Re: [patch] Fix `return' of long/long-long results with no debuginfo

Jan Kratochvil
2009-02-11T21:47:03Z

On Wed, 11 Feb 2009 22:22:41 +0100, Mark Kettenis wrote:
> And I have a (somewhat) vague recollection that there are debug formats or
> compilers where we cant distinguish between functions without a return type
> and functions with "unknown" return type.

As `struct type' for a function type GDB debuginfo readers always create by
make_function_type and this function crashes on NULL `type' we can assume
TYPE_TARGET_TYPE is never NULL.

For DWARF it is clear both according to the standard
3.3.2 Subroutine and Entry Point Return Types
If the subroutine or entry point is a function that returns a value, then its
debugging information entry has a DW_AT_type attribute [...]

and according to the code:
dwarf2read.c:die_type():
/* A missing DW_AT_type represents a void type. */

Made the condition in the patch more explicit now. Removed the original
TYPE_CODE_VOID code emulating the K&R behavior to more follow the modern
ANSI/ISO C compilers refuse `return value;' in a void-returning function
without having typeless/`int'-defaulted functions permitted.

> From: Daniel Jacobowitz <drow@false.org>
> > Everything we pick will be wrong some of the time, but IMO "long" is
> > maximally useful. "long long" on 32-bit platforms is going to pick
> > up garbage from the next register for int or void * returns.

Garbage would be seen by caller of a `long long' returning callee where GDB
would return something smaller (like `int'). As GDB writes (not "reads") the
register it just will write zero to one excessive but unused register. This
register must be callee-saved (not caller-saved) by the ABI supporting `long
long' return type and GDB already asserts the return type is
RETURN_VALUE_REGISTER_CONVENTION.

If GDB would return only the `long' size there would be no way how to do the
`return' command on a debuginfo-less function natively returning `long long'.

> The right solution of course is to distribute libraries with debug
> sumbols. But in cases where that is impossible, would it be an idea
> to use the type of the return value expression given by the user
> instead of int as a fallback?

Any return value gets retyped to `long long' which is intepreted by the caller
even as lower-sized types (not guaranteed by ABI but at lest checked now by
new gdb.base/return-nodebug.exp).

> In the case at hand, "return -1" will still fail, but it would be
> possible to do return (void *)-1 to do what the user wanted.

`return -1' is checked by gdb.base/return-nodebug.exp and it works.

Thanks,
Jan

gdb/
2009-02-11 Jan Kratochvil <jan.kratochvil@redhat.com>

* stack.c (return_command): Default to the builtin_long_long or at
least builtin_long types now. Assert TYPE_TARGET_TYPE of a typed
function is non-NULL. Error on return value for void returning
functions.

gdb/testsuite/
2009-02-11 Jan Kratochvil <jan.kratochvil@redhat.com>

* gdb.base/return-nodebug.exp, gdb.base/return-nodebug.c: New.
* gdb.base/return2.exp (try to return value in a void function): New.

--- gdb/stack.c 11 Feb 2009 16:07:28 -0000 1.185
+++ gdb/stack.c 11 Feb 2009 21:39:57 -0000
@@ -1805,9 +1805,23 @@ return_command (char *retval_exp, int fr
/* Cast return value to the return type of the function. Should
the cast fail, this call throws an error. */
if (thisfun != NULL)
- return_type = TYPE_TARGET_TYPE (SYMBOL_TYPE (thisfun));
- if (return_type == NULL)
- return_type = builtin_type (get_frame_arch (thisframe))->builtin_int;
+ {
+ return_type = TYPE_TARGET_TYPE (SYMBOL_TYPE (thisfun));
+ gdb_assert (return_type != NULL);
+ if (TYPE_CODE (return_type) == TYPE_CODE_VOID)
+ error (_("Function %s does not return any value"),
+ SYMBOL_PRINT_NAME (thisfun));
+ }
+ else
+ {
+ return_type
+ = builtin_type (get_frame_arch (thisframe))->builtin_long_long;
+
+ /* Hopefull each ABI returns at least `long' type in register. */
+ if (using_struct_return (NULL, return_type))
+ return_type
+ = builtin_type (get_frame_arch (thisframe))->builtin_long;
+ }
CHECK_TYPEDEF (return_type);
return_value = value_cast (return_type, return_value);

@@ -1816,14 +1830,7 @@ return_command (char *retval_exp, int fr
if (value_lazy (return_value))
value_fetch_lazy (return_value);

- if (TYPE_CODE (return_type) == TYPE_CODE_VOID)
- /* If the return-type is "void", don't try to find the
- return-value's location. However, do still evaluate the
- return expression so that, even when the expression result
- is discarded, side effects such as "return i++" still
- occur. */
- return_value = NULL;
- else if (thisfun != NULL
+ if (thisfun != NULL
&& using_struct_return (SYMBOL_TYPE (thisfun), return_type))
{
query_prefix = "\
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ gdb/testsuite/gdb.base/return-nodebug.c 11 Feb 2009 21:40:00 -0000
@@ -0,0 +1,40 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+ Copyright 2009 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/>. */
+
+#include <stdio.h>
+
+static TYPE
+func (void)
+{
+ return (TYPE) -1;
+}
+
+static void
+marker (void)
+{
+}
+
+int
+main (void)
+{
+ printf ("result=" FORMAT "\n", CAST func ());
+
+ /* Cannot `next' with no debug info. */
+ marker ();
+
+ return 0;
+}
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ gdb/testsuite/gdb.base/return-nodebug.exp 11 Feb 2009 21:40:00 -0000
@@ -0,0 +1,52 @@
+# Copyright (C) 2009 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/>.
+
+proc do_test {typenospace} {
+ global pf_prefix
+ set old_prefix $pf_prefix
+ lappend pf_prefix "$typenospace:"
+
+ if {[runto "func"]} {
+ # Test return from a function with no debug info (symbol; still it may
+ # have a minimal-symbol) if it does not crash.
+ gdb_test "return -1" "#0 .* main \\(.*" \
+ "return from function with no debug info" \
+ "Make selected stack frame return now\\? \\(y or n\\) " "y"
+
+ # And if it returned the full width of the result.
+ gdb_test "adv marker" "\r\nresult=-1\r\n.* in marker \\(.*" \
+ "full width of the returned result"
+ }
+
+ set pf_prefix $old_prefix
+}
+
+foreach case {{{char} %d (int)} \
+ {{short} %d (int)} \
+ {{int} %d} \
+ {{long} %ld} \
+ {{long long} %lld}} {
+ set type [lindex $case 0]
+ set format [lindex $case 1]
+ set cast [lindex $case 2]
+
+ set typeesc [string map {{ } {\ }} $type]
+ set typenospace [string map {{ } -} $type]
+
+ if {[prepare_for_testing return-nodebug.exp "return-nodebug-$typenospace" "return-nodebug.c" \
+ [list "additional_flags=-DFORMAT=\"$format\" -DTYPE=$typeesc -DCAST=$cast"]] == 0} {
+ do_test $typenospace
+ }
+}
--- gdb/testsuite/gdb.base/return2.exp 3 Jan 2009 05:58:03 -0000 1.11
+++ gdb/testsuite/gdb.base/return2.exp 11 Feb 2009 21:40:00 -0000
@@ -70,6 +70,8 @@ proc return_void { } {
"set break on void_func"
gdb_test "continue" "Breakpoint.* void_func.*" \
"continue to void_func"
+ gdb_test "return 23" "Function void_func does not return any value" \
+ "try to return value in a void function"
send_gdb "return \n"
gdb_expect {
-re "Make void_func return now.*y or n. $" {