io_uring: Fix ->data corruption on re-enqueue

work->data and work->list are shared in union. io_wq_assign_next() sets
->data if a req having a linked_timeout, but then io-wq may want to use
work->list, e.g. to do re-enqueue of a request, so corrupting ->data.

->data is not necessary, just remove it and extract linked_timeout
through @link_list.

Fixes: 60cf46ae ("io-wq: hash dependent work")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.h

@@ -63,10 +63,7 @@ static inline void wq_node_del(struct io_wq_work_list *list,
 } while (0)
 
 struct io_wq_work {
-	union {
-		struct io_wq_work_node list;
-		void *data;
-	};
+	struct io_wq_work_node list;
 	void (*func)(struct io_wq_work **);
 	struct files_struct *files;
 	struct mm_struct *mm;

fs/io_uring.c

@@ -1567,9 +1567,10 @@ static void io_free_req(struct io_kiocb *req)
 
 static void io_link_work_cb(struct io_wq_work **workptr)
 {
-	struct io_wq_work *work = *workptr;
-	struct io_kiocb *link = work->data;
+	struct io_kiocb *req = container_of(*workptr, struct io_kiocb, work);
+	struct io_kiocb *link;
 
+	link = list_first_entry(&req->link_list, struct io_kiocb, link_list);
 	io_queue_linked_timeout(link);
 	io_wq_submit_work(workptr);
 }
@@ -1584,10 +1585,8 @@ static void io_wq_assign_next(struct io_wq_work **workptr, struct io_kiocb *nxt)
 
 	*workptr = &nxt->work;
 	link = io_prep_linked_timeout(nxt);
-	if (link) {
+	if (link)
 		nxt->work.func = io_link_work_cb;
-		nxt->work.data = link;
-	}
 }
 
 /*