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SubjectRe: [PATCH] posix-timer: don't call idr_find() w/ negative ID
On Wed, 20 Feb 2013 13:01:16 -0800
Tejun Heo <tj@kernel.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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