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[binutils-gdb] Fix breakpoints in ifunc after inferior resolved it (@got.plt symbol creation)
- From: Pedro Alves <palves at sourceware dot org>
- To: gdb-cvs at sourceware dot org
- Date: 26 Apr 2018 12:15:49 -0000
- Subject: [binutils-gdb] Fix breakpoints in ifunc after inferior resolved it (@got.plt symbol creation)
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=02e169e2dac9b0354162eb0e7ee3fc95134b232d
commit 02e169e2dac9b0354162eb0e7ee3fc95134b232d
Author: Pedro Alves <palves@redhat.com>
Date: Thu Apr 26 13:02:26 2018 +0100
Fix breakpoints in ifunc after inferior resolved it (@got.plt symbol creation)
Setting a breakpoint on an ifunc symbol after the ifunc has already
been resolved by the inferior should result in creating a breakpoint
location at the ifunc target. However, that's not what happens on
current Fedora:
(gdb) n
53 i = gnu_ifunc (1); /* break-at-call */
(gdb)
54 assert (i == 2);
(gdb) b gnu_ifunc
Breakpoint 2 at gnu-indirect-function resolver at 0x7ffff7bd36ee
(gdb) info breakpoints
Num Type Disp Enb Address What
2 STT_GNU_IFUNC resolver keep y 0x00007ffff7bd36ee <gnu_ifunc+4>
The problem is that elf_gnu_ifunc_resolve_by_got never manages to
resolve an ifunc target. The reason is that GDB never actually
creates the internal got.plt symbols:
(gdb) p 'gnu_ifunc@got.plt'
No symbol "gnu_ifunc@got.plt" in current context.
and this is because GDB expects that rela.plt has relocations for
.plt, while it actually has relocations for .got.plt:
Relocation section [10] '.rela.plt' for section [22] '.got.plt' at offset 0x570 contains 2 entries:
Offset Type Value Addend Name
0x0000000000601018 X86_64_JUMP_SLOT 000000000000000000 +0 __assert_fail
0x0000000000601020 X86_64_JUMP_SLOT 000000000000000000 +0 gnu_ifunc
Using an older system on the GCC compile farm (machine gcc15, an
x86-64 running Debian 6.0.8, with GNU ld 2.20.1), we see that it used
to be that we'd get a .rela.plt section for .plt:
Relocation section [ 9] '.rela.plt' for section [11] '.plt' at offset 0x578 contains 3 entries:
Offset Type Value Addend Name
0x0000000000600cc0 X86_64_JUMP_SLOT 000000000000000000 +0 __assert_fail
0x0000000000600cc8 X86_64_JUMP_SLOT 000000000000000000 +0 __libc_start_main
0x0000000000600cd0 X86_64_JUMP_SLOT 000000000000000000 +0 gnu_ifunc
Those offsets did point into .got.plt, as seen with objdump -h:
20 .got.plt 00000030 0000000000600ca8 0000000000600ca8 00000ca8 2**3
CONTENTS, ALLOC, LOAD, DATA
I also tested on gcc110 on the compile farm (PPC64 running CentOS
7.4.1708, with GNU ld 2.25.1), and there we see instead:
Relocation section [ 9] '.rela.plt' for section [23] '.plt' at offset 0x5d0 contains 4 entries:
Offset Type Value Addend Name
0x0000000010020148 PPC64_JMP_SLOT 000000000000000000 +0 __libc_start_main
0x0000000010020160 PPC64_JMP_SLOT 000000000000000000 +0 __gmon_start__
0x0000000010020178 PPC64_JMP_SLOT 000000000000000000 +0 __assert_fail
0x0000000010020190 PPC64_JMP_SLOT 000000000000000000 +0 gnu_ifunc
But note that those offsets point into .plt, not .got.plt, as seen
with objdump -h:
22 .plt 00000078 0000000010020130 0000000010020130 00010130 2**3
ALLOC
This commit makes us support all the different combinations above.
With that addressed, we now get:
(gdb) p 'gnu_ifunc@got.plt'
$1 = (<text from jump slot in .got.plt, no debug info>) 0x400753 <final>
And setting a breakpoint on the ifunc finds the ifunc target:
(gdb) b gnu_ifunc
Breakpoint 2 at 0x400753
(gdb) info breakpoints
Num Type Disp Enb Address What
2 breakpoint keep y 0x0000000000400753 <final>
gdb/ChangeLog:
2018-04-26 Pedro Alves <palves@redhat.com>
* elfread.c (elf_rel_plt_read): Look for relocations for .got.plt too.
Diff:
---
gdb/ChangeLog | 4 ++++
gdb/elfread.c | 57 ++++++++++++++++++++++++++++++++++++++++-----------------
2 files changed, 44 insertions(+), 17 deletions(-)
diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index a5cb3bb..3266d7f 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2018-04-26 Pedro Alves <palves@redhat.com>
+
+ * elfread.c (elf_rel_plt_read): Look for relocations for .got.plt too.
+
2018-04-25 Pedro Alves <palves@redhat.com>
* infcmd.c (kill_command): Print the pid as string, not the whole
diff --git a/gdb/elfread.c b/gdb/elfread.c
index 2607890..16a692d 100644
--- a/gdb/elfread.c
+++ b/gdb/elfread.c
@@ -535,8 +535,7 @@ elf_rel_plt_read (minimal_symbol_reader &reader,
{
bfd *obfd = objfile->obfd;
const struct elf_backend_data *bed = get_elf_backend_data (obfd);
- asection *plt, *relplt, *got_plt;
- int plt_elf_idx;
+ asection *relplt, *got_plt;
bfd_size_type reloc_count, reloc;
struct gdbarch *gdbarch = get_objfile_arch (objfile);
struct type *ptr_type = builtin_type (gdbarch)->builtin_data_ptr;
@@ -545,11 +544,6 @@ elf_rel_plt_read (minimal_symbol_reader &reader,
if (objfile->separate_debug_objfile_backlink)
return;
- plt = bfd_get_section_by_name (obfd, ".plt");
- if (plt == NULL)
- return;
- plt_elf_idx = elf_section_data (plt)->this_idx;
-
got_plt = bfd_get_section_by_name (obfd, ".got.plt");
if (got_plt == NULL)
{
@@ -559,12 +553,25 @@ elf_rel_plt_read (minimal_symbol_reader &reader,
return;
}
+ /* Depending on system, we may find jump slots in a relocation
+ section for either .got.plt or .plt. */
+ asection *plt = bfd_get_section_by_name (obfd, ".plt");
+ int plt_elf_idx = (plt != NULL) ? elf_section_data (plt)->this_idx : -1;
+
+ int got_plt_elf_idx = elf_section_data (got_plt)->this_idx;
+
/* This search algorithm is from _bfd_elf_canonicalize_dynamic_reloc. */
for (relplt = obfd->sections; relplt != NULL; relplt = relplt->next)
- if (elf_section_data (relplt)->this_hdr.sh_info == plt_elf_idx
- && (elf_section_data (relplt)->this_hdr.sh_type == SHT_REL
- || elf_section_data (relplt)->this_hdr.sh_type == SHT_RELA))
- break;
+ {
+ const auto &this_hdr = elf_section_data (relplt)->this_hdr;
+
+ if (this_hdr.sh_type == SHT_REL || this_hdr.sh_type == SHT_RELA)
+ {
+ if (this_hdr.sh_info == plt_elf_idx
+ || this_hdr.sh_info == got_plt_elf_idx)
+ break;
+ }
+ }
if (relplt == NULL)
return;
@@ -573,6 +580,17 @@ elf_rel_plt_read (minimal_symbol_reader &reader,
std::string string_buffer;
+ /* Does ADDRESS reside in SECTION of OBFD? */
+ auto within_section = [obfd] (asection *section, CORE_ADDR address)
+ {
+ if (section == NULL)
+ return false;
+
+ return (bfd_get_section_vma (obfd, section) <= address
+ && (address < bfd_get_section_vma (obfd, section)
+ + bfd_get_section_size (section)));
+ };
+
reloc_count = relplt->size / elf_section_data (relplt)->this_hdr.sh_entsize;
for (reloc = 0; reloc < reloc_count; reloc++)
{
@@ -585,10 +603,15 @@ elf_rel_plt_read (minimal_symbol_reader &reader,
name = bfd_asymbol_name (*relplt->relocation[reloc].sym_ptr_ptr);
address = relplt->relocation[reloc].address;
- /* Does the pointer reside in the .got.plt section? */
- if (!(bfd_get_section_vma (obfd, got_plt) <= address
- && address < bfd_get_section_vma (obfd, got_plt)
- + bfd_get_section_size (got_plt)))
+ asection *msym_section;
+
+ /* Does the pointer reside in either the .got.plt or .plt
+ sections? */
+ if (within_section (got_plt, address))
+ msym_section = got_plt;
+ else if (within_section (plt, address))
+ msym_section = plt;
+ else
continue;
/* We cannot check if NAME is a reference to mst_text_gnu_ifunc as in
@@ -600,8 +623,8 @@ elf_rel_plt_read (minimal_symbol_reader &reader,
msym = record_minimal_symbol (reader, string_buffer.c_str (),
string_buffer.size (),
- true, address, mst_slot_got_plt, got_plt,
- objfile);
+ true, address, mst_slot_got_plt,
+ msym_section, objfile);
if (msym)
SET_MSYMBOL_SIZE (msym, ptr_size);
}