Determining User Access on a Linux Filesystem with „Classic Permissions“


Looking at a Linux filesystem, checking if a certain file or directory is accessible for reading, writing or executing by certain users or groups poses interesting challenges.

Let the basic and seemingly simple question be: „Given a user X and a file Y, can it be determined if X has access to Y, and if yes, how can it be determined?“ A simple answer was: „Let X try to access Y, and if it does not work, X does not have that kind of access.“ However, this may not be feasible: The users, files and directories in question may not exist yet. More generally, access by users to files and directories should be predictable; appropriate access restrictions should be placed in advance, not after exposing possibly sensitive information. Also, certain types of access, such as deleting a file or directory, can not be simulated in a safe manner.

Moreover, a test procedure that just involves „trying to access the file“ may be incomplete: Just because the way the test procedure has attempted access did not succeed, that does not mean that there is no procedure at all by which the user in question can access the file.

This article investigates Linux filesystems that implement the semantics of „classic UNIX permissions“ in an effort to find more exhaustive methods of determining access.