extensions: add kernel-rust extension for Rust support in kernel builds

Add extension that enables CONFIG_RUST in kernel menuconfig and
configures the build environment (rustc, rustfmt, bindgen, rust-src)
using versioned APT packages from noble-security/noble-updates.

Handles env -i in run_kernel_make_internal by passing tool paths
via RUSTC, RUSTFMT, BINDGEN make params and RUST_LIB_SRC env var.

Includes optional RUST_KERNEL_SAMPLES=yes for building sample
Rust modules (rust_minimal, rust_print, rust_driver_faux) as a
toolchain smoke test.

Tested: kernel 6.19 build for rockchip64 on aarch64, both with
and without Docker.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 2 people

extensions/kernel-rust.sh

@@ -0,0 +1,219 @@
+# Enable Rust support for Linux kernel compilation.
+#
+# Installs Rust toolchain via rustup into ${SRC}/cache/tools/rustup/ and
+# configures the build environment so that CONFIG_RUST appears in kernel
+# menuconfig and gets enabled automatically.
+#
+# The toolchain is cached by a hash of (RUST_VERSION, BINDGEN_VERSION, arch,
+# RUST_EXTRA_COMPONENTS, RUST_EXTRA_CARGO_CRATES). Changing any of these
+# triggers a full reinstall on the next build.
+#
+# Other extensions can request additional rustup components or cargo crates:
+#   RUST_EXTRA_COMPONENTS+=("clippy" "llvm-tools")
+#   RUST_EXTRA_CARGO_CRATES+=("mdbook" "cargo-deb@2.11.0")
+#
+# Usage:  ./compile.sh kernel-config BOARD=... BRANCH=... ENABLE_EXTENSIONS="kernel-rust"
+#
+# References:
+#   https://docs.kernel.org/rust/quick-start.html
+#   https://docs.kernel.org/rust/general-information.html
+#   https://rust-for-linux.com/rust-version-policy
+#   https://rust-lang.github.io/rustup/installation/index.html
+
+# Rust toolchain version installed via rustup.
+# Kernel >= 6.12 requires rustc >= 1.78. See rust-version-policy above.
+RUST_VERSION="${RUST_VERSION:-1.85.0}"
+
+# bindgen-cli version installed via cargo.
+# APT bindgen 0.66.1 panics on kernel >= 6.19 headers (FromBytesWithNulError
+# in codegen/mod.rs). Fixed in >= 0.69.
+BINDGEN_VERSION="${BINDGEN_VERSION:-0.71.1}"
+
+# Enable Rust sample kernel modules for toolchain smoke testing.
+# Set to "yes" to build rust_minimal, rust_print, rust_driver_faux as modules.
+# Can also be set via command line: RUST_KERNEL_SAMPLES=yes
+RUST_KERNEL_SAMPLES="${RUST_KERNEL_SAMPLES:-no}"
+
+# Extra rustup components to install (e.g. clippy, llvm-tools).
+# Other extensions can append: RUST_EXTRA_COMPONENTS+=("clippy")
+declare -g -a RUST_EXTRA_COMPONENTS=()
+
+# Extra cargo crates to install. Supports "name" or "name@version" syntax.
+# Other extensions can append: RUST_EXTRA_CARGO_CRATES+=("mdbook" "cargo-deb@2.11.0")
+declare -g -a RUST_EXTRA_CARGO_CRATES=()
+
+# Resolved tool paths, set by host_dependencies_ready, used by custom_kernel_make_params.
+declare -g RUST_TOOL_RUSTC=""
+declare -g RUST_TOOL_RUSTFMT=""
+declare -g RUST_TOOL_BINDGEN=""
+declare -g RUST_TOOL_SYSROOT=""
+
+function add_host_dependencies__add_rust_compiler() {
+	display_alert "Adding Rust kernel build dependencies" "${EXTENSION}" "info"
+	# bindgen needs libclang for dlopen; available on all target distros.
+	EXTRA_BUILD_DEPS+=" libclang-dev "
+}
+
+# Download rustup-init binary for the current architecture.
+# Follows the project pattern: curl → .tmp → mv → chmod.
+_download_rustup_init() {
+	local target_dir="$1"
+	local target_triple
+	case "${BASH_VERSINFO[5]}" in
+		*aarch64*) target_triple="aarch64-unknown-linux-gnu" ;;
+		*x86_64*) target_triple="x86_64-unknown-linux-gnu" ;;
+		*riscv64*) target_triple="riscv64gc-unknown-linux-gnu" ;;
+		*) exit_with_error "Unsupported architecture for rustup" "${BASH_VERSINFO[5]}" ;;
+	esac
+
+	local url="https://static.rust-lang.org/rustup/dist/${target_triple}/rustup-init"
+	local dest="${target_dir}/rustup-init"
+
+	display_alert "Downloading rustup-init" "${target_triple}" "info"
+	curl --proto '=https' --tlsv1.2 -sSf -o "${dest}.tmp" "${url}"
+	mv "${dest}.tmp" "${dest}"
+	chmod +x "${dest}"
+}
+
+# Install or reuse cached Rust toolchain in ${SRC}/cache/tools/rustup/.
+_prepare_rust_toolchain() {
+	local rust_cache_dir="${SRC}/cache/tools/rustup"
+	mkdir -p "${rust_cache_dir}"
+
+	local rustup_home="${rust_cache_dir}/rustup-home"
+	local cargo_home="${rust_cache_dir}/cargo-home"
+
+	# Content-addressable cache: hash of version config + architecture + extras
+	local cache_key="${RUST_VERSION}|${BINDGEN_VERSION}|${BASH_VERSINFO[5]}"
+	cache_key+="|${RUST_EXTRA_COMPONENTS[*]}|${RUST_EXTRA_CARGO_CRATES[*]}"
+	local cache_hash
+	cache_hash="$(echo -n "${cache_key}" | sha256sum | cut -c1-16)"
+	local marker="${rust_cache_dir}/.marker-${cache_hash}"
+
+	if [[ -f "${marker}" ]]; then
+		display_alert "Rust toolchain cache hit" "${cache_hash}" "cachehit"
+		return 0
+	fi
+
+	# Remove stale markers from previous versions
+	rm -f "${rust_cache_dir}"/.marker-*
+
+	display_alert "Installing Rust toolchain" "rustc ${RUST_VERSION}, bindgen ${BINDGEN_VERSION}" "info"
+
+	# Download rustup-init
+	do_with_retries 3 _download_rustup_init "${rust_cache_dir}"
+
+	# Install minimal toolchain; SKIP_PATH_CHECK suppresses warnings about
+	# system rustc in /usr/bin (e.g. from mtkflash in Docker images).
+	RUSTUP_HOME="${rustup_home}" CARGO_HOME="${cargo_home}" \
+		RUSTUP_INIT_SKIP_PATH_CHECK=yes \
+		"${rust_cache_dir}/rustup-init" -y \
+		--profile minimal \
+		--default-toolchain "${RUST_VERSION}" \
+		--no-modify-path
+
+	# Components: rustfmt (not in minimal profile) + rust-src (kernel needs it) + extras
+	local -a components=(rustfmt rust-src "${RUST_EXTRA_COMPONENTS[@]}")
+	display_alert "Installing rustup components" "${components[*]}" "info"
+	RUSTUP_HOME="${rustup_home}" CARGO_HOME="${cargo_home}" \
+		"${cargo_home}/bin/rustup" component add "${components[@]}"
+
+	# Cargo crates: bindgen-cli (kernel needs it) + extras
+	# Supports "name" or "name@version" syntax.
+	local -a crates=("bindgen-cli@${BINDGEN_VERSION}" "${RUST_EXTRA_CARGO_CRATES[@]}")
+	local crate
+	for crate in "${crates[@]}"; do
+		display_alert "Installing cargo crate" "${crate}" "info"
+		RUSTUP_HOME="${rustup_home}" CARGO_HOME="${cargo_home}" \
+			"${cargo_home}/bin/cargo" install --locked "${crate}"
+	done
+
+	# Mark cache as valid only after everything succeeds
+	touch "${marker}"
+	display_alert "Rust toolchain installed" "${cache_hash}" "info"
+}
+
+# Resolve absolute paths to Rust tool binaries.
+# Uses direct paths into the toolchain (not rustup proxies), so that
+# env -i in run_kernel_make_internal() does not need RUSTUP_HOME set.
+_resolve_rust_tool_paths() {
+	local rustup_home="${SRC}/cache/tools/rustup/rustup-home"
+	local cargo_home="${SRC}/cache/tools/rustup/cargo-home"
+
+	RUST_TOOL_SYSROOT="$(RUSTUP_HOME="${rustup_home}" CARGO_HOME="${cargo_home}" \
+		"${cargo_home}/bin/rustc" --print sysroot)"
+
+	# Direct binaries inside the toolchain, bypassing rustup proxy
+	RUST_TOOL_RUSTC="${RUST_TOOL_SYSROOT}/bin/rustc"
+	RUST_TOOL_RUSTFMT="${RUST_TOOL_SYSROOT}/bin/rustfmt"
+	RUST_TOOL_BINDGEN="${cargo_home}/bin/bindgen"
+}
+
+function host_dependencies_ready__add_rust_compiler() {
+	_prepare_rust_toolchain
+	_resolve_rust_tool_paths
+
+	# Verify all tools are executable
+	local tool_name tool_path
+	for tool_name in RUST_TOOL_RUSTC RUST_TOOL_RUSTFMT RUST_TOOL_BINDGEN; do
+		tool_path="${!tool_name}"
+		[[ -x "${tool_path}" ]] || exit_with_error "Required Rust tool '${tool_name}' not found at ${tool_path}" "${EXTENSION}"
+	done
+
+	display_alert "Rust toolchain ready" \
+		"rustc $(${RUST_TOOL_RUSTC} --version | awk '{print $2}'), bindgen $(${RUST_TOOL_BINDGEN} --version 2>&1 | awk '{print $2}')" "info"
+}
+
+function artifact_kernel_version_parts__add_rust_version() {
+	# Include Rust toolchain version in artifact hash so that changing
+	# RUST_VERSION or BINDGEN_VERSION triggers a kernel rebuild.
+	local cache_key="${RUST_VERSION}|${BINDGEN_VERSION}"
+	local short
+	short="$(echo -n "${cache_key}" | sha256sum | cut -c1-4)"
+
+	artifact_version_parts["_R"]="rust${short}"
+
+	# Add to order array if not already present
+	local found=0 entry
+	for entry in "${artifact_version_part_order[@]}"; do
+		[[ "${entry}" == *"-_R" ]] && found=1 && break
+	done
+	if [[ "${found}" -eq 0 ]]; then
+		artifact_version_part_order+=("0086-_R")
+	fi
+}
+
+function custom_kernel_config__add_rust_compiler() {
+	# https://docs.kernel.org/rust/quick-start.html
+	opts_y+=("RUST")
+
+	# Build sample Rust modules for toolchain smoke testing
+	if [[ "${RUST_KERNEL_SAMPLES}" == "yes" ]]; then
+		display_alert "Enabling Rust sample modules" "${EXTENSION}" "info"
+		opts_y+=("SAMPLES") # Parent menu for all kernel samples
+		opts_y+=("SAMPLES_RUST")
+		opts_m+=("SAMPLE_RUST_MINIMAL")
+		opts_m+=("SAMPLE_RUST_PRINT")
+		opts_m+=("SAMPLE_RUST_DRIVER_FAUX")
+	fi
+}
+
+function custom_kernel_make_params__add_rust_compiler() {
+	# run_kernel_make_internal uses "env -i" which clears all environment
+	# variables, so we pass Rust paths explicitly via make parameters.
+	# Using direct toolchain binaries (not rustup proxies) avoids needing
+	# RUSTUP_HOME in the env -i context.
+
+	common_make_params_quoted+=("RUSTC=${RUST_TOOL_RUSTC}")
+	common_make_params_quoted+=("RUSTFMT=${RUST_TOOL_RUSTFMT}")
+	common_make_params_quoted+=("BINDGEN=${RUST_TOOL_BINDGEN}")
+
+	# Rust standard library source path for kernel build
+	local rust_lib_src="${RUST_TOOL_SYSROOT}/lib/rustlib/src/rust/library"
+	if [[ -d "${rust_lib_src}" ]]; then
+		display_alert "Rust library source" "${rust_lib_src}" "info"
+		common_make_envs+=("RUST_LIB_SRC='${rust_lib_src}'")
+	else
+		display_alert "Rust library source not found" "CONFIG_RUST will not appear in menuconfig" "wrn"
+	fi
+}