Creating and manipulating filesystems on Linux typically involves deep kernel expertise, a barrier for many developers. However, the Linux kernel provides a powerful interface called FUSE, which stands for Filesystem in Userspace. This allows anyone to develop fully functional filesystems simply as a user-space program, without needing to touch kernel code.
The beauty of FUSE is that it abstracts the complex kernel-level interactions. Instead of writing intricate kernel modules, you implement a set of functions in a user-space application that act as callbacks. When the kernel receives a standard filesystem operation (like opening a file, reading, writing, or listing a directory) for a filesystem mounted via FUSE, it passes that request to your user-space program’s corresponding callback function. Your program then performs whatever action is needed – perhaps reading from another location, generating data on the fly, or even interacting with a network service – and returns the result back to the kernel, which then returns it to the requesting application.
This capability opens up fascinating possibilities beyond just standard disk-based filesystems. One compelling example is the creation of a decoy filesystem. Imagine a scenario where you want to protect sensitive directories or perhaps set up a simple honeypot. You can use FUSE to create a virtual filesystem that mounts over a sensitive path. When an attacker or unauthorized process tries to access files within this path, the FUSE filesystem intercepts the requests.
Instead of providing real data, the decoy filesystem can present fabricated, harmless content. For instance, a request to read a file named secrets.txt might trigger your FUSE program to return a string like “This is a fake file. Nothing to see here!” A directory listing (ls) might show plausible-sounding but non-existent filenames. Writes could be silently ignored or logged as suspicious activity. This virtual filesystem layer acts as a shield, misleading intruders into thinking they have accessed legitimate data while the actual sensitive information remains untouched elsewhere.
Building such a decoy filesystem involves using a FUSE library (available in various languages like C, Python, Go, etc.) and implementing the necessary callback functions. You’d need handlers for getattr (to define file/directory properties like size, permissions), readdir (to list directory contents), open, read, write, and potentially others depending on the desired complexity and realism of the decoy. The core logic resides in these functions, determining what information is presented based on the requested path and operation.
Using FUSE to build a decoy filesystem is a practical demonstration of its flexibility. It showcases how developers can craft custom data interfaces and add layers of security or misdirection directly from user space. This powerful feature of Linux allows for creative solutions in data handling, security monitoring, and system customization, proving that filesystem innovation isn’t solely the domain of kernel developers.
Source: https://www.linuxtechi.com/create-decoy-file-system-in-linux/
