From 6b30c39a5e333cfb1475acf0f73de6ef954eea8a Mon Sep 17 00:00:00 2001 From: Hungyu Lin Date: Tue, 20 Oct 2026 21:01:57 +0000 Subject: [PATCH] compat: avoid double rounding in times() do_sys_times() already converts CPU time to USER_HZ ticks. The compat path converts each value back to jiffies before converting it to COMPAT_USER_HZ. This round trip can lose precision even when the two userspace rates match. With HZ=250 and USER_HZ=COMPAT_USER_HZ=100, one user tick becomes two jiffies and then zero compat ticks. Every odd tick count loses one tick, so any of the four tms fields can underreport CPU time by an additional 10 ms. Return the value directly when the native and compat units match. Keep the existing conversion for differing rates. Link: https://lore.kernel.org/20261020210157.52057-1-dennylin0707@gmail.com Fixes: f06febc96ba8 ("timers: fix itimer/many thread hang") Signed-off-by: Hungyu Lin Signed-off-by: Andrew Morton Reviewed-by: Andrew Morton Cc: Arnd Bergmann Cc: Thomas Gleixner Cc: --- kernel/sys.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/kernel/sys.c b/kernel/sys.c index 35b538ba843c..755bfa5ccd92 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -1077,6 +1077,9 @@ SYSCALL_DEFINE1(times, struct tms __user *, tbuf) #ifdef CONFIG_COMPAT static compat_clock_t clock_t_to_compat_clock_t(clock_t x) { + if (USER_HZ == COMPAT_USER_HZ) + return x; + return compat_jiffies_to_clock_t(clock_t_to_jiffies(x)); }