Error when creating a logicalunit from a regular file

Hello,

I am trying to setup tgt on a CentOS 7 system.    I created a target,
and enabled "ALL" initiators to login.  When I try to create a logical
unit from a regular file (which I believe is supported and should
work), I get the error message "tgtadm: invalid request".   In the
logs, I see an error message related to "Permission denied".  Can
someone please help or give me pointers to debug this issue.  Please
CC me in the response.


# tgtadm --mode target --op new --tid 1 --targetname target1
# tgtadm --mode target --op bind --tid 1 -I ALL
# tgtadm --mode target --op show
Target 1: target1
    System information:
        Driver: iscsi
        State: ready
    I_T nexus information:
    LUN information:
        LUN: 0
            Type: controller
            SCSI ID: IET     00010000
            SCSI SN: beaf10
            Size: 0 MB, Block size: 1
            Online: Yes
            Removable media: No
            Prevent removal: No
            Readonly: No
            SWP: No
            Thin-provisioning: No
            Backing store type: null
            Backing store path: None
            Backing store flags:
    Account information:
    ACL information:
        ALL


# ls -l /tmp/tgt_disk2_dd
-rwxrwxrwx. 1 root root 1048576 Jun  2 14:18 /tmp/tgt_disk2_dd

# tgtadm --lld iscsi  --mode logicalunit --op new --tid 1 --lun 2  -b
/tmp/tgt_disk2_dd
tgtadm: invalid request


I see the following log entries:

Jun 02 14:39:46 unknown0800275C3F01 tgtd[5505]: tgtd: device_mgmt(246)
sz:23 params:path=/tmp/tgt_disk2_dd
Jun 02 14:39:46 unknown0800275C3F01 tgtd[5505]: tgtd: bs_thread_open(412) 16
Jun 02 14:39:46 unknown0800275C3F01 tgtd[5505]: tgtd:
backed_file_open(92) Could not open /tmp/tgt_disk2_dd, Permission
denied
Jun 02 14:39:46 unknown0800275C3F01 tgtd[5505]: tgtd:
backed_file_open(92) Could not open /tmp/tgt_disk2_dd, Permission
denied


Thank you very much in advance,
Ahmed.