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, 83 insertions, 0 deletions

diff --git a/crypto/bpf_crypto_skcipher.c b/crypto/bpf_crypto_skcipher.c
new file mode 100644
index 000000000..a88798d3e
--- /dev/null
+++ b/crypto/bpf_crypto_skcipher.c
@@ -0,0 +1,83 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2024 Meta, Inc */
+#include <linux/types.h>
+#include <linux/module.h>
+#include <linux/bpf_crypto.h>
+#include <crypto/skcipher.h>
+
+static void *bpf_crypto_lskcipher_alloc_tfm(const char *algo)
+{
+	return crypto_alloc_lskcipher(algo, 0, 0);
+}
+
+static void bpf_crypto_lskcipher_free_tfm(void *tfm)
+{
+	crypto_free_lskcipher(tfm);
+}
+
+static int bpf_crypto_lskcipher_has_algo(const char *algo)
+{
+	return crypto_has_skcipher(algo, CRYPTO_ALG_TYPE_LSKCIPHER, CRYPTO_ALG_TYPE_MASK);
+}
+
+static int bpf_crypto_lskcipher_setkey(void *tfm, const u8 *key, unsigned int keylen)
+{
+	return crypto_lskcipher_setkey(tfm, key, keylen);
+}
+
+static u32 bpf_crypto_lskcipher_get_flags(void *tfm)
+{
+	return crypto_lskcipher_get_flags(tfm);
+}
+
+static unsigned int bpf_crypto_lskcipher_ivsize(void *tfm)
+{
+	return crypto_lskcipher_ivsize(tfm);
+}
+
+static unsigned int bpf_crypto_lskcipher_statesize(void *tfm)
+{
+	return crypto_lskcipher_statesize(tfm);
+}
+
+static int bpf_crypto_lskcipher_encrypt(void *tfm, const u8 *src, u8 *dst,
+					unsigned int len, u8 *siv)
+{
+	return crypto_lskcipher_encrypt(tfm, src, dst, len, siv);
+}
+
+static int bpf_crypto_lskcipher_decrypt(void *tfm, const u8 *src, u8 *dst,
+					unsigned int len, u8 *siv)
+{
+	return crypto_lskcipher_decrypt(tfm, src, dst, len, siv);
+}
+
+static const struct bpf_crypto_type bpf_crypto_lskcipher_type = {
+	.alloc_tfm	= bpf_crypto_lskcipher_alloc_tfm,
+	.free_tfm	= bpf_crypto_lskcipher_free_tfm,
+	.has_algo	= bpf_crypto_lskcipher_has_algo,
+	.setkey		= bpf_crypto_lskcipher_setkey,
+	.encrypt	= bpf_crypto_lskcipher_encrypt,
+	.decrypt	= bpf_crypto_lskcipher_decrypt,
+	.ivsize		= bpf_crypto_lskcipher_ivsize,
+	.statesize	= bpf_crypto_lskcipher_statesize,
+	.get_flags	= bpf_crypto_lskcipher_get_flags,
+	.owner		= THIS_MODULE,
+	.name		= "skcipher",
+};
+
+static int __init bpf_crypto_skcipher_init(void)
+{
+	return bpf_crypto_register_type(&bpf_crypto_lskcipher_type);
+}
+
+static void __exit bpf_crypto_skcipher_exit(void)
+{
+	int err = bpf_crypto_unregister_type(&bpf_crypto_lskcipher_type);
+	WARN_ON_ONCE(err);
+}
+
+module_init(bpf_crypto_skcipher_init);
+module_exit(bpf_crypto_skcipher_exit);
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Symmetric key cipher support for BPF");