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/cast5.h b/include/crypto/cast5.h
new file mode 100644
index 000000000..3d4ed4ea9
--- /dev/null
+++ b/include/crypto/cast5.h
@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _CRYPTO_CAST5_H
+#define _CRYPTO_CAST5_H
+
+#include <linux/types.h>
+#include <linux/crypto.h>
+#include <crypto/cast_common.h>
+
+#define CAST5_BLOCK_SIZE 8
+#define CAST5_MIN_KEY_SIZE 5
+#define CAST5_MAX_KEY_SIZE 16
+
+struct cast5_ctx {
+	u32 Km[16];
+	u8 Kr[16];
+	int rr;	/* rr ? rounds = 12 : rounds = 16; (rfc 2144) */
+};
+
+int cast5_setkey(struct crypto_tfm *tfm, const u8 *key, unsigned int keylen);
+
+void __cast5_encrypt(struct cast5_ctx *ctx, u8 *dst, const u8 *src);
+void __cast5_decrypt(struct cast5_ctx *ctx, u8 *dst, const u8 *src);
+
+#endif