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