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, 40 insertions, 0 deletions

diff --git a/include/crypto/internal/rng.h b/include/crypto/internal/rng.h
new file mode 100644
index 000000000..e0711b6a5
--- /dev/null
+++ b/include/crypto/internal/rng.h
@@ -0,0 +1,40 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * RNG: Random Number Generator  algorithms under the crypto API
+ *
+ * Copyright (c) 2008 Neil Horman <nhorman@tuxdriver.com>
+ * Copyright (c) 2015 Herbert Xu <herbert@gondor.apana.org.au>
+ */
+
+#ifndef _CRYPTO_INTERNAL_RNG_H
+#define _CRYPTO_INTERNAL_RNG_H
+
+#include <crypto/algapi.h>
+#include <crypto/rng.h>
+
+int crypto_register_rng(struct rng_alg *alg);
+void crypto_unregister_rng(struct rng_alg *alg);
+int crypto_register_rngs(struct rng_alg *algs, int count);
+void crypto_unregister_rngs(struct rng_alg *algs, int count);
+
+#if defined(CONFIG_CRYPTO_RNG) || defined(CONFIG_CRYPTO_RNG_MODULE)
+int crypto_del_default_rng(void);
+#else
+static inline int crypto_del_default_rng(void)
+{
+	return 0;
+}
+#endif
+
+static inline void *crypto_rng_ctx(struct crypto_rng *tfm)
+{
+	return crypto_tfm_ctx(&tfm->base);
+}
+
+static inline void crypto_rng_set_entropy(struct crypto_rng *tfm,
+					  const u8 *data, unsigned int len)
+{
+	crypto_rng_alg(tfm)->set_ent(tfm, data, len);
+}
+
+#endif