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, 244 insertions, 0 deletions

diff --git a/crypto/cmac.c b/crypto/cmac.c
new file mode 100644
index 000000000..83e58937f
--- /dev/null
+++ b/crypto/cmac.c
@@ -0,0 +1,244 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * CMAC: Cipher Block Mode for Authentication
+ *
+ * Copyright © 2013 Jussi Kivilinna <jussi.kivilinna@iki.fi>
+ *
+ * Based on work by:
+ *  Copyright © 2013 Tom St Denis <tstdenis@elliptictech.com>
+ * Based on crypto/xcbc.c:
+ *  Copyright © 2006 USAGI/WIDE Project,
+ *   Author: Kazunori Miyazawa <miyazawa@linux-ipv6.org>
+ */
+
+#include <crypto/internal/cipher.h>
+#include <crypto/internal/hash.h>
+#include <crypto/utils.h>
+#include <linux/err.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+
+/*
+ * +------------------------
+ * | <parent tfm>
+ * +------------------------
+ * | cmac_tfm_ctx
+ * +------------------------
+ * | consts (block size * 2)
+ * +------------------------
+ */
+struct cmac_tfm_ctx {
+	struct crypto_cipher *child;
+	__be64 consts[];
+};
+
+static int crypto_cmac_digest_setkey(struct crypto_shash *parent,
+				     const u8 *inkey, unsigned int keylen)
+{
+	struct cmac_tfm_ctx *ctx = crypto_shash_ctx(parent);
+	unsigned int bs = crypto_shash_blocksize(parent);
+	__be64 *consts = ctx->consts;
+	u64 _const[2];
+	int i, err = 0;
+	u8 msb_mask, gfmask;
+
+	err = crypto_cipher_setkey(ctx->child, inkey, keylen);
+	if (err)
+		return err;
+
+	/* encrypt the zero block */
+	memset(consts, 0, bs);
+	crypto_cipher_encrypt_one(ctx->child, (u8 *)consts, (u8 *)consts);
+
+	switch (bs) {
+	case 16:
+		gfmask = 0x87;
+		_const[0] = be64_to_cpu(consts[1]);
+		_const[1] = be64_to_cpu(consts[0]);
+
+		/* gf(2^128) multiply zero-ciphertext with u and u^2 */
+		for (i = 0; i < 4; i += 2) {
+			msb_mask = ((s64)_const[1] >> 63) & gfmask;
+			_const[1] = (_const[1] << 1) | (_const[0] >> 63);
+			_const[0] = (_const[0] << 1) ^ msb_mask;
+
+			consts[i + 0] = cpu_to_be64(_const[1]);
+			consts[i + 1] = cpu_to_be64(_const[0]);
+		}
+
+		break;
+	case 8:
+		gfmask = 0x1B;
+		_const[0] = be64_to_cpu(consts[0]);
+
+		/* gf(2^64) multiply zero-ciphertext with u and u^2 */
+		for (i = 0; i < 2; i++) {
+			msb_mask = ((s64)_const[0] >> 63) & gfmask;
+			_const[0] = (_const[0] << 1) ^ msb_mask;
+
+			consts[i] = cpu_to_be64(_const[0]);
+		}
+
+		break;
+	}
+
+	return 0;
+}
+
+static int crypto_cmac_digest_init(struct shash_desc *pdesc)
+{
+	int bs = crypto_shash_blocksize(pdesc->tfm);
+	u8 *prev = shash_desc_ctx(pdesc);
+
+	memset(prev, 0, bs);
+	return 0;
+}
+
+static int crypto_cmac_digest_update(struct shash_desc *pdesc, const u8 *p,
+				     unsigned int len)
+{
+	struct crypto_shash *parent = pdesc->tfm;
+	struct cmac_tfm_ctx *tctx = crypto_shash_ctx(parent);
+	struct crypto_cipher *tfm = tctx->child;
+	int bs = crypto_shash_blocksize(parent);
+	u8 *prev = shash_desc_ctx(pdesc);
+
+	do {
+		crypto_xor(prev, p, bs);
+		crypto_cipher_encrypt_one(tfm, prev, prev);
+		p += bs;
+		len -= bs;
+	} while (len >= bs);
+	return len;
+}
+
+static int crypto_cmac_digest_finup(struct shash_desc *pdesc, const u8 *src,
+				    unsigned int len, u8 *out)
+{
+	struct crypto_shash *parent = pdesc->tfm;
+	struct cmac_tfm_ctx *tctx = crypto_shash_ctx(parent);
+	struct crypto_cipher *tfm = tctx->child;
+	int bs = crypto_shash_blocksize(parent);
+	u8 *prev = shash_desc_ctx(pdesc);
+	unsigned int offset = 0;
+
+	crypto_xor(prev, src, len);
+	if (len != bs) {
+		prev[len] ^= 0x80;
+		offset += bs;
+	}
+	crypto_xor(prev, (const u8 *)tctx->consts + offset, bs);
+	crypto_cipher_encrypt_one(tfm, out, prev);
+	return 0;
+}
+
+static int cmac_init_tfm(struct crypto_shash *tfm)
+{
+	struct shash_instance *inst = shash_alg_instance(tfm);
+	struct cmac_tfm_ctx *ctx = crypto_shash_ctx(tfm);
+	struct crypto_cipher_spawn *spawn;
+	struct crypto_cipher *cipher;
+
+	spawn = shash_instance_ctx(inst);
+	cipher = crypto_spawn_cipher(spawn);
+	if (IS_ERR(cipher))
+		return PTR_ERR(cipher);
+
+	ctx->child = cipher;
+
+	return 0;
+}
+
+static void cmac_exit_tfm(struct crypto_shash *tfm)
+{
+	struct cmac_tfm_ctx *ctx = crypto_shash_ctx(tfm);
+	crypto_free_cipher(ctx->child);
+}
+
+static int cmac_create(struct crypto_template *tmpl, struct rtattr **tb)
+{
+	struct shash_instance *inst;
+	struct crypto_cipher_spawn *spawn;
+	struct crypto_alg *alg;
+	u32 mask;
+	int err;
+
+	err = crypto_check_attr_type(tb, CRYPTO_ALG_TYPE_SHASH, &mask);
+	if (err)
+		return err;
+
+	inst = kzalloc(sizeof(*inst) + sizeof(*spawn), GFP_KERNEL);
+	if (!inst)
+		return -ENOMEM;
+	spawn = shash_instance_ctx(inst);
+
+	err = crypto_grab_cipher(spawn, shash_crypto_instance(inst),
+				 crypto_attr_alg_name(tb[1]), 0, mask);
+	if (err)
+		goto err_free_inst;
+	alg = crypto_spawn_cipher_alg(spawn);
+
+	switch (alg->cra_blocksize) {
+	case 16:
+	case 8:
+		break;
+	default:
+		err = -EINVAL;
+		goto err_free_inst;
+	}
+
+	err = crypto_inst_setname(shash_crypto_instance(inst), tmpl->name, alg);
+	if (err)
+		goto err_free_inst;
+
+	inst->alg.base.cra_priority = alg->cra_priority;
+	inst->alg.base.cra_blocksize = alg->cra_blocksize;
+	inst->alg.base.cra_ctxsize = sizeof(struct cmac_tfm_ctx) +
+				     alg->cra_blocksize * 2;
+	inst->alg.base.cra_flags = CRYPTO_AHASH_ALG_BLOCK_ONLY |
+				   CRYPTO_AHASH_ALG_FINAL_NONZERO;
+
+	inst->alg.digestsize = alg->cra_blocksize;
+	inst->alg.descsize = alg->cra_blocksize;
+	inst->alg.init = crypto_cmac_digest_init;
+	inst->alg.update = crypto_cmac_digest_update;
+	inst->alg.finup = crypto_cmac_digest_finup;
+	inst->alg.setkey = crypto_cmac_digest_setkey;
+	inst->alg.init_tfm = cmac_init_tfm;
+	inst->alg.exit_tfm = cmac_exit_tfm;
+
+	inst->free = shash_free_singlespawn_instance;
+
+	err = shash_register_instance(tmpl, inst);
+	if (err) {
+err_free_inst:
+		shash_free_singlespawn_instance(inst);
+	}
+	return err;
+}
+
+static struct crypto_template crypto_cmac_tmpl = {
+	.name = "cmac",
+	.create = cmac_create,
+	.module = THIS_MODULE,
+};
+
+static int __init crypto_cmac_module_init(void)
+{
+	return crypto_register_template(&crypto_cmac_tmpl);
+}
+
+static void __exit crypto_cmac_module_exit(void)
+{
+	crypto_unregister_template(&crypto_cmac_tmpl);
+}
+
+module_init(crypto_cmac_module_init);
+module_exit(crypto_cmac_module_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("CMAC keyed hash algorithm");
+MODULE_ALIAS_CRYPTO("cmac");
+MODULE_IMPORT_NS("CRYPTO_INTERNAL");