Re: [RFC PATCH v3 1/3] tools build: Add Rust library build infrastructure and zlib-rs feature test

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] tools build: feature_check_code passes CLANG_CROSS_FLAGS to g++ in CXXFLAGS
--

commit b53b88b2adbac2b7788068d159976b5144e79923
Author: Ian Rogers <irogers@google.com>

tools build: Add Rust library build infrastructure and zlib-rs feature test

This commit adds shared Rust library build and feature detection rules under
tools/build/Makefile.rust, along with cross-compilation target mappings and a
feature test for zlib-rs.

> diff --git a/tools/build/Makefile.feature b/tools/build/Makefile.feature
> index 4d37d11f674c5..ecf27626d493e 100644
> --- a/tools/build/Makefile.feature
> +++ b/tools/build/Makefile.feature
> @@ -5,10 +5,11 @@ ifneq ($(OUTPUT),)
>    OUTPUT_FEATURES = $(OUTPUT)feature/
>    $(shell mkdir -p $(OUTPUT_FEATURES))
>  endif
> +RUST_OUTPUT ?= $(abspath $(or $(OUTPUT),$(CURDIR)))/
>  
>  feature_check = $(eval $(feature_check_code))
>  define feature_check_code
> -  feature-$(1) := $(shell $(MAKE) OUTPUT=$(OUTPUT_FEATURES) CC="$(CC)" CXX="$(CXX)" CFLAGS="$(EXTRA_CFLAGS) $(FEATURE_CHECK_CFLAGS-$(1))" CXXFLAGS="$(EXTRA_CXXFLAGS) $(FEATURE_CHECK_CXXFLAGS-$(1))" LDFLAGS="$(LDFLAGS) $(FEATURE_CHECK_LDFLAGS-$(1))" -C $(feature_dir) $(OUTPUT_FEATURES)test-$1.bin >/dev/null 2>/dev/null && echo 1 || echo 0)
> +  feature-$(1) := $(shell $(MAKE) OUTPUT=$(OUTPUT_FEATURES) RUST_OUTPUT="$(RUST_OUTPUT)" CC="$(CC)" CXX="$(CXX)" CARGO="$(CARGO)" RUSTC="$(RUSTC)" $(if $(PKG_CONFIG),PKG_CONFIG="$(PKG_CONFIG)") CFLAGS="$(CLANG_CROSS_FLAGS) $(EXTRA_CFLAGS) $(FEATURE_CHECK_CFLAGS-$(1))" CXXFLAGS="$(CLANG_CROSS_FLAGS) $(EXTRA_CXXFLAGS) $(FEATURE_CHECK_CXXFLAGS-$(1))" RUST_FLAGS="$(RUST_FLAGS)" LDFLAGS="$(LDFLAGS) $(FEATURE_CHECK_LDFLAGS-$(1))" -C $(feature_dir) $(OUTPUT_FEATURES)test-$1.bin >/dev/null 2>/dev/null && echo 1 || echo 0)
>  endef

[Severity: Medium]
Does prepending $(CLANG_CROSS_FLAGS) to CXXFLAGS in feature_check_code break
C++ feature detection (such as cxa-demangle, llvm, and llvm-perf) when
cross-compiling with CC=clang and the default CXX=$(CROSS_COMPILE)g++?

In tools/scripts/Makefile.include, CLANG_CROSS_FLAGS is populated whenever
CC_NO_CLANG is 0 and CROSS_COMPILE is set, and is only appended to CFLAGS and
AFLAGS while CXX defaults to $(CROSS_COMPILE)g++ (unless LLVM is set or CXX is
explicitly overridden):

tools/scripts/Makefile.include:
    $(call allow-override,CC,$(CROSS_COMPILE)gcc)
    ...
    $(call allow-override,CXX,$(CROSS_COMPILE)g++)
    ...
    CLANG_CROSS_FLAGS := --target=$(CLANG_TARGET)
    GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)gcc 2>/dev/null))
    ifneq ($(GCC_TOOLCHAIN_DIR),)
    CLANG_CROSS_FLAGS += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
    CLANG_CROSS_FLAGS += --sysroot=$(shell $(CROSS_COMPILE)gcc -print-sysroot)
    CLANG_CROSS_FLAGS += --gcc-toolchain=$(realpath $(GCC_TOOLCHAIN_DIR)/..)
    endif # GCC_TOOLCHAIN_DIR
    endif # CLANG_CROSS_FLAGS
    CFLAGS += $(CLANG_CROSS_FLAGS)
    AFLAGS += $(CLANG_CROSS_FLAGS)

As noted in tools/perf/Makefile.config (commit c8592f8da838 ("perf build: Fix
CXX clang flag injection when only CC=clang is set")), CC being clang does not
imply CXX is clang++. When $(CROSS_COMPILE)g++ is invoked for a C++ feature
check via $(BUILDXX), it rejects Clang-only flags in CLANG_CROSS_FLAGS such as
--target=... and --gcc-toolchain=..., causing the feature check to fail.

Also, this change to CFLAGS and CXXFLAGS in tools/build/Makefile.feature is
not mentioned in the commit message. Should CLANG_CROSS_FLAGS only be passed
in CXXFLAGS when CXX is actually clang++, or is this change intended for a
separate patch?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261105180923.3441359-1-irogers@google.com?part=1