[PATCH] compat: Avoid double rounding in times()

[Hungyu Lin <dennylin0707@gmail.com>]

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.

Fixes: f06febc96ba8 ("timers: fix itimer/many thread hang")
Cc: stable@vger.kernel.org
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
 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));
 }
 

base-commit: f47d223627f80cb7a1eb14c585fe6cb35de5e984
-- 
2.43.0