io_uring: fix -ENOENT issue with linked timer with short timeout

If you prep a read (for example) that needs to get punted to async
context with a timer, if the timeout is sufficiently short, the timer
request will get completed with -ENOENT as it could not find the read.

The issue is that we prep and start the timer before we start the read.
Hence the timer can trigger before the read is even started, and the end
result is then that the timer completes with -ENOENT, while the read
starts instead of being cancelled by the timer.

Fix this by splitting the linked timer into two parts:

1) Prep and validate the linked timer
2) Start timer

The read is then started between steps 1 and 2, so we know that the
timer will always have a consistent view of the read request state.
Reported-by: NHrvoje Zeba <zeba.hrvoje@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -855,7 +855,7 @@ static void io_req_link_next(struct io_kiocb *req, struct io_kiocb **nxtptr)
 	 */
 	nxt = list_first_entry_or_null(&req->link_list, struct io_kiocb, list);
 	while (nxt) {
-		list_del(&nxt->list);
+		list_del_init(&nxt->list);
 		if (!list_empty(&req->link_list)) {
 			INIT_LIST_HEAD(&nxt->link_list);
 			list_splice(&req->link_list, &nxt->link_list);
@@ -2688,13 +2688,17 @@ static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer)
 	 */
 	if (!list_empty(&req->list)) {
 		prev = list_entry(req->list.prev, struct io_kiocb, link_list);
-		list_del_init(&req->list);
+		if (refcount_inc_not_zero(&prev->refs))
+			list_del_init(&req->list);
+		else
+			prev = NULL;
 	}
 
 	spin_unlock_irqrestore(&ctx->completion_lock, flags);
 
 	if (prev) {
 		io_async_find_and_cancel(ctx, req, prev->user_data, NULL);
+		io_put_req(prev);
 	} else {
 		io_cqring_add_event(req, -ETIME);
 		io_put_req(req);
@@ -2702,78 +2706,84 @@ static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer)
 	return HRTIMER_NORESTART;
 }
 
-static int io_queue_linked_timeout(struct io_kiocb *req, struct io_kiocb *nxt)
+static void io_queue_linked_timeout(struct io_kiocb *req, struct timespec64 *ts,
+				    enum hrtimer_mode *mode)
 {
-	const struct io_uring_sqe *sqe = nxt->submit.sqe;
-	enum hrtimer_mode mode;
-	struct timespec64 ts;
-	int ret = -EINVAL;
+	struct io_ring_ctx *ctx = req->ctx;
 
-	if (sqe->ioprio || sqe->buf_index || sqe->len != 1 || sqe->off)
-		goto err;
-	if (sqe->timeout_flags & ~IORING_TIMEOUT_ABS)
-		goto err;
-	if (get_timespec64(&ts, u64_to_user_ptr(sqe->addr))) {
-		ret = -EFAULT;
-		goto err;
+	/*
+	 * If the list is now empty, then our linked request finished before
+	 * we got a chance to setup the timer
+	 */
+	spin_lock_irq(&ctx->completion_lock);
+	if (!list_empty(&req->list)) {
+		req->timeout.timer.function = io_link_timeout_fn;
+		hrtimer_start(&req->timeout.timer, timespec64_to_ktime(*ts),
+				*mode);
 	}
+	spin_unlock_irq(&ctx->completion_lock);
 
-	req->flags |= REQ_F_LINK_TIMEOUT;
-
-	if (sqe->timeout_flags & IORING_TIMEOUT_ABS)
-		mode = HRTIMER_MODE_ABS;
-	else
-		mode = HRTIMER_MODE_REL;
-	hrtimer_init(&nxt->timeout.timer, CLOCK_MONOTONIC, mode);
-	nxt->timeout.timer.function = io_link_timeout_fn;
-	hrtimer_start(&nxt->timeout.timer, timespec64_to_ktime(ts), mode);
-	ret = 0;
-err:
 	/* drop submission reference */
-	io_put_req(nxt);
-
-	if (ret) {
-		struct io_ring_ctx *ctx = req->ctx;
+	io_put_req(req);
+}
 
-		/*
-		 * Break the link and fail linked timeout, parent will get
-		 * failed by the regular submission path.
-		 */
-		list_del(&nxt->list);
-		io_cqring_fill_event(nxt, ret);
-		trace_io_uring_fail_link(req, nxt);
-		io_commit_cqring(ctx);
-		io_put_req(nxt);
-		ret = -ECANCELED;
-	}
+static int io_validate_link_timeout(const struct io_uring_sqe *sqe,
+				    struct timespec64 *ts)
+{
+	if (sqe->ioprio || sqe->buf_index || sqe->len != 1 || sqe->off)
+		return -EINVAL;
+	if (sqe->timeout_flags & ~IORING_TIMEOUT_ABS)
+		return -EINVAL;
+	if (get_timespec64(ts, u64_to_user_ptr(sqe->addr)))
+		return -EFAULT;
 
-	return ret;
+	return 0;
 }
 
-static inline struct io_kiocb *io_get_linked_timeout(struct io_kiocb *req)
+static struct io_kiocb *io_prep_linked_timeout(struct io_kiocb *req,
+					       struct timespec64 *ts,
+					       enum hrtimer_mode *mode)
 {
 	struct io_kiocb *nxt;
+	int ret;
 
 	if (!(req->flags & REQ_F_LINK))
 		return NULL;
 
 	nxt = list_first_entry_or_null(&req->link_list, struct io_kiocb, list);
-	if (nxt && nxt->submit.sqe->opcode == IORING_OP_LINK_TIMEOUT)
-		return nxt;
+	if (!nxt || nxt->submit.sqe->opcode != IORING_OP_LINK_TIMEOUT)
+		return NULL;
 
-	return NULL;
+	ret = io_validate_link_timeout(nxt->submit.sqe, ts);
+	if (ret) {
+		list_del_init(&nxt->list);
+		io_cqring_add_event(nxt, ret);
+		io_double_put_req(nxt);
+		return ERR_PTR(-ECANCELED);
+	}
+
+	if (nxt->submit.sqe->timeout_flags & IORING_TIMEOUT_ABS)
+		*mode = HRTIMER_MODE_ABS;
+	else
+		*mode = HRTIMER_MODE_REL;
+
+	req->flags |= REQ_F_LINK_TIMEOUT;
+	hrtimer_init(&nxt->timeout.timer, CLOCK_MONOTONIC, *mode);
+	return nxt;
 }
 
 static int __io_queue_sqe(struct io_kiocb *req)
 {
+	enum hrtimer_mode mode;
 	struct io_kiocb *nxt;
+	struct timespec64 ts;
 	int ret;
 
-	nxt = io_get_linked_timeout(req);
-	if (unlikely(nxt)) {
-		ret = io_queue_linked_timeout(req, nxt);
-		if (ret)
-			goto err;
+	nxt = io_prep_linked_timeout(req, &ts, &mode);
+	if (IS_ERR(nxt)) {
+		ret = PTR_ERR(nxt);
+		nxt = NULL;
+		goto err;
 	}
 
 	ret = __io_submit_sqe(req, NULL, true);
@@ -2803,14 +2813,25 @@ static int __io_queue_sqe(struct io_kiocb *req)
 			 * submit reference when the iocb is actually submitted.
 			 */
 			io_queue_async_work(req);
+
+			if (nxt)
+				io_queue_linked_timeout(nxt, &ts, &mode);
+
 			return 0;
 		}
 	}
 
-	/* drop submission reference */
 err:
+	/* drop submission reference */
 	io_put_req(req);
 
+	if (nxt) {
+		if (!ret)
+			io_queue_linked_timeout(nxt, &ts, &mode);
+		else
+			io_put_req(nxt);
+	}
+
 	/* and drop final reference, if we failed */
 	if (ret) {
 		io_cqring_add_event(req, ret);