selftests: ublk: add kernel selftests for ublk

Both ublk driver and userspace heavily depends on io_uring subsystem,
and tools/testing/selftests/ should be the best place for holding this
cross-subsystem tests.

Add basic read/write IO test over this ublk null disk, and make sure ublk
working.

More tests will be added.

Signed-off-by: Ming Lei <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

MAINTAINERS

···
 F:	Documentation/block/ublk.rst
 F:	drivers/block/ublk_drv.c
 F:	include/uapi/linux/ublk_cmd.h
+F:	tools/testing/selftests/ublk/
 
 UBSAN
 M:	Kees Cook <[e-mail]>

tools/testing/selftests/Makefile

···
 TARGETS += tmpfs
 TARGETS += tpm2
 TARGETS += tty
+TARGETS += ublk
 TARGETS += uevent
 TARGETS += user_events
 TARGETS += vDSO

tools/testing/selftests/ublk/.gitignore

···
+kublk
+/tools
+*-verify.state

tools/testing/selftests/ublk/Makefile

···
+# SPDX-License-Identifier: GPL-2.0
+
+CFLAGS += -O3 -Wl,-no-as-needed -Wall -I $(top_srcdir)
+LDLIBS += -lpthread -lm -luring
+
+TEST_PROGS := test_null_01.sh
+
+TEST_GEN_PROGS_EXTENDED = kublk
+
+include ../lib.mk
+
+$(TEST_GEN_PROGS_EXTENDED): kublk.c null.c

tools/testing/selftests/ublk/config

···
+CONFIG_BLK_DEV_UBLK=m

tools/testing/selftests/ublk/kublk.c

···
+/* SPDX-License-Identifier: MIT */
+/*
+ * Description: uring_cmd based ublk
+ */
+
+#include "kublk.h"
+
+unsigned int ublk_dbg_mask = UBLK_LOG;
+static const struct ublk_tgt_ops *tgt_ops_list[] = {
+	&null_tgt_ops,
+};
+
+static const struct ublk_tgt_ops *ublk_find_tgt(const char *name)
+{
+	const struct ublk_tgt_ops *ops;
+	int i;
+
+	if (name == NULL)
+		return NULL;
+
+	for (i = 0; sizeof(tgt_ops_list) / sizeof(ops); i++)
+		if (strcmp(tgt_ops_list[i]->name, name) == 0)
+			return tgt_ops_list[i];
+	return NULL;
+}
+
+static inline int ublk_setup_ring(struct io_uring *r, int depth,
+		int cq_depth, unsigned flags)
+{
+	struct io_uring_params p;
+
+	memset(&p, 0, sizeof(p));
+	p.flags = flags | IORING_SETUP_CQSIZE;
+	p.cq_entries = cq_depth;
+
+	return io_uring_queue_init_params(depth, r, &p);
+}
+
+static void ublk_ctrl_init_cmd(struct ublk_dev *dev,
+		struct io_uring_sqe *sqe,
+		struct ublk_ctrl_cmd_data *data)
+{
+	struct ublksrv_ctrl_dev_info *info = &dev->dev_info;
+	struct ublksrv_ctrl_cmd *cmd = (struct ublksrv_ctrl_cmd *)ublk_get_sqe_cmd(sqe);
+
+	sqe->fd = dev->ctrl_fd;
+	sqe->opcode = IORING_OP_URING_CMD;
+	sqe->ioprio = 0;
+
+	if (data->flags & CTRL_CMD_HAS_BUF) {
+		cmd->addr = data->addr;
+		cmd->len = data->len;
+	}
+
+	if (data->flags & CTRL_CMD_HAS_DATA)
+		cmd->data[0] = data->data[0];
+
+	cmd->dev_id = info->dev_id;
+	cmd->queue_id = -1;
+
+	ublk_set_sqe_cmd_op(sqe, data->cmd_op);
+
+	io_uring_sqe_set_data(sqe, cmd);
+}
+
+static int __ublk_ctrl_cmd(struct ublk_dev *dev,
+		struct ublk_ctrl_cmd_data *data)
+{
+	struct io_uring_sqe *sqe;
+	struct io_uring_cqe *cqe;
+	int ret = -EINVAL;
+
+	sqe = io_uring_get_sqe(&dev->ring);
+	if (!sqe) {
+		ublk_err("%s: can't get sqe ret %d\n", __func__, ret);
+		return ret;
+	}
+
+	ublk_ctrl_init_cmd(dev, sqe, data);
+
+	ret = io_uring_submit(&dev->ring);
+	if (ret < 0) {
+		ublk_err("uring submit ret %d\n", ret);
+		return ret;
+	}
+
+	ret = io_uring_wait_cqe(&dev->ring, &cqe);
+	if (ret < 0) {
+		ublk_err("wait cqe: %s\n", strerror(-ret));
+		return ret;
+	}
+	io_uring_cqe_seen(&dev->ring, cqe);
+
+	return cqe->res;
+}
+
+static int ublk_ctrl_stop_dev(struct ublk_dev *dev)
+{
+	struct ublk_ctrl_cmd_data data = {
+		.cmd_op	= UBLK_CMD_STOP_DEV,
+	};
+
+	return __ublk_ctrl_cmd(dev, &data);
+}
+
+static int ublk_ctrl_start_dev(struct ublk_dev *dev,
+		int daemon_pid)
+{
+	struct ublk_ctrl_cmd_data data = {
+		.cmd_op	= UBLK_U_CMD_START_DEV,
+		.flags	= CTRL_CMD_HAS_DATA,
+	};
+
+	dev->dev_info.ublksrv_pid = data.data[0] = daemon_pid;
+
+	return __ublk_ctrl_cmd(dev, &data);
+}
+
+static int ublk_ctrl_add_dev(struct ublk_dev *dev)
+{
+	struct ublk_ctrl_cmd_data data = {
+		.cmd_op	= UBLK_U_CMD_ADD_DEV,
+		.flags	= CTRL_CMD_HAS_BUF,
+		.addr = (__u64) (uintptr_t) &dev->dev_info,
+		.len = sizeof(struct ublksrv_ctrl_dev_info),
+	};
+
+	return __ublk_ctrl_cmd(dev, &data);
+}
+
+static int ublk_ctrl_del_dev(struct ublk_dev *dev)
+{
+	struct ublk_ctrl_cmd_data data = {
+		.cmd_op = UBLK_U_CMD_DEL_DEV,
+		.flags = 0,
+	};
+
+	return __ublk_ctrl_cmd(dev, &data);
+}
+
+static int ublk_ctrl_get_info(struct ublk_dev *dev)
+{
+	struct ublk_ctrl_cmd_data data = {
+		.cmd_op	= UBLK_U_CMD_GET_DEV_INFO,
+		.flags	= CTRL_CMD_HAS_BUF,
+		.addr = (__u64) (uintptr_t) &dev->dev_info,
+		.len = sizeof(struct ublksrv_ctrl_dev_info),
+	};
+
+	return __ublk_ctrl_cmd(dev, &data);
+}
+
+static int ublk_ctrl_set_params(struct ublk_dev *dev,
+		struct ublk_params *params)
+{
+	struct ublk_ctrl_cmd_data data = {
+		.cmd_op	= UBLK_U_CMD_SET_PARAMS,
+		.flags	= CTRL_CMD_HAS_BUF,
+		.addr = (__u64) (uintptr_t) params,
+		.len = sizeof(*params),
+	};
+	params->len = sizeof(*params);
+	return __ublk_ctrl_cmd(dev, &data);
+}
+
+static int ublk_ctrl_get_params(struct ublk_dev *dev,
+		struct ublk_params *params)
+{
+	struct ublk_ctrl_cmd_data data = {
+		.cmd_op	= UBLK_CMD_GET_PARAMS,
+		.flags	= CTRL_CMD_HAS_BUF,
+		.addr = (__u64)params,
+		.len = sizeof(*params),
+	};
+
+	params->len = sizeof(*params);
+
+	return __ublk_ctrl_cmd(dev, &data);
+}
+
+static int ublk_ctrl_get_features(struct ublk_dev *dev,
+		__u64 *features)
+{
+	struct ublk_ctrl_cmd_data data = {
+		.cmd_op	= UBLK_U_CMD_GET_FEATURES,
+		.flags	= CTRL_CMD_HAS_BUF,
+		.addr = (__u64) (uintptr_t) features,
+		.len = sizeof(*features),
+	};
+
+	return __ublk_ctrl_cmd(dev, &data);
+}
+
+static const char *ublk_dev_state_desc(struct ublk_dev *dev)
+{
+	switch (dev->dev_info.state) {
+	case UBLK_S_DEV_DEAD:
+		return "DEAD";
+	case UBLK_S_DEV_LIVE:
+		return "LIVE";
+	case UBLK_S_DEV_QUIESCED:
+		return "QUIESCED";
+	default:
+		return "UNKNOWN";
+	};
+}
+
+static void ublk_ctrl_dump(struct ublk_dev *dev)
+{
+	struct ublksrv_ctrl_dev_info *info = &dev->dev_info;
+	struct ublk_params p;
+	int ret;
+
+	ret = ublk_ctrl_get_params(dev, &p);
+	if (ret < 0) {
+		ublk_err("failed to get params %m\n");
+		return;
+	}
+
+	ublk_log("dev id %d: nr_hw_queues %d queue_depth %d block size %d dev_capacity %lld\n",
+			info->dev_id, info->nr_hw_queues, info->queue_depth,
+			1 << p.basic.logical_bs_shift, p.basic.dev_sectors);
+	ublk_log("\tmax rq size %d daemon pid %d flags 0x%llx state %s\n",
+			info->max_io_buf_bytes, info->ublksrv_pid, info->flags,
+			ublk_dev_state_desc(dev));
+	fflush(stdout);
+}
+
+static void ublk_ctrl_deinit(struct ublk_dev *dev)
+{
+	close(dev->ctrl_fd);
+	free(dev);
+}
+
+static struct ublk_dev *ublk_ctrl_init(void)
+{
+	struct ublk_dev *dev = (struct ublk_dev *)calloc(1, sizeof(*dev));
+	struct ublksrv_ctrl_dev_info *info = &dev->dev_info;
+	int ret;
+
+	dev->ctrl_fd = open(CTRL_DEV, O_RDWR);
+	if (dev->ctrl_fd < 0) {
+		free(dev);
+		return NULL;
+	}
+
+	info->max_io_buf_bytes = UBLK_IO_MAX_BYTES;
+
+	ret = ublk_setup_ring(&dev->ring, UBLK_CTRL_RING_DEPTH,
+			UBLK_CTRL_RING_DEPTH, IORING_SETUP_SQE128);
+	if (ret < 0) {
+		ublk_err("queue_init: %s\n", strerror(-ret));
+		free(dev);
+		return NULL;
+	}
+	dev->nr_fds = 1;
+
+	return dev;
+}
+
+static int __ublk_queue_cmd_buf_sz(unsigned depth)
+{
+	int size =  depth * sizeof(struct ublksrv_io_desc);
+	unsigned int page_sz = getpagesize();
+
+	return round_up(size, page_sz);
+}
+
+static int ublk_queue_max_cmd_buf_sz(void)
+{
+	return __ublk_queue_cmd_buf_sz(UBLK_MAX_QUEUE_DEPTH);
+}
+
+static int ublk_queue_cmd_buf_sz(struct ublk_queue *q)
+{
+	return __ublk_queue_cmd_buf_sz(q->q_depth);
+}
+
+static void ublk_queue_deinit(struct ublk_queue *q)
+{
+	int i;
+	int nr_ios = q->q_depth;
+
+	io_uring_unregister_ring_fd(&q->ring);
+
+	if (q->ring.ring_fd > 0) {
+		io_uring_unregister_files(&q->ring);
+		close(q->ring.ring_fd);
+		q->ring.ring_fd = -1;
+	}
+
+	if (q->io_cmd_buf)
+		munmap(q->io_cmd_buf, ublk_queue_cmd_buf_sz(q));
+
+	for (i = 0; i < nr_ios; i++)
+		free(q->ios[i].buf_addr);
+}
+
+static int ublk_queue_init(struct ublk_queue *q)
+{
+	struct ublk_dev *dev = q->dev;
+	int depth = dev->dev_info.queue_depth;
+	int i, ret = -1;
+	int cmd_buf_size, io_buf_size;
+	unsigned long off;
+	int ring_depth = dev->tgt.sq_depth, cq_depth = dev->tgt.cq_depth;
+
+	q->tgt_ops = dev->tgt.ops;
+	q->state = 0;
+	q->q_depth = depth;
+	q->cmd_inflight = 0;
+	q->tid = gettid();
+
+	cmd_buf_size = ublk_queue_cmd_buf_sz(q);
+	off = UBLKSRV_CMD_BUF_OFFSET + q->q_id * ublk_queue_max_cmd_buf_sz();
+	q->io_cmd_buf = (char *)mmap(0, cmd_buf_size, PROT_READ,
+			MAP_SHARED | MAP_POPULATE, dev->fds[0], off);
+	if (q->io_cmd_buf == MAP_FAILED) {
+		ublk_err("ublk dev %d queue %d map io_cmd_buf failed %m\n",
+				q->dev->dev_info.dev_id, q->q_id);
+		goto fail;
+	}
+
+	io_buf_size = dev->dev_info.max_io_buf_bytes;
+	for (i = 0; i < q->q_depth; i++) {
+		q->ios[i].buf_addr = NULL;
+		q->ios[i].flags = UBLKSRV_NEED_FETCH_RQ | UBLKSRV_IO_FREE;
+
+		if (q->state & UBLKSRV_NO_BUF)
+			continue;
+
+		if (posix_memalign((void **)&q->ios[i].buf_addr,
+					getpagesize(), io_buf_size)) {
+			ublk_err("ublk dev %d queue %d io %d posix_memalign failed %m\n",
+					dev->dev_info.dev_id, q->q_id, i);
+			goto fail;
+		}
+	}
+
+	ret = ublk_setup_ring(&q->ring, ring_depth, cq_depth,
+			IORING_SETUP_COOP_TASKRUN);
+	if (ret < 0) {
+		ublk_err("ublk dev %d queue %d setup io_uring failed %d\n",
+				q->dev->dev_info.dev_id, q->q_id, ret);
+		goto fail;
+	}
+
+	io_uring_register_ring_fd(&q->ring);
+
+	ret = io_uring_register_files(&q->ring, dev->fds, dev->nr_fds);
+	if (ret) {
+		ublk_err("ublk dev %d queue %d register files failed %d\n",
+				q->dev->dev_info.dev_id, q->q_id, ret);
+		goto fail;
+	}
+
+	return 0;
+ fail:
+	ublk_queue_deinit(q);
+	ublk_err("ublk dev %d queue %d failed\n",
+			dev->dev_info.dev_id, q->q_id);
+	return -ENOMEM;
+}
+
+static int ublk_dev_prep(struct ublk_dev *dev)
+{
+	int dev_id = dev->dev_info.dev_id;
+	char buf[64];
+	int ret = 0;
+
+	snprintf(buf, 64, "%s%d", UBLKC_DEV, dev_id);
+	dev->fds[0] = open(buf, O_RDWR);
+	if (dev->fds[0] < 0) {
+		ret = -EBADF;
+		ublk_err("can't open %s, ret %d\n", buf, dev->fds[0]);
+		goto fail;
+	}
+
+	if (dev->tgt.ops->init_tgt)
+		ret = dev->tgt.ops->init_tgt(dev);
+
+	return ret;
+fail:
+	close(dev->fds[0]);
+	return ret;
+}
+
+static void ublk_dev_unprep(struct ublk_dev *dev)
+{
+	if (dev->tgt.ops->deinit_tgt)
+		dev->tgt.ops->deinit_tgt(dev);
+	close(dev->fds[0]);
+}
+
+int ublk_queue_io_cmd(struct ublk_queue *q, struct ublk_io *io, unsigned tag)
+{
+	struct ublksrv_io_cmd *cmd;
+	struct io_uring_sqe *sqe;
+	unsigned int cmd_op = 0;
+	__u64 user_data;
+
+	/* only freed io can be issued */
+	if (!(io->flags & UBLKSRV_IO_FREE))
+		return 0;
+
+	/* we issue because we need either fetching or committing */
+	if (!(io->flags &
+		(UBLKSRV_NEED_FETCH_RQ | UBLKSRV_NEED_COMMIT_RQ_COMP)))
+		return 0;
+
+	if (io->flags & UBLKSRV_NEED_COMMIT_RQ_COMP)
+		cmd_op = UBLK_U_IO_COMMIT_AND_FETCH_REQ;
+	else if (io->flags & UBLKSRV_NEED_FETCH_RQ)
+		cmd_op = UBLK_U_IO_FETCH_REQ;
+
+	if (io_uring_sq_space_left(&q->ring) < 1)
+		io_uring_submit(&q->ring);
+
+	sqe = ublk_queue_alloc_sqe(q);
+	if (!sqe) {
+		ublk_err("%s: run out of sqe %d, tag %d\n",
+				__func__, q->q_id, tag);
+		return -1;
+	}
+
+	cmd = (struct ublksrv_io_cmd *)ublk_get_sqe_cmd(sqe);
+
+	if (cmd_op == UBLK_U_IO_COMMIT_AND_FETCH_REQ)
+		cmd->result = io->result;
+
+	/* These fields should be written once, never change */
+	ublk_set_sqe_cmd_op(sqe, cmd_op);
+	sqe->fd		= 0;	/* dev->fds[0] */
+	sqe->opcode	= IORING_OP_URING_CMD;
+	sqe->flags	= IOSQE_FIXED_FILE;
+	sqe->rw_flags	= 0;
+	cmd->tag	= tag;
+	cmd->q_id	= q->q_id;
+	if (!(q->state & UBLKSRV_NO_BUF))
+		cmd->addr	= (__u64) (uintptr_t) io->buf_addr;
+	else
+		cmd->addr	= 0;
+
+	user_data = build_user_data(tag, _IOC_NR(cmd_op), 0, 0);
+	io_uring_sqe_set_data64(sqe, user_data);
+
+	io->flags = 0;
+
+	q->cmd_inflight += 1;
+
+	ublk_dbg(UBLK_DBG_IO_CMD, "%s: (qid %d tag %u cmd_op %u) iof %x stopping %d\n",
+			__func__, q->q_id, tag, cmd_op,
+			io->flags, !!(q->state & UBLKSRV_QUEUE_STOPPING));
+	return 1;
+}
+
+static void ublk_submit_fetch_commands(struct ublk_queue *q)
+{
+	int i = 0;
+
+	for (i = 0; i < q->q_depth; i++)
+		ublk_queue_io_cmd(q, &q->ios[i], i);
+}
+
+static int ublk_queue_is_idle(struct ublk_queue *q)
+{
+	return !io_uring_sq_ready(&q->ring) && !q->io_inflight;
+}
+
+static int ublk_queue_is_done(struct ublk_queue *q)
+{
+	return (q->state & UBLKSRV_QUEUE_STOPPING) && ublk_queue_is_idle(q);
+}
+
+static inline void ublksrv_handle_tgt_cqe(struct ublk_queue *q,
+		struct io_uring_cqe *cqe)
+{
+	unsigned tag = user_data_to_tag(cqe->user_data);
+
+	if (cqe->res < 0 && cqe->res != -EAGAIN)
+		ublk_err("%s: failed tgt io: res %d qid %u tag %u, cmd_op %u\n",
+			__func__, cqe->res, q->q_id,
+			user_data_to_tag(cqe->user_data),
+			user_data_to_op(cqe->user_data));
+
+	if (q->tgt_ops->tgt_io_done)
+		q->tgt_ops->tgt_io_done(q, tag, cqe);
+}
+
+static void ublk_handle_cqe(struct io_uring *r,
+		struct io_uring_cqe *cqe, void *data)
+{
+	struct ublk_queue *q = container_of(r, struct ublk_queue, ring);
+	unsigned tag = user_data_to_tag(cqe->user_data);
+	unsigned cmd_op = user_data_to_op(cqe->user_data);
+	int fetch = (cqe->res != UBLK_IO_RES_ABORT) &&
+		!(q->state & UBLKSRV_QUEUE_STOPPING);
+	struct ublk_io *io;
+
+	if (cqe->res < 0 && cqe->res != -ENODEV)
+		ublk_err("%s: res %d userdata %llx queue state %x\n", __func__,
+				cqe->res, cqe->user_data, q->state);
+
+	ublk_dbg(UBLK_DBG_IO_CMD, "%s: res %d (qid %d tag %u cmd_op %u target %d) stopping %d\n",
+			__func__, cqe->res, q->q_id, tag, cmd_op,
+			is_target_io(cqe->user_data),
+			(q->state & UBLKSRV_QUEUE_STOPPING));
+
+	/* Don't retrieve io in case of target io */
+	if (is_target_io(cqe->user_data)) {
+		ublksrv_handle_tgt_cqe(q, cqe);
+		return;
+	}
+
+	io = &q->ios[tag];
+	q->cmd_inflight--;
+
+	if (!fetch) {
+		q->state |= UBLKSRV_QUEUE_STOPPING;
+		io->flags &= ~UBLKSRV_NEED_FETCH_RQ;
+	}
+
+	if (cqe->res == UBLK_IO_RES_OK) {
+		assert(tag < q->q_depth);
+		if (q->tgt_ops->queue_io)
+			q->tgt_ops->queue_io(q, tag);
+	} else {
+		/*
+		 * COMMIT_REQ will be completed immediately since no fetching
+		 * piggyback is required.
+		 *
+		 * Marking IO_FREE only, then this io won't be issued since
+		 * we only issue io with (UBLKSRV_IO_FREE | UBLKSRV_NEED_*)
+		 *
+		 * */
+		io->flags = UBLKSRV_IO_FREE;
+	}
+}
+
+static int ublk_reap_events_uring(struct io_uring *r)
+{
+	struct io_uring_cqe *cqe;
+	unsigned head;
+	int count = 0;
+
+	io_uring_for_each_cqe(r, head, cqe) {
+		ublk_handle_cqe(r, cqe, NULL);
+		count += 1;
+	}
+	io_uring_cq_advance(r, count);
+
+	return count;
+}
+
+static int ublk_process_io(struct ublk_queue *q)
+{
+	int ret, reapped;
+
+	ublk_dbg(UBLK_DBG_QUEUE, "dev%d-q%d: to_submit %d inflight cmd %u stopping %d\n",
+				q->dev->dev_info.dev_id,
+				q->q_id, io_uring_sq_ready(&q->ring),
+				q->cmd_inflight,
+				(q->state & UBLKSRV_QUEUE_STOPPING));
+
+	if (ublk_queue_is_done(q))
+		return -ENODEV;
+
+	ret = io_uring_submit_and_wait(&q->ring, 1);
+	reapped = ublk_reap_events_uring(&q->ring);
+
+	ublk_dbg(UBLK_DBG_QUEUE, "submit result %d, reapped %d stop %d idle %d\n",
+			ret, reapped, (q->state & UBLKSRV_QUEUE_STOPPING),
+			(q->state & UBLKSRV_QUEUE_IDLE));
+
+	return reapped;
+}
+
+static void *ublk_io_handler_fn(void *data)
+{
+	struct ublk_queue *q = data;
+	int dev_id = q->dev->dev_info.dev_id;
+	int ret;
+
+	ret = ublk_queue_init(q);
+	if (ret) {
+		ublk_err("ublk dev %d queue %d init queue failed\n",
+				dev_id, q->q_id);
+		return NULL;
+	}
+	ublk_dbg(UBLK_DBG_QUEUE, "tid %d: ublk dev %d queue %d started\n",
+			q->tid, dev_id, q->q_id);
+
+	/* submit all io commands to ublk driver */
+	ublk_submit_fetch_commands(q);
+	do {
+		if (ublk_process_io(q) < 0)
+			break;
+	} while (1);
+
+	ublk_dbg(UBLK_DBG_QUEUE, "ublk dev %d queue %d exited\n", dev_id, q->q_id);
+	ublk_queue_deinit(q);
+	return NULL;
+}
+
+static void ublk_set_parameters(struct ublk_dev *dev)
+{
+	int ret;
+
+	ret = ublk_ctrl_set_params(dev, &dev->tgt.params);
+	if (ret)
+		ublk_err("dev %d set basic parameter failed %d\n",
+				dev->dev_info.dev_id, ret);
+}
+
+static int ublk_send_dev_event(const struct dev_ctx *ctx, int dev_id)
+{
+	uint64_t id;
+	int evtfd = ctx->_evtfd;
+
+	if (evtfd < 0)
+		return -EBADF;
+
+	if (dev_id >= 0)
+		id = dev_id + 1;
+	else
+		id = ERROR_EVTFD_DEVID;
+
+	if (write(evtfd, &id, sizeof(id)) != sizeof(id))
+		return -EINVAL;
+
+	return 0;
+}
+
+
+static int ublk_start_daemon(const struct dev_ctx *ctx, struct ublk_dev *dev)
+{
+	int ret, i;
+	void *thread_ret;
+	const struct ublksrv_ctrl_dev_info *dinfo = &dev->dev_info;
+
+	ublk_dbg(UBLK_DBG_DEV, "%s enter\n", __func__);
+
+	ret = ublk_dev_prep(dev);
+	if (ret)
+		return ret;
+
+	for (i = 0; i < dinfo->nr_hw_queues; i++) {
+		dev->q[i].dev = dev;
+		dev->q[i].q_id = i;
+		pthread_create(&dev->q[i].thread, NULL,
+				ublk_io_handler_fn,
+				&dev->q[i]);
+	}
+
+	/* everything is fine now, start us */
+	ublk_set_parameters(dev);
+	ret = ublk_ctrl_start_dev(dev, getpid());
+	if (ret < 0) {
+		ublk_err("%s: ublk_ctrl_start_dev failed: %d\n", __func__, ret);
+		goto fail;
+	}
+
+	ublk_ctrl_get_info(dev);
+	ublk_send_dev_event(ctx, dev->dev_info.dev_id);
+
+	/* wait until we are terminated */
+	for (i = 0; i < dinfo->nr_hw_queues; i++)
+		pthread_join(dev->q[i].thread, &thread_ret);
+ fail:
+	ublk_dev_unprep(dev);
+	ublk_dbg(UBLK_DBG_DEV, "%s exit\n", __func__);
+
+	return ret;
+}
+
+static int wait_ublk_dev(char *dev_name, int evt_mask, unsigned timeout)
+{
+#define EV_SIZE (sizeof(struct inotify_event))
+#define EV_BUF_LEN (128 * (EV_SIZE + 16))
+	struct pollfd pfd;
+	int fd, wd;
+	int ret = -EINVAL;
+
+	fd = inotify_init();
+	if (fd < 0) {
+		ublk_dbg(UBLK_DBG_DEV, "%s: inotify init failed\n", __func__);
+		return fd;
+	}
+
+	wd = inotify_add_watch(fd, "/dev", evt_mask);
+	if (wd == -1) {
+		ublk_dbg(UBLK_DBG_DEV, "%s: add watch for /dev failed\n", __func__);
+		goto fail;
+	}
+
+	pfd.fd = fd;
+	pfd.events = POLL_IN;
+	while (1) {
+		int i = 0;
+		char buffer[EV_BUF_LEN];
+		ret = poll(&pfd, 1, 1000 * timeout);
+
+		if (ret == -1) {
+			ublk_err("%s: poll inotify failed: %d\n", __func__, ret);
+			goto rm_watch;
+		} else if (ret == 0) {
+			ublk_err("%s: poll inotify timeout\n", __func__);
+			ret = -ETIMEDOUT;
+			goto rm_watch;
+		}
+
+		ret = read(fd, buffer, EV_BUF_LEN);
+		if (ret < 0) {
+			ublk_err("%s: read inotify fd failed\n", __func__);
+			goto rm_watch;
+		}
+
+		while (i < ret) {
+			struct inotify_event *event = (struct inotify_event *)&buffer[i];
+
+			ublk_dbg(UBLK_DBG_DEV, "%s: inotify event %x %s\n",
+					__func__, event->mask, event->name);
+			if (event->mask & evt_mask) {
+				if (!strcmp(event->name, dev_name)) {
+					ret = 0;
+					goto rm_watch;
+				}
+			}
+			i += EV_SIZE + event->len;
+		}
+	}
+rm_watch:
+	inotify_rm_watch(fd, wd);
+fail:
+	close(fd);
+	return ret;
+}
+
+static int ublk_stop_io_daemon(const struct ublk_dev *dev)
+{
+	int daemon_pid = dev->dev_info.ublksrv_pid;
+	int dev_id = dev->dev_info.dev_id;
+	char ublkc[64];
+	int ret = 0;
+
+	/* daemon may be dead already */
+	if (kill(daemon_pid, 0) < 0)
+		goto wait;
+
+	/*
+	 * Wait until ublk char device is closed, when our daemon is shutdown
+	 */
+	snprintf(ublkc, sizeof(ublkc), "%s%d", "ublkc", dev_id);
+	ret = wait_ublk_dev(ublkc, IN_CLOSE_WRITE, 10);
+	/* double check and inotify may not be 100% reliable */
+	if (ret == -ETIMEDOUT)
+		/* the daemon doesn't exist now if kill(0) fails */
+		ret = kill(daemon_pid, 0) < 0;
+wait:
+	waitpid(daemon_pid, NULL, 0);
+	ublk_dbg(UBLK_DBG_DEV, "%s: pid %d dev_id %d ret %d\n",
+			__func__, daemon_pid, dev_id, ret);
+
+	return ret;
+}
+
+static int __cmd_dev_add(const struct dev_ctx *ctx)
+{
+	unsigned nr_queues = ctx->nr_hw_queues;
+	const char *tgt_type = ctx->tgt_type;
+	unsigned depth = ctx->queue_depth;
+	__u64 features;
+	const struct ublk_tgt_ops *ops;
+	struct ublksrv_ctrl_dev_info *info;
+	struct ublk_dev *dev;
+	int dev_id = ctx->dev_id;
+	int ret;
+
+	ops = ublk_find_tgt(tgt_type);
+	if (!ops) {
+		ublk_err("%s: no such tgt type, type %s\n",
+				__func__, tgt_type);
+		return -ENODEV;
+	}
+
+	if (nr_queues > UBLK_MAX_QUEUES || depth > UBLK_QUEUE_DEPTH) {
+		ublk_err("%s: invalid nr_queues or depth queues %u depth %u\n",
+				__func__, nr_queues, depth);
+		return -EINVAL;
+	}
+
+	dev = ublk_ctrl_init();
+	if (!dev) {
+		ublk_err("%s: can't alloc dev id %d, type %s\n",
+				__func__, dev_id, tgt_type);
+		return -ENOMEM;
+	}
+
+	/* kernel doesn't support get_features */
+	ret = ublk_ctrl_get_features(dev, &features);
+	if (ret < 0)
+		return -EINVAL;
+
+	if (!(features & UBLK_F_CMD_IOCTL_ENCODE))
+		return -ENOTSUP;
+
+	info = &dev->dev_info;
+	info->dev_id = ctx->dev_id;
+	info->nr_hw_queues = nr_queues;
+	info->queue_depth = depth;
+	info->flags = ctx->flags;
+	dev->tgt.ops = ops;
+	dev->tgt.sq_depth = depth;
+	dev->tgt.cq_depth = depth;
+
+	ret = ublk_ctrl_add_dev(dev);
+	if (ret < 0) {
+		ublk_err("%s: can't add dev id %d, type %s ret %d\n",
+				__func__, dev_id, tgt_type, ret);
+		goto fail;
+	}
+
+	ret = ublk_start_daemon(ctx, dev);
+	ublk_dbg(UBLK_DBG_DEV, "%s: daemon exit %d\b", ret);
+
+fail:
+	if (ret < 0)
+		ublk_send_dev_event(ctx, -1);
+	ublk_ctrl_deinit(dev);
+	return ret;
+}
+
+static int __cmd_dev_list(struct dev_ctx *ctx);
+
+static int cmd_dev_add(struct dev_ctx *ctx)
+{
+	int res;
+
+	ctx->_evtfd = eventfd(0, 0);
+	if (ctx->_evtfd < 0) {
+		ublk_err("%s: failed to create eventfd %s\n", __func__, strerror(errno));
+		exit(-1);
+	}
+
+	setsid();
+	res = fork();
+	if (res == 0) {
+		__cmd_dev_add(ctx);
+		exit(EXIT_SUCCESS);
+	} else if (res > 0) {
+		uint64_t id;
+
+		res = read(ctx->_evtfd, &id, sizeof(id));
+		close(ctx->_evtfd);
+		if (res == sizeof(id) && id != ERROR_EVTFD_DEVID) {
+			ctx->dev_id = id - 1;
+			return __cmd_dev_list(ctx);
+		}
+		exit(EXIT_FAILURE);
+	} else {
+		return res;
+	}
+}
+
+static int __cmd_dev_del(struct dev_ctx *ctx)
+{
+	int number = ctx->dev_id;
+	struct ublk_dev *dev;
+	int ret;
+
+	dev = ublk_ctrl_init();
+	dev->dev_info.dev_id = number;
+
+	ret = ublk_ctrl_get_info(dev);
+	if (ret < 0)
+		goto fail;
+
+	ret = ublk_ctrl_stop_dev(dev);
+	if (ret < 0)
+		ublk_err("%s: stop dev %d failed ret %d\n", __func__, number, ret);
+
+	ret = ublk_stop_io_daemon(dev);
+	if (ret < 0)
+		ublk_err("%s: stop daemon id %d dev %d, ret %d\n",
+				__func__, dev->dev_info.ublksrv_pid, number, ret);
+	ublk_ctrl_del_dev(dev);
+fail:
+	if (ret >= 0)
+		ret = ublk_ctrl_get_info(dev);
+	ublk_ctrl_deinit(dev);
+
+	return (ret >= 0) ? 0 : ret;
+}
+
+static int cmd_dev_del(struct dev_ctx *ctx)
+{
+	int i;
+
+	if (ctx->dev_id >= 0 || !ctx->all)
+		return __cmd_dev_del(ctx);
+
+	for (i = 0; i < 255; i++) {
+		ctx->dev_id = i;
+		__cmd_dev_del(ctx);
+	}
+	return 0;
+}
+
+static int __cmd_dev_list(struct dev_ctx *ctx)
+{
+	struct ublk_dev *dev = ublk_ctrl_init();
+	int ret;
+
+	if (!dev)
+		return -ENODEV;
+
+	dev->dev_info.dev_id = ctx->dev_id;
+
+	ret = ublk_ctrl_get_info(dev);
+	if (ret < 0) {
+		if (ctx->logging)
+			ublk_err("%s: can't get dev info from %d: %d\n",
+					__func__, ctx->dev_id, ret);
+	} else {
+		ublk_ctrl_dump(dev);
+	}
+
+	ublk_ctrl_deinit(dev);
+
+	return ret;
+}
+
+static int cmd_dev_list(struct dev_ctx *ctx)
+{
+	int i;
+
+	if (ctx->dev_id >= 0 || !ctx->all)
+		return __cmd_dev_list(ctx);
+
+	ctx->logging = false;
+	for (i = 0; i < 255; i++) {
+		ctx->dev_id = i;
+		__cmd_dev_list(ctx);
+	}
+	return 0;
+}
+
+static int cmd_dev_get_features(void)
+{
+#define const_ilog2(x) (63 - __builtin_clzll(x))
+	static const char *feat_map[] = {
+		[const_ilog2(UBLK_F_SUPPORT_ZERO_COPY)] = "ZERO_COPY",
+		[const_ilog2(UBLK_F_URING_CMD_COMP_IN_TASK)] = "COMP_IN_TASK",
+		[const_ilog2(UBLK_F_NEED_GET_DATA)] = "GET_DATA",
+		[const_ilog2(UBLK_F_USER_RECOVERY)] = "USER_RECOVERY",
+		[const_ilog2(UBLK_F_USER_RECOVERY_REISSUE)] = "RECOVERY_REISSUE",
+		[const_ilog2(UBLK_F_UNPRIVILEGED_DEV)] = "UNPRIVILEGED_DEV",
+		[const_ilog2(UBLK_F_CMD_IOCTL_ENCODE)] = "CMD_IOCTL_ENCODE",
+		[const_ilog2(UBLK_F_USER_COPY)] = "USER_COPY",
+		[const_ilog2(UBLK_F_ZONED)] = "ZONED",
+		[const_ilog2(UBLK_F_USER_RECOVERY_FAIL_IO)] = "RECOVERY_FAIL_IO",
+	};
+	struct ublk_dev *dev;
+	__u64 features = 0;
+	int ret;
+
+	dev = ublk_ctrl_init();
+	if (!dev) {
+		fprintf(stderr, "ublksrv_ctrl_init failed id\n");
+		return -EOPNOTSUPP;
+	}
+
+	ret = ublk_ctrl_get_features(dev, &features);
+	if (!ret) {
+		int i;
+
+		printf("ublk_drv features: 0x%llx\n", features);
+
+		for (i = 0; i < sizeof(features) * 8; i++) {
+			const char *feat;
+
+			if (!((1ULL << i)  & features))
+				continue;
+			if (i < sizeof(feat_map) / sizeof(feat_map[0]))
+				feat = feat_map[i];
+			else
+				feat = "unknown";
+			printf("\t%-20s: 0x%llx\n", feat, 1ULL << i);
+		}
+	}
+
+	return ret;
+}
+
+static int cmd_dev_help(char *exe)
+{
+	printf("%s add -t [null] [-q nr_queues] [-d depth] [-n dev_id]\n", exe);
+	printf("\t default: nr_queues=2(max 4), depth=128(max 128), dev_id=-1(auto allocation)\n");
+	printf("%s del [-n dev_id] -a \n", exe);
+	printf("\t -a delete all devices -n delete specified device\n");
+	printf("%s list [-n dev_id] -a \n", exe);
+	printf("\t -a list all devices, -n list specified device, default -a \n");
+	printf("%s features\n", exe);
+	return 0;
+}
+
+int main(int argc, char *argv[])
+{
+	static const struct option longopts[] = {
+		{ "all",		0,	NULL, 'a' },
+		{ "type",		1,	NULL, 't' },
+		{ "number",		1,	NULL, 'n' },
+		{ "queues",		1,	NULL, 'q' },
+		{ "depth",		1,	NULL, 'd' },
+		{ "debug_mask",		1,	NULL,  0  },
+		{ "quiet",		0,	NULL,  0  },
+		{ 0, 0, 0, 0 }
+	};
+	int option_idx, opt;
+	const char *cmd = argv[1];
+	struct dev_ctx ctx = {
+		.queue_depth	=	128,
+		.nr_hw_queues	=	2,
+		.dev_id		=	-1,
+		.tgt_type	=	"unknown",
+	};
+	int ret = -EINVAL, i;
+
+	if (argc == 1)
+		return ret;
+
+	optind = 2;
+	while ((opt = getopt_long(argc, argv, "t:n:d:q:a",
+				  longopts, &option_idx)) != -1) {
+		switch (opt) {
+		case 'a':
+			ctx.all = 1;
+			break;
+		case 'n':
+			ctx.dev_id = strtol(optarg, NULL, 10);
+			break;
+		case 't':
+			if (strlen(optarg) < sizeof(ctx.tgt_type))
+				strcpy(ctx.tgt_type, optarg);
+			break;
+		case 'q':
+			ctx.nr_hw_queues = strtol(optarg, NULL, 10);
+			break;
+		case 'd':
+			ctx.queue_depth = strtol(optarg, NULL, 10);
+			break;
+		case 0:
+			if (!strcmp(longopts[option_idx].name, "debug_mask"))
+				ublk_dbg_mask = strtol(optarg, NULL, 16);
+			if (!strcmp(longopts[option_idx].name, "quiet"))
+				ublk_dbg_mask = 0;
+			break;
+		}
+	}
+
+	i = optind;
+	while (i < argc && ctx.nr_files < MAX_BACK_FILES) {
+		ctx.files[ctx.nr_files++] = argv[i++];
+	}
+
+	if (!strcmp(cmd, "add"))
+		ret = cmd_dev_add(&ctx);
+	else if (!strcmp(cmd, "del"))
+		ret = cmd_dev_del(&ctx);
+	else if (!strcmp(cmd, "list")) {
+		ctx.all = 1;
+		ret = cmd_dev_list(&ctx);
+	} else if (!strcmp(cmd, "help"))
+		ret = cmd_dev_help(argv[0]);
+	else if (!strcmp(cmd, "features"))
+		ret = cmd_dev_get_features();
+	else
+		cmd_dev_help(argv[0]);
+
+	return ret;
+}

tools/testing/selftests/ublk/kublk.h

···
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef KUBLK_INTERNAL_H
+#define KUBLK_INTERNAL_H
+
+#include <unistd.h>
+#include <stdlib.h>
+#include <assert.h>
+#include <stdio.h>
+#include <stdarg.h>
+#include <string.h>
+#include <pthread.h>
+#include <getopt.h>
+#include <limits.h>
+#include <poll.h>
+#include <sys/syscall.h>
+#include <sys/mman.h>
+#include <sys/ioctl.h>
+#include <sys/inotify.h>
+#include <sys/wait.h>
+#include <sys/eventfd.h>
+#include <liburing.h>
+#include <linux/ublk_cmd.h>
+
+#define __maybe_unused __attribute__((unused))
+#define MAX_BACK_FILES   4
+#ifndef min
+#define min(a, b) ((a) < (b) ? (a) : (b))
+#endif
+
+/****************** part 1: libublk ********************/
+
+#define CTRL_DEV		"/dev/ublk-control"
+#define UBLKC_DEV		"/dev/ublkc"
+#define UBLKB_DEV		"/dev/ublkb"
+#define UBLK_CTRL_RING_DEPTH            32
+#define ERROR_EVTFD_DEVID 	-2
+
+/* queue idle timeout */
+#define UBLKSRV_IO_IDLE_SECS		20
+
+#define UBLK_IO_MAX_BYTES               65536
+#define UBLK_MAX_QUEUES                 4
+#define UBLK_QUEUE_DEPTH                128
+
+#define UBLK_DBG_DEV            (1U << 0)
+#define UBLK_DBG_QUEUE          (1U << 1)
+#define UBLK_DBG_IO_CMD         (1U << 2)
+#define UBLK_DBG_IO             (1U << 3)
+#define UBLK_DBG_CTRL_CMD       (1U << 4)
+#define UBLK_LOG                (1U << 5)
+
+struct ublk_dev;
+struct ublk_queue;
+
+struct dev_ctx {
+	char tgt_type[16];
+	unsigned long flags;
+	unsigned nr_hw_queues;
+	unsigned queue_depth;
+	int dev_id;
+	int nr_files;
+	char *files[MAX_BACK_FILES];
+	unsigned int	logging:1;
+	unsigned int	all:1;
+
+	int _evtfd;
+};
+
+struct ublk_ctrl_cmd_data {
+	__u32 cmd_op;
+#define CTRL_CMD_HAS_DATA	1
+#define CTRL_CMD_HAS_BUF	2
+	__u32 flags;
+
+	__u64 data[2];
+	__u64 addr;
+	__u32 len;
+};
+
+struct ublk_io {
+	char *buf_addr;
+
+#define UBLKSRV_NEED_FETCH_RQ		(1UL << 0)
+#define UBLKSRV_NEED_COMMIT_RQ_COMP	(1UL << 1)
+#define UBLKSRV_IO_FREE			(1UL << 2)
+	unsigned short flags;
+	unsigned short refs;		/* used by target code only */
+
+	int result;
+};
+
+struct ublk_tgt_ops {
+	const char *name;
+	int (*init_tgt)(struct ublk_dev *);
+	void (*deinit_tgt)(struct ublk_dev *);
+
+	int (*queue_io)(struct ublk_queue *, int tag);
+	void (*tgt_io_done)(struct ublk_queue *,
+			int tag, const struct io_uring_cqe *);
+};
+
+struct ublk_tgt {
+	unsigned long dev_size;
+	unsigned int  sq_depth;
+	unsigned int  cq_depth;
+	const struct ublk_tgt_ops *ops;
+	struct ublk_params params;
+	char backing_file[1024 - 8 - sizeof(struct ublk_params)];
+};
+
+struct ublk_queue {
+	int q_id;
+	int q_depth;
+	unsigned int cmd_inflight;
+	unsigned int io_inflight;
+	struct ublk_dev *dev;
+	const struct ublk_tgt_ops *tgt_ops;
+	char *io_cmd_buf;
+	struct io_uring ring;
+	struct ublk_io ios[UBLK_QUEUE_DEPTH];
+#define UBLKSRV_QUEUE_STOPPING	(1U << 0)
+#define UBLKSRV_QUEUE_IDLE	(1U << 1)
+#define UBLKSRV_NO_BUF		(1U << 2)
+	unsigned state;
+	pid_t tid;
+	pthread_t thread;
+};
+
+struct ublk_dev {
+	struct ublk_tgt tgt;
+	struct ublksrv_ctrl_dev_info  dev_info;
+	struct ublk_queue q[UBLK_MAX_QUEUES];
+
+	int fds[2];	/* fds[0] points to /dev/ublkcN */
+	int nr_fds;
+	int ctrl_fd;
+	struct io_uring ring;
+};
+
+#ifndef offsetof
+#define offsetof(TYPE, MEMBER)  ((size_t)&((TYPE *)0)->MEMBER)
+#endif
+
+#ifndef container_of
+#define container_of(ptr, type, member) ({                              \
+	unsigned long __mptr = (unsigned long)(ptr);                    \
+	((type *)(__mptr - offsetof(type, member))); })
+#endif
+
+#define round_up(val, rnd) \
+	(((val) + ((rnd) - 1)) & ~((rnd) - 1))
+
+
+extern unsigned int ublk_dbg_mask;
+extern int ublk_queue_io_cmd(struct ublk_queue *q, struct ublk_io *io, unsigned tag);
+
+static inline int is_target_io(__u64 user_data)
+{
+	return (user_data & (1ULL << 63)) != 0;
+}
+
+static inline __u64 build_user_data(unsigned tag, unsigned op,
+		unsigned tgt_data, unsigned is_target_io)
+{
+	assert(!(tag >> 16) && !(op >> 8) && !(tgt_data >> 16));
+
+	return tag | (op << 16) | (tgt_data << 24) | (__u64)is_target_io << 63;
+}
+
+static inline unsigned int user_data_to_tag(__u64 user_data)
+{
+	return user_data & 0xffff;
+}
+
+static inline unsigned int user_data_to_op(__u64 user_data)
+{
+	return (user_data >> 16) & 0xff;
+}
+
+static inline void ublk_err(const char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	vfprintf(stderr, fmt, ap);
+}
+
+static inline void ublk_log(const char *fmt, ...)
+{
+	if (ublk_dbg_mask & UBLK_LOG) {
+		va_list ap;
+
+		va_start(ap, fmt);
+		vfprintf(stdout, fmt, ap);
+	}
+}
+
+static inline void ublk_dbg(int level, const char *fmt, ...)
+{
+	if (level & ublk_dbg_mask) {
+		va_list ap;
+
+		va_start(ap, fmt);
+		vfprintf(stdout, fmt, ap);
+	}
+}
+
+static inline struct io_uring_sqe *ublk_queue_alloc_sqe(struct ublk_queue *q)
+{
+	unsigned left = io_uring_sq_space_left(&q->ring);
+
+	if (left < 1)
+		io_uring_submit(&q->ring);
+	return io_uring_get_sqe(&q->ring);
+}
+
+static inline void *ublk_get_sqe_cmd(const struct io_uring_sqe *sqe)
+{
+	return (void *)&sqe->cmd;
+}
+
+static inline void ublk_mark_io_done(struct ublk_io *io, int res)
+{
+	io->flags |= (UBLKSRV_NEED_COMMIT_RQ_COMP | UBLKSRV_IO_FREE);
+	io->result = res;
+}
+
+static inline const struct ublksrv_io_desc *ublk_get_iod(const struct ublk_queue *q, int tag)
+{
+	return (struct ublksrv_io_desc *)&(q->io_cmd_buf[tag * sizeof(struct ublksrv_io_desc)]);
+}
+
+static inline void ublk_set_sqe_cmd_op(struct io_uring_sqe *sqe, __u32 cmd_op)
+{
+	__u32 *addr = (__u32 *)&sqe->off;
+
+	addr[0] = cmd_op;
+	addr[1] = 0;
+}
+
+static inline int ublk_complete_io(struct ublk_queue *q, unsigned tag, int res)
+{
+	struct ublk_io *io = &q->ios[tag];
+
+	ublk_mark_io_done(io, res);
+
+	return ublk_queue_io_cmd(q, io, tag);
+}
+
+extern const struct ublk_tgt_ops null_tgt_ops;
+
+#endif

tools/testing/selftests/ublk/null.c

···
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include "kublk.h"
+
+static int ublk_null_tgt_init(struct ublk_dev *dev)
+{
+	const struct ublksrv_ctrl_dev_info *info = &dev->dev_info;
+	unsigned long dev_size = 250UL << 30;
+
+	dev->tgt.dev_size = dev_size;
+	dev->tgt.params = (struct ublk_params) {
+		.types = UBLK_PARAM_TYPE_BASIC,
+		.basic = {
+			.logical_bs_shift	= 9,
+			.physical_bs_shift	= 12,
+			.io_opt_shift		= 12,
+			.io_min_shift		= 9,
+			.max_sectors		= info->max_io_buf_bytes >> 9,
+			.dev_sectors		= dev_size >> 9,
+		},
+	};
+
+	return 0;
+}
+
+static int ublk_null_queue_io(struct ublk_queue *q, int tag)
+{
+	const struct ublksrv_io_desc *iod = ublk_get_iod(q, tag);
+
+	ublk_complete_io(q, tag, iod->nr_sectors << 9);
+	return 0;
+}
+
+const struct ublk_tgt_ops null_tgt_ops = {
+	.name = "null",
+	.init_tgt = ublk_null_tgt_init,
+	.queue_io = ublk_null_queue_io,
+};

tools/testing/selftests/ublk/test_common.sh

···
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+_check_root() {
+	local ksft_skip=4
+
+	if [ $UID != 0 ]; then
+		echo please run this as root >&2
+		exit $ksft_skip
+	fi
+}
+
+_remove_ublk_devices() {
+	${UBLK_PROG} del -a
+}
+
+_get_ublk_dev_state() {
+	${UBLK_PROG} list -n "$1" | grep "state" | awk '{print $11}'
+}
+
+_get_ublk_daemon_pid() {
+	${UBLK_PROG} list -n "$1" | grep "pid" | awk '{print $7}'
+}
+
+_prep_test() {
+	_check_root
+	local type=$1
+	shift 1
+	echo "ublk $type: $@"
+}
+
+_show_result()
+{
+	if [ $2 -ne 0 ]; then
+		echo "$1 : [FAIL]"
+	else
+		echo "$1 : [PASS]"
+	fi
+}
+
+_cleanup_test() {
+	${UBLK_PROG} del -n $1
+}
+
+_add_ublk_dev() {
+	local kublk_temp=`mktemp /tmp/kublk-XXXXXX`
+	${UBLK_PROG} add $@ > ${kublk_temp} 2>&1
+	if [ $? -ne 0 ]; then
+		echo "fail to add ublk dev $@"
+		exit -1
+	fi
+	local dev_id=`grep "dev id" ${kublk_temp} | awk -F '[ :]' '{print $3}'`
+	udevadm settle
+	rm -f ${kublk_temp}
+	echo ${dev_id}
+}
+
+export UBLK_PROG=$(pwd)/kublk

tools/testing/selftests/ublk/test_null_01.sh

···
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+. test_common.sh
+
+TID="null_01"
+ERR_CODE=0
+
+_prep_test "null" "basic IO test"
+
+dev_id=`_add_ublk_dev -t null`
+
+# run fio over the two disks
+fio --name=job1 --filename=/dev/ublkb${dev_id} --ioengine=libaio --rw=readwrite --iodepth=32 --size=256M > /dev/null 2>&1
+ERR_CODE=$?
+
+_cleanup_test ${dev_id} "null"
+
+_show_result $TID $ERR_CODE