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, 47 insertions, 0 deletions

diff --git a/include/asm-generic/int-ll64.h b/include/asm-generic/int-ll64.h
new file mode 100644
index 000000000..a248545f1
--- /dev/null
+++ b/include/asm-generic/int-ll64.h
@@ -0,0 +1,47 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * asm-generic/int-ll64.h
+ *
+ * Integer declarations for architectures which use "long long"
+ * for 64-bit types.
+ */
+#ifndef _ASM_GENERIC_INT_LL64_H
+#define _ASM_GENERIC_INT_LL64_H
+
+#include <uapi/asm-generic/int-ll64.h>
+
+
+#ifndef __ASSEMBLY__
+
+typedef __s8  s8;
+typedef __u8  u8;
+typedef __s16 s16;
+typedef __u16 u16;
+typedef __s32 s32;
+typedef __u32 u32;
+typedef __s64 s64;
+typedef __u64 u64;
+
+#define S8_C(x)  x
+#define U8_C(x)  x ## U
+#define S16_C(x) x
+#define U16_C(x) x ## U
+#define S32_C(x) x
+#define U32_C(x) x ## U
+#define S64_C(x) x ## LL
+#define U64_C(x) x ## ULL
+
+#else /* __ASSEMBLY__ */
+
+#define S8_C(x)  x
+#define U8_C(x)  x
+#define S16_C(x) x
+#define U16_C(x) x
+#define S32_C(x) x
+#define U32_C(x) x
+#define S64_C(x) x
+#define U64_C(x) x
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_GENERIC_INT_LL64_H */