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

[Lukasz Majewski <lukma at denx dot de>]

Hi Stepan,

> 06.05.2019 в 16:26:59 +0200 Lukasz Majewski написал:
> 
> > > > > > > I believe that the only 32-bit architecture without
> > > > > > > __NR_clock_settime64 is x32.         
> > > > > > 
> > > > > > Ok, I see. 
> > > > > > 
> > > > > > Please correct me - would it be feasible to just #undef
> > > > > > __ASSYME_TIME64_SYSCALLS for x32 ?        
> > > > > 
> > > > > You'll need to know whether to use __NR_clock_settime64 or
> > > > > __NR_clock_settime in cases where __TIMESIZE == 64 and
> > > > > __WORDSIZE == 32.    
> > > 
> > > Please correct me, but I do have some doubts here.
> > > 
> > > As x32 now uses 64 bit time (and has TIMESIZE==64) - it uses the
> > > clock_settime call (with in-kernel broken tv_nsec padding
> > > clearing - but for this the fix is in its way to upstream).
> > > 
> > > Why does it need to also support clock_settime64 ?   
> > 
> > I was too eager to send the mail.
> > 
> > It is connected with your proposition to use __WORDSIZE for #ifdef
> > on __ASSUME_TIME64_SYSCALLS. 
> > 
> > As x32 has __WORDSIZE=32 (but __TIMESIZE=64), it would fall into
> > "category" of archs using explicit 64 bit calls (i.e.
> > clock_settime64).
> > 
> > However, for it - those shall be replaced with syscalls used up
> > till now (i.e. clock_settime).
> > 
> > Am I right here ?  
> 
> x32 has __WORDSIZE==32 and __TIMESIZE==64.  Future 32-bit
> architectures should have __WORDSIZE==32 and __TIMESIZE==64 too.  And
> the only difference in implementation of clock_settime64 function
> there should be name of syscall (ignoring possible padding clearing
> issues).

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).

> 
> > > > > One way would be by defining __ASSUME_TIME64_SYSCALLS
> > > > > unconditionally on x32 and then defining __NR_clock_settime64
> > > > > to __NR_clock_settime when __ASSUME_TIME64_SYSCALLS is
> > > > > defined while __NR_clock_settime64 isn't.  
> 
> I think now that with this scheme __ASSUME_TIME64_SYSCALLS should be
> defined unconditionally for the __WORDSIZE==64 case too.  With this
> there will be no need to use __TIMESIZE inside clock_settime64
> function.  __TIMESIZE describes interfaces provided by glibc, and has
> no relation with kernel interfaces used by glibc.
> 
> #ifdef __ASSUME_TIME64_SYSCALLS
>   call __NR_clock_settime64 (possibly defined to __NR_clock_settime)
> #else
>   call __NR_clock_settime64 with fallback to __NR_clock_settime on
> ENOSYS #endif

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).



Best regards,

Lukasz Majewski

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