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.
This commit is contained in:
Pedro Alves
parent
e2195274d4
commit
02e169e2da
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@ -1,3 +1,7 @@
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2018-04-26 Pedro Alves <palves@redhat.com>
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* elfread.c (elf_rel_plt_read): Look for relocations for .got.plt too.
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2018-04-25 Pedro Alves <palves@redhat.com>
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* infcmd.c (kill_command): Print the pid as string, not the whole
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@ -535,8 +535,7 @@ elf_rel_plt_read (minimal_symbol_reader &reader,
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{
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bfd *obfd = objfile->obfd;
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const struct elf_backend_data *bed = get_elf_backend_data (obfd);
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asection *plt, *relplt, *got_plt;
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int plt_elf_idx;
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asection *relplt, *got_plt;
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bfd_size_type reloc_count, reloc;
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struct gdbarch *gdbarch = get_objfile_arch (objfile);
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struct type *ptr_type = builtin_type (gdbarch)->builtin_data_ptr;
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@ -545,11 +544,6 @@ elf_rel_plt_read (minimal_symbol_reader &reader,
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if (objfile->separate_debug_objfile_backlink)
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return;
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plt = bfd_get_section_by_name (obfd, ".plt");
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if (plt == NULL)
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return;
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plt_elf_idx = elf_section_data (plt)->this_idx;
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got_plt = bfd_get_section_by_name (obfd, ".got.plt");
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if (got_plt == NULL)
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{
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@ -559,12 +553,25 @@ elf_rel_plt_read (minimal_symbol_reader &reader,
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return;
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}
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/* Depending on system, we may find jump slots in a relocation
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section for either .got.plt or .plt. */
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asection *plt = bfd_get_section_by_name (obfd, ".plt");
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int plt_elf_idx = (plt != NULL) ? elf_section_data (plt)->this_idx : -1;
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int got_plt_elf_idx = elf_section_data (got_plt)->this_idx;
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/* This search algorithm is from _bfd_elf_canonicalize_dynamic_reloc. */
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for (relplt = obfd->sections; relplt != NULL; relplt = relplt->next)
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if (elf_section_data (relplt)->this_hdr.sh_info == plt_elf_idx
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&& (elf_section_data (relplt)->this_hdr.sh_type == SHT_REL
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|| elf_section_data (relplt)->this_hdr.sh_type == SHT_RELA))
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break;
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{
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const auto &this_hdr = elf_section_data (relplt)->this_hdr;
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if (this_hdr.sh_type == SHT_REL || this_hdr.sh_type == SHT_RELA)
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{
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if (this_hdr.sh_info == plt_elf_idx
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|| this_hdr.sh_info == got_plt_elf_idx)
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break;
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}
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}
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if (relplt == NULL)
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return;
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@ -573,6 +580,17 @@ elf_rel_plt_read (minimal_symbol_reader &reader,
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std::string string_buffer;
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/* Does ADDRESS reside in SECTION of OBFD? */
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auto within_section = [obfd] (asection *section, CORE_ADDR address)
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{
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if (section == NULL)
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return false;
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return (bfd_get_section_vma (obfd, section) <= address
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&& (address < bfd_get_section_vma (obfd, section)
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+ bfd_get_section_size (section)));
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};
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reloc_count = relplt->size / elf_section_data (relplt)->this_hdr.sh_entsize;
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for (reloc = 0; reloc < reloc_count; reloc++)
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{
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@ -585,10 +603,15 @@ elf_rel_plt_read (minimal_symbol_reader &reader,
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name = bfd_asymbol_name (*relplt->relocation[reloc].sym_ptr_ptr);
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address = relplt->relocation[reloc].address;
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/* Does the pointer reside in the .got.plt section? */
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if (!(bfd_get_section_vma (obfd, got_plt) <= address
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&& address < bfd_get_section_vma (obfd, got_plt)
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+ bfd_get_section_size (got_plt)))
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asection *msym_section;
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/* Does the pointer reside in either the .got.plt or .plt
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sections? */
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if (within_section (got_plt, address))
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msym_section = got_plt;
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else if (within_section (plt, address))
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msym_section = plt;
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else
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continue;
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/* We cannot check if NAME is a reference to mst_text_gnu_ifunc as in
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@ -600,8 +623,8 @@ elf_rel_plt_read (minimal_symbol_reader &reader,
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msym = record_minimal_symbol (reader, string_buffer.c_str (),
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string_buffer.size (),
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true, address, mst_slot_got_plt, got_plt,
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objfile);
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true, address, mst_slot_got_plt,
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msym_section, objfile);
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if (msym)
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SET_MSYMBOL_SIZE (msym, ptr_size);
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}
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