binutils-gdb/gdb/testsuite/gdb.base/flexible-array-member.c

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gdb: fix value_subscript when array upper bound is not known Since commit 7c6f27129631 ("gdb: make get_discrete_bounds check for non-constant range bounds"), subscripting flexible array member fails: struct no_size { int n; int items[]; }; (gdb) p *ns $1 = {n = 3, items = 0x5555555592a4} (gdb) p ns->items[0] Cannot access memory at address 0xfffe555b733a0164 (gdb) p *((int *) 0x5555555592a4) $2 = 101 <--- we would expect that (gdb) p &ns->items[0] $3 = (int *) 0xfffe5559ee829a24 <--- wrong address Since the flexible array member (items) has an unspecified size, the array type created for it in the DWARF doesn't have dimensions (this is with gcc 9.3.0, Ubuntu 20.04): 0x000000a4: DW_TAG_array_type DW_AT_type [DW_FORM_ref4] (0x00000038 "int") DW_AT_sibling [DW_FORM_ref4] (0x000000b3) 0x000000ad: DW_TAG_subrange_type DW_AT_type [DW_FORM_ref4] (0x00000031 "long unsigned int") This causes GDB to create a range type (TYPE_CODE_RANGE) with a defined constant low bound (dynamic_prop with kind PROP_CONST) and an undefined high bound (dynamic_prop with kind PROP_UNDEFINED). value_subscript gets both bounds of that range using get_discrete_bounds. Before commit 7c6f27129631, get_discrete_bounds didn't check the kind of the dynamic_props and would just blindly read them as if they were PROP_CONST. It would return 0 for the high bound, because we zero-initialize the range_bounds structure. And it didn't really matter in this case, because the returned high bound wasn't used in the end. Commit 7c6f27129631 changed get_discrete_bounds to return a failure if either the low or high bound is not a constant, to make sure we don't read a dynamic prop that isn't a PROP_CONST as a PROP_CONST. This change made get_discrete_bounds start to return a failure for that range, and as a result would not set *lowp and *highp. And since value_subscript doesn't check get_discrete_bounds' return value, it just carries on an uses an uninitialized value for the low bound. If value_subscript did check the return value of get_discrete_bounds, we would get an error message instead of a bogus value. But it would still be a bug, as we wouldn't be able to print the flexible array member's elements. Looking at value_subscript, we see that the low bound is always needed, but the high bound is only needed if !c_style. So, change value_subscript to use get_discrete_low_bound and get_discrete_high_bound separately. This fixes the case described above, where the low bound is known but the high bound isn't (and is not needed). This restores the original behavior without accessing a dynamic_prop in a wrong way. A test is added. In addition to the case described above, a case with an array member of size 0 is added, which is a GNU C extension that existed before flexible array members were introduced. That case currently fails when compiled with gcc <= 8. gcc <= 8 produces DWARF similar to the one shown above, while gcc 9 adds a DW_AT_count of 0 in there, which makes the high bound known. A case where an array member of size 0 is the only member of the struct is also added, as that was how PR 28675 was originally reported, and it's an interesting corner case that I think could trigger other funny bugs. Question about the implementation: in value_subscript, I made it such that if the low or high bound is unknown, we fall back to zero. That effectively makes it the same as it was before 7c6f27129631. But should we instead error() out? gdb/ChangeLog: PR 26875, PR 26901 * gdbtypes.c (get_discrete_low_bound): Make non-static. (get_discrete_high_bound): Make non-static. * gdbtypes.h (get_discrete_low_bound): New declaration. (get_discrete_high_bound): New declaration. * valarith.c (value_subscript): Only fetch high bound if necessary. gdb/testsuite/ChangeLog: PR 26875, PR 26901 * gdb.base/flexible-array-member.c: New test. * gdb.base/flexible-array-member.exp: New test. Change-Id: I832056f80e6c56f621f398b4780d55a3a1e299d7
2020-12-10 02:52:12 +08:00
/* This testcase is part of GDB, the GNU debugger.
Copyright 2020-2021 Free Software Foundation, Inc.
gdb: fix value_subscript when array upper bound is not known Since commit 7c6f27129631 ("gdb: make get_discrete_bounds check for non-constant range bounds"), subscripting flexible array member fails: struct no_size { int n; int items[]; }; (gdb) p *ns $1 = {n = 3, items = 0x5555555592a4} (gdb) p ns->items[0] Cannot access memory at address 0xfffe555b733a0164 (gdb) p *((int *) 0x5555555592a4) $2 = 101 <--- we would expect that (gdb) p &ns->items[0] $3 = (int *) 0xfffe5559ee829a24 <--- wrong address Since the flexible array member (items) has an unspecified size, the array type created for it in the DWARF doesn't have dimensions (this is with gcc 9.3.0, Ubuntu 20.04): 0x000000a4: DW_TAG_array_type DW_AT_type [DW_FORM_ref4] (0x00000038 "int") DW_AT_sibling [DW_FORM_ref4] (0x000000b3) 0x000000ad: DW_TAG_subrange_type DW_AT_type [DW_FORM_ref4] (0x00000031 "long unsigned int") This causes GDB to create a range type (TYPE_CODE_RANGE) with a defined constant low bound (dynamic_prop with kind PROP_CONST) and an undefined high bound (dynamic_prop with kind PROP_UNDEFINED). value_subscript gets both bounds of that range using get_discrete_bounds. Before commit 7c6f27129631, get_discrete_bounds didn't check the kind of the dynamic_props and would just blindly read them as if they were PROP_CONST. It would return 0 for the high bound, because we zero-initialize the range_bounds structure. And it didn't really matter in this case, because the returned high bound wasn't used in the end. Commit 7c6f27129631 changed get_discrete_bounds to return a failure if either the low or high bound is not a constant, to make sure we don't read a dynamic prop that isn't a PROP_CONST as a PROP_CONST. This change made get_discrete_bounds start to return a failure for that range, and as a result would not set *lowp and *highp. And since value_subscript doesn't check get_discrete_bounds' return value, it just carries on an uses an uninitialized value for the low bound. If value_subscript did check the return value of get_discrete_bounds, we would get an error message instead of a bogus value. But it would still be a bug, as we wouldn't be able to print the flexible array member's elements. Looking at value_subscript, we see that the low bound is always needed, but the high bound is only needed if !c_style. So, change value_subscript to use get_discrete_low_bound and get_discrete_high_bound separately. This fixes the case described above, where the low bound is known but the high bound isn't (and is not needed). This restores the original behavior without accessing a dynamic_prop in a wrong way. A test is added. In addition to the case described above, a case with an array member of size 0 is added, which is a GNU C extension that existed before flexible array members were introduced. That case currently fails when compiled with gcc <= 8. gcc <= 8 produces DWARF similar to the one shown above, while gcc 9 adds a DW_AT_count of 0 in there, which makes the high bound known. A case where an array member of size 0 is the only member of the struct is also added, as that was how PR 28675 was originally reported, and it's an interesting corner case that I think could trigger other funny bugs. Question about the implementation: in value_subscript, I made it such that if the low or high bound is unknown, we fall back to zero. That effectively makes it the same as it was before 7c6f27129631. But should we instead error() out? gdb/ChangeLog: PR 26875, PR 26901 * gdbtypes.c (get_discrete_low_bound): Make non-static. (get_discrete_high_bound): Make non-static. * gdbtypes.h (get_discrete_low_bound): New declaration. (get_discrete_high_bound): New declaration. * valarith.c (value_subscript): Only fetch high bound if necessary. gdb/testsuite/ChangeLog: PR 26875, PR 26901 * gdb.base/flexible-array-member.c: New test. * gdb.base/flexible-array-member.exp: New test. Change-Id: I832056f80e6c56f621f398b4780d55a3a1e299d7
2020-12-10 02:52:12 +08:00
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 <stdlib.h>
struct no_size
{
int n;
int items[];
};
struct zero_size
{
int n;
int items[0];
};
struct zero_size_only
{
int items[0];
};
struct no_size *ns;
struct zero_size *zs;
struct zero_size_only *zso;
static void
break_here (void)
{
}
int
main (void)
{
ns = (struct no_size *) malloc (sizeof (*ns) + 3 * sizeof (int));
zs = (struct zero_size *) malloc (sizeof (*zs) + 3 * sizeof (int));
zso = (struct zero_size_only *) malloc (sizeof (*zso) + 3 * sizeof (int));
ns->n = 3;
ns->items[0] = 101;
ns->items[1] = 102;
ns->items[2] = 103;
zs->n = 3;
zs->items[0] = 201;
zs->items[1] = 202;
zs->items[2] = 203;
zso->items[0] = 301;
zso->items[1] = 302;
zso->items[2] = 303;
break_here ();
return 0;
}