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/arch/powerpc/boot/of.h b/arch/powerpc/boot/of.h
new file mode 100644
index 000000000..31b2f5dfd
--- /dev/null
+++ b/arch/powerpc/boot/of.h
@@ -0,0 +1,47 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _PPC_BOOT_OF_H_
+#define _PPC_BOOT_OF_H_
+
+#include "swab.h"
+
+typedef void *phandle;
+typedef u32 ihandle;
+
+void of_init(void *promptr);
+int of_call_prom(const char *service, int nargs, int nret, ...);
+unsigned int of_claim(unsigned long virt, unsigned long size,
+	unsigned long align);
+void *of_vmlinux_alloc(unsigned long size);
+void of_exit(void);
+void *of_finddevice(const char *name);
+int of_getprop(const void *phandle, const char *name, void *buf,
+	       const int buflen);
+int of_setprop(const void *phandle, const char *name, const void *buf,
+	       const int buflen);
+
+/* Console functions */
+void of_console_init(void);
+
+typedef u16			__be16;
+typedef u32			__be32;
+typedef u64			__be64;
+
+#ifdef __LITTLE_ENDIAN__
+#define cpu_to_be16(x) swab16(x)
+#define be16_to_cpu(x) swab16(x)
+#define cpu_to_be32(x) swab32(x)
+#define be32_to_cpu(x) swab32(x)
+#define cpu_to_be64(x) swab64(x)
+#define be64_to_cpu(x) swab64(x)
+#else
+#define cpu_to_be16(x) (x)
+#define be16_to_cpu(x) (x)
+#define cpu_to_be32(x) (x)
+#define be32_to_cpu(x) (x)
+#define cpu_to_be64(x) (x)
+#define be64_to_cpu(x) (x)
+#endif
+
+#define PROM_ERROR (-1u)
+
+#endif /* _PPC_BOOT_OF_H_ */