[PATCH] posix-timer: don't call idr_find() w/ negative ID

[Tejun Heo <tj@kernel.org>]

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.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
Sasha, can you please test whether this makes the warning go away?

Thanks.

 kernel/posix-timers.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/posix-timers.c b/kernel/posix-timers.c
index b51bb08..92465f9 100644
--- 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) {