AddFS

A FUSE-Filesystem where entries can be added but not be removed or overwritten.

This programm uses FUSE to mount an existing directory in some kind of append-only (or better WORM) mode.
Inside the mountpoint new files/folders can be created (and written to once) but not be deleted/modified/truncated/moved.

How to use

sudo addfs [flags] <sourcedir> <mountpoint>

Why?

Well, let's assume you have created log-files or backups and want to protect them in-case your user-account gets compromised. If you write them to an addfs-mountpoint the files will be safe from deletion/modification by regular users, but you are still able to create new logfiles/backups without the need to be root.

This isn't terribly usefull on a local machine, but pretty handy when used on a remote server (ssh/ftp). This way a user-account can upload new files via scp but not modify or delete previously uploaded files. You succesfully created a remote write-once-read-many storage!

Installation

Binary Download:

sudo curl -L -o /usr/bin/addfs https://github.com/dbaumgarten/addfs/releases/download/latest/addfs
sudo chmod +x /usr/bin/addfs

From source:

go install github.com/dbaumgarten/addfs

FAQ

Can't a user just modify the actual files outside the mount-point?

Only if he has access to the actual files. Just take away his write&execute-permissions for the actual directory. As addfs runs as root the user can still write to the directory via the mountpoint but not directly.

Does this mean root owns all the files?

No all creates files/folders belong to the creating user and not to root. Therefore the creating user can read his files and set permissions for his files as usual.

What if I really need to delete/modify an existing file?

Pass --allowRootMutation when running addfs and the root-user will be able to do all the otherwise forbidden operations.

Can I exclude certain files/folders from the write-protection?

Sure, there is --mutableFiles for this. You can specify multiple regular expressions and all files/folders matching at least one of the expressions can be modified like on a "normal" filesystem.
Example (All files ending on .tmp or .swp are mutable):

sudo addfs --mutableFiles '.*\.tmp' --mutableFiles '.*\.swp' /foo /bar

Who can read the files?

Inside the mountpoint standard linux file-permissions are active. You can only read/execute a file if you have the necessary permissions on said file.

Who can create files?

Anyone with w+x access to the mountpoint.

Tests

There is a bash-script (integration_test.sh) to test the functionality of the programm.
To run it some prequesites must be met:

• Be a user that is not root
• Be able to use sudo to become root
• Have a working FUSE-Installation

If all this is done:

./integration_test.sh