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, 186 insertions, 0 deletions

diff --git a/crypto/rsa_helper.c b/crypto/rsa_helper.c
new file mode 100644
index 000000000..94266f290
--- /dev/null
+++ b/crypto/rsa_helper.c
@@ -0,0 +1,186 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * RSA key extract helper
+ *
+ * Copyright (c) 2015, Intel Corporation
+ * Authors: Tadeusz Struk <tadeusz.struk@intel.com>
+ */
+#include <linux/kernel.h>
+#include <linux/export.h>
+#include <linux/err.h>
+#include <linux/fips.h>
+#include <crypto/internal/rsa.h>
+#include "rsapubkey.asn1.h"
+#include "rsaprivkey.asn1.h"
+
+int rsa_get_n(void *context, size_t hdrlen, unsigned char tag,
+	      const void *value, size_t vlen)
+{
+	struct rsa_key *key = context;
+	const u8 *ptr = value;
+	size_t n_sz = vlen;
+
+	/* invalid key provided */
+	if (!value || !vlen)
+		return -EINVAL;
+
+	if (fips_enabled) {
+		while (n_sz && !*ptr) {
+			ptr++;
+			n_sz--;
+		}
+
+		/* In FIPS mode only allow key size 2K and higher */
+		if (n_sz < 256) {
+			pr_err("RSA: key size not allowed in FIPS mode\n");
+			return -EINVAL;
+		}
+	}
+
+	key->n = value;
+	key->n_sz = vlen;
+
+	return 0;
+}
+
+int rsa_get_e(void *context, size_t hdrlen, unsigned char tag,
+	      const void *value, size_t vlen)
+{
+	struct rsa_key *key = context;
+
+	/* invalid key provided */
+	if (!value || !key->n_sz || !vlen || vlen > key->n_sz)
+		return -EINVAL;
+
+	key->e = value;
+	key->e_sz = vlen;
+
+	return 0;
+}
+
+int rsa_get_d(void *context, size_t hdrlen, unsigned char tag,
+	      const void *value, size_t vlen)
+{
+	struct rsa_key *key = context;
+
+	/* invalid key provided */
+	if (!value || !key->n_sz || !vlen || vlen > key->n_sz)
+		return -EINVAL;
+
+	key->d = value;
+	key->d_sz = vlen;
+
+	return 0;
+}
+
+int rsa_get_p(void *context, size_t hdrlen, unsigned char tag,
+	      const void *value, size_t vlen)
+{
+	struct rsa_key *key = context;
+
+	/* invalid key provided */
+	if (!value || !vlen || vlen > key->n_sz)
+		return -EINVAL;
+
+	key->p = value;
+	key->p_sz = vlen;
+
+	return 0;
+}
+
+int rsa_get_q(void *context, size_t hdrlen, unsigned char tag,
+	      const void *value, size_t vlen)
+{
+	struct rsa_key *key = context;
+
+	/* invalid key provided */
+	if (!value || !vlen || vlen > key->n_sz)
+		return -EINVAL;
+
+	key->q = value;
+	key->q_sz = vlen;
+
+	return 0;
+}
+
+int rsa_get_dp(void *context, size_t hdrlen, unsigned char tag,
+	       const void *value, size_t vlen)
+{
+	struct rsa_key *key = context;
+
+	/* invalid key provided */
+	if (!value || !vlen || vlen > key->n_sz)
+		return -EINVAL;
+
+	key->dp = value;
+	key->dp_sz = vlen;
+
+	return 0;
+}
+
+int rsa_get_dq(void *context, size_t hdrlen, unsigned char tag,
+	       const void *value, size_t vlen)
+{
+	struct rsa_key *key = context;
+
+	/* invalid key provided */
+	if (!value || !vlen || vlen > key->n_sz)
+		return -EINVAL;
+
+	key->dq = value;
+	key->dq_sz = vlen;
+
+	return 0;
+}
+
+int rsa_get_qinv(void *context, size_t hdrlen, unsigned char tag,
+		 const void *value, size_t vlen)
+{
+	struct rsa_key *key = context;
+
+	/* invalid key provided */
+	if (!value || !vlen || vlen > key->n_sz)
+		return -EINVAL;
+
+	key->qinv = value;
+	key->qinv_sz = vlen;
+
+	return 0;
+}
+
+/**
+ * rsa_parse_pub_key() - decodes the BER encoded buffer and stores in the
+ *                       provided struct rsa_key, pointers to the raw key as is,
+ *                       so that the caller can copy it or MPI parse it, etc.
+ *
+ * @rsa_key:	struct rsa_key key representation
+ * @key:	key in BER format
+ * @key_len:	length of key
+ *
+ * Return:	0 on success or error code in case of error
+ */
+int rsa_parse_pub_key(struct rsa_key *rsa_key, const void *key,
+		      unsigned int key_len)
+{
+	return asn1_ber_decoder(&rsapubkey_decoder, rsa_key, key, key_len);
+}
+EXPORT_SYMBOL_GPL(rsa_parse_pub_key);
+
+/**
+ * rsa_parse_priv_key() - decodes the BER encoded buffer and stores in the
+ *                        provided struct rsa_key, pointers to the raw key
+ *                        as is, so that the caller can copy it or MPI parse it,
+ *                        etc.
+ *
+ * @rsa_key:	struct rsa_key key representation
+ * @key:	key in BER format
+ * @key_len:	length of key
+ *
+ * Return:	0 on success or error code in case of error
+ */
+int rsa_parse_priv_key(struct rsa_key *rsa_key, const void *key,
+		       unsigned int key_len)
+{
+	return asn1_ber_decoder(&rsaprivkey_decoder, rsa_key, key, key_len);
+}
+EXPORT_SYMBOL_GPL(rsa_parse_priv_key);