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

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Sat, Jul 8, 2017 at 4:12 PM, Tulio Magno Quites Machado Filho
<tuliom@linux.vnet.ibm.com> wrote:
> "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?
>

To use IFUNC in static executables, apply_irel should be called before
any functions with IFUNC implementation is called.  At the moment,
a few functions are used before apply_irel is called.  To address it,
we can move apply_irel forward.  Call it later makes it worse.


H.J.