Linux kernel

By default, the latest LTS linux kernel is installed (Linux Kernel Version History).

Configuration

You can choose your kernel simply by setting the boot.kernelPackages option

For example by adding this to /etc/nixos/configuration.nix:

boot.kernelPackages = pkgs.linuxPackages_latest;

And rebuild your system and reboot to use your new kernel:

$ sudo nixos-rebuild boot
$ sudo reboot

List available kernels

You can list available kernels using nix repl (previously nix-repl) by typing the package name and using the tab completion:

$ nix repl

nix-repl> :l <nixpkgs>
Added 12607 variables.

nix-repl> pkgs.linuxPackages
pkgs.linuxPackages                           pkgs.linuxPackages_latest
pkgs.linuxPackages-libre                     pkgs.linuxPackages_latest-libre
pkgs.linuxPackages-rt                        pkgs.linuxPackages_latest_hardened
pkgs.linuxPackages-rt_5_4                    pkgs.linuxPackages_latest_xen_dom0
pkgs.linuxPackages-rt_5_6                    pkgs.linuxPackages_latest_xen_dom0_hardened
pkgs.linuxPackages-rt_latest                 pkgs.linuxPackages_mptcp
pkgs.linuxPackagesFor                        pkgs.linuxPackages_rpi0
pkgs.linuxPackages_4_14                      pkgs.linuxPackages_rpi1
pkgs.linuxPackages_4_19                      pkgs.linuxPackages_rpi2
pkgs.linuxPackages_4_4                       pkgs.linuxPackages_rpi3
pkgs.linuxPackages_4_9                       pkgs.linuxPackages_rpi4
pkgs.linuxPackages_5_4                       pkgs.linuxPackages_testing
pkgs.linuxPackages_5_8                       pkgs.linuxPackages_testing_bcachefs
pkgs.linuxPackages_5_9                       pkgs.linuxPackages_testing_hardened
pkgs.linuxPackages_custom                    pkgs.linuxPackages_xen_dom0
pkgs.linuxPackages_custom_tinyconfig_kernel  pkgs.linuxPackages_xen_dom0_hardened
pkgs.linuxPackages_hardened                  pkgs.linuxPackages_zen
pkgs.linuxPackages_hardkernel_4_14
pkgs.linuxPackages_hardkernel_latest

Custom kernel modules

Note that if you deviate from the default kernel version, you should also take extra care that extra kernel modules must match the same version. The safest way to do this is to use config.boot.kernelPackages to select the correct module set:

{ config, ... }:
{
  boot.extraModulePackages = with config.boot.kernelPackages; [ wireguard ];
}

Custom kernel commandline

The config attribute boot.kernelParams can be set to supply the Linux kernel with additional command line arguments at boot time.

{ pkgs, config, ... }:

{
  boot.kernelParams = [ /* list of command line arguments */ ];
}

Custom configuration

It is sometimes desirable to change the configuration of your kernel, while keeping the kernel version itself managed through Nixpkgs. To do so, you can add the configuration to a dummy boot.kernelPatches, which will then be merged and applied to the current kernel. As with kernel configuration with NixOS, drop the CONFIG_ prefix from the kernel configuration names.

This example is from the boot.crashDump.enable option:

{
      boot.kernelPatches = [ {
        name = "crashdump-config";
        patch = null;
        extraConfig = ''
                CRASH_DUMP y
                DEBUG_INFO y
                PROC_VMCORE y
                LOCKUP_DETECTOR y
                HARDLOCKUP_DETECTOR y
              '';
        } ];
}

Debugging a failed configuration

As dependencies between kernel configurations items need to be addressed manually, you can inspect the intermediate nix config file after for instance the error

note: keeping build directory '/tmp/nix-build-linux-config-4.19.0-mptcp_v0.94.1.drv-0'

by opening /tmp/nix-build-linux-config-4.19.0-mptcp_v0.94.1.drv-0/.attr-0.

Developing kernel modules

See also: NixOS Manual, 12.2. Developing kernel modules

If you work on an out-of-tree kernel module the workflow could look as follow:

#include <linux/module.h>
#define MODULE_NAME "hello"
static int __init hello_init(void)
{
    printk(KERN_INFO "hello world!\n");
    return 0;
}
static void __exit hello_cleanup(void) {
    printk(KERN_INFO "bye world!\n");
}
module_init(hello_init);
module_exit(hello_cleanup);

obj-m += hello.o

$ nix-shell '<nixpkgs>' -A linux.dev
$ make -C $(nix-build -E '(import <nixpkgs> {}).linux.dev' --no-out-link)/lib/modules/*/build M=$(pwd) modules
$ insmod ./hello.ko
$ dmesg | grep hello
[   82.027229] hello world!

Load modules in the kernel

As far as I understand, if you developed a kernel module, you should end up with having some .ko files inside a subfolder inside $out/lib/modules/${kernel.modDirVersion}. Now, if you want to make your module loadable inside the kernel by modprobe, you should do:

boot.extraModulePackages = [ yourmodulename ];

Then, the user can load it using:

$ sudo modprobe yourmodulename

or unload it using

$ sudo modprobe -r yourmodulename

However, if you want to autoload your module at startup in stage 2, you need to do:

boot.kernelModules = [ "yourmodulename" ];

and the module will be automatically loaded after a reboot. If you want instead to load it at stage 1 (before the root is even mounted), you need to add it to boot.initrd.availableKernelModules and boot.initrd.kernelModules.

Note that if you don't reboot, you can still load manually the module using modprobe yourmodulename>, and to automatically enable a module during configuration switch/reboot, you can put modprobe yourmodulename || true inside the script of a systemctl service (it is for example what does wireguard).

Finally, if you want to define some options by default (used when you load manually a module using modprobe, or when the system boots), you can specify them in:

boot.extraModprobeConfig = ''
  options yourmodulename optionA=valueA optionB=valueB
'';

make menuconfig

It is (currently) not possible to run make menuconfig in the checked out linux kernel sources. This is because ncurses is not part of your working environment when you start it with nix-shell '<nixpkgs>' -A linuxPackages.kernel.

This nix-shell hack adds ncurses as a build dependency to the kernel:

$ nix-shell -E 'with import <nixpkgs> {}; linux.overrideAttrs (o: {nativeBuildInputs=o.nativeBuildInputs ++ [ pkgconfig ncurses ];})'
[nix-shell] $ unpackPhase && cd linux-*
[nix-shell] $ patchPhase
[nix-shell] $ make menuconfig

(thanks to sphalerite)

make xconfig

Similarly to make menuconfig, you need to import qt in the environment:

$ nix-shell -E 'with import <nixpkgs> {}; linux.overrideAttrs (o: {nativeBuildInputs=o.nativeBuildInputs ++ [ pkgconfig qt5.qtbase ];})'

If the source was unpacked and an initial config exists, you can run make xconfig KCONFIG_CONFIG=build/.config

Requesting a change in the default nixos kernel configuration

Please provide a comparison with other distributions' kernel: - arch: https://git.archlinux.org/svntogit/packages.git/tree/trunk/config?h=packages/linux - debian: https://salsa.debian.org/kernel-team/linux/blob/master/debian/config/config and the ARCH specific ones

Booting a kernel from a custom source

The following example shows how to configure NixOS to compile and boot a kernel from a custom source, and with custom configuration options.

{ pkgs, ... }:

{
  boot.kernelPackages = let
      linux_sgx_pkg = { fetchurl, buildLinux, ... } @ args:

        buildLinux (args // rec {
          version = "5.4.0-rc3";
          modDirVersion = version;

          src = fetchurl {
            url = "https://github.com/jsakkine-intel/linux-sgx/archive/v23.tar.gz";
            sha256 = "11rwlwv7s071ia889dk1dgrxprxiwgi7djhg47vi56dj81jgib20";
          };
          kernelPatches = [];

          extraConfig = ''
            INTEL_SGX y
          '';

          extraMeta.branch = "5.4";
        } // (args.argsOverride or {}));
      linux_sgx = pkgs.callPackage linux_sgx_pkg{};
    in 
      pkgs.recurseIntoAttrs (pkgs.linuxPackagesFor linux_sgx);
}

Packaging

Packaging an out-of-tree kernel Module

There are a couple of steps that you will most likely need to do a couple of things. Here is an annotated example:

{ stdenv, fetchFromGitHub, kernel, kmod }:

stdenv.mkDerivation rec {
  name = "v4l2loopback-dc-${version}-${kernel.version}";
  version = "1.6";

  src = fetchFromGitHub {
    owner = "aramg";
    repo = "droidcam";
    rev = "v${version}";
    sha256 = "1d9qpnmqa3pfwsrpjnxdz76ipk4w37bbxyrazchh4vslnfc886fx";
  };

  sourceRoot = "source/linux/v4l2loopback";
  hardeningDisable = [ "pic" ];                                             # 1
  nativeBuildInputs = kernel.moduleBuildDependencies;                       # 2

  makeFlags = [
    "KERNELRELEASE=${kernel.modDirVersion}"                                 # 3
    "KERNEL_DIR=${kernel.dev}/lib/modules/${kernel.modDirVersion}/build"    # 4
    "INSTALL_MOD_PATH=$(out)"                                               # 5
  ];

  meta = with stdenv.lib; {
    description = "A kernel module to create V4L2 loopback devices";
    homepage = "https://github.com/aramg/droidcam";
    license = licenses.gpl2;
    maintainers = [ maintainers.makefu ];
    platforms = platforms.linux;
  };
}

1. for kernel modules it is normally necessary to disable pic and sometimes format hardening

2. most of the time you will need to set the moduleBuildDependencies to build a module

3. this kernel module requires the release to be set, it can be found in ${kernel.modDirVersion}

4. You need to find out how the build environment (Makefile in general) finds the kernel tree. This is sometimes KDIR and sometimes KERNEL_DIR.

5. When you get the error mkdir: cannot create directory '/lib': Permission denied then it may be necessary to set INSTALL_MOD_PATH. This can be done by setting via makeFlags or by setting it directly in the derivation.

See also

Kernel Debugging with QEMU