[gdb] Fix heap-use-after-free in typename_concat

[gdb] Fix heap-use-after-free in typename_concat

When running gdb using AddressSanitizer, and loading a cc1plus binary built
with profiledbootstrap and -flto, we run into a heap-use-after-free error:
...
$ LD_PRELOAD=/usr/lib64/libasan.so.3 ./gdb -batch cc1plus
==26855==ERROR: AddressSanitizer: heap-use-after-free on address \
  0x62100ad8a8b0 at pc 0x7f13803cc9e3 bp 0x7ffe55b0d090 sp 0x7ffe55b0c840
READ of size 47 at 0x62100ad8a8b0 thread T0
    #0 0x7f13803cc9e2  (/usr/lib64/libasan.so.3+0x3e9e2)
    #1 0x5e7a0d in typename_concat gdb/dwarf2read.c:22661
    #2 0x5c6437 in partial_die_full_name gdb/dwarf2read.c:8876
    #3 0x5c6555 in add_partial_symbol gdb/dwarf2read.c:8893
    #4 0x5c6ecf in add_partial_subprogram gdb/dwarf2read.c:9156
    #5 0x5c5e90 in scan_partial_symbols gdb/dwarf2read.c:8668
    #6 0x5c6c0a in add_partial_namespace gdb/dwarf2read.c:9081
    #7 0x5c5f99 in scan_partial_symbols gdb/dwarf2read.c:8702
    #8 0x5c48b6 in process_psymtab_comp_unit_reader gdb/dwarf2read.c:8056
    #9 0x5c3c1f in init_cutu_and_read_dies gdb/dwarf2read.c:7689
    #10 0x5c4c03 in process_psymtab_comp_unit gdb/dwarf2read.c:8140
    #11 0x5c58a2 in dwarf2_build_psymtabs_hard gdb/dwarf2read.c:8500
    #12 0x5c0d03 in dwarf2_build_psymtabs(objfile*) gdb/dwarf2read.c:6337
    #13 0x612359 in read_psyms gdb/elfread.c:1311
    #14 0x798a64 in require_partial_symbols(objfile*, int) gdb/psymtab.c:115
    #15 0x867d7b in read_symbols gdb/symfile.c:821
    #16 0x8683d9 in syms_from_objfile_1 gdb/symfile.c:1000
    #17 0x8684a1 in syms_from_objfile gdb/symfile.c:1017
    #18 0x868873 in symbol_file_add_with_addrs gdb/symfile.c:1124
    #19 0x868b0a in symbol_file_add_from_bfd(bfd*, char const*, \
                    enum_flags<symfile_add_flag>, std::vector<other_sections, \
		    std::allocator<other_sections> >*, \
		    enum_flags<objfile_flag>, objfile*) gdb/symfile.c:1204
    #20 0x868b64 in symbol_file_add(char const*, \
                    enum_flags<symfile_add_flag>, \
		    std::vector<other_sections, \
		    std::allocator<other_sections> >*, \
		    enum_flags<objfile_flag>) gdb/symfile.c:1217
    #21 0x868c39 in symbol_file_add_main_1 gdb/symfile.c:1240
    #22 0x868bd0 in symbol_file_add_main(char const*, \
                    enum_flags<symfile_add_flag>) gdb/symfile.c:1231
    #23 0x71f1b2 in symbol_file_add_main_adapter gdb/main.c:395
    #24 0x71f10e in catch_command_errors gdb/main.c:372
    #25 0x71ff5f in captured_main_1 gdb/main.c:1043
    #26 0x72045d in captured_main gdb/main.c:1163
    #27 0x7204c8 in gdb_main(captured_main_args*) gdb/main.c:1188
    #28 0x40fd7d in main gdb/gdb.c:32
    #29 0x7f137e300f49 in __libc_start_main (/lib64/libc.so.6+0x20f49)
    #30 0x40fc89 in _start (/data/gdb_versions/devel/build/gdb/gdb+0x40fc89)

0x62100ad8a8b0 is located 944 bytes inside of 4064-byte region \
  [0x62100ad8a500,0x62100ad8b4e0)
freed by thread T0 here:
    #0 0x7f13804523a0 in __interceptor_free (/usr/lib64/libasan.so.3+0xc43a0)
    #1 0x435e44 in xfree<void> gdb/common/common-utils.h:60
    #2 0xa82c25 in call_freefun libiberty/obstack.c:103
    #3 0xa83098 in _obstack_free libiberty/obstack.c:280
    #4 0x4367da in auto_obstack::~auto_obstack() gdb/gdb_obstack.h:101
    #5 0x5ed72c in dwarf2_cu::~dwarf2_cu() gdb/dwarf2read.c:25341
    #6 0x5fb5bb in std::default_delete<dwarf2_cu>::operator()(dwarf2_cu*) const \
                   /usr/include/c++/7/bits/unique_ptr.h:78
    #7 0x5f7334 in std::unique_ptr<dwarf2_cu, \
                   std::default_delete<dwarf2_cu> >::~unique_ptr() \
		   /usr/include/c++/7/bits/unique_ptr.h:268
    #8 0x5c3ce5 in init_cutu_and_read_dies gdb/dwarf2read.c:7624
    #9 0x5c4c03 in process_psymtab_comp_unit gdb/dwarf2read.c:8140
    #10 0x5c58a2 in dwarf2_build_psymtabs_hard gdb/dwarf2read.c:8500
    #11 0x5c0d03 in dwarf2_build_psymtabs(objfile*) gdb/dwarf2read.c:6337
    #12 0x612359 in read_psyms gdb/elfread.c:1311
    #13 0x798a64 in require_partial_symbols(objfile*, int) gdb/psymtab.c:115
    #14 0x867d7b in read_symbols gdb/symfile.c:821
    #15 0x8683d9 in syms_from_objfile_1 gdb/symfile.c:1000
    #16 0x8684a1 in syms_from_objfile gdb/symfile.c:1017
    #17 0x868873 in symbol_file_add_with_addrs gdb/symfile.c:1124
    #18 0x868b0a in symbol_file_add_from_bfd(bfd*, char const*, \
                    enum_flags<symfile_add_flag>, std::vector<other_sections,  \
		    std::allocator<other_sections> >*, \
		    enum_flags<objfile_flag>, objfile*) gdb/symfile.c:1204
    #19 0x868b64 in symbol_file_add(char const*, \
                    enum_flags<symfile_add_flag>, std::vector<other_sections, \
		    std::allocator<other_sections> >*, \
		    enum_flags<objfile_flag>) gdb/symfile.c:1217
    #20 0x868c39 in symbol_file_add_main_1 gdb/symfile.c:1240
    #21 0x868bd0 in symbol_file_add_main(char const*, \
                    enum_flags<symfile_add_flag>) gdb/symfile.c:1231
    #22 0x71f1b2 in symbol_file_add_main_adapter gdb/main.c:395
    #23 0x71f10e in catch_command_errors gdb/main.c:372
    #24 0x71ff5f in captured_main_1 gdb/main.c:1043
    #25 0x72045d in captured_main gdb/main.c:1163
    #26 0x7204c8 in gdb_main(captured_main_args*) gdb/main.c:1188
    #27 0x40fd7d in main gdb/gdb.c:32
    #28 0x7f137e300f49 in __libc_start_main (/lib64/libc.so.6+0x20f49)

previously allocated by thread T0 here:
    #0 0x7f13804526b8 in __interceptor_malloc (/usr/lib64/libasan.so.3+0xc46b8)
    #1 0x5114b5 in xmalloc gdb/common/common-utils.c:44
    #2 0xa82bd5 in call_chunkfun libiberty/obstack.c:94
    #3 0xa82eda in _obstack_newchunk libiberty/obstack.c:206
    #4 0x477310 in allocate_on_obstack::operator new(unsigned long, obstack*) \
                   gdb/gdb_obstack.h:117
    #5 0x5dea8c in load_partial_dies gdb/dwarf2read.c:18571
    #6 0x5c487f in process_psymtab_comp_unit_reader gdb/dwarf2read.c:8054
    #7 0x5c3c1f in init_cutu_and_read_dies gdb/dwarf2read.c:7689
    #8 0x5c4c03 in process_psymtab_comp_unit gdb/dwarf2read.c:8140
    #9 0x5c58a2 in dwarf2_build_psymtabs_hard gdb/dwarf2read.c:8500
    #10 0x5c0d03 in dwarf2_build_psymtabs(objfile*) gdb/dwarf2read.c:6337
    #11 0x612359 in read_psyms gdb/elfread.c:1311
    #12 0x798a64 in require_partial_symbols(objfile*, int) gdb/psymtab.c:115
    #13 0x867d7b in read_symbols gdb/symfile.c:821
    #14 0x8683d9 in syms_from_objfile_1 gdb/symfile.c:1000
    #15 0x8684a1 in syms_from_objfile gdb/symfile.c:1017
    #16 0x868873 in symbol_file_add_with_addrs gdb/symfile.c:1124
    #17 0x868b0a in symbol_file_add_from_bfd(bfd*, char const*, \
                    enum_flags<symfile_add_flag>, \
		    std::vector<other_sections, \
		    std::allocator<other_sections> >*, \
		    enum_flags<objfile_flag>, objfile*) gdb/symfile.c:1204
    #18 0x868b64 in symbol_file_add(char const*, enum_flags<symfile_add_flag>, \
                    std::vector<other_sections, \
		    std::allocator<other_sections> >*, \
		    enum_flags<objfile_flag>) gdb/symfile.c:1217
    #19 0x868c39 in symbol_file_add_main_1 gdb/symfile.c:1240
    #20 0x868bd0 in symbol_file_add_main(char const*, \
                    enum_flags<symfile_add_flag>) gdb/symfile.c:1231
    #21 0x71f1b2 in symbol_file_add_main_adapter gdb/main.c:395
    #22 0x71f10e in catch_command_errors gdb/main.c:372
    #23 0x71ff5f in captured_main_1 gdb/main.c:1043
    #24 0x72045d in captured_main gdb/main.c:1163
    #25 0x7204c8 in gdb_main(captured_main_args*) gdb/main.c:1188
    #26 0x40fd7d in main gdb/gdb.c:32
    #27 0x7f137e300f49 in __libc_start_main (/lib64/libc.so.6+0x20f49)
...

This error happens as follows.

The function find_partial_die has a cu argument, but returns a pdi which may
or may not be from that cu:
...
/* Find a partial DIE at OFFSET, which may or may not be in CU,
   except in the case of .debug_types DIEs which do not reference
   outside their CU (they do however referencing other types via
   DW_FORM_ref_sig8).  */

static struct partial_die_info *
find_partial_die (sect_offset sect_off, int offset_in_dwz, struct dwarf2_cu *cu)
...

So the pdi returned by find_partial_die here in partial_die_parent_scope may
be from another cu:
...
partial_die_parent_scope (struct partial_die_info *pdi,
                          struct dwarf2_cu *cu)
{
  const char *grandparent_scope;
  struct partial_die_info *parent, *real_pdi;

  /* We need to look at our parent DIE; if we have a DW_AT_specification,
     then this means the parent of the specification DIE.  */

  real_pdi = pdi;
  while (real_pdi->has_specification)
    real_pdi = find_partial_die (real_pdi->spec_offset,
                                 real_pdi->spec_is_dwz, cu);

  parent = real_pdi->die_parent;
...
in which case both real_pdi and parent will be not from cu, but from another
one, say cu2.

Subsequently, cu's comp_unit_obstack is used to set parent->scope:
...
        parent->scope = typename_concat (&cu->comp_unit_obstack,
                                         grandparent_scope,
                                         parent->name, 0, cu);
...

So, we use cu->comp_unit_obstack to assign a value to the scope field of
a pdi belonging to cu2, and when cu is deleted, the scope field points to a
freed value.

Fix this by making find_partial_die return the cu corresponding to the
returned pdi, and handling this at the call sites.

Tested on x86_64-linux.

gdb/ChangeLog:

2019-05-17  Tom de Vries  <tdevries@suse.de>

	PR gdb/24094
	* dwarf2read.c (struct cu_partial_die_info): New struct.
	(find_partial_die): Return cu_partial_die_info.
	(partial_die_parent_scope, guess_partial_die_structure_name)
	(partial_die_info::fixup): Handle new return type of find_partial_die.