+ compat-avoid-double-rounding-in-times.patch added to mm-nonmm-unstable branch

+ compat-avoid-double-rounding-in-times.patch added to mm-nonmm-unstable branch

[Andrew Morton]

The patch titled
     Subject: compat: avoid double rounding in times()
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     compat-avoid-double-rounding-in-times.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/compat-avoid-double-rounding-in-times.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Hungyu Lin <dennylin0707@gmail.com>
Subject: compat: avoid double rounding in times()
Date: Tue, 20 Oct 2026 21:01:57 +0000

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 <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>
---

 kernel/sys.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/kernel/sys.c~compat-avoid-double-rounding-in-times
+++ a/kernel/sys.c
@@ -1077,6 +1077,9 @@ SYSCALL_DEFINE1(times, struct tms __user
 #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));
 }
 
_

Patches currently in -mm which might be from dennylin0707@gmail.com are

compat-avoid-double-rounding-in-times.patch