[gdb/symtab] Fix segfault on invalid debug info

While looking at PR symtab/31478 (a problem in the cooked indexer with invalid
dwarf) it occurred to me that I could trigger a similar problem using:
...
  Compilation Unit @ offset 0xb2:
   Length:        0x1f (32-bit)
   Version:       4
   Abbrev Offset: 0x6c
   Pointer Size:  8
 <0><bd>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <be>   DW_AT_language    : 2	(non-ANSI C)
 <1><bf>: Abbrev Number: 2 (DW_TAG_subprogram)
    <c0>   DW_AT_low_pc      : 0x4004a7
    <c8>   DW_AT_high_pc     : 0x4004b2
    <d0>   DW_AT_specification: <0xd5>
 <1><d4>: Abbrev Number: 0
  Compilation Unit @ offset 0xd5:
   Length:        0x7 (32-bit)
   Version:       4
   Abbrev Offset: 0x7f
   Pointer Size:  8
...
and indeed I get:
...
$ gdb -q -batch outputs/gdb.dwarf2/dw2-inter-cu-error-2/dw2-inter-cu-error-2

Fatal signal: Segmentation fault
...

The problem is that we're calling prepare_one_comp_unit with cu == nullptr and
comp_unit_die == nullptr here in cooked_indexer::ensure_cu_exists:
...
      cutu_reader new_reader (per_cu, per_objfile, nullptr, nullptr, false,
                              m_index_storage->get_abbrev_cache ());

      prepare_one_comp_unit (new_reader.cu, new_reader.comp_unit_die,
                             language_minimal);
...

Fix this by bailing out for various types of dummy CUs:
...
      if (new_reader.dummy_p || new_reader.comp_unit_die == nullptr
	  || !new_reader.comp_unit_die->has_children)
	return nullptr;
...

Also make sure in scan_attributes that this triggers a dwarf error:
...
$ gdb -q -batch dw2-inter-cu-error-2
DWARF Error: cannot follow reference to DIE at 0xd5 \
  [in module dw2-inter-cu-error-2]
...

With target board readnow, the test-case triggers an assertion failure in
follow_die_offset, so fix this by throwing the same dwarf error.

While we're at it, make the other check for dummy CUs in
cooked_indexer::ensure_cu_exists more robust by adding an intermediate test
for comp_unit_die:
...
-  if (result->dummy_p || !result->comp_unit_die->has_children)
+  if (result->dummy_p || result->comp_unit_die == nullptr
+      || !result->comp_unit_die->has_children)
     return nullptr;
...

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

1 files changed, 51 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.dwarf2/dw2-inter-cu-error-2.exp b/gdb/testsuite/gdb.dwarf2/dw2-inter-cu-error-2.exp
new file mode 100644
index 0000000..585fd54
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-inter-cu-error-2.exp
@@ -0,0 +1,51 @@
+# Copyright 2024 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Check that GDB doesn't crash on invalid dwarf, specifically an inter-CU
+# reference pointing to a dummy CU.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+require dwarf2_support
+
+standard_testfile main.c .S
+
+# Create the DWARF.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    declare_labels label1
+
+    cu {} {
+	compile_unit {{language @DW_LANG_C}} {
+	    subprogram {
+		{MACRO_AT_range { main }}
+		{DW_AT_specification %$label1}
+	    }
+	}
+    }
+
+    label1: cu {} {
+    }
+}
+
+if [prepare_for_testing "failed to prepare" $testfile \
+	[list $asm_file $srcfile] {nodebug}] {
+    return -1
+}
+
+gdb_assert \
+    { [regexp "DWARF Error: cannot follow reference" $gdb_file_cmd_msg] } \
+    "Error message"