Re: [RFC 0/9] Remove legacy platform bits and cleanups

[Stefan Liebler <stli@linux.ibm.com>]

On 07.06.24 14:18, Florian Weimer wrote:
> * Stefan Liebler:
> 
>> - "elf: Remove LD_HWCAP_MASK / tunable glibc.cpu.hwcap_mask"
>> The environment variable LD_HWCAP_MASK / glibc-tunable
>> glibc.cpu.hwcap_mask is not used anymore after removal in
>> elf/dl-cache.c:search_cache() despite of sparc32 where it is used in
>> elf_machine_matches_host().
> 
> Are you sure about that?  It is used to update GLRO(dl_hwcap), and that
> _rtld_global_ro member is consulted in various places.  Its value is
> also returned by getauxval (AT_HWCAP).
> 
> Thanks,
> Florian
> 
I've had a look, but did not found the place where this update happens.

I've also tried out this:
cat tst-LD_HWCAP_MASK.c
#include <stdio.h>
#include <stdlib.h>
#include <sys/auxv.h>

int
main (int argc, char *argv[])
{
  printf ("LD_HWCAP_MASK=\"%s\" GLIBC_TUNABLES=\"%s\" %s\n"
	  "=> getauxval(AT_HWCAP)= 0x%lx\n",
	  getenv("LD_HWCAP_MASK") ?: "", getenv("GLIBC_TUNABLES") ?: "",
	  argv[0], getauxval (AT_HWCAP));
  return EXIT_SUCCESS;
}


LD_HWCAP_MASK="" GLIBC_TUNABLES="" ./tst-LD_HWCAP_MASK-s390x.old
=> getauxval(AT_HWCAP)= 0x7fffff

LD_HWCAP_MASK="0x0" GLIBC_TUNABLES="" ./tst-LD_HWCAP_MASK-s390x.old
=> getauxval(AT_HWCAP)= 0x7fffff

LD_HWCAP_MASK="0xFFFFFFFF" GLIBC_TUNABLES="" ./tst-LD_HWCAP_MASK-s390x.old
=> getauxval(AT_HWCAP)= 0x7fffff

LD_HWCAP_MASK="" GLIBC_TUNABLES="glibc.cpu.hwcap_mask=0x0"
./tst-LD_HWCAP_MASK-s390x.old
=> getauxval(AT_HWCAP)= 0x7fffff

LD_HWCAP_MASK="" GLIBC_TUNABLES="glibc.cpu.hwcap_mask=0xFFFFFFFF"
./tst-LD_HWCAP_MASK-s390x.old
=> getauxval(AT_HWCAP)= 0x7fffff



There is the e.g. GLIBC_TUNABLES=glibc.cpu.hwcaps= which let you
disable/enable the hwcaps used for glibc internal ifuncs.

Before I've implemented GLIBC_TUNABLES=glibc.cpu.hwcaps= for s390x, I've
asked on libc-alpha and Szabolcs Nagy answered this:
(see full question/answer:
Questions regarding manipulation of IFUNC selection and tunables like
glibc.cpu.hwcap_mask
https://sourceware.org/pipermail/libc-alpha/2022-August/141648.html
)
iirc LD_HWCAP_MASK was purely for the old hwcap based
lib path lookup.
...
i think hwcap_mask was not for completely hiding hwcaps
like that.


Attached to the old question is also a program which dumps the hwcaps
passed to s390x-ifunc-resolver. But it was also not changed by the
environment variables.


Please also have a look, if you find the update of GLRO(dl_hwcap).

Thanks,
Stefan