On Wed, 20 Feb 2013 13:37:01 -0800
Tejun Heo <t...@kernel.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...
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
