Project: LinuxKit (Provisioning / Automation & Configuration)
File: kernels.md

LinuxKit kernel images are distributed as hub images which contain the kernel, kernel modules, kernel config file, and optionally, kernel headers to compile kernel modules against. The repository containing the official LinuxKit kernels is at . The LinuxKit kernels are based on the latest stable releases and are updated frequently to include bug and security fixes. For some kernels we do carry additional patches, which are mostly back-ported fixes from newer kernels. The full kernel source with patches can be found on . We publish the following kernel images: primary kernel debug kernel tools for the specific kernel build - bcc and perf builder image for the specific kernel build, useful for compiling compatible kernel modules Each kernel image is tagged with: the full kernel version, e.g. `linuxkit/kernel:6.6.13`. This is a multi-arch index, and should be used whenever possible. the full kernel version plus hash of the files it was created from (git tree hash of the `./kernel` directory), e.g. `6.6.13-c0d96951e9892a7447a8e7965d2d6bd7e621c3fd`. This is a multi-arch index. the full kernel version plus architecture, e.g. `linuxkit/kernel:6.6.13-amd64` or `linuxkit/kernel:6.6.13-arm64`. Each of these is architecture specific. the full kernel version plus hash of the files it was created from (git tree hash of the `./kernel` directory) plus architecture, e.g. `6.6.13-c0d96951e9892a7447a8e7965d2d6bd7e621c3fd-arm64`. With each kernel image, we also publish kernels with additional debugging enabled. These have the same image name and the same tags as the primary kernel, with the `-dbg` suffix added immediately after the version. E.g. `linuxkit/kernel:6.6.13-dbg` `linuxkit/kernel:6.6.13-dbg-c0d96951e9892a7447a8e7965d2d6bd7e621c3fd` `linuxkit/kernel:6.6.13-dbg-amd64` `linuxkit/kernel:6.6.13-dbg-c0d96951e9892a7447a8e7965d2d6bd7e621c3fd-amd64` With each kernel image, we also publish images with various tools. As of this writing, those tools are `perf` and `bcc`. The tools images are named `linuxkit/kernel-<tool>`, followed by the same tags as the primary kernel. For example: `linuxkit/kernel-perf:6.6.13` `linuxkit/kernel-perf:6.6.13-c0d96951e9892a7447a8e7965d2d6bd7e621c3fd` `linuxkit/kernel-perf:6.6.13-amd64` `linuxkit/kernel-perf:6.6.13-c0d96951e9892a7447a8e7965d2d6bd7e621c3fd-amd64` In addition to the official images, there are also some which repackage kernels packages from some Linux distributions into LinuxKit kernel packages. These are mostly provided for testing purposes. Note now linuxkit also embraces Preempt-RT Linux kernel to support more use cases for the promising IoT scenarios. All -rt patches are grabbed from https://www.kernel.org/pub/linux/kernel/projects/rt/. But so far we just enable it over 4.14.x. Most kernel modules are autoloaded with `mdev` but if you need to `modprobe` a module manually you can use the `modprobe` package in the `onboot` section like this: ``` name: modprobe image: linuxkit/modprobe:<hash> command: ["modprobe", "-a", "iscsitcp", "dmmultipath"] ``` This section describes how to build external (out-of-tree) kernel modules. You need the following to build external modules. All of these are to be built for a specific version of the kernel. For the examples, we will assume 5.10.104; replace with your desired version. source available to your modules - you need to get those on your own kernel development headers - available in the `linuxkit/kernel` image as `kernel-dev.tar`, e.g. `linuxkit/kernel:5.10.104` OS with sources and compiler - this must* be the exact same version as that used to compile the kernel As described above, the `linuxkit/kernel` images include `kernel-dev.tar` which contains the headers and other files required to compile kernel modules against the specific version of the kernel. Currently, the headers are not included in the initial RAM disk, but it is possible to compile custom modules offline and then include the modules in the initial RAM disk. The source is available as the same name as the `linuxkit/kernel` image, with the addition of `-builder` on the tag. For example: `linuxkit/kernel:5.10.92` has builder `linuxkit/kernel:5.10.92-builder` `linuxkit/kernel:5.15.15` has builder `linuxkit/kernel:5.15.15-builder` With the above in hand, you can create a multi-stage `Dockerfile` build to compile your

There is an , but basically one can use a multi-stage build to compile the kernel modules: ```dockerfile FROM linuxkit/kernel:5.10.104 AS ksrc FROM linuxkit/kernel:5.10.104-builder AS build RUN apk add build-base COPY --from=ksrc /kernel-dev.tar / RUN tar xf kernel-dev.tar ``` To use the kernel module, we recommend adding a final stage to the Dockerfile above, which copies the kernel module from the `build` stage and performs a `insmod` as the entry point. You can add this package to the `onboot` section in your YAML file. contains an example for the configuration. As described above, the OS builder is referenced via `<kernel-image>-builder`, e.g. `linuxkit/kernel:5.15.15-builder`. As a fallback, in case the `-builder` image is not available or you cannot access it from your development environment, you have 3 total places to determine the correct version of the OS image with sources and compiler: `-builder` tag added to the kernel version, e.g. `linuxkit/kernel:5.10.104-builder` labels on the kernel image, e.g. `docker inspect linuxkit/kernel:5.10.104 | jq -r '.[].Config.Labels["org.mobyproject.linuxkit.kernel.buildimage"]'` `/kernel-builder` file in the kernel image You should use `-builder` tag as the `AS build` in your `Dockerfile`, but you can use the direct source, extracted from the labels or `/kernel-builder` file in the kernel image, in the `AS build`. For example, in the case of `5.10.104`, the label and `/kernel-builder` file show `linuxkit/alpine:2be490394653b7967c250e86fd42cef88de428ba`, so you can use either `linuxkit/alpine:2be490394653b7967c250e86fd42cef88de428ba` or `linuxkit/kernel:5.10.104-builder` to build the modules. Thus, the following are equivalent: ```dockerfile FROM linuxkit/kernel:5.10.104 AS ksrc FROM linuxkit/kernel:5.10.104-builder AS build ``` ```dockerfile FROM linuxkit/kernel:5.10.104 AS ksrc FROM linuxkit/alpine:2be490394653b7967c250e86fd42cef88de428ba AS build ``` This section describes how to build kernels, and how to modify existing ones. Throughout the document, the terms used are: kernel version: actual semver version of a kernel, e.g. `6.6.13` or `5.15.27` kernel series: major.minor version of a kernel, e.g. `6.6.x` or `5.15.x` Throughout this document, the architecture used is the kernel-recognized one, available on most systems as `uname -m`, e.g. `aarch64` or `x86_64`. You may be familiar with the alpine or golang one, e.g. `amd64` or `amd64`, which are not used here. Note: After changing and committing any changes to the kernel directory or any subdirectories, you must update tests, examples and other dependencies. This is done via: ```bash make update-kernel-yamls ``` Each series of kernels has a dedicated directory in , e.g. or . Variants, like rt kernels, have their own directory as well, e.g. . However, for variants, the patches from both the common kernel, e.g. , and the variant, e.g. , are applied, and the configs from both are combined. Within the series-dedicated directory, there are: kernel config file for each architecture named `config-<arch>`, e.g. , one per target architecture. optional patches directory, e.g. , which contains patches to apply to the kernel source The config file and patches are applied during the kernel build process. Note: We try to keep the differences between kernel versions and architectures to a minimum, so if you make changes to one configuration also try to apply it to the others. The script can be used to compare kernel config files. For example: ```sh ../scripts/kconfig-split.py 5.15.x/config-aarch64 5.15.x/config-x86_64 ``` creates a file with the common and the x86_64 and arm64 specific config options for the 5.15.x kernel series. Note: The CI pipeline does not push out kernel images. Anyone modifying a kernel should: Follow the steps below for the desired changes and commit them. Run appropriate `make build` or variants to ensure that it works. Open a PR with the changes. This may fail, as the CI pipeline may not have access to the modified kernels. A maintainer should run `make push` to push out the images. Run (or rerun) the

The targets and variants for building are as follows: `make build` - make all kernels in the version list and their variants `make build-<version>` - make all variants of a specific kernel version `make buildkernel-<version>` - make all variants of a specific kernel version `make buildplainkernel-<version>` - make just the provided version's kernel `make builddebugkernel-<version>` - make just the provided version's debug kernel `make buildtools-<version>` - make just the provided version's tools To push: `make push` - push all kernels in the version list and their variants `make push-<version>` - push all variants of a specific kernel version Finally, for convenience: `make list` - list all kernels in the version list By default, it builds for all supported architectures. To build just for a specific architecture: ```sh make build ARCH=amd64 ``` The variable `ARCH` should use the golang variants only, i.e. `amd64` and `arm64`. To build for multiple architectures, call it multiple times: ```sh make build ARCH=amd64 make build ARCH=arm64 ``` When building for a specific architecture, the build process will use your local Docker, passing it `--platforms` for the architecture. If you have a builder on a different architecture, e.g. you are running on an Apple Silicon Mac (arm64) and want to build for `x86_64` without emulating (which can be very slow), you can use the `BUILDER` variable: ```sh make build ARCH=x86_64 BUILDER=remote-amd64-builder ``` Builder also supports a builder pattern. If `BUILDER` contains the string `{{.Arch}}`, it will be replaced with the architecture being built. For example: ```sh make build ARCH=x86_64 BUILDER=remote-{{.Arch}}-builder make build ARCH=aarch64 BUILDER=remote-{{.Arch}}-builder ``` will build `x86_64` on `remote-amd64-builder` and `aarch64` on `remote-arm64-builder`. Finally, if no `BUILDER` is specified, the build will look for a builder named `linuxkit-linux-{{.Arch}}-builder`, e.g. `linuxkit-linux-amd64-builder` or `linuxkit-linux-arm64-builder`. If that builder does not exist, it will fall back to your local Docker setup. The process of modifying the kernel configuration is as follows: Create a `linuxkit/kconfig` container image: `make kconfig`. This is not pushed out. Run a container based on `linuxkit/kconfig`. In the container, modify the config to suit your needs using normal kernel tools like `make defconfig` or `make menuconfig`. Save the config from the image. The `linuxkit/kconfig` image contains the patched sources for all support kernels and architectures in `/linux-<major>.<minor>.<rev>`. The kernel source also has the kernel config copied to the default kernel config location, so that `make menuconfig` and `make defconfig` work correctly. Run the container as follows: ```sh docker run --rm -ti -v $(pwd):/src linuxkit/kconfig ``` This will give you a interactive shell where you can modify the kernel configuration you want, while mounting the directory, so that you can save the modified config. To create or modify the config, you must cd to the correct directory, e.g. ```sh cd /linux-6.6.13 cd /linux-5.15.27 ``` Now you can build the config. When `make defconfig` or `make menuconfig` is done, the modified config file will be in `.config`; save the file back to `/src`, e.g. ```sh cp .config /src/6.6.x/config-x86_64 ``` You can also configure other architectures other than the native one. For example to configure the arm64 kernel on x86_64, use: ```sh make ARCH=arm64 defconfig make ARCH=arm64 oldconfig # or menuconfig ``` Note that the generated file must be final. When you actually build the kernel, it will check that running `make defconfig` will have no changes. If there are changes, the build will fail. The easiest way to check it is to rerun `make defconfig` inside the kconfig container. Finish your creation of the config file, as above. Copy the `.config` file to the target location, as above. Copy the `.config` file to the source location for defconfig, e.g. `cp .config arch/x86/configs/x8664config` or

config /linux/arch/arm64/configs/defconfig` Run `make defconfig` again, and check that there are no changes, e.g. `diff .config arch/x86/configs/x8664config` or `diff .config /linux/arch/arm64/configs/defconfig` If there are no differences, then you can commit the new config file. Finally, test that you can build the kernel with that config as `make build-<version>`, e.g. `make build-5.15.148`. If you want to add a new kernel version within an existing series, e.g. `5.15.27` already exists and you want to add (or replace it with) `5.15.148`, apply the following process. Determine the series, i.e. the kernel major.minor version, followed by `x`. E.g. for `5.15.148`, the series is `5.15.x`. Modify the `KERNEL_VERSION` in the `build-args` file in the series directory to the new version. E.g. `5.15.x/build-args`. Create a new `linuxkit/kconfig` container image: `make kconfig`. This is not pushed out. Run a container based on `linuxkit/kconfig`. ```sh docker run --rm -ti -v $(pwd):/src linuxkit/kconfig ``` In the container, change directory to the kernel source directory for the new version, e.g. `cd /linux-5.15.148`. Run `make defconfig` to create the default config file. If the config file has changed, copy it out of the container and check it in, e.g. `cp .config /src/5.15.x/config-x86_64`. Repeat for other architectures. Commit the changed config files. Test that you can build the kernel with that config as `make build-<version>`, e.g. `make build-5.15.148`. To add a new kernel series, you need to: Create new directory for the series, e.g. `6.7.x` Create config files for each architecture in that directory Optionally, create a `patches/` subdirectory in that directory with any patches to add Create a `build-args` file in that directory with at least the following settings: ```bash KERNEL_VERSION=<version> KERNEL_SERIES=<series> BUILD_IMAGE=linuxkit/alpine:<builder> ``` Since the last major series likely is the best basis for the new one, subject to additional modifications, you can use the previous one as a starting point. Make the directory for the new series, e.g. `mkdir 7.0.x` Create a new `linuxkit/kconfig` container image: `make kconfig`. This is not pushed out. Run a container based on `linuxkit/kconfig`. ```sh docker run --rm -ti -v $(pwd):/src linuxkit/kconfig ``` In the container, change directory to the kernel source directory for the new version, e.g. `cd /linux-7.0.5`. Copy the existing config file for the previous series, e.g. `cp /src/6.6.x/config-x86_64 .config`. Run `make oldconfig` to create the config file for the new series from the old one. Answer any questions. Save the newly generated config file `.config` to the source directory, e.g. `cp .config /src/7.0.x/config-x86_64`. Repeat for other architectures. Commit the new config files. Test that you can build the kernel with that config as `make build-<version>`, e.g. `make build-7.0.5`. In addition, there are tests that are applied to a specific kernel version, notably the tests in . You will need to add a new test case for the new series, copying an existing one and modifying it as needed. To build and test locally modified kernels, e.g., to try a different kernel config or new patches, the existing kernel build system in the directory can be re-used. For example, assuming the current 4.9 kernel is 4.9.33, you can build a local kernel with: ```sh make build_4.9.x ``` This will create a local kernel image called `linuxkit/kernel:4.9.33-<hash>-dirty` assuming you haven't committed you local changes. You can then use this in your YAML file as: ``` kernel: image: linuxkit/kernel:4.9.33-<hash>-dirty ``` If you have committed your local changes, the `-dirty` will not be

Then you can also override the Hub organisation to use the image elsewhere with (and also disable image signing): ```sh make ORG=<your hub org> ``` The image will be uploaded to Hub and can be use in a YAML file as `<your hub org>/kernel:4.9.33` or as `<your hub org>/kernel:4.9.33-<hash>`. The kernel build system has some provision to allow local customisation to the build. If you want to override/add some kernel config options, you can add a file called `config-4.9.x-x86_64-foo` and then invoke the build with `make EXTRA=-foo build_4.9.x-foo` and this will build an image with the additional kernel config options enabled. If you want additional patches being applied, just copy them to the `patches-4.X.x` and the build process will pick them up. We may apply patches to the Linux kernel used in LinuxKit, primarily to cherry-pick some upstream patches or to add some additional functionality, not yet accepted upstream. Patches are located in `kernel/patches-<kernel version>` and should follow these rules: Patches must be in `git am` format, i.e. they should contain a complete and sensible commit message. Patches must contain a Developer's Certificate of Origin. Patch files must have a numeric prefix to ensure the ordering in which they are applied. If patches are cherry-picked, they must be cherry-picked with `-x` to contain the original commit ID. If patches are from a different git tree (other than the stable tree), or from a mailing list posting they should contain an `Origin:` line with a link to the source. This document outlines the recommended procedure to handle patches. The general process is to apply them to a branch of the and then export them with `git format-patch`. If you want to add or remove patches currently used, please also ping @rneugeba on the PR so that we can update our internal Linux tree to ensure that patches are carried forward if we update the kernel in the future. Patches are applied to point releases of the linux stable tree. You need an up-to-date copy of that tree: ```sh git clone git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git ``` Add it as a remote to a clone of the . We use the following variables: `KITSRC`: Base directory of LinuxKit repository `LINUXSRC`: Base directory of Linux stable kernel repository e.g.: ```sh KITSRC=~/src/linuxkit/linuxkit LINUXSRC=~/src/linuxkit/linux ``` to refer to the location of the LinuxKit and Linux kernel trees. There are different ways to do this, but we recommend applying the patches to the current version and then rebase to the new version. We define the following variables to refer to the current base tag and the new tag you want to rebase the patches to: ```sh CURTAG=v4.9.14 NEWTAG=v4.9.15 ``` If you don't already have a branch, it's best to import the current patch set and then rebase: ```sh cd $LINUXSRC git checkout -b ${NEWTAG}-linuxkit ${CURTAG} git am ${KITSRC}/kernel/patches/*.patch git rebase ${NEWTAG}-linuxkit ${NEWTAG} ``` The `git am` should not have any conflicts and if the rebase has conflicts resolve them, then `git add <files>` and `git rebase --continue`. If you already have linux tree with a `${CURTAG}-linuxkit` branch, you can rebase by creating a new branch from the current branch and then rebase: ```sh cd $LINUXSRC git checkout ${CURTAG}-linuxkit git branch ${NEWTAG}-linuxkit ${CURTAG}-linuxkit git rebase --onto ${NEWTAG} ${NEWTAG} ${NEWTAG}-linuxkit ``` Again, resolve any conflicts as described above. If you want to add or remove patches make sure you have an up-to-date branch with the currently applied patches (see above). Then either any normal means (`git cherry-pick -x`, `git am`, or `git commit`, etc) to add new patches. For cherry-picked patches also please add a `Origin:` line after the DCO lines with a reference the git tree the patch was cherry-picked from. If the patch is not cherry-picked try to include as much information in the commit message as possible as to where the patch originated

The canonical form would be to add a `Origin:` line after the DCO lines, e.g.: ``` Origin: https://patchwork.ozlabs.org/patch/622404/ ``` To export patches to LinuxKit, you should use `git format-patch` from the Linux tree, e.g., something along these lines: ```sh cd $LINUXSRC rm $KITSRC/kernel/patches-4.9.x/* git format-patch -o $KITSRC/kernel/patches-4.9.x v4.9.15..HEAD ``` Then, create a PR for LinuxKit. The `kernel-perf` package contains a statically linked `perf` binary under `/usr/bin` which is matched with the kernel of the same tag. The simplest way to use the `perf` utility is to add the package to the `init` section in the YAML file. This adds the binary to the root filesystem. To use the binary, you can either bind mount it into the `getty` or `ssh` service container or you can access the root filesystem from the `getty` container via `nsenter`: ```sh nsenter -m/proc/1/ns/mnt ash ``` Alternatively, you can add the `kernel-perf` package as stage in a multi-stage build to add it to a custom package. The kernel build Makefile has support for building the ZFS kernel modules. Note, the modules are currently not distributed as standard LinuxKit packages and if you wish to use them you have to compile them yourself: ```sh cd kernel make ORG=<foo> pushzfs4.9.x # or different kernel version ``` will build and push a `zfs-kmod-4.9.<version>` image to Docker Hub under the `ORG` specified. This package contains the all the standard kernel modules from the kernel specified plus the `spl` and `zfs` kernel modules, with `depmod` run over them, so they can be `modprobe`ed. To use the modules do something like this in your YAML file: ``` kernel: image: linuxkit/kernel:4.9.<version> cmdline: "console=tty0 console=ttyS0 console=ttyAMA0" init: <foo>/zfs-kmod:4.9.<version> ... ``` Then, you also need to make sure the Alpine `zfs` utilities are available in the container where your want to run `zfs` commands. The Alpine `zfs` utilities are available in `linuxkit/alpine` and the version of the kernel module should match the version of the tools. The container where you run the `zfs` tools might also need `CAPSYSMODULE` to be able to load the kernel modules. All of the linuxkit `.yml` files use the images from `linuxkit/kernel:<tag>`. When updating the kernel, you run commands to update the tests. The updates to any file that contains references to `linuxkit/kernel` in this repository work as follows: Semver tags are replaced by the most recent kernel version. For example, `linuxkit/kernel:5.10.104` will become `6.6.13` when available, and then `6.6.15`, and then `7.0.1`, etc. The highest semver always is used. Semver+hash tags are replaced by the most recent hash and patch version for that series. For example, `linuxkit/kernel:5.10.104-abcdef1234` will become `5.10.104-aaaa54232` (same semver, newer hash), and then `5.10.105-bbbb12345` (newer semver, newer hash), etc. The highest semver+hash always is used. This is not an inherent characteristic of `linuxkit` tool, which never will change your `.yml` files. It is part of the update process for yml files in this repository. The net of the above is the following rule: If you want a reference to a specific kernel series, e.g. a test or example that works only with `5.10.x`, then use a specific hash, e.g. `linuxkit/kernel:5.10.104-abcdef1234`. The hash and patch version will update, but not more. The most common use case for this is kernel version-specific tests. If you want a reference to the most recent kernel, whatever version it is, then use a semver tag, e.g. `linuxkit/kernel:6.6.13`. The most common use case for this is examples that work with any kernel version, which is the vast majority of cases. You can get the current hash by executing the following: ```bash $ cd kernel $ make tag-plain-kernel-<version> $ make tag-plain-kernel-6.6.13 linuxkit/kernel:6.6.13-3a8b3faf92390265b1fbee792b9a3fe14d14c26e ```