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gdb: avoid dereferencing empty str_offsets_base optional in dwarf_decode_macros

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Since 4d7188abfd ("gdbsupport: add debug assertions in
gdb::optional::get"), some macro-related tests fail on Ubuntu 20.04 with
the system gcc 9.3.0 compiler when building with _GLIBCXX_DEBUG.  For
example, gdb.base/info-macros.exp results in:

   (gdb) break -qualified main
   /home/smarchi/src/binutils-gdb/gdb/../gdbsupport/gdb_optional.h:206: internal-error: T& gdb::optional<T>::get() [with T = long unsigned int]: Assertion `this->has_value ()' failed.

The binary contains DWARF 4 debug info and includes a pre-standard
(pre-DWARF 5) .debug_macro section.  The CU doesn't have a
DW_AT_str_offsets_base attribute (which doesn't exist in DWARF 4).  The
field dwarf2_cu::str_offsets_base is therefore empty.  At
dwarf2/read.c:24138, we unconditionally read the value in the optional,
which triggers the assertion shown above.

The same thing happens when building the test program with DWARF 5 with
the same gcc compiler, as that version of gcc doesn't use indirect
string forms, even with DWARF 5.  So it still doesn't add a
DW_AT_str_offsets_base attribute on the CU.

Fix that by propagating down a gdb::optional<ULONGEST> for the str
offsets base instead of ULONGEST.  That value is only used in
dwarf_decode_macro_bytes, when encountering an "strx" macro operation
(DW_MACRO_define_strx or DW_MACRO_undef_strx).  Add a check there that
we indeed have a value in the optional before reading it.  This is
unlikely to happen, but could happen in theory with an erroneous file
that uses DW_MACRO_define_strx but does not provide a
DW_AT_str_offsets_base (in practice, some things would probably have
failed before and stopped processing of debug info).  I tested the
complaint by inverting the condition and using a clang-compiled binary,
which uses the strx operators.  This is the result:

    During symbol reading: use of DW_MACRO_define_strx with unknown string offsets base [in module /home/simark/build/binutils-gdb/gdb/testsuite/outputs/gdb.base/info-macros/info-macros]

The test now passes cleanly with the setup mentioned above, and the
testsuite looks on par with how it was before 4d7188abfd.

Change-Id: I7ebd2724beb7b9b4178872374c2a177aea696e77
This commit is contained in:
Simon Marchi 2021-08-03 11:31:13 -04:00
parent d40947728b
commit f6c4a82abd
3 changed files with 20 additions and 7 deletions
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4
gdb/dwarf2/read.c
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@ -24122,7 +24122,7 @@ dwarf_decode_macros (struct dwarf2_cu *cu, unsigned int offset,
struct dwarf2_section_info *str_offsets_section;
struct dwarf2_section_info *str_section;
ULONGEST str_offsets_base;
gdb::optional<ULONGEST> str_offsets_base;
if (cu->dwo_unit != nullptr)
{
@ -24135,7 +24135,7 @@ dwarf_decode_macros (struct dwarf2_cu *cu, unsigned int offset,
{
str_offsets_section = &per_objfile->per_bfd->str_offsets;
str_section = &per_objfile->per_bfd->str;
str_offsets_base = *cu->str_offsets_base;
str_offsets_base = cu->str_offsets_base;
}
dwarf_decode_macros (per_objfile, builder, section, lh,