linux-kernel - Re: [PATCH] posix-timer: don't call idr

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Message-Id: <20130220132300.ccffde44.akpm@linux-foundation.org>
Date:	Wed, 20 Feb 2013 13:23:00 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Sasha Levin <sasha.levin@...cle.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] posix-timer: don't call idr_find() w/ negative ID

On Wed, 20 Feb 2013 13:01:16 -0800
Tejun Heo <tj@...nel.org> wrote:

> Recent idr updates make idr_find() trigger WARN_ON_ONCE() before
> returning NULL when a negative ID is specified.  Apparently,
> posix-timer::__lock_timer() was depending on idr_find() returning NULL
> on negative ID, thus triggering the new WARN_ON_ONCE().  Make
> __lock_timer() first check whether @timer_id is negative and return
> NULL without invoking idr_find() if so.
> 
> Note that the previous code was theoretically broken.  idr_find()
> masked off the sign bit before performing lookup and if the matching
> IDs were in use, it would have returned pointer for the incorrect
> entry.
> 
> ...
>
> --- a/kernel/posix-timers.c
> +++ b/kernel/posix-timers.c
> @@ -637,6 +637,9 @@ static struct k_itimer *__lock_timer(timer_t timer_id, unsigned long *flags)
>  {
>  	struct k_itimer *timr;
>  
> +	if ((int)timer_id < 0)
> +		return NULL;
> +
>  	rcu_read_lock();
>  	timr = idr_find(&posix_timers_id, (int)timer_id);
>  	if (timr) {

This is a bit risky - if some arch defines timer_t to be a u64 then we
will incorrectly treat 0x0000 0001 ffff ffff as a negative number. 
(That's a lot of timers!)

A fancy way of avoiding this is

	if (timer_id & ((typeof timer_id)1 << (sizeof(timer_id) - 1)))

(approximately ;))

But I think casting to (long) should be good enough?
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