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, 118 insertions, 0 deletions

diff --git a/lib/crypto/chacha-block-generic.c b/lib/crypto/chacha-block-generic.c
new file mode 100644
index 000000000..4a6d62758
--- /dev/null
+++ b/lib/crypto/chacha-block-generic.c
@@ -0,0 +1,118 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * The "hash function" used as the core of the ChaCha stream cipher (RFC7539)
+ *
+ * Copyright (C) 2015 Martin Willi
+ */
+
+#include <crypto/chacha.h>
+#include <linux/bitops.h>
+#include <linux/bug.h>
+#include <linux/export.h>
+#include <linux/kernel.h>
+#include <linux/string.h>
+#include <linux/unaligned.h>
+
+static void chacha_permute(struct chacha_state *state, int nrounds)
+{
+	u32 *x = state->x;
+	int i;
+
+	/* whitelist the allowed round counts */
+	WARN_ON_ONCE(nrounds != 20 && nrounds != 12);
+
+	for (i = 0; i < nrounds; i += 2) {
+		x[0]  += x[4];    x[12] = rol32(x[12] ^ x[0],  16);
+		x[1]  += x[5];    x[13] = rol32(x[13] ^ x[1],  16);
+		x[2]  += x[6];    x[14] = rol32(x[14] ^ x[2],  16);
+		x[3]  += x[7];    x[15] = rol32(x[15] ^ x[3],  16);
+
+		x[8]  += x[12];   x[4]  = rol32(x[4]  ^ x[8],  12);
+		x[9]  += x[13];   x[5]  = rol32(x[5]  ^ x[9],  12);
+		x[10] += x[14];   x[6]  = rol32(x[6]  ^ x[10], 12);
+		x[11] += x[15];   x[7]  = rol32(x[7]  ^ x[11], 12);
+
+		x[0]  += x[4];    x[12] = rol32(x[12] ^ x[0],   8);
+		x[1]  += x[5];    x[13] = rol32(x[13] ^ x[1],   8);
+		x[2]  += x[6];    x[14] = rol32(x[14] ^ x[2],   8);
+		x[3]  += x[7];    x[15] = rol32(x[15] ^ x[3],   8);
+
+		x[8]  += x[12];   x[4]  = rol32(x[4]  ^ x[8],   7);
+		x[9]  += x[13];   x[5]  = rol32(x[5]  ^ x[9],   7);
+		x[10] += x[14];   x[6]  = rol32(x[6]  ^ x[10],  7);
+		x[11] += x[15];   x[7]  = rol32(x[7]  ^ x[11],  7);
+
+		x[0]  += x[5];    x[15] = rol32(x[15] ^ x[0],  16);
+		x[1]  += x[6];    x[12] = rol32(x[12] ^ x[1],  16);
+		x[2]  += x[7];    x[13] = rol32(x[13] ^ x[2],  16);
+		x[3]  += x[4];    x[14] = rol32(x[14] ^ x[3],  16);
+
+		x[10] += x[15];   x[5]  = rol32(x[5]  ^ x[10], 12);
+		x[11] += x[12];   x[6]  = rol32(x[6]  ^ x[11], 12);
+		x[8]  += x[13];   x[7]  = rol32(x[7]  ^ x[8],  12);
+		x[9]  += x[14];   x[4]  = rol32(x[4]  ^ x[9],  12);
+
+		x[0]  += x[5];    x[15] = rol32(x[15] ^ x[0],   8);
+		x[1]  += x[6];    x[12] = rol32(x[12] ^ x[1],   8);
+		x[2]  += x[7];    x[13] = rol32(x[13] ^ x[2],   8);
+		x[3]  += x[4];    x[14] = rol32(x[14] ^ x[3],   8);
+
+		x[10] += x[15];   x[5]  = rol32(x[5]  ^ x[10],  7);
+		x[11] += x[12];   x[6]  = rol32(x[6]  ^ x[11],  7);
+		x[8]  += x[13];   x[7]  = rol32(x[7]  ^ x[8],   7);
+		x[9]  += x[14];   x[4]  = rol32(x[4]  ^ x[9],   7);
+	}
+}
+
+/**
+ * chacha_block_generic - generate one keystream block and increment block counter
+ * @state: input state matrix
+ * @out: output keystream block
+ * @nrounds: number of rounds (20 or 12; 20 is recommended)
+ *
+ * This is the ChaCha core, a function from 64-byte strings to 64-byte strings.
+ * The caller has already converted the endianness of the input.  This function
+ * also handles incrementing the block counter in the input matrix.
+ */
+void chacha_block_generic(struct chacha_state *state,
+			  u8 out[CHACHA_BLOCK_SIZE], int nrounds)
+{
+	struct chacha_state permuted_state = *state;
+	int i;
+
+	chacha_permute(&permuted_state, nrounds);
+
+	for (i = 0; i < ARRAY_SIZE(state->x); i++)
+		put_unaligned_le32(permuted_state.x[i] + state->x[i],
+				   &out[i * sizeof(u32)]);
+
+	state->x[12]++;
+
+	chacha_zeroize_state(&permuted_state);
+}
+EXPORT_SYMBOL(chacha_block_generic);
+
+/**
+ * hchacha_block_generic - abbreviated ChaCha core, for XChaCha
+ * @state: input state matrix
+ * @out: the output words
+ * @nrounds: number of rounds (20 or 12; 20 is recommended)
+ *
+ * HChaCha is the ChaCha equivalent of HSalsa and is an intermediate step
+ * towards XChaCha (see https://cr.yp.to/snuffle/xsalsa-20081128.pdf).  HChaCha
+ * skips the final addition of the initial state, and outputs only certain words
+ * of the state.  It should not be used for streaming directly.
+ */
+void hchacha_block_generic(const struct chacha_state *state,
+			   u32 out[HCHACHA_OUT_WORDS], int nrounds)
+{
+	struct chacha_state permuted_state = *state;
+
+	chacha_permute(&permuted_state, nrounds);
+
+	memcpy(&out[0], &permuted_state.x[0], 16);
+	memcpy(&out[4], &permuted_state.x[12], 16);
+
+	chacha_zeroize_state(&permuted_state);
+}
+EXPORT_SYMBOL(hchacha_block_generic);