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, 77 insertions, 0 deletions

diff --git a/include/crypto/aes-cbc.h b/include/crypto/aes-cbc.h
new file mode 100644
index 000000000..7bae34093
--- /dev/null
+++ b/include/crypto/aes-cbc.h
@@ -0,0 +1,77 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * AES-CBC and AES-CBC-CTS unauthenticated encryption and decryption
+ *
+ * Copyright 2026 Google LLC
+ */
+#ifndef _CRYPTO_AES_CBC_H
+#define _CRYPTO_AES_CBC_H
+
+#include <crypto/aes.h>
+
+/**
+ * aes_cbc_encrypt() - Encrypt data using AES-CBC
+ * @dst: The destination buffer.  Can be in-place or out-of-place.  For other
+ *	 overlaps the behavior is unspecified.
+ * @src: The source data
+ * @len: Number of bytes to encrypt.  Must be a multiple of AES_BLOCK_SIZE.
+ * @iv: The initialization vector.  It is updated with the next value, i.e. the
+ *	last ciphertext block (or left unchanged if @len == 0).
+ * @key: The key, already prepared using aes_preparekey() or aes_prepareenckey()
+ *
+ * This supports incremental encryption.  The length of each chunk must be a
+ * multiple of AES_BLOCK_SIZE, and the updated @iv must be passed in each time.
+ *
+ * Context: Any context.
+ */
+void aes_cbc_encrypt(u8 *dst, const u8 *src, size_t len,
+		     u8 iv[at_least AES_BLOCK_SIZE], aes_encrypt_arg key);
+
+/**
+ * aes_cbc_decrypt() - Decrypt data using AES-CBC
+ * @dst: The destination buffer.  Can be in-place or out-of-place.  For other
+ *	 overlaps the behavior is unspecified.
+ * @src: The source data
+ * @len: Number of bytes to decrypt.  Must be a multiple of AES_BLOCK_SIZE.
+ * @iv: The initialization vector.  It is updated with the next value, i.e. the
+ *	last ciphertext block (or left unchanged if @len == 0).
+ * @key: The key, already prepared using aes_preparekey()
+ *
+ * This supports incremental decryption.  The length of each chunk must be a
+ * multiple of AES_BLOCK_SIZE, and the updated @iv must be passed in each time.
+ *
+ * Context: Any context.
+ */
+void aes_cbc_decrypt(u8 *dst, const u8 *src, size_t len,
+		     u8 iv[at_least AES_BLOCK_SIZE], const struct aes_key *key);
+
+/**
+ * aes_cbc_cts_encrypt() - Encrypt data using AES-CBC-CTS (CS3 variant)
+ * @dst: The destination buffer.  Can be in-place or out-of-place.  For other
+ *	 overlaps the behavior is unspecified.
+ * @src: The source data
+ * @len: Number of bytes to encrypt, at least AES_BLOCK_SIZE
+ * @iv: The initialization vector, clobbered by this function
+ * @key: The key, already prepared using aes_preparekey() or aes_prepareenckey()
+ *
+ * Context: Any context.
+ */
+void aes_cbc_cts_encrypt(u8 *dst, const u8 *src, size_t len,
+			 u8 iv[at_least AES_BLOCK_SIZE], aes_encrypt_arg key);
+
+/**
+ * aes_cbc_cts_decrypt() - Decrypt data using AES-CBC-CTS (CS3 variant)
+ * @dst: The destination buffer.  Can be in-place or out-of-place.  For other
+ *	 overlaps the behavior is unspecified.
+ * @src: The source data
+ * @len: Number of bytes to decrypt, at least AES_BLOCK_SIZE
+ * @iv: The initialization vector, clobbered by this function
+ * @key: The key, already prepared using aes_preparekey()
+ *
+ * Context: Any context.
+ */
+void aes_cbc_cts_decrypt(u8 *dst, const u8 *src, size_t len,
+			 u8 iv[at_least AES_BLOCK_SIZE],
+			 const struct aes_key *key);
+
+#endif /* _CRYPTO_AES_CBC_H */