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, 74 insertions, 0 deletions

diff --git a/lib/crypto/powerpc/poly1305.h b/lib/crypto/powerpc/poly1305.h
new file mode 100644
index 000000000..b8ed098a0
--- /dev/null
+++ b/lib/crypto/powerpc/poly1305.h
@@ -0,0 +1,74 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Poly1305 authenticator algorithm, RFC7539.
+ *
+ * Copyright 2023- IBM Corp. All rights reserved.
+ */
+#include <asm/switch_to.h>
+#include <linux/cpufeature.h>
+#include <linux/jump_label.h>
+#include <linux/kernel.h>
+#include <linux/unaligned.h>
+
+asmlinkage void poly1305_p10le_4blocks(struct poly1305_block_state *state, const u8 *m, u32 mlen);
+asmlinkage void poly1305_64s(struct poly1305_block_state *state, const u8 *m, u32 mlen, int highbit);
+asmlinkage void poly1305_emit_64(const struct poly1305_state *state, const u32 nonce[4], u8 digest[POLY1305_DIGEST_SIZE]);
+
+static __ro_after_init DEFINE_STATIC_KEY_FALSE(have_p10);
+
+static void vsx_begin(void)
+{
+	preempt_disable();
+	enable_kernel_vsx();
+}
+
+static void vsx_end(void)
+{
+	disable_kernel_vsx();
+	preempt_enable();
+}
+
+static void poly1305_block_init(struct poly1305_block_state *dctx,
+				const u8 raw_key[POLY1305_BLOCK_SIZE])
+{
+	if (!static_key_enabled(&have_p10))
+		return poly1305_block_init_generic(dctx, raw_key);
+
+	dctx->h = (struct poly1305_state){};
+	dctx->core_r.key.r64[0] = get_unaligned_le64(raw_key + 0);
+	dctx->core_r.key.r64[1] = get_unaligned_le64(raw_key + 8);
+}
+
+static void poly1305_blocks(struct poly1305_block_state *state, const u8 *src,
+			    unsigned int len, u32 padbit)
+{
+	if (!static_key_enabled(&have_p10))
+		return poly1305_blocks_generic(state, src, len, padbit);
+	vsx_begin();
+	if (len >= POLY1305_BLOCK_SIZE * 4) {
+		poly1305_p10le_4blocks(state, src, len);
+		src += len - (len % (POLY1305_BLOCK_SIZE * 4));
+		len %= POLY1305_BLOCK_SIZE * 4;
+	}
+	while (len >= POLY1305_BLOCK_SIZE) {
+		poly1305_64s(state, src, POLY1305_BLOCK_SIZE, padbit);
+		len -= POLY1305_BLOCK_SIZE;
+		src += POLY1305_BLOCK_SIZE;
+	}
+	vsx_end();
+}
+
+static void poly1305_emit(const struct poly1305_state *state,
+			  u8 digest[POLY1305_DIGEST_SIZE], const u32 nonce[4])
+{
+	if (!static_key_enabled(&have_p10))
+		return poly1305_emit_generic(state, digest, nonce);
+	poly1305_emit_64(state, nonce, digest);
+}
+
+#define poly1305_mod_init_arch poly1305_mod_init_arch
+static void poly1305_mod_init_arch(void)
+{
+	if (cpu_has_feature(CPU_FTR_ARCH_31))
+		static_branch_enable(&have_p10);
+}