Can't expose a fuse based volume to a Docker container

Question

I'm trying to provide my docker container a volume of encrypted file system for internal use. The idea is that the container will write to the volume as usual, but in fact the host will be encrypting the data before writing it to the filesystem.

I'm trying to use EncFS - it works well on the host, e.g:

encfs /encrypted /visible

I can write files to /visible, and those get encrypted. However, when trying to run a container with /visible as the volume, e.g.:

docker run -i -t --privileged -v /visible:/myvolume imagename bash

I do get a volume in the container, but it's on the original /encrypted folder, not going through the EncFS. If I unmount the EncFS from /visible, I can see the files written by the container. Needless to say /encrypted is empty.

Is there a way to have docker mount the volume through EncFS, and not write directly to the folder? In contrast, docker works fine when I use an NFS mount as a volume. It writes to the network device, and not to the local folder on which I mounted the device.

Thanks

Answer 1

I am unable to duplicate your problem locally. If I try to expose an encfs filesystem as a Docker volume, I get an error trying to start the container:

FATA[0003] Error response from daemon: Cannot start container <cid>:
setup mount namespace stat /visible: permission denied 

So it's possible you have something different going on. In any case, this is what solved my problem:

By default, FUSE only permits the user who mounted a filesystem to have access to that filesystem. When you are running a Docker container, that container is initially running as root.

You can use the allow_root or allow_other mount options when you mount the FUSE filesystem. For example:

$ encfs -o allow_root /encrypted /other

Here, allow_root will permit the root user to have acces to the mountpoint, while allow_other will permit anyone to have access to the mountpoint (provided that the Unix permissions on the directory allow them access).

If I mounted by encfs filesytem using allow_root, I can then expose that filesystem as a Docker volume and the contents of that filesystem are correctly visible from inside the container.