From: "eightbitraptor (Matthew Valentine-House)" Date: 2022-09-29T12:50:46+00:00 Subject: [ruby-core:110147] [Ruby master Bug#19028] GCC12 Introduces new warn flags `-Wuse-after-free` Issue #19028 has been updated by eightbitraptor (Matthew Valentine-House). nobu (Nobuyoshi Nakada) wrote in #note-4: > GCC 12 on GitHub Actions doesn���t show that warning. > https://github.com/ruby/ruby/actions/runs/3146751201/jobs/5115549022#step:13:130 > > I���m not sure what differs. mame (Yusuke Endoh) wrote in #note-5: > I am not able to repro with `gcc-12 (Ubuntu 12.1.0-2ubuntu1~22.04) 12.1.0` and x86_64-linux. Apologies. I had a look into this and realised that this warning only occurs when `optflags=-O0`. With `O0` I can successfully repro this on: * `gcc version 12.2.1 20220819 (Red Hat 12.2.1-2) (GCC)` on `Fedora 36` - where I originally saw this warning. * `gcc version 12.1.0 (Ubuntu 12.1.0-2ubuntu1~22.04)` * `gcc version 12.2.0 (Ubuntu 12.2.0-3ubuntu1)` on `Ubuntu Kinetic Kudu (development branch) 22.10` I don't know if we should justify lowering a warning level globally for a warning that only appears in unoptimized code - what do you think? ---------------------------------------- Bug #19028: GCC12 Introduces new warn flags `-Wuse-after-free` https://bugs.ruby-lang.org/issues/19028#change-99404 * Author: eightbitraptor (Matthew Valentine-House) * Status: Open * Priority: Normal * Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN ---------------------------------------- [GCC 12 introduced a new warning flag `-Wuse-after-free`](https://github.com/gcc-mirror/gcc/commit/671a283636de75f7ed638ee6b01ed2d44361b8b6) which attempts to warn about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function Details of the levels are in [the C++ Dialect Options section of the GCC documentation](https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Dialect-Options.html). Compiling with `-Wall` uses the default setting of `2` for this flag. Which warns on the `TRY_WITH_GC` macro defined in `gc.c` ``` gc.c: In function ���objspace_xrealloc���: gc.c:12213:33: warning: pointer ���ptr��� may be used after ���realloc��� [-Wuse-after-free] 12213 | TRY_WITH_GC(new_size, mem = realloc(ptr, new_size)); | ^~~~~~~~~~~~~~~~~~~~~~ gc.c:12123:19: note: in definition of macro ���TRY_WITH_GC��� 12123 | else if ((expr)) { \ | ^~~~ In file included from ./include/ruby/defines.h:72, from ./include/ruby/ruby.h:25, from constant.h:13, from gc.c:97: gc.c:12213:33: note: call to ���realloc��� here 12213 | TRY_WITH_GC(new_size, mem = realloc(ptr, new_size)); | ^~~~~~~~~~~~~~~~~~~~~~ ./include/ruby/backward/2/assume.h:43:46: note: in definition of macro ���RB_LIKELY��� 43 | # define RB_LIKELY(x) (__builtin_expect(!!(x), 1)) | ^ gc.c:12116:13: note: in expansion of macro ���LIKELY��� 12116 | if (LIKELY((expr))) { \ | ^~~~~~ gc.c:12213:5: note: in expansion of macro ���TRY_WITH_GC��� 12213 | TRY_WITH_GC(new_size, mem = realloc(ptr, new_size)); | ^~~~~~~~~~~ ``` My understanding is that if `realloc` returns a null pointer then the memory requested for reallocation is guaranteed to not be touched (according to [the Open Group](https://pubs.opengroup.org/onlinepubs/9699919799/functions/realloc.html) - thank you @nobu for bringing this to my attention). Given that this is a new warning, my proposed solution is to lower the level down to the base level `1`. This will only warn on unconditional calls to deallocation functions or successful calls to realloc. I've opened a PR that sets `-Wuse-after-free=1` only for GCC versions > 11. An alternative approach might be to use `#pragma GCC diagnostic` to suppress this just for GCC but I opted for what I thought was the easiest fix to start with. [Github PR #6465](https://github.com/ruby/ruby/pull/6465) -- https://bugs.ruby-lang.org/ Unsubscribe: