Re: __xstat et al. as compat symbols (was: Re: [PATCH 14/16] linux: Move {f}xstat{at} to compat symbols)

[Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org>]

On 21/10/2020 02:21, Florian Weimer wrote:
> * Adhemerval Zanella via Libc-alpha:
> 
>> diff --git a/sysdeps/unix/sysv/linux/alpha/fxstat64.c b/sysdeps/unix/sysv/linux/alpha/fxstat64.c
>> index 9d6b8eca32..bcfb55050c 100644
>> --- a/sysdeps/unix/sysv/linux/alpha/fxstat64.c
>> +++ b/sysdeps/unix/sysv/linux/alpha/fxstat64.c
>> @@ -22,9 +22,11 @@
>>  #include <kernel_stat.h>
>>  #include <sysdep.h>
>>  #include <xstatconv.h>
>> +#include <shlib-compat.h>
>>  
>>  /* Get information about the file NAME in BUF.  */
>>  int
>> +attribute_compat_text_section
>>  __fxstat64 (int vers, int fd, struct stat64 *buf)
>>  {
>>    switch (vers)
>> @@ -42,4 +44,12 @@ __fxstat64 (int vers, int fd, struct stat64 *buf)
>>        }
>>      }
>>  }
>> -strong_alias (__fxstat64, __fxstat);
>> +
>> +#if SHLIB_COMPAT(libc, GLIBC_2_0, GLIBC_2_33)
>> +strong_alias (__fxstat64, __fxstat_compat)
>> +compat_symbol (libc, __fxstat_compat, __fxstat, GLIBC_2_0);
>> +#endif
>> +
>> +#if SHLIB_COMPAT(libc, GLIBC_2_1, GLIBC_2_33)
>> +compat_symbol (libc, __fxstat64, __fxstat64, GLIBC_2_1);
>> +#endif
> 
> This change breaks a lot of static libraries out there.  Do we really
> want to do this immediately, or should we just stop using these symbols
> from libc_nonshared.a, and defer the compat symbol treatment to a future
> glibc version?

This is the similar the libm finite symbols where my understanding was user
was supposed to rebuild the library.

The issue is only for static linking, by stopping using libc_nonshared.a we
need to provide the versioned stat symbols anyway.  And we still need to handle
new ABIs where adding the old xstat symbols does not make sense (such as
riscv32 or any new 32-bit architecture).

It would required a new compat_symbol directive to export the symbols *just*
for static linking and some building tinkering to just build the xstat symbol
for dynamic object and static linking depending of support ABI version.

If we just defer the compat move, it would add the needless symbols on
newer ABIs that we will need to keep supporting indefinitely (even though they
won't be used anywhere unless user explicily links against a protected symbol)
and we would eventually need to handle the y2038 support on old ABIs (with the
_TIME_SIZE selection) by either adding proper 64-bit time support for by 
removing the xstat for good.