Kubernetes 1.26: Support for Passing Pod fsGroup to CSI Drivers At Mount Time

Authors: Fabio Bertinatto (Red Hat), Hemant Kumar (Red Hat)

Delegation of fsGroup to CSI drivers was first introduced as alpha in Kubernetes 1.22, and graduated to beta in Kubernetes 1.25. For Kubernetes 1.26, we are happy to announce that this feature has graduated to General Availability (GA).

In this release, if you specify a fsGroup in the security context, for a (Linux) Pod, all processes in the pod's containers are part of the additional group that you specified.

In previous Kubernetes releases, the kubelet would always apply the fsGroup ownership and permission changes to files in the volume according to the policy you specified in the Pod's .spec.securityContext.fsGroupChangePolicy field.

Starting with Kubernetes 1.26, CSI drivers have the option to apply the fsGroup settings during volume mount time, which frees the kubelet from changing the permissions of files and directories in those volumes.

How does it work?

CSI drivers that support this feature should advertise the VOLUME_MOUNT_GROUP node capability.

After recognizing this information, the kubelet passes the fsGroup information to the CSI driver during pod startup. This is done through the NodeStageVolumeRequest and NodePublishVolumeRequest CSI calls.

Consequently, the CSI driver is expected to apply the fsGroup to the files in the volume using a mount option. As an example, Azure File CSIDriver utilizes the gid mount option to map the fsGroup information to all the files in the volume.

It should be noted that in the example above the kubelet refrains from directly applying the permission changes into the files and directories in that volume files. Additionally, two policy definitions no longer have an effect: neither .spec.fsGroupPolicy for the CSIDriver object, nor .spec.securityContext.fsGroupChangePolicy for the Pod.

For more details about the inner workings of this feature, check out the enhancement proposal and the CSI Driver fsGroup Support in the CSI developer documentation.

Why is it important?

Without this feature, applying the fsGroup information to files is not possible in certain storage environments.

For instance, Azure File does not support a concept of POSIX-style ownership and permissions of files. The CSI driver is only able to set the file permissions at the volume level.

How do I use it?

This feature should be mostly transparent to users. If you maintain a CSI driver that should support this feature, read CSI Driver fsGroup Support for more information on how to support this feature in your CSI driver.

Existing CSI drivers that do not support this feature will continue to work as usual: they will not receive any fsGroup information from the kubelet. In addition to that, the kubelet will continue to perform the ownership and permissions changes to files for those volumes, according to the policies specified in .spec.fsGroupPolicy for the CSIDriver and .spec.securityContext.fsGroupChangePolicy for the relevant Pod.

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