linux-kernel - Re: [PATCH] prctl: add PR_{SET,GET}_CHILD_SUBREAPER to allow simple process supervision

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Date:	Sat, 07 Jan 2012 11:13:50 -0500
From:	Valdis.Kletnieks@...edu
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Oleg Nesterov <oleg@...hat.com>,
	Lennart Poettering <lennart@...ttering.net>
Subject: Re: [PATCH] prctl: add PR_{SET,GET}_CHILD_SUBREAPER to allow simple process supervision

On Sat, 07 Jan 2012 16:56:37 +0100, Kay Sievers said:
> Resending this, it got lost last year's September.
>
> We still need it to properly implement init-like service managers.

> From: Lennart Poettering <lennart@...ttering.net>
> Subject: prctl: add PR_{SET,GET}_CHILD_SUBREAPER to allow simple process supervision

> Users of this will be the systemd per-user instance, which provides
> init-like functionality for the user's login session and D-Bus, which
> activates bus services on-demand. Both need init-like capabilities
> to be able to properly keep track of the services they start.

> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -552,6 +552,18 @@ struct signal_struct {
>  	int			group_stop_count;
>  	unsigned int		flags; /* see SIGNAL_* flags below */
>
> +	/*
> +	 * PR_SET_CHILD_SUBREAPER marks a process, like a service
> +	 * manager, to re-parent orphan (double-forking) child processes
> +	 * to this process instead of 'init'. The service manager is
> +	 * able to receive SIGCHLD signals and is able to investigate
> +	 * the process until it calls wait(). All children of this
> +	 * process will inherit a flag if they should look for a
> +	 * child_subreaper process at exit.
> +	 */
> +	unsigned int		is_child_subreaper:1;
> +	unsigned int		has_child_subreaper:1;

Is there someplace we can stick these two fields where they won't expand the
signal_struct?  Can we stick them in signal_struct->flags instead? Looks like we've
only burned 3 bits of that unsigned int.  Yes, I know that would complicate the
prctl get/set code.

> +		/* find the first ancestor marked as child_subreaper */
> +		for (reaper = father->real_parent;
> +		     reaper != &init_task;
> +		     reaper = reaper->real_parent) {

I admit being insufficiently caffienated - does this DTRT in a PID namespace? That
&init_task looks fishy to me...

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