Re: [PATCH v2] rust: allow to use INIT_STACK_ALL_ZERO

[Kees Cook <kees@kernel.org>]

On February 10, 2023 1:51:41 PM PST, Andrea Righi <andrea.righi@canonical.com> wrote:
>With CONFIG_INIT_STACK_ALL_ZERO enabled, bindgen passes
>-ftrivial-auto-var-init=zero to clang, that triggers the following
>error:
>
> error: '-ftrivial-auto-var-init=zero' hasn't been enabled; enable it at your own peril for benchmarking purpose only with '-enable-trivial-auto-var-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.
>
>So, make sure bindgen is using this extra option if the major version of
>the libclang used by bindgen is < 17.
>
>In this way we can enable CONFIG_INIT_STACK_ALL_ZERO with CONFIG_RUST
>without triggering any build error.
>
>Link: https://github.com/llvm/llvm-project/issues/44842
>Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
>---
>
>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..c77d7ce96a85 100644
>--- a/rust/Makefile
>+++ b/rust/Makefile
>@@ -269,6 +269,19 @@ BINDGEN_TARGET		:= $(BINDGEN_TARGET_$(SRCARCH))
> # some configurations, with new GCC versions, etc.
> bindgen_extra_c_flags = -w --target=$(BINDGEN_TARGET)
> 
>+# Auto variable zero-initialization requires an additional special option with
>+# clang that is going to be removed sometimes in the future (likely in
>+# clang-17), so make sure to pass this option only if clang supports it
>+# (libclang major version < 17).
>+#
>+# https://github.com/llvm/llvm-project/issues/44842
>+ifdef CONFIG_INIT_STACK_ALL_ZERO
>+libclang_maj_ver=$(shell $(BINDGEN) $(srctree)/scripts/rust_is_available_bindgen_libclang.h 2>&1 | sed -ne 's/.*clang version \([0-9]*\).*/\1/p')
>+ifeq ($(shell expr $(libclang_maj_ver) \< 17), 1)
>+bindgen_extra_c_flags += -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
>+endif
>+endif

This logic already exists in the top-level Makefile. How does -ftrivial-auto-var-init=zero make it into bindgen_c_flags normally? (I.e. why does the legacy -enable... option not?)

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


-- 
Kees Cook