[tip: timers/urgent] posix-cpu-timers: Use u64 multiplication in update_rlimit_cpu()

[tip-bot2 for Zhan Xusheng]

The following commit has been merged into the timers/urgent branch of tip:

Commit-ID:     26aff38fefb1d6cd87e22525f41cc8f1aa61b24f
Gitweb:        https://git.kernel.org/tip/26aff38fefb1d6cd87e22525f41cc8f1aa61b24f
Author:        Zhan Xusheng <zhanxusheng1024@xxxxxxxxx>
AuthorDate:    Tue, 16 Jun 2026 19:20:17 +08:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxx>
CommitterDate: Wed, 17 Jun 2026 17:58:34 +02:00

posix-cpu-timers: Use u64 multiplication in update_rlimit_cpu()

update_rlimit_cpu() converts the RLIMIT_CPU value to nanoseconds with

        u64 nsecs = rlim_new * NSEC_PER_SEC;

On 32-bit kernels both rlim_new (unsigned long) and NSEC_PER_SEC
(1000000000L) are 32-bit, so the multiplication is performed in unsigned
long and truncated for rlim_new > 4 seconds before being widened to u64.

The same file already casts to u64 for the matching computation in
check_process_timers():

        u64 softns = (u64)soft * NSEC_PER_SEC;

As a result, the truncated value is installed into the CPUCLOCK_PROF
expiry cache (nextevt), causing the process CPU timer to be programmed
to fire prematurely for any RLIMIT_CPU soft limit >= 5 seconds. The
actual SIGXCPU/SIGKILL decision in check_process_timers() already casts
to u64 and is therefore correct, so limit enforcement is not broken;
only the expiry-cache programming is wrong. Apply the same cast here so
both paths convert rlim_cur identically.

64-bit kernels are unaffected.

Fixes: 858cf3a8c599 ("timers/itimer: Convert internal cputime_t units to nsec")
Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://patch.msgid.link/20260616112017.1681372-1-zhanxusheng@xxxxxxxxxx
---
 kernel/time/posix-cpu-timers.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 74775b9..5e633d8 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -41,7 +41,7 @@ void posix_cputimers_group_init(struct posix_cputimers *pct, u64 cpu_limit)
  */
 int update_rlimit_cpu(struct task_struct *task, unsigned long rlim_new)
 {
-	u64 nsecs = rlim_new * NSEC_PER_SEC;
+	u64 nsecs = (u64)rlim_new * NSEC_PER_SEC;
 	unsigned long irq_fl;
 
 	if (!lock_task_sighand(task, &irq_fl))