * 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.
> | > +		 * Global init gets no signals it doesn't want.
> | >  		 */
> | > -		if (current == task_child_reaper(current))
> | > +		if (is_global_init(current->group_leader))
> | >  			continue;
> | 
> | ...this breaks exec() from /sbin/init. Note that de_thread() kills other
> | sub-threads with SIGKILL. With this patch de_thread() will hang waiting
> | for other threads to die.
> 
> Again for threaded-init I guess :-(
> 
> Well, we discussed last week about allowing non-root users to clone their
> pid namespace. The user can then create a container-init and this
> process would become immune to signal even by a root user ?

please see below,

> | 
> | I think it is better to not change the current behaviour which is not
> | perfect (buggy), until we actually protect /sbin/init from unwanted
> | signals.
> 
> Can we preserve the existing behavior by checking only the main thread
> of global init (i.e pass in 'current' rather than 'current->group_leader'
> to is_global_init()) ?

Yes, this is what I meant, this is what we have in Linus's tree.
This way a container-init could be killed. This all is not correct,
but we shouldn't replace one bug with another.

Oleg.