binutils-gdb/gdb/compile/compile-object-run.c
Pedro Alves 7022349d5c Stop assuming no-debug-info functions return int
The fact that GDB defaults to assuming that functions return int, when
it has no debug info for the function has been a recurring source of
user confusion.  Recently this came up on the errno pretty printer
discussions.  Shortly after, it came up again on IRC, with someone
wondering why does getenv() in GDB return a negative int:

  (gdb) p getenv("PATH")
  $1 = -6185

This question (with s/getenv/random-other-C-runtime-function) is a FAQ
on IRC.

The reason for the above is:

 (gdb) p getenv
 $2 = {<text variable, no debug info>} 0x7ffff7751d80 <getenv>
 (gdb) ptype getenv
 type = int ()

... which means that GDB truncated the 64-bit pointer that is actually
returned from getent to 32-bit, and then sign-extended it:

 (gdb) p /x -6185
 $6 = 0xffffe7d7

The workaround is to cast the function to the right type, like:

 (gdb) p ((char *(*) (const char *)) getenv) ("PATH")
 $3 = 0x7fffffffe7d7 "/usr/local/bin:/"...

IMO, we should do better than this.

I see the "assume-int" issue the same way I see printing bogus values
for optimized-out variables instead of "<optimized out>" -- I'd much
rather that the debugger tells me "I don't know" and tells me how to
fix it than showing me bogus misleading results, making me go around
tilting at windmills.

If GDB prints a signed integer when you're expecting a pointer or
aggregate, you at least have some sense that something is off, but
consider the case of the function actually returning a 64-bit integer.
For example, compile this without debug info:

 unsigned long long
 function ()
 {
   return 0x7fffffffffffffff;
 }

Currently, with pristine GDB, you get:

 (gdb) p function ()
 $1 = -1                      # incorrect
 (gdb) p /x function ()
 $2 = 0xffffffff              # incorrect

maybe after spending a few hours debugging you suspect something is
wrong with that -1, and do:

 (gdb) ptype function
 type = int ()

and maybe, just maybe, you realize that the function actually returns
unsigned long long.  And you try to fix it with:

(gdb) p /x (unsigned long long) function ()
 $3 = 0xffffffffffffffff      # incorrect

... which still produces the wrong result, because GDB simply applied
int to unsigned long long conversion.  Meaning, it sign-extended the
integer that it extracted from the return of the function, to 64-bits.

and then maybe, after asking around on IRC, you realize you have to
cast the function to a pointer of the right type, and call that.  It
won't be easy, but after a few missteps, you'll get to it:

.....  (gdb) p /x ((unsigned long long(*) ()) function) ()
 $666 = 0x7fffffffffffffff             # finally! :-)


So to improve on the user experience, this patch does the following
(interrelated) things:

 - makes no-debug-info functions no longer default to "int" as return
   type.  Instead, they're left with NULL/"<unknown return type>"
   return type.

    (gdb) ptype getenv
    type = <unknown return type> ()

 - makes calling a function with unknown return type an error.

    (gdb) p getenv ("PATH")
    'getenv' has unknown return type; cast the call to its declared return type

 - and then to make it easier to call the function, makes it possible
   to _only_ cast the return of the function to the right type,
   instead of having to cast the function to a function pointer:

    (gdb) p (char *) getenv ("PATH")                      # now Just Works
    $3 = 0x7fffffffe7d7 "/usr/local/bin:/"...

    (gdb) p ((char *(*) (const char *)) getenv) ("PATH")  # continues working
    $4 = 0x7fffffffe7d7 "/usr/local/bin:/"...

   I.e., it makes GDB default the function's return type to the type
   of the cast, and the function's parameters to the type of the
   arguments passed down.

After this patch, here's what you'll get for the "unsigned long long"
example above:

 (gdb) p function ()
 'function' has unknown return type; cast the call to its declared return type
 (gdb) p /x (unsigned long long) function ()
 $4 = 0x7fffffffffffffff     # correct!

Note that while with "print" GDB shows the name of the function that
has the problem:

  (gdb) p getenv ("PATH")
  'getenv' has unknown return type; cast the call to its declared return type

which can by handy in more complicated expressions, "ptype" does not:

  (gdb) ptype getenv ("PATH")
  function has unknown return type; cast the call to its declared return type

This will be fixed in the next patch.

gdb/ChangeLog:
2017-09-04  Pedro Alves  <palves@redhat.com>

	* ada-lang.c (ada_evaluate_subexp) <TYPE_CODE_FUNC>: Don't handle
	TYPE_GNU_IFUNC specially here.  Throw error if return type is
	unknown.
	* ada-typeprint.c (print_func_type): Handle functions with unknown
	return type.
	* c-typeprint.c (c_type_print_base): Handle functions and methods
	with unknown return type.
	* compile/compile-c-symbols.c (convert_symbol_bmsym)
	<mst_text_gnu_ifunc>: Use nodebug_text_gnu_ifunc_symbol.
	* compile/compile-c-types.c: Include "objfiles.h".
	(convert_func): For functions with unknown return type, warn and
	default to int.
	* compile/compile-object-run.c (compile_object_run): Adjust call
	to call_function_by_hand_dummy.
	* elfread.c (elf_gnu_ifunc_resolve_addr): Adjust call to
	call_function_by_hand.
	* eval.c (evaluate_subexp_standard): Adjust calls to
	call_function_by_hand.  Handle functions and methods with unknown
	return type.  Pass expect_type to call_function_by_hand.
	* f-typeprint.c (f_type_print_base): Handle functions with unknown
	return type.
	* gcore.c (call_target_sbrk): Adjust call to
	call_function_by_hand.
	* gdbtypes.c (objfile_type): Leave nodebug text symbol with NULL
	return type instead of int.  Make nodebug_text_gnu_ifunc_symbol be
	an integer address type instead of nodebug.
	* guile/scm-value.c (gdbscm_value_call): Adjust call to
	call_function_by_hand.
	* infcall.c (error_call_unknown_return_type): New function.
	(call_function_by_hand): New "default_return_type" parameter.
	Pass it down.
	(call_function_by_hand_dummy): New "default_return_type"
	parameter.  Use it instead of defaulting to int.  If there's no
	default and the return type is unknown, throw an error.  If
	there's a default return type, and the called function has no
	debug info, then assume the function is prototyped.
	* infcall.h (call_function_by_hand, call_function_by_hand_dummy):
	New "default_return_type" parameter.
	(error_call_unknown_return_type): New declaration.
	* linux-fork.c (call_lseek): Cast return type of lseek.
	(inferior_call_waitpid, checkpoint_command): Adjust calls to
	call_function_by_hand.
	* linux-tdep.c (linux_infcall_mmap, linux_infcall_munmap): Adjust
	calls to call_function_by_hand.
	* m2-typeprint.c (m2_procedure): Handle functions with unknown
	return type.
	* objc-lang.c (lookup_objc_class, lookup_child_selector)
	(value_nsstring, print_object_command): Adjust calls to
	call_function_by_hand.
	* p-typeprint.c (pascal_type_print_varspec_prefix): Handle
	functions with unknown return type.
	(pascal_type_print_func_varspec_suffix): New function.
	(pascal_type_print_varspec_suffix) <TYPE_CODE_FUNC,
	TYPE_CODE_METHOD>: Use it.
	* python/py-value.c (valpy_call): Adjust call to
	call_function_by_hand.
	* rust-lang.c (rust_evaluate_funcall): Adjust call to
	call_function_by_hand.
	* valarith.c (value_x_binop, value_x_unop): Adjust calls to
	call_function_by_hand.
	* valops.c (value_allocate_space_in_inferior): Adjust call to
	call_function_by_hand.
	* typeprint.c (type_print_unknown_return_type): New function.
	* typeprint.h (type_print_unknown_return_type): New declaration.

gdb/testsuite/ChangeLog:
2017-09-04  Pedro Alves  <palves@redhat.com>

	* gdb.base/break-main-file-remove-fail.exp (test_remove_bp): Cast
	return type of munmap in infcall.
	* gdb.base/break-probes.exp: Cast return type of foo in infcall.
	* gdb.base/checkpoint.exp: Simplify using for loop.  Cast return
	type of ftell in infcall.
	* gdb.base/dprintf-detach.exp (dprintf_detach_test): Cast return
	type of getpid in infcall.
	* gdb.base/infcall-exec.exp: Cast return type of execlp in
	infcall.
	* gdb.base/info-os.exp: Cast return type of getpid in infcall.
	Bail on failure to extract the pid.
	* gdb.base/nodebug.c: #include <stdint.h>.
	(multf, multf_noproto, mult, mult_noproto, add8, add8_noproto):
	New functions.
	* gdb.base/nodebug.exp (test_call_promotion): New procedure.
	Change expected output of print/whatis/ptype with functions with
	no debug info.  Test all supported languages.  Call
	test_call_promotion.
	* gdb.compile/compile.exp: Adjust expected output to expect
	warning.
	* gdb.threads/siginfo-threads.exp: Likewise.
2017-09-04 20:21:13 +01:00

196 lines
6.0 KiB
C

/* Call module for 'compile' command.
Copyright (C) 2014-2017 Free Software Foundation, Inc.
This file is part of GDB.
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 "defs.h"
#include "compile-object-run.h"
#include "value.h"
#include "infcall.h"
#include "objfiles.h"
#include "compile-internal.h"
#include "dummy-frame.h"
#include "block.h"
#include "valprint.h"
#include "compile.h"
/* Helper for do_module_cleanup. */
struct do_module_cleanup
{
/* Boolean to set true upon a call of do_module_cleanup.
The pointer may be NULL. */
int *executedp;
/* .c file OBJFILE was built from. It needs to be xfree-d. */
char *source_file;
/* Copy from struct compile_module. */
enum compile_i_scope_types scope;
void *scope_data;
/* Copy from struct compile_module. */
struct type *out_value_type;
CORE_ADDR out_value_addr;
/* Copy from struct compile_module. */
struct munmap_list *munmap_list_head;
/* objfile_name of our objfile. */
char objfile_name_string[1];
};
/* Cleanup everything after the inferior function dummy frame gets
discarded. */
static dummy_frame_dtor_ftype do_module_cleanup;
static void
do_module_cleanup (void *arg, int registers_valid)
{
struct do_module_cleanup *data = (struct do_module_cleanup *) arg;
struct objfile *objfile;
if (data->executedp != NULL)
{
*data->executedp = 1;
/* This code cannot be in compile_object_run as OUT_VALUE_TYPE
no longer exists there. */
if (data->scope == COMPILE_I_PRINT_ADDRESS_SCOPE
|| data->scope == COMPILE_I_PRINT_VALUE_SCOPE)
{
struct value *addr_value;
struct type *ptr_type = lookup_pointer_type (data->out_value_type);
addr_value = value_from_pointer (ptr_type, data->out_value_addr);
/* SCOPE_DATA would be stale unlesse EXECUTEDP != NULL. */
compile_print_value (value_ind (addr_value), data->scope_data);
}
}
ALL_OBJFILES (objfile)
if ((objfile->flags & OBJF_USERLOADED) == 0
&& (strcmp (objfile_name (objfile), data->objfile_name_string) == 0))
{
free_objfile (objfile);
/* It may be a bit too pervasive in this dummy_frame dtor callback. */
clear_symtab_users (0);
break;
}
/* Delete the .c file. */
unlink (data->source_file);
xfree (data->source_file);
munmap_list_free (data->munmap_list_head);
/* Delete the .o file. */
unlink (data->objfile_name_string);
xfree (data);
}
/* Perform inferior call of MODULE. This function may throw an error.
This function may leave files referenced by MODULE on disk until
the inferior call dummy frame is discarded. This function may throw errors.
Thrown errors and left MODULE files are unrelated events. Caller must no
longer touch MODULE's memory after this function has been called. */
void
compile_object_run (struct compile_module *module)
{
struct value *func_val;
struct frame_id dummy_id;
struct cleanup *cleanups;
struct do_module_cleanup *data;
const char *objfile_name_s = objfile_name (module->objfile);
int dtor_found, executed = 0;
struct symbol *func_sym = module->func_sym;
CORE_ADDR regs_addr = module->regs_addr;
struct objfile *objfile = module->objfile;
data = (struct do_module_cleanup *) xmalloc (sizeof (*data)
+ strlen (objfile_name_s));
data->executedp = &executed;
data->source_file = xstrdup (module->source_file);
strcpy (data->objfile_name_string, objfile_name_s);
data->scope = module->scope;
data->scope_data = module->scope_data;
data->out_value_type = module->out_value_type;
data->out_value_addr = module->out_value_addr;
data->munmap_list_head = module->munmap_list_head;
xfree (module->source_file);
xfree (module);
module = NULL;
TRY
{
struct type *func_type = SYMBOL_TYPE (func_sym);
htab_t copied_types;
int current_arg = 0;
struct value **vargs;
/* OBJFILE may disappear while FUNC_TYPE still will be in use. */
copied_types = create_copied_types_hash (objfile);
func_type = copy_type_recursive (objfile, func_type, copied_types);
htab_delete (copied_types);
gdb_assert (TYPE_CODE (func_type) == TYPE_CODE_FUNC);
func_val = value_from_pointer (lookup_pointer_type (func_type),
BLOCK_START (SYMBOL_BLOCK_VALUE (func_sym)));
vargs = XALLOCAVEC (struct value *, TYPE_NFIELDS (func_type));
if (TYPE_NFIELDS (func_type) >= 1)
{
gdb_assert (regs_addr != 0);
vargs[current_arg] = value_from_pointer
(TYPE_FIELD_TYPE (func_type, current_arg), regs_addr);
++current_arg;
}
if (TYPE_NFIELDS (func_type) >= 2)
{
gdb_assert (data->out_value_addr != 0);
vargs[current_arg] = value_from_pointer
(TYPE_FIELD_TYPE (func_type, current_arg), data->out_value_addr);
++current_arg;
}
gdb_assert (current_arg == TYPE_NFIELDS (func_type));
call_function_by_hand_dummy (func_val,
NULL, TYPE_NFIELDS (func_type), vargs,
do_module_cleanup, data);
}
CATCH (ex, RETURN_MASK_ERROR)
{
/* In the case of DTOR_FOUND or in the case of EXECUTED nothing
needs to be done. */
dtor_found = find_dummy_frame_dtor (do_module_cleanup, data);
if (!executed)
data->executedp = NULL;
gdb_assert (!(dtor_found && executed));
if (!dtor_found && !executed)
do_module_cleanup (data, 0);
throw_exception (ex);
}
END_CATCH
dtor_found = find_dummy_frame_dtor (do_module_cleanup, data);
gdb_assert (!dtor_found && executed);
}