Re: [PATCHv2] Fix float128 IFUNC relocations on ppc64le [BZ #21707]

["Tulio Magno Quites Machado Filho" <tuliom at linux dot vnet dot ibm dot com>]

"H.J. Lu" <hjl.tools@gmail.com> writes:

> On Sat, Jul 8, 2017 at 11:31 AM, Tulio Magno Quites Machado Filho
> <tuliom@linux.vnet.ibm.com> wrote:
>> Changes since version 1:
>>
>>  - Added a testcase.  This is now validating both statically and
>>    dynamically linked executables.
>>  - Fixed an issue in the $(foreach ..) in sysdeps/powerpc/powerpc64le/Makefile.
>>  - Added a comment to csu/libc-start.c
>>  - Added a comment to csu/libc-tls.c
>>
>> -- 8< --
>>
>> The patch proposed by Peter Bergner [1] to libgc in order to fix
>> [BZ #21707] adds a dependency on a symbol provided by the loader,
>> forcing the loader to be linked to tests after libgcc was linked.
>>
>> It also requires to read the thread pointer during IRELA relocations.
>>
>> Tested on powerpc, powerpc64, powerpc64le, s390x and x86_64.
>>
>> [1] https://sourceware.org/ml/libc-alpha/2017-06/msg01383.html
>>
>> 2017-07-08  Tulio Magno Quites Machado Filho  <tuliom@linux.vnet.ibm.com>
>>
>>         [BZ #21707]
>>         * csu/libc-start.c (LIBC_START_MAIN): Perform IREL{,A}
>>         relocations after initializing the TCB on statically linked
>>         executables..
>>         * csu/libc-tls.c (__libc_setup_tls): Add a comment about
>>         IREL{,A} relocations.
>>         * elf/Makefile (tests-static-normal): Add tst-tlsifunc-static.
>>         (tests): Add tst-tlsifunc.
>>         * elf/tst-tlsifunc.c: New file.
>>         * elf/tst-tlsifunc-static.c: Likewise.
>>         * sysdeps/powerpc/powerpc64le/Makefile (f128-loader-link): New
>>         variable.
>>         [$(subdir) = math] (test-float128% test-ifloat128%): Force
>>         linking to the loader after linking to libgcc.
>>         [$(subdir) = wcsmbs stdlib] (bug-strtod bug-strtod2 bug-strtod2)
>>         (tst-strtod-round tst-wcstod-round tst-strtod6 tst-strrom)
>>         (tst-strfrom-locale strfrom-skeleton): Likewise.
>> ---
>>  csu/libc-start.c                     | 11 +++---
>>  csu/libc-tls.c                       |  2 ++
>>  elf/Makefile                         |  5 +--
>>  elf/tst-tlsifunc-static.c            | 19 +++++++++++
>>  elf/tst-tlsifunc.c                   | 66 ++++++++++++++++++++++++++++++++++++
>>  sysdeps/powerpc/powerpc64le/Makefile | 10 ++++++
>>  6 files changed, 107 insertions(+), 6 deletions(-)
>>  create mode 100644 elf/tst-tlsifunc-static.c
>>  create mode 100644 elf/tst-tlsifunc.c
>>
>> diff --git a/csu/libc-start.c b/csu/libc-start.c
>> index c2dd159..84b7f99 100644
>> --- a/csu/libc-start.c
>> +++ b/csu/libc-start.c
>> @@ -188,12 +188,15 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
>>
>>    ARCH_INIT_CPU_FEATURES ();
>>
>> -  /* Perform IREL{,A} relocations.  */
>> -  apply_irel ();
>> -
>>    /* The stack guard goes into the TCB, so initialize it early.  */
>>    __libc_setup_tls ();
>>
>> +  /* Perform IREL{,A} relocations.
>> +     Note: the relocations must happen after TLS initialization so that
>> +     IFUNC resolvers can benefit from thread-local storage, e.g. powerpc's
>> +     hwcap and platform fields available in the TCB.  */
>> +  apply_irel ();
>> +
>>    /* Set up the stack checker's canary.  */
>>    uintptr_t stack_chk_guard = _dl_setup_stack_chk_guard (_dl_random);
>>  # ifdef THREAD_SET_STACK_GUARD
>> @@ -224,7 +227,7 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
>>    __pointer_chk_guard_local = pointer_chk_guard;
>>  # endif
>>
>> -#endif
>> +#endif /* !SHARED  */
>>
>
> apply_irel should be called as early as possible.

Why?  Could you elaborate, please?

-- 
Tulio Magno