kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 130 insertions, 0 deletions

diff --git a/lib/crc/Kconfig b/lib/crc/Kconfig
new file mode 100644
index 000000000..927fc6a6b
--- /dev/null
+++ b/lib/crc/Kconfig
@@ -0,0 +1,130 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+# Kconfig for the kernel's cyclic redundancy check (CRC) library code
+
+config CRC4
+	tristate
+	help
+	  The CRC4 library functions.  Select this if your module uses any of
+	  the functions from <linux/crc4.h>.
+
+config CRC7
+	tristate
+	help
+	  The CRC7 library functions.  Select this if your module uses any of
+	  the functions from <linux/crc7.h>.
+
+config CRC8
+	tristate
+	help
+	  The CRC8 library functions.  Select this if your module uses any of
+	  the functions from <linux/crc8.h>.
+
+config CRC16
+	tristate
+	help
+	  The CRC16 library functions.  Select this if your module uses any of
+	  the functions from <linux/crc16.h>.
+
+config CRC_CCITT
+	tristate
+	help
+	  The CRC-CCITT library functions.  Select this if your module uses any
+	  of the functions from <linux/crc-ccitt.h>.
+
+config CRC_ITU_T
+	tristate
+	help
+	  The CRC-ITU-T library functions.  Select this if your module uses
+	  any of the functions from <linux/crc-itu-t.h>.
+
+config CRC_T10DIF
+	tristate
+	help
+	  The CRC-T10DIF library functions.  Select this if your module uses
+	  any of the functions from <linux/crc-t10dif.h>.
+
+config CRC_T10DIF_ARCH
+	bool
+	depends on CRC_T10DIF && CRC_OPTIMIZATIONS
+	default y if ARM && KERNEL_MODE_NEON
+	default y if ARM64
+	default y if PPC64 && ALTIVEC
+	default y if RISCV && RISCV_ISA_ZBC
+	default y if X86
+
+config CRC32
+	tristate
+	select BITREVERSE
+	help
+	  The CRC32 library functions.  Select this if your module uses any of
+	  the functions from <linux/crc32.h> or <linux/crc32c.h>.
+
+config CRC32_ARCH
+	bool
+	depends on CRC32 && CRC_OPTIMIZATIONS
+	default y if ARM && KERNEL_MODE_NEON
+	default y if ARM64
+	default y if LOONGARCH && 64BIT
+	default y if MIPS && CPU_MIPSR6
+	default y if PPC64 && ALTIVEC
+	default y if RISCV && RISCV_ISA_ZBC
+	default y if S390
+	default y if SPARC64
+	default y if X86
+
+config CRC64
+	tristate
+	help
+	  The CRC64 library functions.  Select this if your module uses any of
+	  the functions from <linux/crc64.h>.
+
+config CRC64_ARCH
+	bool
+	depends on CRC64 && CRC_OPTIMIZATIONS
+	default y if ARM && KERNEL_MODE_NEON && !CPU_BIG_ENDIAN
+	default y if ARM64
+	default y if RISCV && RISCV_ISA_ZBC && 64BIT
+	default y if X86_64
+
+config CRC_OPTIMIZATIONS
+	bool "Enable optimized CRC implementations" if EXPERT
+	depends on !UML
+	default y
+	help
+	  Disabling this option reduces code size slightly by disabling the
+	  architecture-optimized implementations of any CRC variants that are
+	  enabled.  CRC checksumming performance may get much slower.
+
+	  Keep this enabled unless you're really trying to minimize the size of
+	  the kernel.
+
+config CRC_KUNIT_TEST
+	tristate "KUnit tests for CRC functions" if !KUNIT_ALL_TESTS
+	depends on KUNIT && (CRC7 || CRC16 || CRC_T10DIF || CRC32 || CRC64)
+	default KUNIT_ALL_TESTS
+	help
+	  Unit tests for the CRC library functions.
+
+	  This is intended to help people writing architecture-specific
+	  optimized versions.  If unsure, say N.
+
+config CRC_ENABLE_ALL_FOR_KUNIT
+	tristate "Enable all CRC functions for KUnit test"
+	depends on KUNIT
+	select CRC7
+	select CRC16
+	select CRC_T10DIF
+	select CRC32
+	select CRC64
+	help
+	  Enable all CRC functions that have test code in CRC_KUNIT_TEST.
+
+	  Enable this only if you'd like the CRC KUnit test suite to test all
+	  the CRC variants, even ones that wouldn't otherwise need to be built.
+
+config CRC_BENCHMARK
+	bool "Benchmark for the CRC functions"
+	depends on CRC_KUNIT_TEST
+	help
+	  Include benchmarks in the KUnit test suite for the CRC functions.