[gdb/symtab] Fix assert in set_length

When running the included test-case, we run into:
...
(gdb) break _start^M
read.h:309: internal-error: set_length: \
  Assertion `m_length == length' failed.^M
...

The problem is that while there are two CUs:
...
$ readelf -wi debug-names-missing-cu | grep @
  Compilation Unit @ offset 0x0:
  Compilation Unit @ offset 0x2d:
...
the CU table in the .debug_names section only contains the first one:
...
CU table:
[  0] 0x0
...

The incomplete CU table makes create_cus_from_debug_names_list set the size of
the CU at 0x0 to the actual size of both CUs combined.

This eventually leads to the assert, when we read the actual size from the CU
header.

While having an incomplete CU table in a .debug_names section is incorrect,
we need a better failure mode than asserting.

The easiest way to fix this is to set the length to 0 (meaning: unkown) in
create_cus_from_debug_names_list.

This makes the failure mode to accept the incomplete CU table, but to ignore
the missing CU.

It would be nice to instead reject the .debug_names index, and build a
complete CU list, but the point where we find this out is well after
dwarf2_initialize_objfile, so it looks rather intrusive to restart at that
point.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29453

2 files changed, 83 insertions, 1 deletions

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 84faeb4..6c6ca96 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -4655,7 +4655,9 @@ create_cus_from_debug_names_list (dwarf2_per_bfd *per_bfd,
 			 " ignoring .debug_names."));
 	      return false;
 	    }
-	  const ULONGEST length = sect_off_next - sect_off_prev;
+	  /* Note: we're not using length = sect_off_next - sect_off_prev,
+	     to gracefully handle an incomplete CU list.  */
+	  const ULONGEST length = 0;
 	  dwarf2_per_cu_data_up per_cu
 	    = create_cu_from_index_list (per_bfd, &section, is_dwz,
 					 sect_off_prev, length);
diff --git a/gdb/testsuite/gdb.dwarf2/debug-names-missing-cu.exp b/gdb/testsuite/gdb.dwarf2/debug-names-missing-cu.exp
new file mode 100644
index 0000000..f70debd
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/debug-names-missing-cu.exp
@@ -0,0 +1,80 @@
+# Copyright 2022 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/>.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile _start.c debug-names.S
+
+set func_info_vars \
+    [get_func_info _start [list debug additional_flags=-nostartfiles]]
+
+# Create the DWARF.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble {
+    filename $asm_file
+    add_dummy_cus 0
+} {
+    global func_info_vars
+    foreach var $func_info_vars {
+	global $var
+    }
+
+    cu { label cu_label } {
+	compile_unit {{language @DW_LANG_C}} {
+	    subprogram {
+		{DW_AT_name _start}
+		{DW_AT_low_pc $_start_start DW_FORM_addr}
+		{DW_AT_high_pc $_start_end DW_FORM_addr}
+	    }
+	    base_type {
+		{name int}
+		{byte_size 4 sdata}
+		{encoding @DW_ATE_signed}
+	    }
+	}
+    }
+
+    # This CU is missing from the cu list in .debug_names.
+    cu {} {
+    }
+
+    debug_names {} {
+	cu cu_label
+	name _start subprogram cu_label 0xEDDB6232
+	name int base_type cu_label 0xB888030
+    }
+}
+
+if [prepare_for_testing "failed to prepare" $testfile "${asm_file} ${srcfile}" \
+	[list additional_flags=-nostartfiles]] {
+    return -1
+}
+
+# Verify that .debug_names section is not ignored.
+set index [have_index $binfile]
+gdb_assert { [string equal $index "debug_names"] } ".debug_names used"
+
+# Verify that initially no symtab is expanded.
+gdb_test_no_output "maint info symtabs"
+
+# Verify that _start is found in the debuginfo, rather than in the minimal
+# symbols, which would result instead in:
+#   $1 = {<text variable, no debug info>} $hex <_start>
+gdb_test "print _start" " = {void \\(\\)} $hex <_start>"