From 5febf432df1cfa5b25d99b54c32103fafbdd0eb9 Mon Sep 17 00:00:00 2001 From: Miguel Ojeda Date: Sun, 16 Aug 2026 15:32:33 +0200 Subject: kbuild: rust: keep Rust objects out of Clang LTO with inline helpers 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 Cc: Boqun Feng Cc: Alice Ryhl Cc: Matthew Maurer Cc: Josh Poimboeuf Cc: Peter Zijlstra Cc: stable@vger.kernel.org Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust") Acked-by: Gary Guo Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org Signed-off-by: Miguel Ojeda --- lib/clz_ctz.c | 43 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 43 insertions(+) create mode 100644 lib/clz_ctz.c (limited to 'lib/clz_ctz.c') diff --git a/lib/clz_ctz.c b/lib/clz_ctz.c new file mode 100644 index 000000000..8778ec44b --- /dev/null +++ b/lib/clz_ctz.c @@ -0,0 +1,43 @@ +// SPDX-License-Identifier: GPL-2.0-only +/* + * lib/clz_ctz.c + * + * Copyright (C) 2013 Chanho Min + * + * The functions in this file aren't called directly, but are required by + * GCC builtins such as __builtin_ctz, and therefore they can't be removed + * despite appearing unreferenced in kernel source. + * + * __c[lt]z[sd]i2 can be overridden by linking arch-specific versions. + */ + +#include +#include + +int __weak __ctzsi2(int val); +int __weak __attribute_const__ __ctzsi2(int val) +{ + return __ffs(val); +} +EXPORT_SYMBOL(__ctzsi2); + +int __weak __clzsi2(int val); +int __weak __attribute_const__ __clzsi2(int val) +{ + return 32 - fls(val); +} +EXPORT_SYMBOL(__clzsi2); + +int __weak __clzdi2(u64 val); +int __weak __attribute_const__ __clzdi2(u64 val) +{ + return 64 - fls64(val); +} +EXPORT_SYMBOL(__clzdi2); + +int __weak __ctzdi2(u64 val); +int __weak __attribute_const__ __ctzdi2(u64 val) +{ + return __ffs64(val); +} +EXPORT_SYMBOL(__ctzdi2); -- cgit v1.3.1