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, 100 insertions, 0 deletions

diff --git a/lib/crypto/poly1305.c b/lib/crypto/poly1305.c
new file mode 100644
index 000000000..f313ccc4b
--- /dev/null
+++ b/lib/crypto/poly1305.c
@@ -0,0 +1,100 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Poly1305 authenticator algorithm, RFC7539
+ *
+ * Copyright (C) 2015 Martin Willi
+ *
+ * Based on public domain code by Andrew Moon and Daniel J. Bernstein.
+ */
+
+#include <crypto/internal/poly1305.h>
+#include <linux/export.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/string.h>
+#include <linux/unaligned.h>
+
+#ifdef CONFIG_CRYPTO_LIB_POLY1305_ARCH
+#include "poly1305.h" /* $(SRCARCH)/poly1305.h */
+#else
+#define poly1305_block_init	poly1305_block_init_generic
+#define poly1305_blocks		poly1305_blocks_generic
+#define poly1305_emit		poly1305_emit_generic
+#endif
+
+void poly1305_init(struct poly1305_desc_ctx *desc,
+		   const u8 key[POLY1305_KEY_SIZE])
+{
+	desc->s[0] = get_unaligned_le32(key + 16);
+	desc->s[1] = get_unaligned_le32(key + 20);
+	desc->s[2] = get_unaligned_le32(key + 24);
+	desc->s[3] = get_unaligned_le32(key + 28);
+	desc->buflen = 0;
+	poly1305_block_init(&desc->state, key);
+}
+EXPORT_SYMBOL(poly1305_init);
+
+void poly1305_update(struct poly1305_desc_ctx *desc,
+		     const u8 *src, unsigned int nbytes)
+{
+	if (desc->buflen + nbytes >= POLY1305_BLOCK_SIZE) {
+		unsigned int bulk_len;
+
+		if (desc->buflen) {
+			unsigned int l = POLY1305_BLOCK_SIZE - desc->buflen;
+
+			memcpy(&desc->buf[desc->buflen], src, l);
+			src += l;
+			nbytes -= l;
+
+			poly1305_blocks(&desc->state, desc->buf,
+					POLY1305_BLOCK_SIZE, 1);
+			desc->buflen = 0;
+		}
+
+		bulk_len = round_down(nbytes, POLY1305_BLOCK_SIZE);
+		nbytes %= POLY1305_BLOCK_SIZE;
+
+		if (bulk_len) {
+			poly1305_blocks(&desc->state, src, bulk_len, 1);
+			src += bulk_len;
+		}
+	}
+	if (nbytes) {
+		memcpy(&desc->buf[desc->buflen], src, nbytes);
+		desc->buflen += nbytes;
+	}
+}
+EXPORT_SYMBOL(poly1305_update);
+
+void poly1305_final(struct poly1305_desc_ctx *desc, u8 *dst)
+{
+	if (unlikely(desc->buflen)) {
+		desc->buf[desc->buflen++] = 1;
+		memset(desc->buf + desc->buflen, 0,
+		       POLY1305_BLOCK_SIZE - desc->buflen);
+		poly1305_blocks(&desc->state, desc->buf, POLY1305_BLOCK_SIZE,
+				0);
+	}
+
+	poly1305_emit(&desc->state.h, dst, desc->s);
+	*desc = (struct poly1305_desc_ctx){};
+}
+EXPORT_SYMBOL(poly1305_final);
+
+#ifdef poly1305_mod_init_arch
+static int __init poly1305_mod_init(void)
+{
+	poly1305_mod_init_arch();
+	return 0;
+}
+subsys_initcall(poly1305_mod_init);
+
+static void __exit poly1305_mod_exit(void)
+{
+}
+module_exit(poly1305_mod_exit);
+#endif
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Poly1305 authenticator algorithm, RFC7539");