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, 63 insertions, 0 deletions

diff --git a/io_uring/refs.h b/io_uring/refs.h
new file mode 100644
index 000000000..0fe16b67c
--- /dev/null
+++ b/io_uring/refs.h
@@ -0,0 +1,63 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef IOU_REQ_REF_H
+#define IOU_REQ_REF_H
+
+#include <linux/atomic.h>
+#include <linux/io_uring_types.h>
+
+/*
+ * Shamelessly stolen from the mm implementation of page reference checking,
+ * see commit f958d7b528b1 for details.
+ */
+#define req_ref_zero_or_close_to_overflow(req)	\
+	((unsigned int) atomic_read(&(req->refs)) + 127u <= 127u)
+
+static inline bool req_ref_inc_not_zero(struct io_kiocb *req)
+{
+	WARN_ON_ONCE(!(req->flags & REQ_F_REFCOUNT));
+	return atomic_inc_not_zero(&req->refs);
+}
+
+static inline bool req_ref_put_and_test_atomic(struct io_kiocb *req)
+{
+	WARN_ON_ONCE(!(data_race(req->flags) & REQ_F_REFCOUNT));
+	WARN_ON_ONCE(req_ref_zero_or_close_to_overflow(req));
+	return atomic_dec_and_test(&req->refs);
+}
+
+static inline bool req_ref_put_and_test(struct io_kiocb *req)
+{
+	if (likely(!(req->flags & REQ_F_REFCOUNT)))
+		return true;
+
+	WARN_ON_ONCE(req_ref_zero_or_close_to_overflow(req));
+	return atomic_dec_and_test(&req->refs);
+}
+
+static inline void req_ref_get(struct io_kiocb *req)
+{
+	WARN_ON_ONCE(!(req->flags & REQ_F_REFCOUNT));
+	WARN_ON_ONCE(req_ref_zero_or_close_to_overflow(req));
+	atomic_inc(&req->refs);
+}
+
+static inline void req_ref_put(struct io_kiocb *req)
+{
+	WARN_ON_ONCE(!(req->flags & REQ_F_REFCOUNT));
+	WARN_ON_ONCE(req_ref_zero_or_close_to_overflow(req));
+	atomic_dec(&req->refs);
+}
+
+static inline void __io_req_set_refcount(struct io_kiocb *req, int nr)
+{
+	if (!(req->flags & REQ_F_REFCOUNT)) {
+		req->flags |= REQ_F_REFCOUNT;
+		atomic_set(&req->refs, nr);
+	}
+}
+
+static inline void io_req_set_refcount(struct io_kiocb *req)
+{
+	__io_req_set_refcount(req, 1);
+}
+#endif