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Re: Symbol refcounting for STT_GNU_IFUNC symbols
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.