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

["Arnd Bergmann" <arnd@arndb.de>]

On Tue, Oct 20, 2026, at 23:01, Hungyu Lin wrote:
> 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.

Good catch!

> Fixes: f06febc96ba8 ("timers: fix itimer/many thread hang")

That one apparently just moved the code into a separate function, but
the bug was already there before 2.6.12:

+               tmp.tms_utime = compat_jiffies_to_clock_t(cputime_to_jiffies(utime));
+               tmp.tms_stime = compat_jiffies_to_clock_t(cputime_to_jiffies(stime));
+               tmp.tms_cutime = compat_jiffies_to_clock_t(cputime_to_jiffies(cutime));
+               tmp.tms_cstime = compat_jiffies_to_clock_t(cputime_to_jiffies(cstime));


>  #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));

I don't think there are any architectures left that define USER_HZ
and COMPAT_USER_HZ differently. This was true on ia64 with x86 compat,
but that was dropped in v2.6.33 back in 2010.

   Arnd