linuxkit/docs/kernels.md

# Linux kernels

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
[linuxkit/kernels](https://hub.docker.com/r/linuxkit/kernel/).

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 [github](https://github.com/linuxkit/linux). Each kernel
image is tagged with the full kernel version plus the hash of the
files it was created from (git tree hash of the `./kernel`
directory). For convenience, the latest kernel of each stable series
is also available under a shorthand tag, e.g. `linuxkit/kernel:4.9.x`
for the latest `4.9` kernel. For selected kernels (mostly the LTS
kernels and latest stable kernels) we also compile/push kernels with
additional debugging enabled. The hub images for these kernels have
the `_dbg` suffix in the tag. For some kernels, we also provide
matching packages containing the `perf` utility for debugging and
performance tracing.  The perf package is called `kernel-perf` and is
tagged the same way as the kernel packages.

In addition to the official kernel images, LinuxKit offers the ability
to build bootable Linux images with kernels from various
distributions. We mostly offer this mostly for testing
purposes. "Foreign" kernel images are created by re-packing the native
kernel packages into hub images. The hub images are typically tagged
with the kernel version.

In summary, LinuxKit offers a choice of the following kernels:
- [linuxkit/kernel](https://hub.docker.com/r/linuxkit/kernel/): Official LinuxKit kernels.
- [linuxkit/kernel-mainline](https://hub.docker.com/r/linuxkit/kernel-mainline/): Mainline [kernel.org](http://kernel.org) kernels from the [Ubuntu Mainline PPA](http://kernel.ubuntu.com/~kernel-ppa/mainline/).
- [linuxkit/kernel-ubuntu](https://hub.docker.com/r/linuxkit/kernel-ubuntu/): Selected Ubuntu kernels.
- [linuxkit/kernel-debian](https://hub.docker.com/r/linuxkit/kernel-debian/): Selected Debian kernels.
- [linuxkit/kernel-centos](https://hub.docker.com/r/linuxkit/kernel-centos/): Selected CentOS kernels.
- [linuxkit/kernel-fedora](https://hub.docker.com/r/linuxkit/kernel-fedora/): Selected Fedora kernels.


## Compiling kernel modules

The LinuxKit kernel packages 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 include then include the modules in the initial
RAM disk.

There is a [example](../tests/kmod), but basically one can use a
multi-stage build to compile the kernel modules:
```
FROM linuxkit/kernel:4.9.x AS ksrc
# Extract headers and compile module
FROM linuxkit/kernel-compile:1b396c221af673757703258159ddc8539843b02b@sha256:6b32d205bfc6407568324337b707d195d027328dbfec554428ea93e7b0a8299b AS build
COPY --from=ksrc /kernel-dev.tar /
RUN tar xf kernel-dev.tar

# copy module source code and compile
```

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. [kmod.yml](../tests/kmod/kmod.yml) contains an example for the
configuration.


## Building and using custom kernels

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
[`./kernel`](./kernel) can be re-used. For example, assuming the
current 4.9 kernel is 4.9.28, you can build a local kernel with:
```
make build_4.9.28 HASH=foo
```
This will create a local kernel image called
`linuxkit/kernel:4.9.28-foo` which you can use in your YAML file as:
```
kernel:
  image: "linuxkit/kernel:4.9.28-foo"
```

If you have more substantial changes, or require a different kernel
version, it's best to replicate the kernel build system and change the
Docker Hub organisation to your own.


## Working with Linux kernel patches for LinuxKit

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
[Linux stable tree](https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/)
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.


### Preparation

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 [LinuxKit clone](https://github.com/linuxkit/linux).

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.


### Updating the patches to a new kernel version

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.


### Adding/Removing patches

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
from. The canonical form would be to add a `Origin:` line after the
DCO lines, e.g.:
```
Origin: https://patchwork.ozlabs.org/patch/622404/
```

### Export patches to LinuxKit

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.


## Using `perf`

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`:
```
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.