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

[Stefan Liebler]

On 11/13/2017 09:13 PM, H.J. Lu wrote:
> On Mon, Nov 13, 2017 at 5:58 AM, H.J. Lu <hjl.tools@gmail.com> 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.
>>
> 
> BTW, i386 linker checks invalid PLT usage for IFUNC:
> 
> [hjl@gnu-6 tmp]$ cat x.s
> .text
> .globl bar
> .type bar, @function
> bar:
> jmp foo
> .size bar, .-bar
> .hidden foo
> .type foo, %gnu_indirect_function
> .globl foo
> foo:
> ret
> .size foo, .-foo
> [hjl@gnu-6 tmp]$ gcc -m32 -c x.s
> [hjl@gnu-6 tmp]$ ld -m elf_i386 -shared x.o
> ld: x.o: unsupported non-PIC call to IFUNC `foo'
> ld: final link failed: Nonrepresentable section on output
> [hjl@gnu-6 tmp]$ gcc  -c x.s
> [hjl@gnu-6 tmp]$ ld  -shared x.o
> [hjl@gnu-6 tmp]$
> 
> since i386 uses a different relocation,  R_386_PLT32, for PLT.    We can
> catch this issue at link-time.  Can you update 390 linker to do something
> similar?  The relevant linker commit is
> 

Thanks for this hint. I've forwarded this mail to Andreas Krebbel. He 
will have a look into binutils.

> commit 74437ea28fb611d4c88077b486fd7c0a8b4c2a25
> Author: H.J. Lu <hjl.tools@gmail.com>
> Date:   Mon Aug 29 08:12:59 2016 -0700
> 
>      i386: Issue an error on non-PIC call to IFUNC in PIC object
> 
>      On i386, IFUNC function must be called via PLT.  Since PLT in PIC
>      object uses EBX register, R_386_PLT32 relocation must be used to
>      call IFUNC function even when IFUNC function is defined locally.
>      Linker should issue an error when R_386_PC32 relocation is used
>      to call IFUNC function.
> 
>      Since PR ld/19784 tests doesn't use PLT relocation to local IFUNC
>      function, they are moved to the x86-64 test directory.
> 
>      bfd/
> 
>              PR ld/14961
>              PR ld/20515
>              * elf32-i386.c (elf_i386_check_relocs): Issue an error when
>              R_386_PC32 relocation is used to call IFUNC function in PIC
>              object.
> 
>