Avoid crash on missing dwz file

If DWARF contains a reference to a "dwz" file, but there is no
.gnu_debugaltlink section, then gdb will crash.  This happens because
dwarf2_get_dwz_file will return NULL, but some callers do not expect
this.

This patch changes dwarf2_get_dwz_file so that callers can require a
dwz file.  Then, it updates the callers that are attempting to process
references to the dwz file to require one.

This includes a new testcase.  The dwarf.exp changes don't handle the
new forms exactly correctly -- they are only handled well enough to
let this test case complete.

gdb/ChangeLog
2021-03-06  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.h (dwarf2_get_dwz_file): Add 'require' parameter.
	* dwarf2/read.c (dwarf2_get_dwz_file): Add 'require' parameter.
	(get_abbrev_section_for_cu, read_attribute_value)
	(get_debug_line_section): Update.
	* dwarf2/macro.c (dwarf_decode_macro_bytes): Update.

gdb/testsuite/ChangeLog
2021-03-06  Tom Tromey  <tom@tromey.com>

	* lib/dwarf.exp (_handle_DW_FORM): Treat DW_FORM_GNU_ref_alt and
	DW_FORM_GNU_strp_alt like DW_FORM_sec_offset.
	* gdb.dwarf2/dwznolink.exp: New file.

3 files changed, 68 insertions, 2 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index b7c2e72..ecbb768 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2021-03-06  Tom Tromey  <tom@tromey.com>
+
+	* lib/dwarf.exp (_handle_DW_FORM): Treat DW_FORM_GNU_ref_alt and
+	DW_FORM_GNU_strp_alt like DW_FORM_sec_offset.
+	* gdb.dwarf2/dwznolink.exp: New file.
+
 2021-03-05  Mark Wielaard  <mark@klomp.org>
 
 	* lib/valgrind.exp (vgdb_start): Add --wait=1 to vgdbcmd.
diff --git a/gdb/testsuite/gdb.dwarf2/dwznolink.exp b/gdb/testsuite/gdb.dwarf2/dwznolink.exp
new file mode 100644
index 0000000..98976d7
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dwznolink.exp
@@ -0,0 +1,60 @@
+# Copyright 2021 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
+}
+
+# No remote host testing either.
+if {[is_remote host]} {
+    return 0
+}
+
+standard_testfile main.c dwznolink.S
+
+set asm_file [standard_output_file $srcfile2]
+
+# The DWARF should contain a reference to a supplementary ("dwz")
+# file, but the section that links to the file should be missing.  At
+# one point, this caused gdb crashes.
+Dwarf::assemble $asm_file {
+    cu {} {
+	compile_unit {{language @DW_LANG_C}} {
+	    constant {
+		{name 0 DW_FORM_GNU_strp_alt}
+		{type 97 DW_FORM_GNU_ref_alt}
+		{const_value 99 data1}
+	    }
+	}
+    }
+}
+
+# We can't use prepare_for_testing here because we need to check the
+# 'file' command's output.
+if {[build_executable $testfile.exp $testfile \
+	 [list $srcfile $asm_file] {nodebug quiet}]} {
+    return -1
+}
+
+gdb_exit
+gdb_start
+gdb_reinitialize_dir $srcdir/$subdir
+
+gdb_test "file -readnow $binfile" \
+    "could not read '.gnu_debugaltlink' section" \
+    "file $testfile"
diff --git a/gdb/testsuite/lib/dwarf.exp b/gdb/testsuite/lib/dwarf.exp
index c1c07be..f8fbd38 100644
--- a/gdb/testsuite/lib/dwarf.exp
+++ b/gdb/testsuite/lib/dwarf.exp
@@ -456,6 +456,8 @@ namespace eval Dwarf {
 		_op .${size}byte $value
 	    }
 
+	    DW_FORM_GNU_ref_alt -
+	    DW_FORM_GNU_strp_alt -
 	    DW_FORM_sec_offset {
 		variable _cu_offset_size
 		_op .${_cu_offset_size}byte $value
@@ -553,8 +555,6 @@ namespace eval Dwarf {
 
 	    DW_FORM_GNU_addr_index -
 	    DW_FORM_GNU_str_index -
-	    DW_FORM_GNU_ref_alt -
-	    DW_FORM_GNU_strp_alt -
 
 	    default {
 		error "unhandled form $form"