Re: Symbol refcounting for STT_GNU_IFUNC symbols

[Stephen Clarke <stephen dot clarke at st dot com>]

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.