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, 247 insertions, 0 deletions

diff --git a/crypto/ecdh.c b/crypto/ecdh.c
new file mode 100644
index 000000000..9f0b93b31
--- /dev/null
+++ b/crypto/ecdh.c
@@ -0,0 +1,247 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* ECDH key-agreement protocol
+ *
+ * Copyright (c) 2016, Intel Corporation
+ * Authors: Salvator Benedetto <salvatore.benedetto@intel.com>
+ */
+
+#include <linux/module.h>
+#include <crypto/internal/ecc.h>
+#include <crypto/internal/kpp.h>
+#include <crypto/kpp.h>
+#include <crypto/ecdh.h>
+#include <linux/scatterlist.h>
+
+struct ecdh_ctx {
+	unsigned int curve_id;
+	unsigned int ndigits;
+	u64 private_key[ECC_MAX_DIGITS];
+};
+
+static inline struct ecdh_ctx *ecdh_get_ctx(struct crypto_kpp *tfm)
+{
+	return kpp_tfm_ctx(tfm);
+}
+
+static int ecdh_set_secret(struct crypto_kpp *tfm, const void *buf,
+			   unsigned int len)
+{
+	struct ecdh_ctx *ctx = ecdh_get_ctx(tfm);
+	struct ecdh params;
+	int ret = 0;
+
+	if (crypto_ecdh_decode_key(buf, len, &params) < 0 ||
+	    params.key_size > sizeof(u64) * ctx->ndigits)
+		return -EINVAL;
+
+	memset(ctx->private_key, 0, sizeof(ctx->private_key));
+
+	if (!params.key || !params.key_size)
+		return ecc_gen_privkey(ctx->curve_id, ctx->ndigits,
+				       ctx->private_key);
+
+	ecc_digits_from_bytes(params.key, params.key_size,
+			      ctx->private_key, ctx->ndigits);
+
+	if (ecc_is_key_valid(ctx->curve_id, ctx->ndigits,
+			     ctx->private_key, params.key_size) < 0) {
+		memzero_explicit(ctx->private_key, params.key_size);
+		ret = -EINVAL;
+	}
+
+	return ret;
+}
+
+static int ecdh_compute_value(struct kpp_request *req)
+{
+	struct crypto_kpp *tfm = crypto_kpp_reqtfm(req);
+	struct ecdh_ctx *ctx = ecdh_get_ctx(tfm);
+	u64 *public_key;
+	u64 *shared_secret = NULL;
+	void *buf;
+	size_t copied, nbytes, public_key_sz;
+	int ret = -ENOMEM;
+
+	nbytes = ctx->ndigits << ECC_DIGITS_TO_BYTES_SHIFT;
+	/* Public part is a point thus it has both coordinates */
+	public_key_sz = 2 * nbytes;
+
+	public_key = kmalloc(public_key_sz, GFP_KERNEL);
+	if (!public_key)
+		return -ENOMEM;
+
+	if (req->src) {
+		shared_secret = kmalloc(nbytes, GFP_KERNEL);
+		if (!shared_secret)
+			goto free_pubkey;
+
+		/* from here on it's invalid parameters */
+		ret = -EINVAL;
+
+		/* must have exactly two points to be on the curve */
+		if (public_key_sz != req->src_len)
+			goto free_all;
+
+		copied = sg_copy_to_buffer(req->src,
+					   sg_nents_for_len(req->src,
+							    public_key_sz),
+					   public_key, public_key_sz);
+		if (copied != public_key_sz)
+			goto free_all;
+
+		ret = crypto_ecdh_shared_secret(ctx->curve_id, ctx->ndigits,
+						ctx->private_key, public_key,
+						shared_secret);
+
+		buf = shared_secret;
+	} else {
+		ret = ecc_make_pub_key(ctx->curve_id, ctx->ndigits,
+				       ctx->private_key, public_key);
+		buf = public_key;
+		nbytes = public_key_sz;
+	}
+
+	if (ret < 0)
+		goto free_all;
+
+	/* might want less than we've got */
+	nbytes = min_t(size_t, nbytes, req->dst_len);
+	copied = sg_copy_from_buffer(req->dst, sg_nents_for_len(req->dst,
+								nbytes),
+				     buf, nbytes);
+	if (copied != nbytes)
+		ret = -EINVAL;
+
+	/* fall through */
+free_all:
+	kfree_sensitive(shared_secret);
+free_pubkey:
+	kfree(public_key);
+	return ret;
+}
+
+static unsigned int ecdh_max_size(struct crypto_kpp *tfm)
+{
+	struct ecdh_ctx *ctx = ecdh_get_ctx(tfm);
+
+	/* Public key is made of two coordinates, add one to the left shift */
+	return ctx->ndigits << (ECC_DIGITS_TO_BYTES_SHIFT + 1);
+}
+
+static int ecdh_nist_p192_init_tfm(struct crypto_kpp *tfm)
+{
+	struct ecdh_ctx *ctx = ecdh_get_ctx(tfm);
+
+	ctx->curve_id = ECC_CURVE_NIST_P192;
+	ctx->ndigits = ECC_CURVE_NIST_P192_DIGITS;
+
+	return 0;
+}
+
+static struct kpp_alg ecdh_nist_p192 = {
+	.set_secret = ecdh_set_secret,
+	.generate_public_key = ecdh_compute_value,
+	.compute_shared_secret = ecdh_compute_value,
+	.max_size = ecdh_max_size,
+	.init = ecdh_nist_p192_init_tfm,
+	.base = {
+		.cra_name = "ecdh-nist-p192",
+		.cra_driver_name = "ecdh-nist-p192-generic",
+		.cra_priority = 100,
+		.cra_module = THIS_MODULE,
+		.cra_ctxsize = sizeof(struct ecdh_ctx),
+	},
+};
+
+static int ecdh_nist_p256_init_tfm(struct crypto_kpp *tfm)
+{
+	struct ecdh_ctx *ctx = ecdh_get_ctx(tfm);
+
+	ctx->curve_id = ECC_CURVE_NIST_P256;
+	ctx->ndigits = ECC_CURVE_NIST_P256_DIGITS;
+
+	return 0;
+}
+
+static struct kpp_alg ecdh_nist_p256 = {
+	.set_secret = ecdh_set_secret,
+	.generate_public_key = ecdh_compute_value,
+	.compute_shared_secret = ecdh_compute_value,
+	.max_size = ecdh_max_size,
+	.init = ecdh_nist_p256_init_tfm,
+	.base = {
+		.cra_name = "ecdh-nist-p256",
+		.cra_driver_name = "ecdh-nist-p256-generic",
+		.cra_priority = 100,
+		.cra_module = THIS_MODULE,
+		.cra_ctxsize = sizeof(struct ecdh_ctx),
+	},
+};
+
+static int ecdh_nist_p384_init_tfm(struct crypto_kpp *tfm)
+{
+	struct ecdh_ctx *ctx = ecdh_get_ctx(tfm);
+
+	ctx->curve_id = ECC_CURVE_NIST_P384;
+	ctx->ndigits = ECC_CURVE_NIST_P384_DIGITS;
+
+	return 0;
+}
+
+static struct kpp_alg ecdh_nist_p384 = {
+	.set_secret = ecdh_set_secret,
+	.generate_public_key = ecdh_compute_value,
+	.compute_shared_secret = ecdh_compute_value,
+	.max_size = ecdh_max_size,
+	.init = ecdh_nist_p384_init_tfm,
+	.base = {
+		.cra_name = "ecdh-nist-p384",
+		.cra_driver_name = "ecdh-nist-p384-generic",
+		.cra_priority = 100,
+		.cra_module = THIS_MODULE,
+		.cra_ctxsize = sizeof(struct ecdh_ctx),
+	},
+};
+
+static bool ecdh_nist_p192_registered;
+
+static int __init ecdh_init(void)
+{
+	int ret;
+
+	/* NIST p192 will fail to register in FIPS mode */
+	ret = crypto_register_kpp(&ecdh_nist_p192);
+	ecdh_nist_p192_registered = ret == 0;
+
+	ret = crypto_register_kpp(&ecdh_nist_p256);
+	if (ret)
+		goto nist_p256_error;
+
+	ret = crypto_register_kpp(&ecdh_nist_p384);
+	if (ret)
+		goto nist_p384_error;
+
+	return 0;
+
+nist_p384_error:
+	crypto_unregister_kpp(&ecdh_nist_p256);
+
+nist_p256_error:
+	if (ecdh_nist_p192_registered)
+		crypto_unregister_kpp(&ecdh_nist_p192);
+	return ret;
+}
+
+static void __exit ecdh_exit(void)
+{
+	if (ecdh_nist_p192_registered)
+		crypto_unregister_kpp(&ecdh_nist_p192);
+	crypto_unregister_kpp(&ecdh_nist_p256);
+	crypto_unregister_kpp(&ecdh_nist_p384);
+}
+
+module_init(ecdh_init);
+module_exit(ecdh_exit);
+MODULE_ALIAS_CRYPTO("ecdh");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("ECDH generic algorithm");