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

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Date:	Wed, 20 Feb 2013 14:05:51 -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 13:37:01 -0800
Tejun Heo <tj@...nel.org> wrote:

> Hello, Andrew.
> 
> On Wed, Feb 20, 2013 at 01:23:00PM -0800, Andrew Morton wrote:
> > > @@ -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?
> 
> Sans WARN_ON_ONCE(), the code would behave the same as before, which
> in turn, from what I can tell, is the behavior the code intended to
> implement before idr_alloc() conversion.
> 
> If timer_id is being allocated from idr, a valid id can never go over
> INT_MAX and returning NULL for any ID above that is the correct
> behavior, I think.  If timer_t is larger than int, both (int) and
> (long) castings wouldn't be useful.  Both will miss (1LU << 33) + 1
> and idr_find() will end up looking for 1.
> 
> If we want to be strict, we would have to do, I think,
> 
> 	if ((unsigned long long)timer_t > INT_MAX)
> 
> hopefully with some comments.  That said, if I'm grepping it right,
> all archs define timer_t as int, so maybe we're just being paranoid.
> 

Sure, it's unlikely to cause a problem in practice.  Maybe five years
from now, after idr has been cleaned up and switched to 64-bit, we've
left a little hand grenade for someone.  It would be good to
future-safe it in some fashion.

I wonder if we should add some generic facility to handle this:

/*
 * Query whether an unsigned type is `negative' when we don't know its size
 */
#define msb_is_set(v)	{ implementation goes here ;) }

Maybe not justified, dunno...
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