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

[H.J. Lu]

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.
>
> 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.