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, 196 insertions, 0 deletions

diff --git a/crypto/twofish_generic.c b/crypto/twofish_generic.c
new file mode 100644
index 000000000..368018cfa
--- /dev/null
+++ b/crypto/twofish_generic.c
@@ -0,0 +1,196 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Twofish for CryptoAPI
+ *
+ * Originally Twofish for GPG
+ * By Matthew Skala <mskala@ansuz.sooke.bc.ca>, July 26, 1998
+ * 256-bit key length added March 20, 1999
+ * Some modifications to reduce the text size by Werner Koch, April, 1998
+ * Ported to the kerneli patch by Marc Mutz <Marc@Mutz.com>
+ * Ported to CryptoAPI by Colin Slater <hoho@tacomeat.net>
+ *
+ * The original author has disclaimed all copyright interest in this
+ * code and thus put it in the public domain. The subsequent authors 
+ * have put this under the GNU General Public License.
+ *
+ * This code is a "clean room" implementation, written from the paper
+ * _Twofish: A 128-Bit Block Cipher_ by Bruce Schneier, John Kelsey,
+ * Doug Whiting, David Wagner, Chris Hall, and Niels Ferguson, available
+ * through http://www.counterpane.com/twofish.html
+ *
+ * For background information on multiplication in finite fields, used for
+ * the matrix operations in the key schedule, see the book _Contemporary
+ * Abstract Algebra_ by Joseph A. Gallian, especially chapter 22 in the
+ * Third Edition.
+ */
+
+#include <linux/unaligned.h>
+#include <crypto/algapi.h>
+#include <crypto/twofish.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/types.h>
+#include <linux/errno.h>
+#include <linux/bitops.h>
+
+/* Macros to compute the g() function in the encryption and decryption
+ * rounds.  G1 is the straight g() function; G2 includes the 8-bit
+ * rotation for the high 32-bit word. */
+
+#define G1(a) \
+     (ctx->s[0][(a) & 0xFF]) ^ (ctx->s[1][((a) >> 8) & 0xFF]) \
+   ^ (ctx->s[2][((a) >> 16) & 0xFF]) ^ (ctx->s[3][(a) >> 24])
+
+#define G2(b) \
+     (ctx->s[1][(b) & 0xFF]) ^ (ctx->s[2][((b) >> 8) & 0xFF]) \
+   ^ (ctx->s[3][((b) >> 16) & 0xFF]) ^ (ctx->s[0][(b) >> 24])
+
+/* Encryption and decryption Feistel rounds.  Each one calls the two g()
+ * macros, does the PHT, and performs the XOR and the appropriate bit
+ * rotations.  The parameters are the round number (used to select subkeys),
+ * and the four 32-bit chunks of the text. */
+
+#define ENCROUND(n, a, b, c, d) \
+   x = G1 (a); y = G2 (b); \
+   x += y; y += x + ctx->k[2 * (n) + 1]; \
+   (c) ^= x + ctx->k[2 * (n)]; \
+   (c) = ror32((c), 1); \
+   (d) = rol32((d), 1) ^ y
+
+#define DECROUND(n, a, b, c, d) \
+   x = G1 (a); y = G2 (b); \
+   x += y; y += x; \
+   (d) ^= y + ctx->k[2 * (n) + 1]; \
+   (d) = ror32((d), 1); \
+   (c) = rol32((c), 1); \
+   (c) ^= (x + ctx->k[2 * (n)])
+
+/* Encryption and decryption cycles; each one is simply two Feistel rounds
+ * with the 32-bit chunks re-ordered to simulate the "swap" */
+
+#define ENCCYCLE(n) \
+   ENCROUND (2 * (n), a, b, c, d); \
+   ENCROUND (2 * (n) + 1, c, d, a, b)
+
+#define DECCYCLE(n) \
+   DECROUND (2 * (n) + 1, c, d, a, b); \
+   DECROUND (2 * (n), a, b, c, d)
+
+/* Macros to convert the input and output bytes into 32-bit words,
+ * and simultaneously perform the whitening step.  INPACK packs word
+ * number n into the variable named by x, using whitening subkey number m.
+ * OUTUNPACK unpacks word number n from the variable named by x, using
+ * whitening subkey number m. */
+
+#define INPACK(n, x, m) \
+   x = get_unaligned_le32(in + (n) * 4) ^ ctx->w[m]
+
+#define OUTUNPACK(n, x, m) \
+   x ^= ctx->w[m]; \
+   put_unaligned_le32(x, out + (n) * 4)
+
+
+
+/* Encrypt one block.  in and out may be the same. */
+static void twofish_encrypt(struct crypto_tfm *tfm, u8 *out, const u8 *in)
+{
+	struct twofish_ctx *ctx = crypto_tfm_ctx(tfm);
+
+	/* The four 32-bit chunks of the text. */
+	u32 a, b, c, d;
+	
+	/* Temporaries used by the round function. */
+	u32 x, y;
+
+	/* Input whitening and packing. */
+	INPACK (0, a, 0);
+	INPACK (1, b, 1);
+	INPACK (2, c, 2);
+	INPACK (3, d, 3);
+	
+	/* Encryption Feistel cycles. */
+	ENCCYCLE (0);
+	ENCCYCLE (1);
+	ENCCYCLE (2);
+	ENCCYCLE (3);
+	ENCCYCLE (4);
+	ENCCYCLE (5);
+	ENCCYCLE (6);
+	ENCCYCLE (7);
+	
+	/* Output whitening and unpacking. */
+	OUTUNPACK (0, c, 4);
+	OUTUNPACK (1, d, 5);
+	OUTUNPACK (2, a, 6);
+	OUTUNPACK (3, b, 7);
+	
+}
+
+/* Decrypt one block.  in and out may be the same. */
+static void twofish_decrypt(struct crypto_tfm *tfm, u8 *out, const u8 *in)
+{
+	struct twofish_ctx *ctx = crypto_tfm_ctx(tfm);
+  
+	/* The four 32-bit chunks of the text. */
+	u32 a, b, c, d;
+	
+	/* Temporaries used by the round function. */
+	u32 x, y;
+	
+	/* Input whitening and packing. */
+	INPACK (0, c, 4);
+	INPACK (1, d, 5);
+	INPACK (2, a, 6);
+	INPACK (3, b, 7);
+	
+	/* Encryption Feistel cycles. */
+	DECCYCLE (7);
+	DECCYCLE (6);
+	DECCYCLE (5);
+	DECCYCLE (4);
+	DECCYCLE (3);
+	DECCYCLE (2);
+	DECCYCLE (1);
+	DECCYCLE (0);
+
+	/* Output whitening and unpacking. */
+	OUTUNPACK (0, a, 0);
+	OUTUNPACK (1, b, 1);
+	OUTUNPACK (2, c, 2);
+	OUTUNPACK (3, d, 3);
+
+}
+
+static struct crypto_alg alg = {
+	.cra_name           =   "twofish",
+	.cra_driver_name    =   "twofish-generic",
+	.cra_priority       =   100,
+	.cra_flags          =   CRYPTO_ALG_TYPE_CIPHER,
+	.cra_blocksize      =   TF_BLOCK_SIZE,
+	.cra_ctxsize        =   sizeof(struct twofish_ctx),
+	.cra_module         =   THIS_MODULE,
+	.cra_u              =   { .cipher = {
+	.cia_min_keysize    =   TF_MIN_KEY_SIZE,
+	.cia_max_keysize    =   TF_MAX_KEY_SIZE,
+	.cia_setkey         =   twofish_setkey,
+	.cia_encrypt        =   twofish_encrypt,
+	.cia_decrypt        =   twofish_decrypt } }
+};
+
+static int __init twofish_mod_init(void)
+{
+	return crypto_register_alg(&alg);
+}
+
+static void __exit twofish_mod_fini(void)
+{
+	crypto_unregister_alg(&alg);
+}
+
+module_init(twofish_mod_init);
+module_exit(twofish_mod_fini);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION ("Twofish Cipher Algorithm");
+MODULE_ALIAS_CRYPTO("twofish");
+MODULE_ALIAS_CRYPTO("twofish-generic");