Variant Symlink Filesystem

This module provides a Variant Symlink Filesystem for linux similar to the one implemented in FreeBSD.

A Variant Symlink is a symlink that is based off of an environment variable's value. If the environment variable is not present, the symlink simply looks like a broken link, and cannot be followed. If however the process contains the specified environment variable, it will be used to determine the path that the symlink will resolve to.

Building

Simply clone the repo, and inside the directory run the following:

$ make

This should build the Filesystem as a module.

Usage

Once the kernel module has been built, simply load it, and create a mount point at the desired location:

$ sudo insmod varsymfs.ko
$ sudo mount -o ENV_VAR -t varsymfs none /opt/pq/0

This will create a mount point under the directory '/opt/pq/0/' which will contain a single symlink 'resolve'. The symlink will resolve to the value of the environment variable ENV_VAR. If the environment variable is not set, the symlink will simply appear to be a broken symlink.

Demo

guestm@lvm:~/work/varsymfs/varsymfs-new [0]
$ ls -l /opt/pq/0
total 0
lrwxrwxrwx 1 root root 0 Oct  9 09:49 resolve
guestm@lvm:~/work/varsymfs/varsymfs-new [0]
$ export ENV_VAR=~/pq/opt/pq/0/chassis/
next/ test/
guestm@lvm:~/work/varsymfs/varsymfs-new [0]
$ export ENV_VAR=~/pq/opt/pq/0/chassis/test/
guestm@lvm:~/work/varsymfs/varsymfs-new [0]
$ exec bash
guestm@lvm:~/work/varsymfs/varsymfs-new [0]
$ ls -l /opt/pq/0
total 0
lrwxrwxrwx 1 root root 0 Oct  9 09:49 resolve -> /home/guestm/pq/opt/pq/0/chassis/test/
guestm@lvm:~/work/varsymfs/varsymfs-new [0]

Thanks

• Justin Swartz ( [email protected] ) For providing me with code to do the environment variable lookup from inside a kernel module.