Hi everyone,

We encountered an issue with KRBD mounting after mapping it to the host with 
read-only option.
We try to pinpoint where the problem is, but not able to do it.

The image is mounted well if we map it without the "read-only" option.
This leads to an issue that the pod in k8s cannot use the snapshotted 
persistent volume created by ceph-csi rbd provisioner.
Thank you for reading.

I have reported the bug here: Bug #50234: krbd failed to mount after map image 
with read-only option - Ceph - Ceph<https://tracker.ceph.com/issues/50234>

Context
- Using admin keyring
- Linux Kernel: 3.10.0-1160.15.2.el7.x86_64
- Linux Distribution: Red Hat Enterprise Linux Server 7.8 (Maipo)
- Ceph version: "ceph version 14.2.8 (2d095e947a02261ce61424021bb43bd3022d35cb) 
nautilus (stable)"

rbd image 'csi-vol-85919409-9797-11eb-80ba-720b2b57c790':
        size 10 GiB in 2560 objects
        order 22 (4 MiB objects)
        snapshot_count: 0
        id: 533a03bba388ea
        block_name_prefix: rbd_data.533a03bba388ea
        format: 2
        features: layering
        op_features:
        flags:
        create_timestamp: Wed Apr  7 13:51:02 2021
        access_timestamp: Wed Apr  7 13:51:02 2021
        modify_timestamp: Wed Apr  7 13:51:02 2021

Bug Reproduction
# Map RBD image WITH read-only option, CANNOT mount with both readonly or 
readwrite option
sudo rbd device map -p k8s-sharedpool 
csi-vol-85919409-9797-11eb-80ba-720b2b57c790 -ro
  /dev/rbd0
sudo mount -v -r -t ext4 /dev/rbd0 /mnt/test1
  mount: cannot mount /dev/rbd0 read-only

sudo mount -v -r -t ext4 /dev/rbd0 /mnt/test1
  mount: /dev/rbd0 is write-protected, mounting read-only
  mount: cannot mount /dev/rbd0 read-only

# Map RBD image WITHOUT read-only option, CAN mount with both readonly or 
readwrite option
sudo rbd device map -p k8s-sharedpool 
csi-vol-85919409-9797-11eb-80ba-720b2b57c790
  /dev/rbd0
sudo mount -v -r -t ext4 /dev/rbd0 /mnt/test1

  mount: /mnt/test1 does not contain SELinux labels.
       You just mounted an file system that supports labels which does not
       contain labels, onto an SELinux box. It is likely that confined
       applications will generate AVC messages and not be allowed access to
       this file system.  For more details see restorecon(8) and mount(8).
  mount: /dev/rbd0 mounted on /mnt/test1.

sudo mount -v -t ext4 /dev/rbd0 /mnt/test1
  mount: /mnt/test1 does not contain SELinux labels.
       You just mounted an file system that supports labels which does not
       contain labels, onto an SELinux box. It is likely that confined
       applications will generate AVC messages and not be allowed access to
       this file system.  For more details see restorecon(8) and mount(8).
  mount: /dev/rbd0 mounted on /mnt/test1.

With my best regards,
Son Hai Ha


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