Date:   Fri, 10 Feb 2023 14:43:56 -0800
From:   Kees Cook <kees@kernel.org>
To:     Andrea Righi <andrea.righi@canonical.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>
CC:     Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        bjorn3_gh@protonmail.com, Kees Cook <keescook@chromium.org>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] rust: allow to use INIT_STACK_ALL_ZERO
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On February 10, 2023 1:51:41 PM PST, Andrea Righi <andrea=2Erighi@canonical=
=2Ecom> wrote:
>With CONFIG_INIT_STACK_ALL_ZERO enabled, bindgen passes
>-ftrivial-auto-var-init=3Dzero to clang, that triggers the following
>error:
>
> error: '-ftrivial-auto-var-init=3Dzero' hasn't been enabled; enable it a=
t your own peril for benchmarking purpose only with '-enable-trivial-auto-v=
ar-init-zero-knowing-it-will-be-removed-from-clang'
>
>However, this additional option that is currently required by clang is
>going to be removed in the future (as the name of the option suggests),
>likely with clang-17=2E
>
>So, make sure bindgen is using this extra option if the major version of
>the libclang used by bindgen is < 17=2E
>
>In this way we can enable CONFIG_INIT_STACK_ALL_ZERO with CONFIG_RUST
>without triggering any build error=2E
>
>Link: https://github=2Ecom/llvm/llvm-project/issues/44842
>Signed-off-by: Andrea Righi <andrea=2Erighi@canonical=2Ecom>
>---
>
>Changes in v2:
> - check the version of libclang used by bindgen to determine if we need
>   to pass the additional clang option
>
> rust/Makefile | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
>diff --git a/rust/Makefile b/rust/Makefile
>index ff70c4c916f8=2E=2Ec77d7ce96a85 100644
>--- a/rust/Makefile
>+++ b/rust/Makefile
>@@ -269,6 +269,19 @@ BINDGEN_TARGET		:=3D $(BINDGEN_TARGET_$(SRCARCH))
> # some configurations, with new GCC versions, etc=2E
> bindgen_extra_c_flags =3D -w --target=3D$(BINDGEN_TARGET)
>=20
>+# Auto variable zero-initialization requires an additional special optio=
n with
>+# clang that is going to be removed sometimes in the future (likely in
>+# clang-17), so make sure to pass this option only if clang supports it
>+# (libclang major version < 17)=2E
>+#
>+# https://github=2Ecom/llvm/llvm-project/issues/44842
>+ifdef CONFIG_INIT_STACK_ALL_ZERO
>+libclang_maj_ver=3D$(shell $(BINDGEN) $(srctree)/scripts/rust_is_availab=
le_bindgen_libclang=2Eh 2>&1 | sed -ne 's/=2E*clang version \([0-9]*\)=2E*/=
\1/p')
>+ifeq ($(shell expr $(libclang_maj_ver) \< 17), 1)
>+bindgen_extra_c_flags +=3D -enable-trivial-auto-var-init-zero-knowing-it=
-will-be-removed-from-clang
>+endif
>+endif

This logic already exists in the top-level Makefile=2E How does -ftrivial-=
auto-var-init=3Dzero make it into bindgen_c_flags normally? (I=2Ee=2E why d=
oes the legacy -enable=2E=2E=2E option not?)

>+
> bindgen_c_flags =3D $(filter-out $(bindgen_skip_c_flags), $(c_flags)) \
> 	$(bindgen_extra_c_flags)
> endif


--=20
Kees Cook