Documentation: dev-tools: add container.rst page

Add a dev-tools/container.rst documentation page for the
scripts/container tool.  This covers the basic usage with additional
information about environment variables and user IDs.  It also
includes a number of practical examples with a reference to the
experimental kernel.org toolchain images.

Update MAINTAINERS accordingly with a reference to the added file.

Signed-off-by: Guillaume Tucker <gtucker@gtucker.io>
Reviewed-by: Onur Özkan <work@onurozkan.dev>
Tested-by: Nicolas Schier <nsc@kernel.org>
Acked-by: Nicolas Schier <nsc@kernel.org>
Link: https://patch.msgid.link/af886533cc5cbdd6ef1d909793b79a1ad42c74ca.1769090419.git.gtucker@gtucker.io
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Documentation/dev-tools/container.rst

@@ -0,0 +1,227 @@
+.. 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

Documentation/dev-tools/index.rst

@@ -38,6 +38,7 @@ Documentation/process/debugging/index.rst
    gpio-sloppy-logic-analyzer
    autofdo
    propeller
+   container
 
 
 .. only::  subproject and html

MAINTAINERS

@@ -6383,6 +6383,7 @@ F:	include/linux/console*
 CONTAINER BUILD SCRIPT
 M:	Guillaume Tucker <gtucker@gtucker.io>
 S:	Maintained
+F:	Documentation/dev-tools/container.rst
 F:	scripts/container
 
 CONTEXT TRACKING