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, 117 insertions, 0 deletions

diff --git a/io_uring/tw.h b/io_uring/tw.h
new file mode 100644
index 000000000..3ade5ad57
--- /dev/null
+++ b/io_uring/tw.h
@@ -0,0 +1,117 @@
+// SPDX-License-Identifier: GPL-2.0
+#ifndef IOU_TW_H
+#define IOU_TW_H
+
+#include <linux/sched.h>
+#include <linux/percpu-refcount.h>
+#include <linux/io_uring_types.h>
+
+#include "mpscq.h"
+
+#define IO_LOCAL_TW_DEFAULT_MAX		20
+
+/*
+ * Terminate the request if either of these conditions are true:
+ *
+ * 1) It's being executed by the original task, but that task is marked
+ *    with PF_EXITING as it's exiting.
+ * 2) PF_KTHREAD is set, in which case the invoker of the task_work is
+ *    our fallback task_work.
+ * 3) The ring has been closed and is going away.
+ */
+static inline bool io_should_terminate_tw(struct io_ring_ctx *ctx)
+{
+	return (current->flags & (PF_EXITING | PF_KTHREAD)) || percpu_ref_is_dying(&ctx->refs);
+}
+
+void io_req_task_work_add_remote(struct io_kiocb *req, unsigned flags);
+void tctx_task_work(struct callback_head *cb);
+void io_tctx_fallback_work(struct work_struct *work);
+int io_run_local_work(struct io_ring_ctx *ctx, int min_events, int max_events);
+int io_run_task_work_sig(struct io_ring_ctx *ctx);
+
+__cold void io_cancel_local_task_work(struct io_ring_ctx *ctx);
+int io_run_local_work_locked(struct io_ring_ctx *ctx, int min_events);
+
+void io_req_local_work_add(struct io_kiocb *req, unsigned flags);
+void io_req_normal_work_add(struct io_kiocb *req);
+void tctx_task_work_run(struct io_uring_task *tctx, unsigned int max_entries, unsigned int *count);
+
+static inline void __io_req_task_work_add(struct io_kiocb *req, unsigned flags)
+{
+	if (req->ctx->flags & IORING_SETUP_DEFER_TASKRUN)
+		io_req_local_work_add(req, flags);
+	else
+		io_req_normal_work_add(req);
+}
+
+static inline void io_req_task_work_add(struct io_kiocb *req)
+{
+	__io_req_task_work_add(req, 0);
+}
+
+static inline int io_run_task_work(void)
+{
+	bool ret = false;
+
+	/*
+	 * Always check-and-clear the task_work notification signal. With how
+	 * signaling works for task_work, we can find it set with nothing to
+	 * run. We need to clear it for that case, like get_signal() does.
+	 */
+	if (test_thread_flag(TIF_NOTIFY_SIGNAL))
+		clear_notify_signal();
+	/*
+	 * PF_IO_WORKER never returns to userspace, so check here if we have
+	 * notify work that needs processing.
+	 */
+	if (current->flags & PF_IO_WORKER) {
+		if (test_thread_flag(TIF_NOTIFY_RESUME)) {
+			__set_current_state(TASK_RUNNING);
+			resume_user_mode_work(NULL);
+		}
+		if (current->io_uring) {
+			unsigned int count = 0;
+
+			__set_current_state(TASK_RUNNING);
+			tctx_task_work_run(current->io_uring, UINT_MAX, &count);
+			if (count)
+				ret = true;
+		}
+	}
+	if (task_work_pending(current)) {
+		__set_current_state(TASK_RUNNING);
+		task_work_run();
+		ret = true;
+	}
+
+	return ret;
+}
+
+static inline bool io_local_work_pending(struct io_ring_ctx *ctx)
+{
+	return !mpscq_empty(&ctx->work_list);
+}
+
+static inline bool io_task_work_pending(struct io_ring_ctx *ctx)
+{
+	return task_work_pending(current) || io_local_work_pending(ctx);
+}
+
+static inline void io_tw_lock(struct io_ring_ctx *ctx, io_tw_token_t tw)
+{
+	lockdep_assert_held(&ctx->uring_lock);
+}
+
+static inline bool io_allowed_defer_tw_run(struct io_ring_ctx *ctx)
+{
+	return likely(ctx->submitter_task == current);
+}
+
+static inline bool io_allowed_run_tw(struct io_ring_ctx *ctx)
+{
+	return likely(!(ctx->flags & IORING_SETUP_DEFER_TASKRUN) ||
+		      ctx->submitter_task == current);
+}
+
+#endif