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/lib/crypto/arm/blake2b.h b/lib/crypto/arm/blake2b.h
new file mode 100644
index 000000000..5c7649852
--- /dev/null
+++ b/lib/crypto/arm/blake2b.h
@@ -0,0 +1,40 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * BLAKE2b digest algorithm, NEON accelerated
+ *
+ * Copyright 2020 Google LLC
+ */
+
+#include <asm/neon.h>
+#include <asm/simd.h>
+
+static __ro_after_init DEFINE_STATIC_KEY_FALSE(have_neon);
+
+asmlinkage void blake2b_compress_neon(struct blake2b_ctx *ctx,
+				      const u8 *data, size_t nblocks, u32 inc);
+
+static void blake2b_compress(struct blake2b_ctx *ctx,
+			     const u8 *data, size_t nblocks, u32 inc)
+{
+	if (!static_branch_likely(&have_neon) || !may_use_simd()) {
+		blake2b_compress_generic(ctx, data, nblocks, inc);
+		return;
+	}
+	do {
+		const size_t blocks = min_t(size_t, nblocks,
+					    SZ_4K / BLAKE2B_BLOCK_SIZE);
+
+		scoped_ksimd()
+			blake2b_compress_neon(ctx, data, blocks, inc);
+
+		data += blocks * BLAKE2B_BLOCK_SIZE;
+		nblocks -= blocks;
+	} while (nblocks);
+}
+
+#define blake2b_mod_init_arch blake2b_mod_init_arch
+static void blake2b_mod_init_arch(void)
+{
+	if (elf_hwcap & HWCAP_NEON)
+		static_branch_enable(&have_neon);
+}