gdb: handle case where type alignment is unknown

It was spotted that if type_align returned 0 then it was possible to
trigger a divide by zero exception within GDB.  It turns out this will
only happen in an edge case where GDB is unable to figure out the
alignment of a field within a structure.

The attached test generates some non-standard, probably broken, DWARF,
that triggers this condition, and then fixes this issue by throwing an
exception when this case occurs.

gdb/ChangeLog:

	PR gdb/27847
	* amd64-tdep.c (amd64_has_unaligned_fields): Move call to
	type_align, and spot case where the alignment is unknown.

gdb/testsuite/ChangeLog:

	PR gdb/27847
	* gdb.dwarf2/dw2-weird-type-len.c: New file.
	* gdb.dwarf2/dw2-weird-type-len.exp: New file.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2021-06-07  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	PR gdb/27847
+	* amd64-tdep.c (amd64_has_unaligned_fields): Move call to
+	type_align, and spot case where the alignment is unknown.
+
 2021-06-07  Carl Love  <cel@us.ibm.com>
 
 	* ppc-tdep.h (ppc_insn_prefix_dform): Declare.

gdb/amd64-tdep.c

@@ -554,7 +554,6 @@ amd64_has_unaligned_fields (struct type *type)
 	{
 	  struct type *subtype = check_typedef (type->field (i).type ());
 	  int bitpos = TYPE_FIELD_BITPOS (type, i);
-	  int align = type_align(subtype);
 
 	  /* Ignore static fields, empty fields (for example nested
 	     empty structures), and bitfields (these are handled by
@@ -568,6 +567,10 @@ amd64_has_unaligned_fields (struct type *type)
 	  if (bitpos % 8 != 0)
 	    return true;
 
+	  int align = type_align (subtype);
+	  if (align == 0)
+	    error (_("could not determine alignment of type"));
+
 	  int bytepos = bitpos / 8;
 	  if (bytepos % align != 0)
 	    return true;

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2021-06-07  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	PR gdb/27847
+	* gdb.dwarf2/dw2-weird-type-len.c: New file.
+	* gdb.dwarf2/dw2-weird-type-len.exp: New file.
+
 2021-06-07  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* gdb.base/attach.exp (test_command_line_attach_run): Use

gdb/testsuite/gdb.dwarf2/dw2-weird-type-len.c

@@ -0,0 +1,45 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   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/>.  */
+
+struct foo_t
+{
+  int field : 24;
+};
+
+struct bar_t
+{
+  struct foo_t f;
+};
+
+struct bar_t
+get_bar ()
+{
+  asm ("get_bar_label: .globl get_bar_label");
+  struct bar_t b;
+
+  b.f.field = 0;
+
+  return b;
+}
+
+int
+main ()
+{
+  asm ("main_label: .globl main_label");
+  struct bar_t b = get_bar ();
+  return b.f.field;
+}

gdb/testsuite/gdb.dwarf2/dw2-weird-type-len.exp

@@ -0,0 +1,107 @@
+# 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 x86-64 targets.  It checks for a bug
+# that existed in amd64-tdep.c, and depends on an error being produced
+# from within that file.
+if {![istarget x86_64-*] || ![is_lp64_target]} {
+    return 0
+}
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile .c -dw.S
+
+# Make some DWARF for the test.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    global srcfile
+
+    get_func_info main
+    get_func_info get_bar
+
+    cu {} {
+	DW_TAG_compile_unit {
+	    {DW_AT_language @DW_LANG_C}
+	    {DW_AT_name     $srcfile}
+	    {DW_AT_comp_dir /tmp}
+	} {
+	    declare_labels integer_label foo_t_label bar_t_label
+
+	    foo_t_label: DW_TAG_structure_type {
+		{name foo_t}
+		{byte_size 3 DW_FORM_sdata}
+	    } {
+		member {
+		    {name field}
+		    {type :$integer_label}
+		    {data_member_location 0 DW_FORM_sdata}
+		}
+	    }
+
+	    integer_label: DW_TAG_base_type {
+		{DW_AT_byte_size 3 DW_FORM_sdata}
+		{DW_AT_encoding  @DW_ATE_signed}
+		{DW_AT_name      integer}
+	    }
+
+	    bar_t_label: DW_TAG_structure_type {
+		{name bar_t}
+		{byte_size 3 DW_FORM_sdata}
+	    } {
+		member {
+		    {name f}
+		    {type :$foo_t_label}
+		    {data_member_location 0 DW_FORM_sdata}
+		}
+	    }
+
+	    DW_TAG_subprogram {
+		{name main}
+		{low_pc $main_start addr}
+		{high_pc $main_len data8}
+		{DW_AT_type :$integer_label}
+	    }
+
+	    DW_TAG_subprogram {
+		{name get_bar}
+		{low_pc $get_bar_start addr}
+		{high_pc $get_bar_len data8}
+		{DW_AT_type :$bar_t_label}
+	    }
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+	  [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+if ![runto_main] {
+    return -1
+}
+
+# At one point this would trigger a divide by zero inside GDB.  Now we
+# just get an error message.
+gdb_test "print get_bar ()" "could not determine alignment of type"
+
+# Check GDB is still running.
+gdb_test "p 1 + 2" " = 3"