Re: [PATCH] Use libc_hidden_proto / _def for hidden wchar ifunc symbols.

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

On 11/20/2017 04:59 PM, H.J. Lu wrote:
> On Mon, Nov 20, 2017 at 7:51 AM, Stefan Liebler <stli@linux.vnet.ibm.com> wrote:
>> On 11/13/2017 06:30 PM, H.J. Lu wrote:
>>>
>>> On Mon, Nov 13, 2017 at 9:11 AM, Stefan Liebler <stli@linux.vnet.ibm.com>
>>> wrote:
>>>>
>>>> On 11/13/2017 02:58 PM, H.J. Lu wrote:
>>>>>
>>>>>
>>>>> On Mon, Nov 13, 2017 at 3:55 AM, Florian Weimer <fweimer@redhat.com>
>>>>> wrote:
>>>>>>
>>>>>>
>>>>>> On 11/13/2017 11:16 AM, Stefan Liebler wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> This patch adds the libc_hidden_proto / libc_hidden_def construct.
>>>>>>> Then the __GI_* symbols are the default-ifunc-variants which can be
>>>>>>> called
>>>>>>> without PLT.
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> attribute_hidden and *_hidden_{proto,def} conflict on some
>>>>>> architectures.
>>>>>> You need to remove attribute_hidden as part of this change.
>>>>>
>>>>>
>>>>>
>>>>> That is true.  On i686, a hidden IFUNC function inside libc.so must be
>>>>> compiled
>>>>> with -fPIC via PLT since EBX must be loaded with GOT first.   This
>>>>> isn't an issue
>>>>> for x86-64 since PLT uses PC-relative addressing.  In this case, we
>>>>> should remove
>>>>> hidden attribute, instead of using __GI_* symbols, if we sill want to
>>>>> use
>>>>> IFUNC
>>>>> inside libc.so.
>>
>> For the moment, I propose to remove the attribute_hidden in order to fix
>> those internal IFUNC calls.
>> Is the attached patch okay to commit?
> 
> LGTM.
> 
Committed.

>>>>
>>>> On s390/s390x, just removing attribute_hidden in wchar.h is fine, too.
>>>> Then
>>>> r12 will be setup with the GOT-pointer and IFUNC will be used inside
>>>> libc.so.
>>>>
>>>> What was your original intention? Getting rid of "PLT" or "PLT and IFUNC"
>>>> for calls inside libc.so?
>>>
>>>
>>> The original intention is to remove PLT.  But it doesn't work for targets
>>> which
>>> need to set up a special register for PLT which is required by IFUNC.
>>>
>>>>>
>>>>> Now I have question, is there a way to apply attribute_hidden to a
>>>>> function
>>>>> depending on architecture? For example, we remove attribute_hidden
>>>>> from __wcsnlen, __wcscat, __wcsncpy, __wcpncpy,  __wcschrnul in headers
>>>>> under include/.   For x86, we mark them hidden in a header file under
>>>>> sysdeps/x86?
>>>>
>>>>
>>>> I don't know if duplicating the wchar.h file is such a good idea.
>>>>
>>>
>>> Can we add
>>>
>>> #include <wcharP.h>
>>>
>>> to include/wchar.h and add a dummy sysdeps/generic/wcharP.h?  Then
>>> I can add sysdeps/x86_64/wcharP.h to hide __wcsnlen, __wcscat, __wcsncpy,
>>> _wcpncpy,  __wcschrnul
>>>
>>>
>> @H.J. Lu: Can you propose a separate patch for architecture dependent
>> wcharP.h files?
> 
> Sure.  I will submit one after your patch is checked in.
> 
>