Kernel Module Flake

A nix flake dedicated to making the developer tooling around kernel module development easier. There are two ways to use this flake:

1. Clone this flake and start hacking.
2. Add this flake as an input to your own flake and use the provided scripts and builder functions.

The first way is recommended if you just want to get up and running immediately and start hacking. The second way is for someone who wants to integrate this into their own kernel module development process without keeping a thousand lines of Nix up to date.

Features

• Compile a minimal kernel designed for debugging (Time to build on Ryzen 5 3600 - 6 cores):

• Enabled by default Rust support & the ability to switch to the Rust-For-Linux branch
• Nix functions for building Rust and C kernel modules
• QEMU VM support using Nix's built in functions for generating an initramfs
• Remote GDB debugging through the VM
• Comes with an editor that is configured to have language diagnostics for C and Rust development (can be disabled)
• Out of tree rust-analyzer support

Cloning the flake

Get started by cloning this repository.

git clone [email protected]:jordanisaacs/kernel-module-flake
cd kernel-module-flake

# nix develop .# or direnv allow to get into the dev environment

runvm   # Calls QEMU with the necessary commands, uses sudo for enabling kvm

#### Inside QEMU
# insmod module/helloworld.ko   # Load the kernel module
# rmmod module/helloworld.ko    # Unload the module
#### C^A+X to exit
#### In another terminal while the VM is running
# rungdb                        # Connect to the VM with remote GDB debugging
### (GDB)
## lx-symbols-nix               # Runs lx-symbols with the nix store paths of the modules
####


cd helloworld
bear -- make            # generate the compile_commands.json
vim helloworld.c        # Start editing!

# exit and then nix develop .# or just direnv reload
# to rebuild and update the runvm command

Flake as an input

The lib.builders output of the flake exposes all the components as Nix builder functions. You can use them to compile your own kernel, configfile, initramfs, and generate the runvm and rungdb commands. An example of how the functions are used is below. See the flake.nix file for more details, and the build directory for the arguments that can be passed to the builders.

{
   inputs.kernelFlake.url = "github:jordanisaacs/kernel-module-flake";

   outputs =  {
     self,
     nixpkgs,
     kernelFlake
   }: let
     system = "x86_64-system";
     pkgs = nixpkgs.legacyPackages.${system};

     kernelLib = kernelFlake.lib.builders {inherit pkgs;};

     buildRustModule = buildLib.buildRustModule {inherit kernel;};
     buildCModule = buildLib.buildCModule {inherit kernel;};

     configfile = buildLib.buildKernelConfig {
       generateConfigFlags = {};
       structuredExtraConfig = {};

       inherit kernel nixpkgs;
     };

     kernel = buildLib.buildKernel {
       inherit configfile;

       src = ./kernel-src;
       version = "";
       modDirVersion = "";
     };

     modules = [exampleModule];

     initramfs = buildLib.buildInitramfs {
       inherit kernel modules;
     };

     exampleModule = buildCModule { name = "example-module"; src = ./.; };

     runQemu = buildLib.buildQemuCmd {inherit kernel initramfs;};
     runGdb = buildLib.buildGdbCmd {inherit kernel modules;};
   in { };
}

How it works

Kernel Build

A custom kernel is built according to Chris Done's Build and run minimal Linux / Busybox systems in Qemu. Extra config is added which I got through Kaiwan Billimoria's Linux Kernel Programming.

First a derivation is built for the .config file. It is generated using a modified version of the configfile derivation in the generic kernel builder (also known as the buildLinux function). This modified derivation is required to remove the NixOS distribution default configuration. More documentation is in the build/c-module.nix the flake.

Compiling the kernel is the same as Nix but modified to not remove any of the source files from the dev output. This is because they are necessary for things such as gdb debugging, and rust development.

Then a new package set called linuxDev is then added as an overlay using linuxPackagesFor.

Rust Support

Rust support is enabled by default using kernel version 6.1. You can disable Rust support and the kernel will build without it by setting enableRust = false in flake.nix. Note that you cannot do much with Rust in 6.1 so there is a second option to use Rust For Linux's branch. This is disabled by default and can be turn on by setting useRustForLinux = true in flake.nix. It will change from building the rust/rust_out_of_tree.rs module to the rfl_rust/rust_out_of_tree.rs by default. The enableRust option can be passed

Kernel Modules

The kernel modules are built using nix. You can build them manually with nix build .#helloworld and nix build .#rustOutOfTree. They are copied into the initramfs for you. There is a buildCModule and buildRustModule function exposed for building your own modules (build/rust-module.nix and build/c-module.nix).

eBPF Support

eBPF is enabled by default. This makes the initrd much larger due to needing python for bcc, and the compile time of the linux kernel longer. You can disable it by setting enableBPF = false in flake.nix.

Remote GDB

Remote GDB debugging is activated through the rungdb command (build/run-gdb.nix). It wraps GDB to provide the kernel source in the search path, loads vmlinux, sources the kernel gdb scripts, and then connects to the VM. An alias is provided lx-symbols-nix that runs the lx-symbols command with all the provided modules' nix store paths as search directories.

initramfs

The initial ram disk is built using the new make-initrd-ng. It is called through its nix wrapper which safely copies the nix store packages needed over. To see how to include modules and other options see the builder, build/initramfs.nix.

Neovim Editor

A neovim editor is provided that is set up for Nix, C (CCLS), and Rust (Rust-Analyzer). See my neovim-flake for more details on how the configuration works. It is enabled by default but can be disabled in flake.nix by setting enableEditor = false.

C

Clang-format was copied over from the linux source tree. To get CCLS working correctly call bear -- make to get a compile_commands.json. Then open up C files.

Rust

The flake is configured to build the kernel with a rust-project.json but it is not usable to out of tree modules. A script is run that parses the kernel's rust-project.json and generates one for the module itself. It is accessed with make rust-analyzer. Credit to thepacketgeek for the script. Additionally, rust-analyzer is designed to use cargo check for diagnostics. There is an opt-out to use rustc outputs which is configured within the editor's rust-analyzer configuration.

Direnv

If you have nix-direnv enabled a shell with everything you need should open when you cd into the directory after calling direnv allow