# Linux kernel {#sec-linux-kernel}

The Nix expressions to build the Linux kernel are in [`pkgs/os-specific/linux/kernel`](https://github.com/NixOS/nixpkgs/blob/master/pkgs/os-specific/linux/kernel).

The function [`pkgs.buildLinux`](https://github.com/NixOS/nixpkgs/blob/d77bda728d5041c1294a68fb25c79e2d161f62b9/pkgs/os-specific/linux/kernel/generic.nix) builds a kernel with [common configuration values](https://github.com/NixOS/nixpkgs/blob/d77bda728d5041c1294a68fb25c79e2d161f62b9/pkgs/os-specific/linux/kernel/common-config.nix).
This is the preferred option unless you have a very specific use case.
Most kernels packaged in Nixpkgs are built that way, and it will also generate kernels suitable for NixOS.
[`pkgs.linuxManualConfig`](https://github.com/NixOS/nixpkgs/blob/d77bda728d5041c1294a68fb25c79e2d161f62b9/pkgs/os-specific/linux/kernel/build.nix) requires a complete configuration to be passed.
It has fewer additional features than `pkgs.buildLinux`, which provides common configuration values and exposes the `features` attribute, as explained below.

Both functions have an argument `kernelPatches` which should be a list of `{name, patch, extraConfig}` attribute sets, where `name` is the name of the patch (which is included in the kernel’s `meta.description` attribute), `patch` is the patch itself (possibly compressed), and `extraConfig` (optional) is a string specifying extra options to be concatenated to the kernel configuration file (`.config`).

The kernel derivation created with `pkgs.buildLinux` exports an attribute `features` specifying whether optional functionality is or isn’t enabled. This is used in NixOS to implement kernel-specific behaviour.

If you are using a kernel packaged in Nixpkgs, you can customize it by overriding its arguments. For details on how each argument affects the generated kernel, refer to [the `pkgs.buildLinux` source code](https://github.com/NixOS/nixpkgs/blob/d77bda728d5041c1294a68fb25c79e2d161f62b9/pkgs/os-specific/linux/kernel/generic.nix).

:::{.example #ex-overriding-kernel-derivation}

# Overriding the kernel derivation

Assuming you are using the kernel from `pkgs.linux_latest`:

```nix
pkgs.linux_latest.override {
  ignoreConfigErrors = true;
  autoModules = false;
  kernelPreferBuiltin = true;
  structuredExtraConfig = with lib.kernel; {
    DEBUG_KERNEL = yes;
    FRAME_POINTER = yes;
    KGDB = yes;
    KGDB_SERIAL_CONSOLE = yes;
    DEBUG_INFO = yes;
  };
}
```

:::

## Manual kernel configuration {#sec-manual-kernel-configuration}

Sometimes it may not be desirable to use kernels built with `pkgs.buildLinux`, especially if most of the common configuration has to be altered or disabled to achieve a kernel as expected by the target use case.
An example of this is building a kernel for use in a VM or micro VM. You can use `pkgs.linuxPackages_custom` in these cases. It requires the `src`, `version`, and `configfile` attributes to be specified.

:::{.example #ex-using-linux-manual-config}

# Using `pkgs.linuxPackages_custom` with a specific source, version, and config file

```nix
{ pkgs, ... }:
pkgs.linuxPackages_custom {
  version = "6.1.55";
  src = pkgs.fetchurl {
    url = "https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-${version}.tar.xz";
    hash = "sha256-qH4kHsFdU0UsTv4hlxOjdp2IzENrW5jPbvsmLEr/FcA=";
  };
  configfile = ./path_to_config_file;
}
```

If necessary, the version string can be slightly modified to explicitly mark it as a custom version. If you do so, ensure the `modDirVersion` attribute matches the source's version, otherwise the build will fail.

```nix
{ pkgs, ... }:
pkgs.linuxPackages_custom {
  version = "6.1.55-custom";
  modDirVersion = "6.1.55";
  src = pkgs.fetchurl {
    url = "https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-${modDirVersion}.tar.xz";
    hash = "sha256-qH4kHsFdU0UsTv4hlxOjdp2IzENrW5jPbvsmLEr/FcA=";
  };
  configfile = ./path_to_config_file;
}
```

:::

Additional attributes can be used with `linuxManualConfig` for further customisation instead of `linuxPackages_custom`. You're encouraged to read [the `pkgs.linuxManualConfig` source code](https://github.com/NixOS/nixpkgs/blob/d77bda728d5041c1294a68fb25c79e2d161f62b9/pkgs/os-specific/linux/kernel/build.nix) to understand how to use them.

To edit the `.config` file for Linux X.Y from within Nix, proceed as follows:

```ShellSession
$ nix-shell '<nixpkgs>' -A linuxKernel.kernels.linux_X_Y.configEnv
$ unpackPhase
$ cd linux-*
$ make nconfig
```

## Developing kernel modules {#sec-linux-kernel-developing-modules}

When developing kernel modules it's often convenient to run the edit-compile-run loop as quickly as possible.
See the snippet below as an example.

:::{.example #ex-edit-compile-run-kernel-modules}

# Edit-compile-run loop when developing `mellanox` drivers

```ShellSession
$ nix-build '<nixpkgs>' -A linuxPackages.kernel.dev
$ nix-shell '<nixpkgs>' -A linuxPackages.kernel
$ unpackPhase
$ cd linux-*
$ make -C $dev/lib/modules/*/build M=$(pwd)/drivers/net/ethernet/mellanox modules
# insmod ./drivers/net/ethernet/mellanox/mlx5/core/mlx5_core.ko
```

:::

## Maintainer information {#sec-linux-kernel-maintainer-information}

### Updating kernels {#sec-linux-updates}

Updating all kernels can be done with the following script:

```ShellSession
$ pkgs/os-specific/linux/kernel/update.sh
```

The change gets submitted like this:

* File a PR against `staging-nixos`.
  * Add a `backport release-XX.XX` label for an automated backport.
    We don't expect many other changes on that branch to require a backport, hence there's no such branch for stable.
    By using an additional PR, we get the automatic backport against stable without manual cherry-picks.
* Merge into `staging-nixos`.
* File as PR from `staging-nixos` against `master`.
* When all status checks are green, merge.

### Add a new (major) version of the Linux kernel {#sec-linux-add-new-kernel-version}

* When running `./pkgs/os-specific/linux/kernel/update.sh`, new kernel majors get discovered automatically.
* Prepare all Nix expressions for the new kernel
  * Instantiate the new kernel in `pkgs/top-level/linux-kernels.nix` in the `kernels`-section.
    ```nix
    {
      linux_X_Y = callPackage ../os-specific/linux/kernel/mainline.nix {
        branch = "X.Y";
        kernelPatches = [
          # any new patches required (it makes to look which patches are used by its predecessor)
        ];
      };
    }
    ```
  * Instantiate the package-set in `vanillaPackages`:
    ```nix
    {
      linux_X_Y = recurseIntoAttrs (packagesFor kernels.linux_X_Y);
    }
    ```
  * Update `linux_latest` to the new attribute.
* __SQUASH__ the changes into the `linux: init at …` commit.
* If a new hardened is available:
  * Instantiate a `linux_X_Y_hardened = hardenedKernelsFor kernels.linux_X_Y { };` in `kernels` and
    `linux_X_Y_hardened = hardenedKernelFor kernels.linux_X_Y { };` in the `packages`-section.
  * Make sure to remove the hardened variant of the previous kernel version unless it's LTS.
    We only support the latest and latest LTS version of hardened.
* If no new hardened kernel is available:
  * Keep the previously latest kernel until its mainline counterpart gets removed.
    After that `linux_hardened` points to the latest LTS supported by hardened.
* __SQUASH__ the changes into the `linux_X_Y_hardened: init at …` commit.
