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 ];
}

Note that wireguard has been merged into the mainline kernel, so on newer (21.05+) versions of NixOS with the default kernel wireguard is no longer an option available.

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,^[1]^[2] 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
              '';
        } ];
}

Another way to build a kernel with custom configuration is to create a overlay and use either extraConfig or structuredExtraConfig. This allows for more flexibility, since you can basically override any kernel option available here ^[3]. Using structuredExtraConfig is recommended since there is some sanity check if the options you're passing to your kernel is correct. Also, setting ignoreConfigErrors is useful to avoid error: unused option during build.

{
  nixpkgs = {
    overlays = [
      (self: super: {
        linuxZenWMuQSS = pkgs.linuxPackagesFor (pkgs.linux_zen.override {
          structuredExtraConfig = with lib.kernel; {
            SCHED_MUQSS = yes;
          };
          ignoreConfigErrors = true;
        });
      })
    ];
  };
}

Pinning a kernel version

boot.kernelPackages = pkgs.linuxPackagesFor (pkgs.linux_4_19.override {
    argsOverride = rec {
      src = pkgs.fetchurl {
            url = "mirror://kernel/linux/kernel/v4.x/linux-${version}.tar.xz";
            sha256 = "0ibayrvrnw2lw7si78vdqnr20mm1d3z0g6a0ykndvgn5vdax5x9a";
      };
      version = "4.19.60";
      modDirVersion = "4.19.60";
      };
  });

Debugging a failed configuration

As dependencies between kernel configurations items need to be addressed manually, use --keep-failed to 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.

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 ++ [ pkg-config 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 ++ [ pkg-config 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://github.com/archlinux/svntogit-packages/blob/packages/linux/trunk/config - 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);
}

Out-of-tree kernel modules

Packaging out-of-tree kernel modules

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

{ stdenv, lib, 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" "format" ];                                             # 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 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 necessary to disable pic in compiler hardenings as the kernel need different compiler flags.

2. In addition to other dependencies in nativeBuildInputs you should include kernel.moduleBuildDependencies as this propagates additional libraries required during the build.

3. Some kernel modules try guess the kernel version based on the running kernel via uname. Usually they save this information in a makefile variable like KERNELRELEASE. If this is the case you can override the kernel version via makeFlags. The right kernel version string 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. Lastly it is required to give the kernel build system the right location where to install the kernel module. This is done by setting INSTALL_MOD_PATH to $out Otherwise an error like mkdir: cannot create directory '/lib': Permission denied is generated.

Developing out-of-tree 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!

If wishing to develop out-of-tree kernel modules for kernels aside from the default variant shipped under pkgs.linuxPackages, a flow similar to the example below will be used. The example below illustrates a flow for building kernel modules for pkgs.linuxPackages_latest.

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

Loading out-of-tree kernel modules

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
'';

Cross-compiling Linux from source

Save the following as shell.nix for your nix-shell:

{ pkgs ? import <nixpkgs> {} }:
let
  # more platforms are defined here: https://github.com/NixOS/nixpkgs/blob/master/lib/systems/examples.nix
  aarch64 = pkgs.pkgsCross.aarch64-multiplatform;
in
aarch64.linux.overrideAttrs (old: {
  # the override is optional if you need for example more build dependencies
  nativeBuildInputs = old.nativeBuildInputs ++ [ pkgs.gllvm ];
}

Than you can run the following commands (in the nix-shell $makeFlags will contain the necessary cross-compiling arguments)

$ nix-shell
# to configure
$ nix-shell> make $makeFlags defconfig -j $(nproc)
# to build
$ nix-shell> make $makeFlags -j $(nproc)

Compiling Linux with clang

Save this as shell.nix

{ pkgs ? import <nixpkgs> {} }:
pkgs.linux.override {
   # needs to be clang11Stdenv or newer
   stdenv = clang11Stdenv;
}

Than you can use:

$ nix-shell
# to configure
$ nix-shell> make $makeFlags defconfig -j $(nproc)
# to build
$ nix-shell> make $makeFlags -j $(nproc)

Overriding kernel packages

In order to override linuxPackages, use the extend attribute. Example:

linuxPackages.extend (self: super: {
  xpadneo = super.xpadneo.overrideAttrs (o: rec {
    src = pkgs.fetchFromGitHub {
      # Custom override goes here.
    };
  });
});

Here is a fully worked example to enable debugging for the zfsUnstable module:

 nixpkgs.overlays = [
    (self: super: {
      linuxPackages = super.linuxPackages.extend (lpself: lpsuper: {
        zfsUnstable = super.linuxPackages.zfsUnstable.overrideAttrs (oldAttrs: {
          configureFlags = oldAttrs.configureFlags ++ [ "--enable-debug" ];
        });
      });
    })
  ];

Troubleshooting

Build fails

Too high ram usage

turn off `DEBUG_INFO_BTF`

References

[1] #nixos 2018-05-09

[2] nixos/modules/misc/crashdump.nix#L63-L73

[3] pkgs/os-specific/linux/kernel/generic.nix#L11-L56

[1]

[2]

[3]