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, 133 insertions, 0 deletions

diff --git a/crypto/blowfish_generic.c b/crypto/blowfish_generic.c
new file mode 100644
index 000000000..f3c5f9b09
--- /dev/null
+++ b/crypto/blowfish_generic.c
@@ -0,0 +1,133 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Cryptographic API.
+ *
+ * Blowfish Cipher Algorithm, by Bruce Schneier.
+ * http://www.counterpane.com/blowfish.html
+ *
+ * Adapted from Kerneli implementation.
+ *
+ * Copyright (c) Herbert Valerio Riedel <hvr@hvrlab.org>
+ * Copyright (c) Kyle McMartin <kyle@debian.org>
+ * Copyright (c) 2002 James Morris <jmorris@intercode.com.au>
+ */
+
+#include <crypto/algapi.h>
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/mm.h>
+#include <linux/unaligned.h>
+#include <linux/types.h>
+#include <crypto/blowfish.h>
+
+/*
+ * Round loop unrolling macros, S is a pointer to a S-Box array
+ * organized in 4 unsigned longs at a row.
+ */
+#define GET32_3(x) (((x) & 0xff))
+#define GET32_2(x) (((x) >> (8)) & (0xff))
+#define GET32_1(x) (((x) >> (16)) & (0xff))
+#define GET32_0(x) (((x) >> (24)) & (0xff))
+
+#define bf_F(x) (((S[GET32_0(x)] + S[256 + GET32_1(x)]) ^ \
+		S[512 + GET32_2(x)]) + S[768 + GET32_3(x)])
+
+#define ROUND(a, b, n) ({ b ^= P[n]; a ^= bf_F(b); })
+
+static void bf_encrypt(struct crypto_tfm *tfm, u8 *dst, const u8 *src)
+{
+	struct bf_ctx *ctx = crypto_tfm_ctx(tfm);
+	const u32 *P = ctx->p;
+	const u32 *S = ctx->s;
+	u32 yl = get_unaligned_be32(src);
+	u32 yr = get_unaligned_be32(src + 4);
+
+	ROUND(yr, yl, 0);
+	ROUND(yl, yr, 1);
+	ROUND(yr, yl, 2);
+	ROUND(yl, yr, 3);
+	ROUND(yr, yl, 4);
+	ROUND(yl, yr, 5);
+	ROUND(yr, yl, 6);
+	ROUND(yl, yr, 7);
+	ROUND(yr, yl, 8);
+	ROUND(yl, yr, 9);
+	ROUND(yr, yl, 10);
+	ROUND(yl, yr, 11);
+	ROUND(yr, yl, 12);
+	ROUND(yl, yr, 13);
+	ROUND(yr, yl, 14);
+	ROUND(yl, yr, 15);
+
+	yl ^= P[16];
+	yr ^= P[17];
+
+	put_unaligned_be32(yr, dst);
+	put_unaligned_be32(yl, dst + 4);
+}
+
+static void bf_decrypt(struct crypto_tfm *tfm, u8 *dst, const u8 *src)
+{
+	struct bf_ctx *ctx = crypto_tfm_ctx(tfm);
+	const u32 *P = ctx->p;
+	const u32 *S = ctx->s;
+	u32 yl = get_unaligned_be32(src);
+	u32 yr = get_unaligned_be32(src + 4);
+
+	ROUND(yr, yl, 17);
+	ROUND(yl, yr, 16);
+	ROUND(yr, yl, 15);
+	ROUND(yl, yr, 14);
+	ROUND(yr, yl, 13);
+	ROUND(yl, yr, 12);
+	ROUND(yr, yl, 11);
+	ROUND(yl, yr, 10);
+	ROUND(yr, yl, 9);
+	ROUND(yl, yr, 8);
+	ROUND(yr, yl, 7);
+	ROUND(yl, yr, 6);
+	ROUND(yr, yl, 5);
+	ROUND(yl, yr, 4);
+	ROUND(yr, yl, 3);
+	ROUND(yl, yr, 2);
+
+	yl ^= P[1];
+	yr ^= P[0];
+
+	put_unaligned_be32(yr, dst);
+	put_unaligned_be32(yl, dst + 4);
+}
+
+static struct crypto_alg alg = {
+	.cra_name		=	"blowfish",
+	.cra_driver_name	=	"blowfish-generic",
+	.cra_priority		=	100,
+	.cra_flags		=	CRYPTO_ALG_TYPE_CIPHER,
+	.cra_blocksize		=	BF_BLOCK_SIZE,
+	.cra_ctxsize		=	sizeof(struct bf_ctx),
+	.cra_module		=	THIS_MODULE,
+	.cra_u			=	{ .cipher = {
+	.cia_min_keysize	=	BF_MIN_KEY_SIZE,
+	.cia_max_keysize	=	BF_MAX_KEY_SIZE,
+	.cia_setkey		=	blowfish_setkey,
+	.cia_encrypt		=	bf_encrypt,
+	.cia_decrypt		=	bf_decrypt } }
+};
+
+static int __init blowfish_mod_init(void)
+{
+	return crypto_register_alg(&alg);
+}
+
+static void __exit blowfish_mod_fini(void)
+{
+	crypto_unregister_alg(&alg);
+}
+
+module_init(blowfish_mod_init);
+module_exit(blowfish_mod_fini);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Blowfish Cipher Algorithm");
+MODULE_ALIAS_CRYPTO("blowfish");
+MODULE_ALIAS_CRYPTO("blowfish-generic");