According to a previous post I made here: https://security.stackexchange.com/questions/180170/protecting-folder-contents-from-processes, it should be technically possible to implement a system that utilizes the standard Discretionary Access Control model that the linux kernel natively supports, in which a list of processes of my choosing will be the only ones capable of reading a specific mounted directory.
Specifically I wish to have my file manager (Nautilus) be able to read from the contents of my mounted directory and display to me the folders and files inside as it normally should, but have any other process be unable to do so.
To that end, I created a new group for that specific purpose, I changed the executable /usr/bin/nautilus 's group from "root" to that special group and I also set the setgid flag on that file so that the processes initiating from that file will have their effective group ID the one of the group which is meant to have access to that mounted folder. Basically nautilus' effective running group as a process will be the special group.
Finally, I went to the mounted folder and did a "chgrp -R special_group ." and "chmod -R 770 .". The owner of the that folder is "root".
All fine and dandy, except that now I'm locked out of the mounted folder. Nautilus, for whatever reason, cannot access this folder and gives me generic "Permission denied". Worse, I cannot access my own user's Trash folder because of the same error.
Am I doing something completely wrong here?