[Resolved] Unable to add a loop device using ublk

[harshad1024]

Unale to add loop device using ublk. It fails to open the ublk-control device. Not sure if anyting special needs to be done by user at this point of time. ./disk01.img is present in the path. This is confirmed. Please confirm what is the process/steps to create a basic simple loop using ublk.

$ ublk add -t loop -f ./disk01.img
conrol dev /dev/ublk-control can't be opened

This is the deugging info (only important parts):

(gdb) s
cmd_dev_add (argv=0x7fffffffe308, argc=5) at ublksrv_tgt.cpp:479
479		struct ublksrv_dev_data data = {0};
...
(gdb) p data
$15 = {dev_id = -1, max_io_buf_bytes = 524288, nr_hw_queues = 1, queue_depth = 256, tgt_type = 0x7fffffffe5ef "loop", tgt_ops = 0x0, tgt_argc = 0, tgt_argv = 0x0, 
  run_dir = 0x40608c "/var/run/ublksrvd", flags = 0, ublksrv_flags = 1, reserved = {0, 0, 0, 0, 0, 0, 0}}

This is within ublksrv_ctrl_init()

(gdb) s
ublksrv_ctrl_init (data=0x7fffffffe040) at ublksrv_cmd.c:97
97		struct ublksrv_ctrl_dev *dev = (struct ublksrv_ctrl_dev *)calloc(1,
(gdb) n
102		dev->ctrl_fd = open(CTRL_DEV, O_RDWR);
(gdb) n
103		if (dev->ctrl_fd < 0) {
(gdb) p dev
$18 = (struct ublksrv_ctrl_dev *) 0x41bf50
(gdb) p *dev
$19 = {ring = {sq = {khead = 0x0, ktail = 0x0, kring_mask = 0x0, kring_entries = 0x0, kflags = 0x0, kdropped = 0x0, array = 0x0, sqes = 0x0, sqe_head = 0, sqe_tail = 0, 
      ring_sz = 0, ring_ptr = 0x0, ring_mask = 0, ring_entries = 0, pad = {0, 0}}, cq = {khead = 0x0, ktail = 0x0, kring_mask = 0x0, kring_entries = 0x0, kflags = 0x0, 
      koverflow = 0x0, cqes = 0x0, ring_sz = 0, ring_ptr = 0x0, ring_mask = 0, ring_entries = 0, pad = {0, 0}}, flags = 0, ring_fd = 0, features = 0, enter_ring_fd = 0, 
    int_flags = 0 '\000', pad = "\000\000", pad2 = 0}, ctrl_fd = -1, bs_shift = 0, dev_info = {nr_hw_queues = 0, queue_depth = 0, state = 0, pad0 = 0, 
    max_io_buf_bytes = 0, dev_id = 0, ublksrv_pid = 0, pad1 = 0, flags = 0, ublksrv_flags = 0, reserved0 = 0, reserved1 = 0, reserved2 = 0}, tgt_type = 0x0, tgt_ops = 0x0, 
  run_dir = 0x0, tgt_argc = 0, tgt_argv = 0x0, queues_cpuset = 0x0}
(gdb) p dev->ctrl_fd 
$20 = -1
(gdb) n
104			fprintf(stderr, "conrol dev %s can't be opened\n", CTRL_DEV);

System details:

uname -m = x86_64                                                               
uname -r = 5.19.8-200.fc36.x86_64 

$ ldd /usr/local/sbin/ublk   
        linux-vdso.so.1 (0x00007ffd1e879000)
	libublksrv.so.0 => /usr/local/lib/libublksrv.so.0 (0x00007fe837bb0000)
	liburing.so.2 => /lib/liburing.so.2 (0x00007fe837ba3000)
	libstdc++.so.6 => /lib64/libstdc++.so.6 (0x00007fe837800000)
	libm.so.6 => /lib64/libm.so.6 (0x00007fe837ac5000)
	libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007fe837aa5000)
	libc.so.6 => /lib64/libc.so.6 (0x00007fe837400000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fe837bec000)

[harshad1024]

I was missing the ublk_drv module.

# ls /dev/*blk*
ls: cannot access '/dev/*blk*': No such file or directory
# insmod ./ublk_drv.ko
# ls  /dev/*blk*
/dev/ublk-control

[harshad1024]

Done. Readme nees a little rework to make it newbie friendly :). I will try to update the readme.

# ublk add -d 2 -t loop -f ./disk01.img 
dev id 0: nr_hw_queues 1 queue_depth 2 block size 4096 dev_capacity 14000
	max rq size 524288 daemon pid 1539 flags 0x0 state LIVE
	queue 0: tid 1542 affinity(0 1 )
	target {"backing_file":"./disk01.img","dev_size":7168000,"direct_io":1,"name":"loop","type":1}