From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id 11C0C386F818; Wed, 9 Dec 2020 18:53:07 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 11C0C386F818 From: "cvs-commit at gcc dot gnu.org" To: gdb-prs@sourceware.org Subject: [Bug gdb/26901] Array subscript fails with flexible array member without size Date: Wed, 09 Dec 2020 18:53:07 +0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gdb X-Bugzilla-Component: gdb X-Bugzilla-Version: 10.1 X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: cvs-commit at gcc dot gnu.org X-Bugzilla-Status: NEW X-Bugzilla-Resolution: X-Bugzilla-Priority: P2 X-Bugzilla-Assigned-To: unassigned at sourceware dot org X-Bugzilla-Target-Milestone: 10.2 X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: Message-ID: In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Bugzilla-URL: http://sourceware.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-BeenThere: gdb-prs@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gdb-prs mailing list List-Unsubscribe: , List-Archive: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 09 Dec 2020 18:53:08 -0000 https://sourceware.org/bugzilla/show_bug.cgi?id=3D26901 --- Comment #3 from cvs-commit at gcc dot gnu.org --- The master branch has been updated by Simon Marchi : https://sourceware.org/git/gitweb.cgi?p=3Dbinutils-gdb.git;h=3D5b56203a7cad= d545b33713e98e274e582242e090 commit 5b56203a7cadd545b33713e98e274e582242e090 Author: Simon Marchi Date: Wed Dec 9 13:52:12 2020 -0500 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 =3D {n =3D 3, items =3D 0x5555555592a4} (gdb) p ns->items[0] Cannot access memory at address 0xfffe555b733a0164 (gdb) p *((int *) 0x5555555592a4) $2 =3D 101 <--- we would expect that (gdb) p &ns->items[0] $3 =3D (int *) 0xfffe5559ee829a24 <--- wrong address Since the flexible array member (items) has an unspecified size, the ar= ray 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 <=3D 8. gcc <=3D 8 produces DWA= RF 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 --=20 You are receiving this mail because: You are on the CC list for the bug.=