feat: Support `VOLUME_MOUNT_GROUP` Capability

[s4ke]

Something that would be useful for us, is something that we already PR'ed to the existing Docker volume plugin by @costela (see https://github.com/costela/docker-volume-hetzner) : support for automatic chowning to uid:gid after volume creation so that we can run containers with users other than root.

I am not very familiar with the CSI spec yet, but I was wondering if this exists already as a feature in csi-driver or whether this would require some extra work.

[apricote]

Setting the group ownership is part of the csi-spec through the capability VOLUME_MOUNT_GROUP.

We do not yet support this capability.

[s4ke]

Ah, interesting. If we (as in everyone interested in this project) were to implement this after we manage to get the driver working on swarm, when in the life-cycle would we do this the best?

[apricote]

What do you mean by life-cycle? This is unrelated to Docker Swarm support, and can be implemented in parallel. I do not know whether the docker swarm CSI implementation supports this capability though.

[s4ke]

As I said, still a bit new to the CSI spec. Sorry, I rephrased the above comment which makes the statement now a bit confusing.

Essentially I am interested where in the code and therefore when in the process of creating a new volume such a thing would have to be added.

[apricote]

No problem ;)

The csi-spec describes this, you can search for VOLUME_MOUNT_GROUP to find the docs for it. Another source that describes this is the kubernetes-csi book, and while it contains a lot of kubernetes specific details, it is also a good secondary source for the csi-spec: https://kubernetes-csi.github.io/docs/support-fsgroup.html#overview-1

To summarize the spec, the field is passed in the calls CreateVolume, NodeStageVolume (not implemented by us) and NodePublishVolume.

As we do the actual mount in NodePublishVolume, this would be the right place to implement the new functionality:

https://github.com/hetznercloud/csi-driver/blob/0b82720f1ae26c915f9df7a2efe5ae87f24fc18f/driver/node.go#L52-L88

which then calls to LinuxMountService.Publish (line 84):

https://github.com/hetznercloud/csi-driver/blob/0b82720f1ae26c915f9df7a2efe5ae87f24fc18f/volumes/mount.go#L55

Additionally, you would need to add the capability to the list of supported capabilities here:

https://github.com/hetznercloud/csi-driver/blob/0b82720f1ae26c915f9df7a2efe5ae87f24fc18f/driver/identity.go#L46

[s4ke]

So I guess this could work with a simple solution similar to what we did in the docker volume plugin then?

https://github.com/costela/docker-volume-hetzner/blob/master/driver.go#L102

[apricote]

I think so, but we can probably skip the additional mount/unmount and just run chown after mounting it.

[s4ke]

Just as a sidenote - since we are Swarm users obviously, this is not something that I can work on until the Swarm plugin works. So if anyone wants to take over, go for it!

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[schmidp]

I am currently looking at this but I am not a go developer so some of my questions might be stupid:

1. you mention adding the capability to GetPluginCapabilities. Too me it looks like it should be added to: NodeGetCapabilities instead:

{
	Type: &proto.NodeServiceCapability_Rpc{
		Rpc: &proto.NodeServiceCapability_RPC{
			Type: proto.NodeServiceCapability_RPC_VOLUME_MOUNT_GROUP,
		},
	},
},

2. A plan of action could be:

   1. Get the volumeMountGroup in NodePublishVolume:
   2. Add it to a new field in MountOpts
   3. Check if the gid is already in the mount flags and handle that case
      • https://github.com/SynologyOpenSource/synology-csi/blob/78c814d565755bed6bcad874add84b1c3247441e/pkg/driver/nodeserver.go#L222
      • https://github.com/kubernetes-sigs/azurefile-csi-driver/blob/aaf6117e1c1db319610f9e07578602dd7bb1d201/pkg/azurefile/nodeserver.go#L266
   4. Check what fsGroupChangePolicy says and in case set ownership recursively: https://github.com/kubernetes-sigs/azurefile-csi-driver/blob/aaf6117e1c1db319610f9e07578602dd7bb1d201/pkg/azurefile/nodeserver.go#L366
   5. in mount.go set the correct gid
   6. .... things I am misisng
   7. it works :)

[apricote]

Hey @schmidp,

you mention adding the capability to GetPluginCapabilities. Too me it looks like it should be added to: NodeGetCapabilities instead:

You are correct, it needs to be in the NodeGetCapabilities response.

A plan of action could be:

Sounds good, we should also add some unit/integrations for the helper functions. For e2e tests we can rely on the kubernetes test suite which already has tests around this. Once we add the capability, these tests should automatically use the new functionality instead of the kubelet-native alternative.

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