gdb: use gdb_bfd_count_sections in macho_symfile_offsets

When loading a mach-o (macOS) executable and trying to set a breakpoint,
a GDB built with ASan or -D_GLIBCXX_DEBUG will crash with an
out-of-bound vector access.  This can be reproduced on Linux using the
repro files in bug 28017 [1]:

    $ ./gdb -nx --data-directory=data-directory -q repro/test -ex "b main" -batch
    /usr/include/c++/11.1.0/debug/vector:445:
    In function:
        std::__debug::vector<_Tp, _Allocator>::const_reference
        std::__debug::vector<_Tp,
        _Allocator>::operator[](std::__debug::vector<_Tp,
        _Allocator>::size_type) const [with _Tp = long unsigned int; _Allocator
        = std::allocator<long unsigned int>; std::__debug::vector<_Tp,
        _Allocator>::const_reference = const long unsigned int&;
        std::__debug::vector<_Tp, _Allocator>::size_type = long unsigned int]

    Error: attempt to subscript container with out-of-bounds index 13, but
    container only holds 13 elements.

    Objects involved in the operation:
        sequence "this" @ 0x0x61300000a590 {
          type = std::__debug::vector<unsigned long, std::allocator<unsigned long> >;
        }

The out-of-bound access happens here:

    #0  0x00007ffff6405d22 in raise () from /usr/lib/libc.so.6
    #1  0x00007ffff63ef862 in abort () from /usr/lib/libc.so.6
    #2  0x00007ffff664e21e in __gnu_debug::_Error_formatter::_M_error() const [clone .cold] from /usr/lib/libstdc++.so.6
    #3  0x000055555699e5ff in std::__debug::vector<unsigned long, std::allocator<unsigned long> >::operator[] (this=0x61300000a590, __n=13) at /usr/include/c++/11.1.0/debug/vector:445
    #4  0x0000555556a58c17 in objfile::section_offset (this=0x61300000a4c0, section=0x55555bbe4ac0 <_bfd_std_section>) at /home/simark/src/binutils-gdb/gdb/objfiles.h:644
    #5  0x0000555556a58cac in obj_section::offset (this=0x62100016d2a8) at /home/simark/src/binutils-gdb/gdb/objfiles.h:838
    #6  0x0000555556a58cfa in obj_section::addr (this=0x62100016d2a8) at /home/simark/src/binutils-gdb/gdb/objfiles.h:850
    #7  0x000055555779f5f7 in sort_cmp (sect1=0x62100016d2a8, sect2=0x62100016d170) at /home/simark/src/binutils-gdb/gdb/objfiles.c:902
    #8  0x00005555577aae35 in __gnu_cxx::__ops::_Iter_comp_iter<bool (*)(obj_section const*, obj_section const*)>::operator()<obj_section**, obj_section**> (this=0x7fffffffa9e0, __it1=0x60c000015970, __it2=0x60c000015940) at /usr/include/c++/11.1.0/bits/predefined_ops.h:158
    #9  0x00005555577aa2b8 in std::__insertion_sort<obj_section**, __gnu_cxx::__ops::_Iter_comp_iter<bool (*)(obj_section const*, obj_section const*)> > (__first=0x60c000015940, __last=0x60c0000159c0, __comp=...) at /usr/include/c++/11.1.0/bits/stl_algo.h:1826
    #10 0x00005555577a8e26 in std::__final_insertion_sort<obj_section**, __gnu_cxx::__ops::_Iter_comp_iter<bool (*)(obj_section const*, obj_section const*)> > (__first=0x60c000015940, __last=0x60c0000159c0, __comp=...) at /usr/include/c++/11.1.0/bits/stl_algo.h:1871
    #11 0x00005555577a723c in std::__sort<obj_section**, __gnu_cxx::__ops::_Iter_comp_iter<bool (*)(obj_section const*, obj_section const*)> > (__first=0x60c000015940, __last=0x60c0000159c0, __comp=...) at /usr/include/c++/11.1.0/bits/stl_algo.h:1957
    #12 0x00005555577a50f4 in std::sort<obj_section**, bool (*)(obj_section const*, obj_section const*)> (__first=0x60c000015940, __last=0x60c0000159c0, __comp=0x55555779f4e7 <sort_cmp(obj_section const*, obj_section const*)>) at /usr/include/c++/11.1.0/bits/stl_algo.h:4875
    #13 0x00005555577a147e in update_section_map (pspace=0x61200001d2c0, pmap=0x6030000d40b0, pmap_size=0x6030000d40b8) at /home/simark/src/binutils-gdb/gdb/objfiles.c:1165
    #14 0x00005555577a19a0 in find_pc_section (pc=0x100003fa0) at /home/simark/src/binutils-gdb/gdb/objfiles.c:1212
    #15 0x00005555576dd39e in lookup_minimal_symbol_by_pc_section (pc_in=0x100003fa0, section=0x0, prefer=lookup_msym_prefer::TEXT, previous=0x0) at /home/simark/src/binutils-gdb/gdb/minsyms.c:750
    #16 0x00005555576de552 in lookup_minimal_symbol_by_pc (pc=0x100003fa0) at /home/simark/src/binutils-gdb/gdb/minsyms.c:986
    #17 0x0000555557d44b54 in find_pc_sect_line (pc=0x100003fa0, section=0x62100016d170, notcurrent=0) at /home/simark/src/binutils-gdb/gdb/symtab.c:3163
    #18 0x0000555557d489fa in find_function_start_sal_1 (func_addr=0x100003fa0, section=0x62100016d170, funfirstline=true) at /home/simark/src/binutils-gdb/gdb/symtab.c:3650
    #19 0x0000555557d49015 in find_function_start_sal (sym=0x621000191670, funfirstline=true) at /home/simark/src/binutils-gdb/gdb/symtab.c:3706
    #20 0x0000555557485283 in symbol_to_sal (result=0x7fffffffbb30, funfirstline=1, sym=0x621000191670) at /home/simark/src/binutils-gdb/gdb/linespec.c:4460
    #21 0x00005555574728c2 in convert_linespec_to_sals (state=0x7fffffffc390, ls=0x7fffffffc3e0) at /home/simark/src/binutils-gdb/gdb/linespec.c:2335
    #22 0x0000555557475a8e in parse_linespec (parser=0x7fffffffc360, arg=0x60200007a550 "main", match_type=symbol_name_match_type::WILD) at /home/simark/src/binutils-gdb/gdb/linespec.c:2716
    #23 0x0000555557479027 in event_location_to_sals (parser=0x7fffffffc360, location=0x606000097be0) at /home/simark/src/binutils-gdb/gdb/linespec.c:3173
    #24 0x00005555574798f7 in decode_line_full (location=0x606000097be0, flags=1, search_pspace=0x0, default_symtab=0x0, default_line=0, canonical=0x7fffffffcca0, select_mode=0x0, filter=0x0) at /home/simark/src/binutils-gdb/gdb/linespec.c:3253
    #25 0x0000555556b4949f in parse_breakpoint_sals (location=0x606000097be0, canonical=0x7fffffffcca0) at /home/simark/src/binutils-gdb/gdb/breakpoint.c:9134
    #26 0x0000555556b6ce95 in create_sals_from_location_default (location=0x606000097be0, canonical=0x7fffffffcca0, type_wanted=bp_breakpoint) at /home/simark/src/binutils-gdb/gdb/breakpoint.c:13819
    #27 0x0000555556b645a6 in bkpt_create_sals_from_location (location=0x606000097be0, canonical=0x7fffffffcca0, type_wanted=bp_breakpoint) at /home/simark/src/binutils-gdb/gdb/breakpoint.c:12631
    #28 0x0000555556b4badf in create_breakpoint (gdbarch=0x621000152d10, location=0x606000097be0, cond_string=0x0, thread=0, extra_string=0x0, force_condition=false, parse_extra=1, tempflag=0, type_wanted=bp_breakpoint, ignore_count=0, pending_break_support=AUTO_BOOLEAN_AUTO, ops=0x55555bd728a0 <bkpt_breakpoint_ops>, from_tty=0, enabled=1, internal=0, flags=0) at /home/simark/src/binutils-gdb/gdb/breakpoint.c:9410
    #29 0x0000555556b4d3b1 in break_command_1 (arg=0x7fffffffe291 "", flag=0, from_tty=0) at /home/simark/src/binutils-gdb/gdb/breakpoint.c:9590
    #30 0x0000555556b4dc1b in break_command (arg=0x7fffffffe28d "main", from_tty=0) at /home/simark/src/binutils-gdb/gdb/breakpoint.c:9660
    #31 0x0000555556d24ca9 in do_const_cfunc (c=0x61100003a240, args=0x7fffffffe28d "main", from_tty=0) at /home/simark/src/binutils-gdb/gdb/cli/cli-decode.c:102
    #32 0x0000555556d2fcd3 in cmd_func (cmd=0x61100003a240, args=0x7fffffffe28d "main", from_tty=0) at /home/simark/src/binutils-gdb/gdb/cli/cli-decode.c:2160
    #33 0x0000555557e84e93 in execute_command (p=0x7fffffffe290 "n", from_tty=0) at /home/simark/src/binutils-gdb/gdb/top.c:674
    #34 0x00005555575a9933 in catch_command_errors (command=0x555557e84043 <execute_command(char const*, int)>, arg=0x7fffffffe28b "b main", from_tty=0, do_bp_actions=true) at /home/simark/src/binutils-gdb/gdb/main.c:523
    #35 0x00005555575a9fdb in execute_cmdargs (cmdarg_vec=0x7fffffffd910, file_type=CMDARG_FILE, cmd_type=CMDARG_COMMAND, ret=0x7fffffffd5b0) at /home/simark/src/binutils-gdb/gdb/main.c:618
    #36 0x00005555575ad48a in captured_main_1 (context=0x7fffffffdd00) at /home/simark/src/binutils-gdb/gdb/main.c:1322
    #37 0x00005555575ada9c in captured_main (data=0x7fffffffdd00) at /home/simark/src/binutils-gdb/gdb/main.c:1343
    #38 0x00005555575adb31 in gdb_main (args=0x7fffffffdd00) at /home/simark/src/binutils-gdb/gdb/main.c:1368
    #39 0x000055555681e179 in main (argc=8, argv=0x7fffffffde78) at /home/simark/src/binutils-gdb/gdb/gdb.c:32

The section being dealt with at that moment is the special *COM*
section:

    (top-gdb) p section.name
    $1 = 0x55555a1bbe60 "*COM*"
    (top-gdb) p section
    $2 = (bfd_section *) 0x55555bbe4ac0 <_bfd_std_section>

I'm not too sure what this section is for, but this is one of four
special BFD sections that GDB puts after the regular sections in the
objfile::sections and objfile::section_offsets lists.  You can check
gdb_bfd_section_index to see how they are handled.
gdb_bfd_count_sections returns "+ 4" to account for those sections.

The problem is that macho_symfile_offsets uses bfd_count_sections
instead of gdb_bfd_count_sections when allocating the
objfile::section_offsets vector.  The vector will therefore contain,
say, 13 elements instead of 17.  When trying to access the section
offset of the *COM* section, the first after the regular sections, we
access section_offsets[13], which is out of bounds.

Fix that by using gdb_bfd_count_sections instead of bfd_count_sections.
I'm fairly confident that this is correct, as this is what
default_symfile_offsets does.

With this patch, the command shown above terminates normally:

    $ ./gdb -nx --data-directory=data-directory -q repro/test -ex "b main" -batch
    Breakpoint 1 at 0x100003fad: file test.c, line 2.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=28017

gdb/ChangeLog:

	PR gdb/28017
	* machoread.c (macho_symfile_offsets): Use
	gdb_bfd_count_sections to allocate objfile::section_offsets.

Change-Id: Ic3a56f46f7232e9f24581f8255fc1ab981935450

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2021-06-28  Simon Marchi  <[email protected]>
+
+	PR gdb/28017
+	* machoread.c (macho_symfile_offsets): Use
+	gdb_bfd_count_sections to allocate objfile::section_offsets.
+
 2021-06-28  Simon Marchi  <[email protected]>
 
 	* objfiles.h (struct obj_section): Move down.

gdb/machoread.c

@@ -907,7 +907,7 @@ macho_symfile_offsets (struct objfile *objfile,
   struct obj_section *osect;
 
   /* Allocate section_offsets.  */
-  objfile->section_offsets.assign (bfd_count_sections (objfile->obfd), 0);
+  objfile->section_offsets.assign (gdb_bfd_count_sections (objfile->obfd), 0);
 
   /* This code is run when we first add the objfile with
      symfile_add_with_addrs_or_offsets, when "addrs" not "offsets" are