Re: [PATCH v2 2/7] y2038: Introduce __ASSUME_64BIT_TIME define

[Stepan Golosunov <stepan at golosunov dot pp dot ru>]

06.05.2019 в 23:14:44 +0200 Lukasz Majewski написал:
> For ARM, x86 there shall be call to clock_settime64 syscall
> 
> For x32 there shall be call to clock_settime syscall (which is
> supporting 64 bit anyway - despite the ignoring possible padding
> clearing issue).

> I rather thought about something like:
> 
> __clock_settime64 (clockid_t clock_id, const struct __timespec64 *tp)
> {
>   <overflow check>
> 
> #ifdef __ASSUME_TIME64_SYSCALLS
> # ifdef __NR_clock_settime64
>   int ret = INLINE_SYSCALL_CALL (clock_settime64, clock_id, tp);
>   if (ret == 0 || errno != ENOSYS)
> 	return ret;
> # endif
>   /* Fall back to syscall supporting 32bit struct timespec.  */
>   struct timespec ts32;
>   valid_timespec64_to_timespec (tp, &ts32);
>   return INLINE_SYSCALL_CALL (clock_settime, clock_id, &ts32);
> #else
>   /* 64 bit machines + x32 */
>   return INLINE_SYSCALL_CALL (clock_settime, clock_id, tp);
> #endif
> }
> 
> In the above pseudo code we assume that __ASSUME_TIME64_SYSCALLS is
> #undef'ed for x32 (so it is treated as a 'special case' - in the same
> way as x86_64).

Usually the point of __ASSUME_* macros is to avoid compiling old
compatibility code when it's not needed.  That means there should be
no fallback to 32-bit clock_settime when __ASSUME_TIME64_SYSCALLS is
defined.  Even when it's defined on 32-bit arm or x86.

And with your current use of __ASSUME_TIME64_SYSCALLS
__NR_clock_settime64 won't be used on any existing architecture.
Because __ASSUME_TIME64_SYSCALLS won't be defined until Linux 5.0 is
no longer supported.

If you replace

#ifdef __ASSUME_TIME64_SYSCALLS

with

#ifdef __NR_clock_settime64

result would make more sense than what you have now.