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[gdb/exp] Fix printing of out of bounds struct members
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When building gdb with -O0 -fsanitize=address, and running test-case
gdb.ada/uninitialized_vars.exp, I run into:
...
(gdb) info locals
a = 0
z = (a => 1, b => false, c => 2.0)
=================================================================
==66372==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000097f58 at pc 0xffff52c0da1c bp 0xffffc90a1d40 sp 0xffffc90a1d80
READ of size 4 at 0x602000097f58 thread T0
    #0 0xffff52c0da18 in memmove (/lib64/libasan.so.8+0x6da18)
    #1 0xbcab24 in unsigned char* std::__copy_move_backward<false, true, std::random_access_iterator_tag>::__copy_move_b<unsigned char const, unsigned char>(unsigned char const*, unsigned char const*, unsigned char*) /usr/include/c++/13/bits/stl_algobase.h:748
    #2 0xbc9bf4 in unsigned char* std::__copy_move_backward_a2<false, unsigned char const*, unsigned char*>(unsigned char const*, unsigned char const*, unsigned char*) /usr/include/c++/13/bits/stl_algobase.h:769
    #3 0xbc898c in unsigned char* std::__copy_move_backward_a1<false, unsigned char const*, unsigned char*>(unsigned char const*, unsigned char const*, unsigned char*) /usr/include/c++/13/bits/stl_algobase.h:778
    #4 0xbc715c in unsigned char* std::__copy_move_backward_a<false, unsigned char const*, unsigned char*>(unsigned char const*, unsigned char const*, unsigned char*) /usr/include/c++/13/bits/stl_algobase.h:807
    #5 0xbc4e6c in unsigned char* std::copy_backward<unsigned char const*, unsigned char*>(unsigned char const*, unsigned char const*, unsigned char*) /usr/include/c++/13/bits/stl_algobase.h:867
    #6 0xbc2934 in void gdb::copy<unsigned char const, unsigned char>(gdb::array_view<unsigned char const>, gdb::array_view<unsigned char>) gdb/../gdbsupport/array-view.h:223
    #7 0x20e0100 in value::contents_copy_raw(value*, long, long, long) gdb/value.c:1239
    #8 0x20e9830 in value::primitive_field(long, int, type*) gdb/value.c:3078
    #9 0x20e98f8 in value_field(value*, int) gdb/value.c:3095
    #10 0xcafd64 in print_field_values gdb/ada-valprint.c:658
    #11 0xcb0fa0 in ada_val_print_struct_union gdb/ada-valprint.c:857
    #12 0xcb1bb4 in ada_value_print_inner(value*, ui_file*, int, value_print_options const*) gdb/ada-valprint.c:1042
    #13 0xc66e04 in ada_language::value_print_inner(value*, ui_file*, int, value_print_options const*) const (/home/vries/gdb/build/gdb/gdb+0xc66e04)
    #14 0x20ca1e8 in common_val_print(value*, ui_file*, int, value_print_options const*, language_defn const*) gdb/valprint.c:1092
    #15 0x20caabc in common_val_print_checked(value*, ui_file*, int, value_print_options const*, language_defn const*) gdb/valprint.c:1184
    #16 0x196c524 in print_variable_and_value(char const*, symbol*, frame_info_ptr, ui_file*, int) gdb/printcmd.c:2355
    #17 0x1d99ca0 in print_variable_and_value_data::operator()(char const*, symbol*) gdb/stack.c:2308
    #18 0x1dabca0 in gdb::function_view<void (char const*, symbol*)>::bind<print_variable_and_value_data>(print_variable_and_value_data&)::{lambda(gdb::fv_detail::erased_callable, char const*, symbol*)#1}::operator()(gdb::fv_detail::erased_callable, char const*, symbol*) const gdb/../gdbsupport/function-view.h:305
    #19 0x1dabd14 in gdb::function_view<void (char const*, symbol*)>::bind<print_variable_and_value_data>(print_variable_and_value_data&)::{lambda(gdb::fv_detail::erased_callable, char const*, symbol*)#1}::_FUN(gdb::fv_detail::erased_callable, char const*, symbol*) gdb/../gdbsupport/function-view.h:299
    #20 0x1dab34c in gdb::function_view<void (char const*, symbol*)>::operator()(char const*, symbol*) const gdb/../gdbsupport/function-view.h:289
    #21 0x1d9963c in iterate_over_block_locals gdb/stack.c:2240
    #22 0x1d99790 in iterate_over_block_local_vars(block const*, gdb::function_view<void (char const*, symbol*)>) gdb/stack.c:2259
    #23 0x1d9a598 in print_frame_local_vars gdb/stack.c:2380
    #24 0x1d9afac in info_locals_command(char const*, int) gdb/stack.c:2458
    #25 0xfd7b30 in do_simple_func gdb/cli/cli-decode.c:95
    #26 0xfe5a2c in cmd_func(cmd_list_element*, char const*, int) gdb/cli/cli-decode.c:2735
    #27 0x1f03790 in execute_command(char const*, int) gdb/top.c:575
    #28 0x1384080 in command_handler(char const*) gdb/event-top.c:566
    #29 0x1384e2c in command_line_handler(std::unique_ptr<char, gdb::xfree_deleter<char> >&&) gdb/event-top.c:802
    #30 0x1f731e4 in tui_command_line_handler gdb/tui/tui-interp.c:104
    #31 0x1382a58 in gdb_rl_callback_handler gdb/event-top.c:259
    #32 0x21dbb80 in rl_callback_read_char readline/readline/callback.c:290
    #33 0x1382510 in gdb_rl_callback_read_char_wrapper_noexcept gdb/event-top.c:195
    #34 0x138277c in gdb_rl_callback_read_char_wrapper gdb/event-top.c:234
    #35 0x1fe9b40 in stdin_event_handler gdb/ui.c:155
    #36 0x35ff1bc in handle_file_event gdbsupport/event-loop.cc:573
    #37 0x35ff9d8 in gdb_wait_for_event gdbsupport/event-loop.cc:694
    #38 0x35fd284 in gdb_do_one_event(int) gdbsupport/event-loop.cc:264
    #39 0x1768080 in start_event_loop gdb/main.c:408
    #40 0x17684c4 in captured_command_loop gdb/main.c:472
    #41 0x176cfc8 in captured_main gdb/main.c:1342
    #42 0x176d088 in gdb_main(captured_main_args*) gdb/main.c:1361
    #43 0xb73edc in main gdb/gdb.c:39
    #44 0xffff519b09d8 in __libc_start_call_main (/lib64/libc.so.6+0x309d8)
    #45 0xffff519b0aac in __libc_start_main@@GLIBC_2.34 (/lib64/libc.so.6+0x30aac)
    #46 0xb73c2c in _start (/home/vries/gdb/build/gdb/gdb+0xb73c2c)

0x602000097f58 is located 0 bytes after 8-byte region [0x602000097f50,0x602000097f58)
allocated by thread T0 here:
    #0 0xffff52c65218 in calloc (/lib64/libasan.so.8+0xc5218)
    #1 0xcbc278 in xcalloc gdb/alloc.c:97
    #2 0x35f21e8 in xzalloc(unsigned long) gdbsupport/common-utils.cc:29
    #3 0x20de270 in value::allocate_contents(bool) gdb/value.c:937
    #4 0x20edc08 in value::fetch_lazy() gdb/value.c:4033
    #5 0x20dadc0 in value::entirely_covered_by_range_vector(std::vector<range, std::allocator<range> > const&) gdb/value.c:229
    #6 0xcb2298 in value::entirely_optimized_out() gdb/value.h:560
    #7 0x20ca6fc in value_check_printable gdb/valprint.c:1133
    #8 0x20caa8c in common_val_print_checked(value*, ui_file*, int, value_print_options const*, language_defn const*) gdb/valprint.c:1182
    #9 0x196c524 in print_variable_and_value(char const*, symbol*, frame_info_ptr, ui_file*, int) gdb/printcmd.c:2355
    #10 0x1d99ca0 in print_variable_and_value_data::operator()(char const*, symbol*) gdb/stack.c:2308
    #11 0x1dabca0 in gdb::function_view<void (char const*, symbol*)>::bind<print_variable_and_value_data>(print_variable_and_value_data&)::{lambda(gdb::fv_detail::erased_callable, char const*, symbol*)#1}::operator()(gdb::fv_detail::erased_callable, char const*, symbol*) const gdb/../gdbsupport/function-view.h:305
    #12 0x1dabd14 in gdb::function_view<void (char const*, symbol*)>::bind<print_variable_and_value_data>(print_variable_and_value_data&)::{lambda(gdb::fv_detail::erased_callable, char const*, symbol*)#1}::_FUN(gdb::fv_detail::erased_callable, char const*, symbol*) gdb/../gdbsupport/function-view.h:299
    #13 0x1dab34c in gdb::function_view<void (char const*, symbol*)>::operator()(char const*, symbol*) const gdb/../gdbsupport/function-view.h:289
    #14 0x1d9963c in iterate_over_block_locals gdb/stack.c:2240
    #15 0x1d99790 in iterate_over_block_local_vars(block const*, gdb::function_view<void (char const*, symbol*)>) gdb/stack.c:2259
    #16 0x1d9a598 in print_frame_local_vars gdb/stack.c:2380
    #17 0x1d9afac in info_locals_command(char const*, int) gdb/stack.c:2458
    #18 0xfd7b30 in do_simple_func gdb/cli/cli-decode.c:95
    #19 0xfe5a2c in cmd_func(cmd_list_element*, char const*, int) gdb/cli/cli-decode.c:2735
    #20 0x1f03790 in execute_command(char const*, int) gdb/top.c:575
    #21 0x1384080 in command_handler(char const*) gdb/event-top.c:566
    #22 0x1384e2c in command_line_handler(std::unique_ptr<char, gdb::xfree_deleter<char> >&&) gdb/event-top.c:802
    #23 0x1f731e4 in tui_command_line_handler gdb/tui/tui-interp.c:104
    #24 0x1382a58 in gdb_rl_callback_handler gdb/event-top.c:259
    #25 0x21dbb80 in rl_callback_read_char readline/readline/callback.c:290
    #26 0x1382510 in gdb_rl_callback_read_char_wrapper_noexcept gdb/event-top.c:195
    #27 0x138277c in gdb_rl_callback_read_char_wrapper gdb/event-top.c:234
    #28 0x1fe9b40 in stdin_event_handler gdb/ui.c:155
    #29 0x35ff1bc in handle_file_event gdbsupport/event-loop.cc:573

SUMMARY: AddressSanitizer: heap-buffer-overflow (/lib64/libasan.so.8+0x6da18) in memmove
...

The error happens when trying to print either variable y or y2:
...
   type Variable_Record (A : Boolean := True) is record
      case A is
         when True =>
            B : Integer;
         when False =>
            C : Float;
            D : Integer;
      end case;
   end record;
   Y  : Variable_Record := (A => True, B => 1);
   Y2 : Variable_Record := (A => False, C => 1.0, D => 2);
...
when the variables are uninitialized.

The error happens only when printing the entire variable:
...
(gdb) p y.a
$2 = 216
(gdb) p y.b
There is no member named b.
(gdb) p y.c
$3 = 9.18340949e-41
(gdb) p y.d
$4 = 1
(gdb) p y
<AddressSanitizer: heap-buffer-overflow>
...

The error happens as follows:
- field a functions as discriminant, choosing either the b, or c+d variant.
- when y.a happens to be set to 216, as above, gdb interprets this as the
  variable having the c+d variant (which is why trying to print y.b fails).
- when printing y, gdb allocates a value, copies the bytes into it from the
  target, and then prints the value.
- gdb allocates the value using the type size, which is 8.  It's 8 because
  that's what the DW_AT_byte_size indicates.  Note that for valid values of a,
  it gives correct results: if a is 0 (c+d variant), size is 12, if a is 1
  (b variant), size is 8.
- gdb tries to print field d, which is at an 8 byte offset, and that results
  in a out-of-bounds access for the allocated 8-byte value.

Fix this by handling this case in value::contents_copy_raw, such that we have:
...
(gdb) p y
$1 = (a => 24, c => 9.18340949e-41,
      d => <error reading variable: access outside bounds of object>)
...

An alternative (additional) fix could be this: in compute_variant_fields_inner
gdb reads the discriminant y.a to decide which variant is active.  It would be
nice to detect that the value (y.a == 24) is not a valid Boolean, and give up
on choosing a variant altoghether.  However, the situation regarding the
internal type CODE_TYPE_BOOL is currently ambiguous (see PR31282) and it's not
possible to reliably decide what valid values are.

The test-case source file gdb.ada/uninitialized-variable-record/parse.adb is
a reduced version of gdb.ada/uninitialized_vars/parse.adb, so it copies the
copyright years.

Note that the test-case needs gcc-12 or newer, it's unsupported for older gcc
versions. [ So, it would be nice to rewrite it into a dwarf assembly
test-case. ]

The test-case loops over all languages.  This is inherited from an earlier
attempt to fix this, which had language-specific fixes (in print_field_values,
cp_print_value_fields, pascal_object_print_value_fields and
f_language::value_print_inner).  I've left this in, but I suppose it's not
strictly necessary anymore.

Tested on x86_64-linux.

PR exp/31258
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31258
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@vries
vries committed Feb 19, 2024
1 parent d1648ff commit c3e06e6
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122 changes: 122 additions & 0 deletions gdb/testsuite/gdb.ada/uninitialized-variable-record.exp
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Original file line number Diff line number Diff line change
@@ -0,0 +1,122 @@
# Copyright 2024 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 "ada.exp"

require allow_ada_tests

standard_ada_testfile parse

if {[gdb_compile_ada "${srcfile}" "${binfile}" executable {debug}] != "" } {
return -1
}

clean_restart ${testfile}

set bp_location [gdb_get_line_number "START" ${testdir}/parse.adb]
runto "parse.adb:$bp_location"

# Check that we have the expected value for variable y2.

gdb_test "p y2" [string_to_regexp " = (a => false, c => 1.0, d => 2)"]

# Shorthand.

proc set_lang { lang } {
gdb_test_multiple "set language $lang" "" {
-re -wrap "" {
}
}
}

# Calculate the offset of y2.d.

set re_cast [string_to_regexp "(access integer)"]
gdb_test_multiple "print &y2.d - &y2" "" {
-re -wrap " = $re_cast ($hex)" {
set offset_d $expect_out(1,string)
pass $gdb_test_name
}
}

# Try to find a interesting discriminator value, such that at the same time:
# - the d field is part of the variable, and
# - the type size is too small to contain d.

set interesting_discriminator -1
set_lang c
for { set i 0 } { $i < 256 } { incr i } {
with_test_prefix $i {

# Patch in the discriminator value.
gdb_test_multiple "set var *(unsigned char *)(&y2.a)=$i" "" {
-re -wrap "" {
}
}

# Check that we have the variant with fields c+d instead of b.
set have_b 0
gdb_test_multiple "with language ada -- print y2.b" "" {
-re -wrap " = $decimal" {
set have_b 1
}
-re -wrap "" {
}
}
if { $have_b } {
# This is the variant with field b.
continue
}

set size 0
gdb_test_multiple "print sizeof (y2)" "" {
-re -wrap " = (.*)" {
set size $expect_out(1,string)
}
}

if { ! $size } {
continue
}

if { [expr $size > $offset_d] } {
# Field d fits in the size.
continue
}

set interesting_discriminator $i
break
}
}

require {expr $interesting_discriminator != -1}

foreach lang [gdb_supported_languages] {
with_test_prefix $lang {
set_lang $lang

gdb_test_multiple "print y2" "" {
-re -wrap ", d => $decimal.*" {
fail $gdb_test_name
}
-re -wrap ", d = $decimal.*" {
fail $gdb_test_name
}
-re -wrap "" {
pass $gdb_test_name
}
}
}
}
33 changes: 33 additions & 0 deletions gdb/testsuite/gdb.ada/uninitialized-variable-record/parse.adb
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Original file line number Diff line number Diff line change
@@ -0,0 +1,33 @@
-- Copyright 2009-2024 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/>.

-- Based on gdb.ada/uninitialized_vars/parse.adb.

procedure Parse is

type Variable_Record (A : Boolean := True) is record
case A is
when True =>
B : Integer;
when False =>
C : Float;
D : Integer;
end case;
end record;
Y2 : Variable_Record := (A => False, C => 1.0, D => 2);

begin
null; -- START
end Parse;
3 changes: 3 additions & 0 deletions gdb/value.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1229,6 +1229,9 @@ value::contents_copy_raw (struct value *dst, LONGEST dst_offset,
gdb_assert (!dst->bits_any_optimized_out (TARGET_CHAR_BIT * dst_offset,
TARGET_CHAR_BIT * length));

if ((src_offset + copy_length) * unit_size > enclosing_type ()-> length ())
error (_("access outside bounds of object"));

/* Copy the data. */
gdb::array_view<gdb_byte> dst_contents
= dst->contents_all_raw ().slice (dst_offset * unit_size,
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