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, 24 insertions, 0 deletions

diff --git a/include/crypto/cast6.h b/include/crypto/cast6.h
new file mode 100644
index 000000000..38f490cd5
--- /dev/null
+++ b/include/crypto/cast6.h
@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _CRYPTO_CAST6_H
+#define _CRYPTO_CAST6_H
+
+#include <linux/types.h>
+#include <linux/crypto.h>
+#include <crypto/cast_common.h>
+
+#define CAST6_BLOCK_SIZE 16
+#define CAST6_MIN_KEY_SIZE 16
+#define CAST6_MAX_KEY_SIZE 32
+
+struct cast6_ctx {
+	u32 Km[12][4];
+	u8 Kr[12][4];
+};
+
+int __cast6_setkey(struct cast6_ctx *ctx, const u8 *key, unsigned int keylen);
+int cast6_setkey(struct crypto_tfm *tfm, const u8 *key, unsigned int keylen);
+
+void __cast6_encrypt(const void *ctx, u8 *dst, const u8 *src);
+void __cast6_decrypt(const void *ctx, u8 *dst, const u8 *src);
+
+#endif