Symbol refcounting for STT_GNU_IFUNC symbols
H.J. Lu
hjl.tools@gmail.com
Mon Jul 12 23:30:00 GMT 2010
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.
-------------- next part --------------
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.
diff --git a/bfd/elf-ifunc.c b/bfd/elf-ifunc.c
index 0de236f..742e575 100644
--- a/bfd/elf-ifunc.c
+++ b/bfd/elf-ifunc.c
@@ -187,6 +187,15 @@ _bfd_elf_allocate_ifunc_dyn_relocs (struct bfd_link_info *info,
htab = elf_hash_table (info);
+ /* Support garbage collection against STT_GNU_IFUNC symbols. */
+ if (h->plt.refcount == 0 && h->got.refcount == 0)
+ {
+ h->got = htab->init_got_offset;
+ h->plt = htab->init_plt_offset;
+ *head = NULL;
+ return TRUE;
+ }
+
/* Return and discard space for dynamic relocations against it if
it is never referenced in a non-shared object. */
if (!h->ref_regular)
diff --git a/bfd/elf32-i386.c b/bfd/elf32-i386.c
index e85a7e5..ac9bdb6 100644
--- a/bfd/elf32-i386.c
+++ b/bfd/elf32-i386.c
@@ -789,8 +789,6 @@ elf_i386_get_local_sym_hash (struct elf_i386_link_hash_table *htab,
ret->elf.indx = sec->id;
ret->elf.dynstr_index = ELF32_R_SYM (rel->r_info);
ret->elf.dynindx = -1;
- ret->elf.plt.offset = (bfd_vma) -1;
- ret->elf.got.offset = (bfd_vma) -1;
*slot = ret;
}
return &ret->elf;
@@ -1162,6 +1160,12 @@ elf_i386_tls_transition (struct bfd_link_info *info, bfd *abfd,
unsigned int to_type = from_type;
bfd_boolean check = TRUE;
+ /* Skip TLS transition for functions. */
+ if (h != NULL
+ && (h->type == STT_FUNC
+ || h->type == STT_GNU_IFUNC))
+ return TRUE;
+
switch (from_type)
{
case R_386_TLS_GD:
@@ -1819,6 +1823,23 @@ elf_i386_gc_sweep_hook (bfd *abfd,
break;
}
}
+ else
+ {
+ /* A local symbol. */
+ Elf_Internal_Sym *isym;
+
+ isym = bfd_sym_from_r_symndx (&htab->sym_cache,
+ abfd, r_symndx);
+
+ /* Check relocation against local STT_GNU_IFUNC symbol. */
+ if (isym != NULL
+ && ELF32_ST_TYPE (isym->st_info) == STT_GNU_IFUNC)
+ {
+ h = elf_i386_get_local_sym_hash (htab, abfd, rel, FALSE);
+ if (h == NULL)
+ abort ();
+ }
+ }
r_type = ELF32_R_TYPE (rel->r_info);
if (! elf_i386_tls_transition (info, abfd, sec, NULL,
@@ -1845,6 +1866,11 @@ elf_i386_gc_sweep_hook (bfd *abfd,
{
if (h->got.refcount > 0)
h->got.refcount -= 1;
+ if (h->type == STT_GNU_IFUNC)
+ {
+ if (h->plt.refcount > 0)
+ h->plt.refcount -= 1;
+ }
}
else if (local_got_refcounts != NULL)
{
@@ -1867,6 +1893,16 @@ elf_i386_gc_sweep_hook (bfd *abfd,
}
break;
+ case R_386_GOTOFF:
+ if (h != NULL && h->type == STT_GNU_IFUNC)
+ {
+ if (h->got.refcount > 0)
+ h->got.refcount -= 1;
+ if (h->plt.refcount > 0)
+ h->plt.refcount -= 1;
+ }
+ break;
+
default:
break;
}
diff --git a/bfd/elf64-x86-64.c b/bfd/elf64-x86-64.c
index 21524fa..108f257 100644
--- a/bfd/elf64-x86-64.c
+++ b/bfd/elf64-x86-64.c
@@ -603,8 +603,6 @@ elf64_x86_64_get_local_sym_hash (struct elf64_x86_64_link_hash_table *htab,
ret->elf.indx = sec->id;
ret->elf.dynstr_index = ELF64_R_SYM (rel->r_info);
ret->elf.dynindx = -1;
- ret->elf.plt.offset = (bfd_vma) -1;
- ret->elf.got.offset = (bfd_vma) -1;
*slot = ret;
}
return &ret->elf;
@@ -951,6 +949,12 @@ elf64_x86_64_tls_transition (struct bfd_link_info *info, bfd *abfd,
unsigned int to_type = from_type;
bfd_boolean check = TRUE;
+ /* Skip TLS transition for functions. */
+ if (h != NULL
+ && (h->type == STT_FUNC
+ || h->type == STT_GNU_IFUNC))
+ return TRUE;
+
switch (from_type)
{
case R_X86_64_TLSGD:
@@ -1657,6 +1661,24 @@ elf64_x86_64_gc_sweep_hook (bfd *abfd, struct bfd_link_info *info,
break;
}
}
+ else
+ {
+ /* A local symbol. */
+ Elf_Internal_Sym *isym;
+
+ isym = bfd_sym_from_r_symndx (&htab->sym_cache,
+ abfd, r_symndx);
+
+ /* Check relocation against local STT_GNU_IFUNC symbol. */
+ if (isym != NULL
+ && ELF64_ST_TYPE (isym->st_info) == STT_GNU_IFUNC)
+ {
+ h = elf64_x86_64_get_local_sym_hash (htab, abfd, rel,
+ FALSE);
+ if (h == NULL)
+ abort ();
+ }
+ }
r_type = ELF64_R_TYPE (rel->r_info);
if (! elf64_x86_64_tls_transition (info, abfd, sec, NULL,
@@ -1687,6 +1709,11 @@ elf64_x86_64_gc_sweep_hook (bfd *abfd, struct bfd_link_info *info,
h->plt.refcount -= 1;
if (h->got.refcount > 0)
h->got.refcount -= 1;
+ if (h->type == STT_GNU_IFUNC)
+ {
+ if (h->plt.refcount > 0)
+ h->plt.refcount -= 1;
+ }
}
else if (local_got_refcounts != NULL)
{
--- /dev/null 2010-07-09 11:29:32.805426765 -0700
+++ binutils/ld/testsuite/ld-ifunc/ifunc-10-i386.d 2010-07-12 16:07:54.899638448 -0700
@@ -0,0 +1,6 @@
+#ld: -m elf_i386 -e bar --gc-sections
+#as: --32
+#readelf: -r --wide
+#target: x86_64-*-* i?86-*-*
+
+There are no relocations in this file.
--- /dev/null 2010-07-09 11:29:32.805426765 -0700
+++ binutils/ld/testsuite/ld-ifunc/ifunc-10-i386.s 2010-07-12 16:07:02.761538046 -0700
@@ -0,0 +1,20 @@
+ .section .text.foo,"ax",@progbits
+ .type foo, @function
+foo:
+ .global foo
+ movl ifunc@GOT(%ecx), %eax
+ movl ifunc@GOTOFF(%ecx), %eax
+ call ifunc@PLT
+ call ifunc
+ 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
--- /dev/null 2010-07-09 11:29:32.805426765 -0700
+++ binutils/ld/testsuite/ld-ifunc/ifunc-10-x86-64.d 2010-07-12 16:09:27.086846682 -0700
@@ -0,0 +1,6 @@
+#ld: -m elf_x86_64 -e bar --gc-sections
+#as: --64
+#readelf: -r --wide
+#target: x86_64-*-*
+
+There are no relocations in this file.
--- /dev/null 2010-07-09 11:29:32.805426765 -0700
+++ binutils/ld/testsuite/ld-ifunc/ifunc-10-x86-64.s 2010-07-12 16:09:40.831469890 -0700
@@ -0,0 +1,20 @@
+ .section .text.foo,"ax",@progbits
+ .type foo, @function
+foo:
+ .global foo
+ movl ifunc@GOTPCREL(%rip), %eax
+ movl ifunc(%rip), %eax
+ call ifunc@PLT
+ call ifunc
+ 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
--- /dev/null 2010-07-09 11:29:32.805426765 -0700
+++ binutils/ld/testsuite/ld-ifunc/ifunc-11-i386.d 2010-07-12 16:08:07.264477641 -0700
@@ -0,0 +1,6 @@
+#ld: -m elf_i386 -e bar --gc-sections
+#as: --32
+#readelf: -r --wide
+#target: x86_64-*-* i?86-*-*
+
+There are no relocations in this file.
--- /dev/null 2010-07-09 11:29:32.805426765 -0700
+++ binutils/ld/testsuite/ld-ifunc/ifunc-11-i386.s 2010-07-12 16:11:04.141413023 -0700
@@ -0,0 +1,21 @@
+ .section .text.foo,"ax",@progbits
+ .type foo, @function
+foo:
+ .global foo
+ movl ifunc@GOT(%ecx), %eax
+ movl ifunc@GOTOFF(%ecx), %eax
+ call ifunc@PLT
+ call ifunc
+ ret
+
+ .section .text.bar,"ax",@progbits
+ .type bar, @function
+bar:
+ .global bar
+ ret
+
+ .section .text.ifunc,"ax",@progbits
+ .type ifunc, @gnu_indirect_function
+ .global ifunc
+ifunc:
+ ret
--- /dev/null 2010-07-09 11:29:32.805426765 -0700
+++ binutils/ld/testsuite/ld-ifunc/ifunc-11-x86-64.d 2010-07-12 16:09:52.784598599 -0700
@@ -0,0 +1,6 @@
+#ld: -m elf_x86_64 -e bar --gc-sections
+#as: --64
+#readelf: -r --wide
+#target: x86_64-*-*
+
+There are no relocations in this file.
--- /dev/null 2010-07-09 11:29:32.805426765 -0700
+++ binutils/ld/testsuite/ld-ifunc/ifunc-11-x86-64.s 2010-07-12 16:10:06.159537495 -0700
@@ -0,0 +1,21 @@
+ .section .text.foo,"ax",@progbits
+ .type foo, @function
+foo:
+ .global foo
+ movl ifunc@GOTPCREL(%rip), %eax
+ movl ifunc(%rip), %eax
+ call ifunc@PLT
+ call ifunc
+ ret
+
+ .section .text.bar,"ax",@progbits
+ .type bar, @function
+bar:
+ .global bar
+ ret
+
+ .section .text.ifunc,"ax",@progbits
+ .type ifunc, @gnu_indirect_function
+ .global ifunc
+ifunc:
+ ret
More information about the Binutils
mailing list