moss

moss is a Unix-like, Linux-compatible kernel written in Rust and AArch64 assembly.

It features an asynchronous kernel core, a modular architecture abstraction layer, and binary compatibility with Linux userspace applications. Moss is currently capable of running a dynamically linked Arch Linux AArch64 userspace, including bash, BusyBox, coreutils, ps, top, and strace.

Features

Architecture & Memory

• Full support for AArch64.
• A well-defined HAL allowing for easy porting to other architectures (e.g., x86_64, RISC-V).
• Memory Management:
  • Full MMU enablement and page table management.
  • Copy-on-Write (CoW) pages.
  • Safe copy to/from userspace async functions.
  • Kernel and userspace page fault management.
  • Kernel stack-overflow detection.
  • Shared library mapping and relocation.
  • /proc/self/maps support.
  • Buddy allocator for physical addresses and smalloc for boot allocations and tracking memory reservations.
  • A full slab allocator for kernel object allocations, featureing a per-CPU object cache.

Async Core

One of the defining features of moss is its usage of Rust's async/await model within the kernel context:

• All non-trivial system calls are written as async functions, sleep-able functions are prefixed with .await.
• The compiler enforces that spinlocks cannot be held over sleep points, eliminating a common class of kernel deadlocks.
• Any future can be wrapped with the .interruptable() combinator, allowing signals to interrupt the waiting future and appropriate action to be taken.

Process Management

• Full task management including both UP and SMP scheduling via EEVDF and task migration via IPIs.
• Capable of running dynamically linked ELF binaries from Arch Linux.
• Currently implements 105 Linux syscalls
• fork(), execve(), clone(), and full process lifecycle management.
• Job control support (process groups, waitpid, background tasks).
• Signal delivery, masking, and propagation (SIGTERM, SIGSTOP, SIGCONT, SIGCHLD, etc.).
• ptrace support sufficient to run strace on Arch binaries.

VFS & Filesystems

• Virtual File System with full async abstractions.
• Drivers:
  • Ramdisk block device implementation.
  • FAT32 filesystem driver (ro).
  • Ext2/3/4 filesystem driver (read support, partial write support).
  • devfs driver for kernel character device access.
  • tmpfs driver for temporary file storage in RAM (rw).
  • procfs driver for process and kernel information exposure.

libkernel & Testing

moss is built on top of libkernel, a utility library designed to be architecture-agnostic. This allows logic to be tested on a host machine (e.g., x86) before running on bare metal.

• Address Types: Strong typing for VA (Virtual), PA (Physical), and UA (User) addresses.
• Containers: VMA management, generic page-based ring buffer (kbuf), and waker sets.
• Sync Primitives: spinlock, mutex, condvar, per_cpu.
• Test Suite: A comprehensive suite of 230+ tests ensuring functionality across architectures (e.g., validating AArch64 page table parsing logic on an x86 host).
• Userspace Testing, usertest: A dedicated userspace test-suite to validate syscall behavior in the kernel at run-time.

Building and Running

Prerequisites

You will need QEMU for AArch64 emulation, as well as wget, e2fsprogs, and jq for image creation. We use just as a task runner to simplify common commands, but you can also run the underlying commands directly if you prefer.

Additionally, you will need a version of the aarch64-none-elf toolchain installed.

To install aarch64-none-elf on any OS, download the appropriate release of aarch64-none-elf onto your computer, unpack it, then export the bin directory to PATH (Can be done via running: export PATH="~/Downloads/arm-gnu-toolchain-X.X.relX-x86_64-aarch64-none-elf/bin:$PATH", where X is the version number you downloaded onto your machine, in your terminal).

Debian/Ubuntu

sudo apt install qemu-system-aarch64 wget jq e2fsprogs just

macOS

brew install qemu wget jq e2fsprogs just

NixOS

Run the following command:

nix shell nixpkgs#pkgsCross.aarch64-embedded.stdenv.cc nixpkgs#pkgsCross.aarch64-embedded.stdenv.cc.bintools

Running via QEMU

To build the kernel and launch it in QEMU:

just run

If you don't have just installed, you can run the underlying commands directly:

# First time only (to create the image)
./scripts/create-image.sh
# Then, to run the kernel in QEMU
# By default it will launch into bash, which alpine doesn't have, however `ash` and `sh` are both available.
cargo run --release -- /bin/ash

The kernel runs off of moss.img. This image is a minimal alpine rootfs with the addition of a custom usertest binary in /bin/usertest.

Running the Test Suite

Because libkernel is architecturally decoupled, you can run the logic tests on your host machine:

just test-unit

To run the userspace test suite in QEMU:

just test-userspace

or

cargo run -r -- /bin/usertest

If you've made changes to the usertests and want to recreate the image, you can run:

just create-image

or

./scripts/create-image.sh

Roadmap & Status

moss is under active development. Current focus areas include:

• Networking Stack: TCP/IP implementation.
• A fully read/write capable filesystem driver.
• Expanding coverage beyond the current 105 calls.
• systemd bringup.

Non-Goals (for now)

• Binary compatibility beyond AArch64.
• Production hardening.

Moss is an experimental kernel focused on exploring asynchronous design and Linux ABI compatibility in Rust.

Contributing

Contributions are welcome! Whether you are interested in writing a driver, porting to x86, or adding syscalls.

License

Distributed under the MIT License. See LICENSE for more information.