linux-kernel - Re: [PATCH v3 05/11] rlimits: allow setrlimit to non-current tasks

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Message-Id: <20100513155621.51ca77a4.akpm@linux-foundation.org>
Date:	Thu, 13 May 2010 15:56:21 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	adobriyan@...il.com, nhorman@...driver.com, oleg@...hat.com,
	linux-kernel@...r.kernel.org, jirislaby@...il.com
Subject: Re: [PATCH v3 05/11] rlimits: allow setrlimit to non-current tasks

On Mon, 10 May 2010 20:00:45 +0200
Jiri Slaby <jslaby@...e.cz> wrote:

> From: Jiri Slaby <jirislaby@...il.com>
> 
> Add locking to allow setrlimit accept task parameter other than
> current.
> 
> Namely, lock tasklist_lock for read and check whether the task
> structure has sighand non-null. Do all the signal processing under
> that lock still held.
> 
> Signed-off-by: Jiri Slaby <jirislaby@...il.com>
> Cc: Oleg Nesterov <oleg@...hat.com>
> ---
>  kernel/sys.c |   11 ++++++++++-
>  1 files changed, 10 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 7c76f84..eb21661 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1272,6 +1272,7 @@ SYSCALL_DEFINE2(old_getrlimit, unsigned int, resource,
>  
>  #endif
>  
> +/* make sure you are allowed to change @tsk limits before calling this */
>  int do_setrlimit(struct task_struct *tsk, unsigned int resource,
>  		struct rlimit *new_rlim)
>  {
> @@ -1285,9 +1286,16 @@ int do_setrlimit(struct task_struct *tsk, unsigned int resource,
>  	if (resource == RLIMIT_NOFILE && new_rlim->rlim_max > sysctl_nr_open)
>  		return -EPERM;
>  
> +	/* protect tsk->signal and tsk->sighand from disappearing */
> +	read_lock(&tasklist_lock);
> +	if (!tsk->sighand) {
> +		retval = -ESRCH;
> +		goto out;
> +	}
> +
>  	retval = security_task_setrlimit(tsk, resource, new_rlim);

I looked at the amount of code which exists under
security_task_setrlimit() and nearly died.  Please convince me that it
is correct to do all that work under tasklist_lock, and that it is also
maintainable.

> 	update_rlimit_cpu(tsk, new_rlim->rlim_cur);
>  out:
> +	read_unlock(&tasklist_lock);

And this causes task-struct.sighand->siglock to be taken under
tasklist_lock.  Was that a pre-existing ranking, or is it a new one?

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