Documentation: dev-tools: add container.rst page

.. SPDX-License-Identifier: GPL-2.0-only

.. Copyright (C) 2025 Guillaume Tucker

====================

Containerized Builds

====================

The ``container`` tool can be used to run any command in the kernel source tree

from within a container.  Doing so facilitates reproducing builds across

various platforms, for example when a test bot has reported an issue which

requires a specific version of a compiler or an external test suite.  While

this can already be done by users who are familiar with containers, having a

dedicated tool in the kernel tree lowers the barrier to entry by solving common

problems once and for all (e.g. user id management).  It also makes it easier

to share an exact command line leading to a particular result.  The main use

case is likely to be kernel builds but virtually anything can be run: KUnit,

checkpatch etc. provided a suitable image is available.

Options

=======

Command line syntax::

  scripts/container -i IMAGE [OPTION]... CMD...

Available options:

``-e, --env-file ENV_FILE``

    Path to an environment file to load in the container.

``-g, --gid GID``

    Group id to use inside the container.

``-i, --image IMAGE``

    Container image name (required).

``-r, --runtime RUNTIME``

    Container runtime name.  Supported runtimes: ``docker``, ``podman``.

    If not specified, the first one found on the system will be used

    i.e. Podman if present, otherwise Docker.

``-s, --shell``

    Run the container in an interactive shell.

``-u, --uid UID``

    User id to use inside the container.

    If the ``-g`` option is not specified, the user id will also be used for

    the group id.

``-v, --verbose``

    Enable verbose output.

``-h, --help``

    Show the help message and exit.

Usage

=====

It's entirely up to the user to choose which image to use and the ``CMD``

arguments are passed directly as an arbitrary command line to run in the

container.  The tool will take care of mounting the source tree as the current

working directory and adjust the user and group id as needed.

The container image which would typically include a compiler toolchain is

provided by the user and selected via the ``-i`` option.  The container runtime

can be selected with the ``-r`` option, which can be either ``docker`` or

``podman``.  If none is specified, the first one found on the system will be

used while giving priority to Podman.  Support for other runtimes may be added

later depending on their popularity among users.

By default, commands are run non-interactively.  The user can abort a running

container with SIGINT (Ctrl-C).  To run commands interactively with a TTY, the

``--shell`` or ``-s`` option can be used.  Signals will then be received by the

shell directly rather than the parent ``container`` process.  To exit an

interactive shell, use Ctrl-D or ``exit``.

.. note::

   The only host requirement aside from a container runtime is Python 3.10 or

   later.

.. note::

   Out-of-tree builds are not fully supported yet.  The ``O=`` option can

   however already be used with a relative path inside the source tree to keep

   separate build outputs.  A workaround to build outside the tree is to use

   ``mount --bind``, see the examples section further down.

Environment Variables

=====================

Environment variables are not propagated to the container so they have to be

either defined in the image itself or via the ``-e`` option using an

environment file.  In some cases it makes more sense to have them defined in

the Containerfile used to create the image.  For example, a Clang-only compiler

toolchain image may have ``LLVM=1`` defined.

The local environment file is more useful for user-specific variables added

during development.  It is passed as-is to the container runtime so its format

may vary.  Typically, it will look like the output of ``env``.  For example::

  INSTALL_MOD_STRIP=1

  SOME_RANDOM_TEXT=One upon a time

Please also note that ``make`` options can still be passed on the command line,

so while this can't be done since the first argument needs to be the

executable::

  scripts/container -i docker.io/tuxmake/korg-clang LLVM=1 make  # won't work

this will work::

  scripts/container -i docker.io/tuxmake/korg-clang make LLVM=1

User IDs

========

This is an area where the behaviour will vary slightly depending on the

container runtime.  The goal is to run commands as the user invoking the tool.

With Podman, a namespace is created to map the current user id to a different

one in the container (1000 by default).  With Docker, while this is also

possible with recent versions it requires a special feature to be enabled in

the daemon so it's not used here for simplicity.  Instead, the container is run

with the current user id directly.  In both cases, this will provide the same

file permissions for the kernel source tree mounted as a volume.  The only

difference is that when using Docker without a namespace, the user id may not

be the same as the default one set in the image.

Say, we're using an image which sets up a default user with id 1000 and the

current user calling the ``container`` tool has id 1234.  The kernel source

tree was checked out by this same user so the files belong to user 1234.  With

Podman, the container will be running as user id 1000 with a mapping to id 1234

so that the files from the mounted volume appear to belong to id 1000 inside

the container.  With Docker and no namespace, the container will be running

with user id 1234 which can access the files in the volume but not in the user

1000 home directory.  This shouldn't be an issue when running commands only in

the kernel tree but it is worth highlighting here as it might matter for

special corner cases.

.. note::

   Podman's `Docker compatibility

   <https://podman-desktop.io/docs/migrating-from-docker/managing-docker-compatibility>`__

   mode to run ``docker`` commands on top of a Podman backend is more complex

   and not fully supported yet.  As such, Podman will take priority if both

   runtimes are available on the system.

Examples

========

The TuxMake project provides a variety of prebuilt container images available

on `Docker Hub <https://hub.docker.com/u/tuxmake>`__.  Here's the shortest

example to build a kernel using a TuxMake Clang image::

  scripts/container -i docker.io/tuxmake/korg-clang -- make LLVM=1 defconfig

  scripts/container -i docker.io/tuxmake/korg-clang -- make LLVM=1 -j$(nproc)

.. note::

   When running a command with options within the container, it should be

   separated with a double dash ``--`` to not confuse them with the

   ``container`` tool options.  Plain commands with no options don't strictly

   require the double dashes e.g.::

     scripts/container -i docker.io/tuxmake/korg-clang make mrproper

To run ``checkpatch.pl`` in a ``patches`` directory with a generic Perl image::

  scripts/container -i perl:slim-trixie scripts/checkpatch.pl patches/*

As an alternative to the TuxMake images, the examples below refer to

``kernel.org`` images which are based on the `kernel.org compiler toolchains

<https://mirrors.edge.kernel.org/pub/tools/>`__.  These aren't (yet) officially

available in any public registry but users can build their own locally instead

using this `experimental repository

<https://gitlab.com/gtucker/korg-containers>`__ by running ``make

PREFIX=kernel.org/``.

To build just ``bzImage`` using Clang::

  scripts/container -i kernel.org/clang -- make bzImage -j$(nproc)

Same with GCC 15 as a particular version tag::

  scripts/container -i kernel.org/gcc:15 -- make bzImage -j$(nproc)

For an out-of-tree build, a trick is to bind-mount the destination directory to

a relative path inside the source tree::

  mkdir -p $HOME/tmp/my-kernel-build

  mkdir -p build

  sudo mount --bind $HOME/tmp/my-kernel-build build

  scripts/container -i kernel.org/gcc -- make mrproper

  scripts/container -i kernel.org/gcc -- make O=build defconfig

  scripts/container -i kernel.org/gcc -- make O=build -j$(nproc)

To run KUnit in an interactive shell and get the full output::

  scripts/container -s -i kernel.org/gcc:kunit -- \

      tools/testing/kunit/kunit.py \

          run \

          --arch=x86_64 \

          --cross_compile=x86_64-linux-

To just start an interactive shell::

  scripts/container -si kernel.org/gcc bash

To build the HTML documentation, which requires the ``kdocs`` image built with

``make PREFIX=kernel.org/ extra`` as it's not a compiler toolchain::

  scripts/container -i kernel.org/kdocs make htmldocs

   gpio-sloppy-logic-analyzer

   autofdo

   propeller

   container

.. only::  subproject and html

CONTAINER BUILD SCRIPT

M:	Guillaume Tucker <gtucker@gtucker.io>

S:	Maintained

F:	Documentation/dev-tools/container.rst

F:	scripts/container

CONTEXT TRACKING