Operation not permitted when using CSI to mount the user-space file system

[Joson Wong]

I created a pod with runsc, in the pod.yaml file, a volume was mounted using CSI. The backend implementation is a user-space file system, as shown below:
volumeMounts:
- name: quarkfs-storage
mountPath: /home/test_dir
readOnly: false
volumes:
- name: quarkfs-storage
persistentVolumeClaim:
claimName: quarkfs-pvc-1

After the container is started, the "write" operation cannot be performed:

root@gvisor-podtest-1:/home/test_dir# df -h
Filesystem Size Used Avail Use% Mounted on
none 8.0E 58M 8.0E 1% /
none 250G 0 250G 0% /dev
none 250G 0 250G 0% /dev/shm
none 3.5T 278G 3.3T 8% /etc/hosts
none 3.5T 278G 3.3T 8% /etc/hostname
none 300G 9.7G 291G 4% /home/test_dir
none 250G 0 250G 0% /sys/fs/cgroup
none 3.5T 278G 3.3T 8% /etc/resolv.conf
none 3.5T 278G 3.3T 8% /dev/termination-log
none 128M 12K 128M 1% /run/secrets/kubernetes.io/serviceaccount
none 250G 0 250G 0% /tmp
root@gvisor-podtest-1:/home/test_dir# ls
888 999 Megatron-LM930 core data_cache fio-rand-read hostfile nccl-master-0.nccl nccl3-master-0.nccl3 nccl4-master-0.nccl4 nccl5-master-0.nccl5 test.file
root@gvisor-podtest-1:/home/test_dir# touch 777
touch: cannot touch '777': Operation not permitted

How is it happen? Is this normal? How can I perform a write operation under the /home/test_dir directory?