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Re: Symbol refcounting for STT_GNU_IFUNC symbols


On Mon, Jul 5, 2010 at 9:05 AM, Stephen Clarke <stephen.clarke@st.com> wrote:
> H.J. Lu wrote:
>>
>> On Fri, Jul 2, 2010 at 7:47 AM, Stephen Clarke <stephen.clarke@st.com>
>> wrote:
>>>
>>> In bfd/elf32-386.c, elf_i386_check_relocs(), for R_386_GOT32 and
>>> R_386_GOTOFF relocations with a symbol with STT_GNU_IFUNC type, the
>>> h->got.refcount is incremented:
>>>
>>> ? ? ? ? ? ? ? case R_386_GOT32:
>>> ? ? ? ? ? ? ? case R_386_GOTOFF:
>>> ? ? ? ? ? ? ? ? h->got.refcount += 1;
>>>
>>> I guess this is because the mechanism for IFUNC symbols requires a GOT
>>> entry (even for a GOTOFF relocation which does not normally require a
>>> GOT entry).
>>>
>>> However, there is no corresponding decrement in the
>>> elf_i386_gc_sweep_hook() if the relocation is removed.
>>> Is this just a minor (safe) inaccuracy, or is there some reason why the
>>> refcount cannot be decremented in elf_i386_gc_sweep_hook()?
>>>
>>
>> It could be an oversight. ?Do you have an testcase?
>
> I don't have a real testcase ... the question arose from a discussion
> with colleagues about the refcounting in elf32-sh.c ... and since that is
> based on the refcounting in elf32-i386.c, we reviewed that.
> (I know ifunc is not supported yet in elf32-sh.c: our original
> discussion was unrelated to ifunc symbols.)
>
> However, here's a synthetic testcase:
> $ cat test.s
> ? ? ? ?.section .text.foo,"ax",@progbits
> ? ? ? ?.type foo, @function
> foo:
> ? ? ? ?.global foo
> ? ? ? ?movl ifunc@GOT(%ecx), %eax
> ? ? ? ?ret
>
> ? ? ? ?.section .text.bar,"ax",@progbits
> ? ? ? ?.type bar, @function
> bar:
> ? ? ? ?.global bar
> ? ? ? ?ret
>
> ? ? ? ?.section .text.ifunc,"ax",@progbits
> ? ? ? ?.type ifunc, @gnu_indirect_function
> ifunc:
> ? ? ? ?ret
>
> $ as -o test.o test.s
> $ ld -e bar --gc-sections test.o
>
> If I use a recent version of binutils:
> $ ld -v
> GNU ld (GNU Binutils) 2.20.51.20100702
>
> then for the ld command I get segv, i.e.
> $ ld -e bar --gc-sections test.o
> Segmentation fault
>
> but if I hack out the cause of that, then I get an executable in
> which garbage collection has removed all the original sections that
> refer to the ifunc symbol, but there is still a plt entry, got entry
> and ?dynamic relocation for the ifunc symbol.
>
> $ objdump -h -d a.out
>
> a.out: ? ? file format elf32-i386
>
> Sections:
> Idx Name ? ? ? ? ?Size ? ? ?VMA ? ? ? LMA ? ? ? File off ?Algn
> ?0 .rel.plt ? ? ?00000008 ?08048074 ?08048074 ?00000074 ?2**2
> ? ? ? ? ? ? ? ? ?CONTENTS, ALLOC, LOAD, READONLY, DATA
> ?1 .plt ? ? ? ? ?00000010 ?0804807c ?0804807c ?0000007c ?2**2
> ? ? ? ? ? ? ? ? ?CONTENTS, ALLOC, LOAD, READONLY, CODE
> ?2 .text ? ? ? ? 00000001 ?0804808c ?0804808c ?0000008c ?2**0
> ? ? ? ? ? ? ? ? ?CONTENTS, ALLOC, LOAD, READONLY, CODE
> ?3 .got.plt ? ? ?00000010 ?08049090 ?08049090 ?00000090 ?2**2
> ? ? ? ? ? ? ? ? ?CONTENTS, ALLOC, LOAD, DATA
>
> Disassembly of section .plt:
>
> 0804807c <.plt>:
> ?804807c: ? ? ? ff 25 9c 90 04 08 ? ? ? jmp ? ?*0x804909c
> ?8048082: ? ? ? 68 00 00 00 00 ? ? ? ? ?push ? $0x0
> ?8048087: ? ? ? e9 00 00 00 00 ? ? ? ? ?jmp ? ?804808c <bar>
>
> Disassembly of section .text:
>
> 0804808c <bar>:
> ?804808c: ? ? ? c3 ? ? ? ? ? ? ? ? ? ? ?ret
>
>
> Steve.
>
>

I am testing this patch. Totally removing .got.plt section will
be tricky since it is created before garbage collection.
Please open another bug if it is really needed.

Thanks.

-- 
H.J.
---
bfd/

2010-07-12  H.J. Lu  <hongjiu.lu@intel.com>

 	PR ld/11791
	* elf-ifunc.c (_bfd_elf_allocate_ifunc_dyn_relocs): Support
	garbage collection against STT_GNU_IFUNC symbols.

	* elf32-i386.c (elf_i386_get_local_sym_hash): Don't set
	elf.plt.offset/elf.got.offset to -1.
	(elf_i386_tls_transition): Skip TLS transition for functions.
	(elf_i386_gc_sweep_hook): Support STT_GNU_IFUNC symbols.

	* elf64-x86-64.c (elf64_x86_64_get_local_sym_hash): Don't set
	elf.plt.offset/elf.got.offset to -1.
	(elf64_x86_64_tls_transition): Skip TLS transition for functions.
	(elf64_x86_64_gc_sweep_hook): Support STT_GNU_IFUNC symbols.

ld/testsuite/

2010-07-12  H.J. Lu  <hongjiu.lu@intel.com>

 	PR ld/11791
	* ld-ifunc/ifunc-10-i386.d: New.
	* ld-ifunc/ifunc-10-i386.s: Likewise.
	* ld-ifunc/ifunc-10-x86-64.d: Likewise.
	* ld-ifunc/ifunc-10-x86-64.s: Likewise.
	* ld-ifunc/ifunc-11-i386.d: Likewise.
	* ld-ifunc/ifunc-11-i386.s: Likewise.
	* ld-ifunc/ifunc-11-x86-64.d: Likewise.
	* ld-ifunc/ifunc-11-x86-64.s: Likewise.

Attachment: binutils-pr11791-1.patch
Description: Text document


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