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compat: avoid double rounding in times()

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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: f06febc96b ("timers: fix itimer/many thread hang")
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Thomas Gleixner <tglx@kernel.org>
Cc: <stable@vger.kernel.org>
This commit is contained in:
Hungyu Lin 2026-10-20 21:01:57 +00:00 committed by Andrew Morton
parent 26d7e34b42
commit 6b30c39a5e
1 changed files with 3 additions and 0 deletions
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3
kernel/sys.c
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@ -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));
}