linux-kernel - Re: [PATCH 2/2] core: allow setrlimit to non-current tasks

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Date:	Wed, 2 Sep 2009 15:50:24 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jiri Slaby <jirislaby@...il.com>
Cc:	akpm@...ux-foundation.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] core: allow setrlimit to non-current tasks

On 09/02, Jiri Slaby wrote:
>
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1240,20 +1240,28 @@ int setrlimit(struct task_struct *tsk, unsigned int resource,
>  		struct rlimit *new_rlim)
>  {
>  	struct rlimit *old_rlim;
> +	unsigned long flags;
>  	int retval;
>
>  	if (new_rlim->rlim_cur > new_rlim->rlim_max)
>  		return -EINVAL;
> +
> +	if (lock_task_sighand(tsk, &flags) == NULL)
> +		return -ESRCH;

No, sorry, this can't work.

Because we need task_lock() to update rlimits, and ->alloc_lock does not
nest under ->siglock.

Looks like we have to use tasklist_lock, but please don't use _irq, and
please do not check ->signal != NULL. Perhaps it makes sense to take
tasklist only if !same_thread_group(tsk, current) though.

Oh. We really need to make ->signal refcountable.

But there is another minor problem. If we use read_lock(ttasklist), then
the write to /proc/application_pid/limits can race with application doing
sys_setrlimits().

Nothing bad can happen, but this means that "echo ... > /proc/limits" can
be lost. Not good, if admin wants to lower ->rlim_max we should try to ensure
this always works.

Oleg.

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