Re: Glibc 2.31 - time64 with 64-bit kernel and 32-bit userland

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 18/01/2021 17:14, Florian Weimer wrote:
> * Adhemerval Zanella via Libc-help:
> 
>> This is the expected behavior, running this small example:
>>
>> --
>> #include <time.h>
>>
>> int main (int argc, char *argv[])
>> {
>>   clock_gettime (CLOCK_REALTIME, &((struct timespec) {}));
>>   clock_gettime (CLOCK_BOOTTIME, &((struct timespec) {}));
>>   return 0;
>> }
>> --
>>
>> On a aarch64 kernel without 64-bit compat time_t support (4.12.13),
>> this is the generated syscalls:
>>
>> --
>> clock_gettime64(CLOCK_REALTIME, 0xfffebb58) = -1 ENOSYS (Function not implemented)
>> clock_gettime(CLOCK_REALTIME, {tv_sec=1610997887, tv_nsec=516302599}) = 0
>> clock_gettime(CLOCK_BOOTTIME, {tv_sec=2750059, tv_nsec=271773400}) = 0
>> --
>>
>> The catch here the 64-bit kernel when running 32-bit userland will
>> use compat entrypoints to provide the expected kernel ABI. For kernel 
>> older than v5.1, it means that even if kernel is 64-bit and support 
>> 64-bit time_t, the 32-bit syscall entrypoints won't have 64-bit time_t
>> support.
> 
> Ugh.  Is this for ILP32 support?
> 
> Should we just undefined __NR_clock_gettime64 for 64-bit AArch64?

No, this is armhf default binaries.  This situation is similar to
i686 running on x86_64 kernels.

AArch64 (and any other architecture with default 64-bit time_t)
will build Linux clock_gettime.c as:

int
__clock_gettime64 (clockid_t clock_id, struct __timespec64 *tp)
{
  int r;
# define __NR_clock_gettime64 __NR_clock_gettime
  if (true)
    {
      r = INLINE_VSYSCALL (clock_gettime64, 2, clock_id, tp);
      if (r == 0 || errno != ENOSYS)
        return r;
    }
  return r;
}