Should legacy fstat (via __fxstat) and fstat@GLIBC_2.33 call the same syscall?

[Florian Weimer]

* Adhemerval Zanella via Libc-alpha:

> On 14/01/2021 09:00, Florian Weimer via Libc-alpha wrote:
>> Currently, __fxstat may get mapped to the fstat system call, while fstat
>> goes to fstatat.  This makes sandboxing issues less obvious to debug:
>> 
>>   <https://bugs.chromium.org/p/chromium/issues/detail?id=1164975>
>> 
>> Should we change this before the release?  And if yes, in what
>> direction?
>
> Implementing fstat/lstat/stat through fstatat does really simplify
> the code at *lot* for some architectures. And on others and on some
> ABI (32-bit with 64-bit time_t), it would require to use the same
> syscall anyway (statx).
>
> I don't really see any gain in adding back this complexity back on
> stat calls, specially with y2038 support requirement.  

But we currently have this code for the x*stat interfaces.  For example,
in sysdeps/unix/sysv/linux/xstat.c:

__xstat (int vers, const char *name, struct stat *buf)
{
  switch (vers)
    {
    case _STAT_VER_KERNEL:
      {
# if STAT_IS_KERNEL_STAT
        /* New kABIs which uses generic pre 64-bit time Linux ABI,
           e.g. csky, nios2  */
        int r = INLINE_SYSCALL_CALL (fstatat64, AT_FDCWD, name, buf, 0);
        return r ?: stat_overflow (buf);
# else
        /* Old kABIs with old non-LFS support, e.g. arm, i386, hppa, m68k,
           microblaze, s390, sh, powerpc, and sparc32.  */
        return INLINE_SYSCALL_CALL (stat, name, buf);
# endif
      }

    default:
      {
# if STAT_IS_KERNEL_STAT
        return INLINE_SYSCALL_ERROR_RETURN_VALUE (EINVAL);
# else
        struct stat64 buf64;
        int r = INLINE_SYSCALL_CALL (stat64, name, &buf64);
        return r ?: __xstat32_conv (vers, &buf64, buf);
#endif
      }
    }
}

For _STAT_VER_KERNEL, we should be able to call stat directly.

Thanks,
Florian
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