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, 45 insertions, 0 deletions

diff --git a/lib/crypto/arm/sha1.h b/lib/crypto/arm/sha1.h
new file mode 100644
index 000000000..3e2d8c7ca
--- /dev/null
+++ b/lib/crypto/arm/sha1.h
@@ -0,0 +1,45 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * SHA-1 optimized for ARM
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <asm/simd.h>
+
+static __ro_after_init DEFINE_STATIC_KEY_FALSE(have_neon);
+static __ro_after_init DEFINE_STATIC_KEY_FALSE(have_ce);
+
+asmlinkage void sha1_block_data_order(struct sha1_block_state *state,
+				      const u8 *data, size_t nblocks);
+asmlinkage void sha1_transform_neon(struct sha1_block_state *state,
+				    const u8 *data, size_t nblocks);
+asmlinkage void sha1_ce_transform(struct sha1_block_state *state,
+				  const u8 *data, size_t nblocks);
+
+static void sha1_blocks(struct sha1_block_state *state,
+			const u8 *data, size_t nblocks)
+{
+	if (IS_ENABLED(CONFIG_KERNEL_MODE_NEON) &&
+	    static_branch_likely(&have_neon) && likely(may_use_simd())) {
+		scoped_ksimd() {
+			if (static_branch_likely(&have_ce))
+				sha1_ce_transform(state, data, nblocks);
+			else
+				sha1_transform_neon(state, data, nblocks);
+		}
+	} else {
+		sha1_block_data_order(state, data, nblocks);
+	}
+}
+
+#ifdef CONFIG_KERNEL_MODE_NEON
+#define sha1_mod_init_arch sha1_mod_init_arch
+static void sha1_mod_init_arch(void)
+{
+	if (elf_hwcap & HWCAP_NEON) {
+		static_branch_enable(&have_neon);
+		if (elf_hwcap2 & HWCAP2_SHA1)
+			static_branch_enable(&have_ce);
+	}
+}
+#endif /* CONFIG_KERNEL_MODE_NEON */