Recently I thought I had a great use for storing extended attributes. I needed to store the time a file had been updated by my file management system without effecting the actual file modifications times. Storing this information directly in the file metadata would ensure that it would not become out of sync with some external database tracking this information.
I wanted this data to be modifiable only by a specific user or group. Unfortunately, setting an ACL on extended attributes applies to all extended attributes. I didn’t want to disallow users from setting their own extended attributes on these files and folders, so I abandoned the idea.
I wonder if we’ll ever be able to set permissions on specific attributes.