WSL2-Linux-Kernel/Documentation/admin-guide/bootconfig.rst

.. SPDX-License-Identifier: GPL-2.0

.. _bootconfig:

==================
Boot Configuration
==================

:Author: Masami Hiramatsu <mhiramat@kernel.org>

Overview
========

The boot configuration expands the current kernel command line to support
additional key-value data when booting the kernel in an efficient way.
This allows administrators to pass a structured-Key config file.

Config File Syntax
==================

The boot config syntax is a simple structured key-value. Each key consists
of dot-connected-words, and key and value are connected by ``=``. The value
has to be terminated by semi-colon (``;``) or newline (``\n``).
For array value, array entries are separated by comma (``,``). ::

  KEY[.WORD[...]] = VALUE[, VALUE2[...]][;]

Unlike the kernel command line syntax, spaces are OK around the comma and ``=``.

Each key word must contain only alphabets, numbers, dash (``-``) or underscore
(``_``). And each value only contains printable characters or spaces except
for delimiters such as semi-colon (``;``), new-line (``\n``), comma (``,``),
hash (``#``) and closing brace (``}``).

If you want to use those delimiters in a value, you can use either double-
quotes (``"VALUE"``) or single-quotes (``'VALUE'``) to quote it. Note that
you can not escape these quotes.

There can be a key which doesn't have value or has an empty value. Those keys
are used for checking if the key exists or not (like a boolean).

Key-Value Syntax
----------------

The boot config file syntax allows user to merge partially same word keys
by brace. For example::

 foo.bar.baz = value1
 foo.bar.qux.quux = value2

These can be written also in::

 foo.bar {
    baz = value1
    qux.quux = value2
 }

Or more shorter, written as following::

 foo.bar { baz = value1; qux.quux = value2 }

In both styles, same key words are automatically merged when parsing it
at boot time. So you can append similar trees or key-values.

Same-key Values
---------------

It is prohibited that two or more values or arrays share a same-key.
For example,::

 foo = bar, baz
 foo = qux  # !ERROR! we can not re-define same key

If you want to update the value, you must use the override operator
``:=`` explicitly. For example::

 foo = bar, baz
 foo := qux

then, the ``qux`` is assigned to ``foo`` key. This is useful for
overriding the default value by adding (partial) custom bootconfigs
without parsing the default bootconfig.

If you want to append the value to existing key as an array member,
you can use ``+=`` operator. For example::

 foo = bar, baz
 foo += qux

In this case, the key ``foo`` has ``bar``, ``baz`` and ``qux``.

Moreover, sub-keys and a value can coexist under a parent key.
For example, following config is allowed.::

 foo = value1
 foo.bar = value2
 foo := value3 # This will update foo's value.

Note, since there is no syntax to put a raw value directly under a
structured key, you have to define it outside of the brace. For example::

 foo {
     bar = value1
     bar {
         baz = value2
         qux = value3
     }
 }

Also, the order of the value node under a key is fixed. If there
are a value and subkeys, the value is always the first child node
of the key. Thus if user specifies subkeys first, e.g.::

 foo.bar = value1
 foo = value2

In the program (and /proc/bootconfig), it will be shown as below::

 foo = value2
 foo.bar = value1

Comments
--------

The config syntax accepts shell-script style comments. The comments starting
with hash ("#") until newline ("\n") will be ignored.

::

 # comment line
 foo = value # value is set to foo.
 bar = 1, # 1st element
       2, # 2nd element
       3  # 3rd element

This is parsed as below::

 foo = value
 bar = 1, 2, 3

Note that you can not put a comment between value and delimiter(``,`` or
``;``). This means following config has a syntax error ::

 key = 1 # comment
       ,2


/proc/bootconfig
================

/proc/bootconfig is a user-space interface of the boot config.
Unlike /proc/cmdline, this file shows the key-value style list.
Each key-value pair is shown in each line with following style::

 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...]


Boot Kernel With a Boot Config
==============================

There are two options to boot the kernel with bootconfig: attaching the
bootconfig to the initrd image or embedding it in the kernel itself.

Attaching a Boot Config to Initrd
---------------------------------

Since the boot configuration file is loaded with initrd by default,
it will be added to the end of the initrd (initramfs) image file with
padding, size, checksum and 12-byte magic word as below.

[initrd][bootconfig][padding][size(le32)][checksum(le32)][#BOOTCONFIG\n]

The size and checksum fields are unsigned 32bit little endian value.

When the boot configuration is added to the initrd image, the total
file size is aligned to 4 bytes. To fill the gap, null characters
(``\0``) will be added. Thus the ``size`` is the length of the bootconfig
file + padding bytes.

The Linux kernel decodes the last part of the initrd image in memory to
get the boot configuration data.
Because of this "piggyback" method, there is no need to change or
update the boot loader and the kernel image itself as long as the boot
loader passes the correct initrd file size. If by any chance, the boot
loader passes a longer size, the kernel fails to find the bootconfig data.

To do this operation, Linux kernel provides ``bootconfig`` command under
tools/bootconfig, which allows admin to apply or delete the config file
to/from initrd image. You can build it by the following command::

 # make -C tools/bootconfig

To add your boot config file to initrd image, run bootconfig as below
(Old data is removed automatically if exists)::

 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z

To remove the config from the image, you can use -d option as below::

 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z

Then add "bootconfig" on the normal kernel command line to tell the
kernel to look for the bootconfig at the end of the initrd file.

Embedding a Boot Config into Kernel
-----------------------------------

If you can not use initrd, you can also embed the bootconfig file in the
kernel by Kconfig options. In this case, you need to recompile the kernel
with the following configs::

 CONFIG_BOOT_CONFIG_EMBED=y
 CONFIG_BOOT_CONFIG_EMBED_FILE="/PATH/TO/BOOTCONFIG/FILE"

``CONFIG_BOOT_CONFIG_EMBED_FILE`` requires an absolute path or a relative
path to the bootconfig file from source tree or object tree.
The kernel will embed it as the default bootconfig.

Just as when attaching the bootconfig to the initrd, you need ``bootconfig``
option on the kernel command line to enable the embedded bootconfig.

Note that even if you set this option, you can override the embedded
bootconfig by another bootconfig which attached to the initrd.

Kernel parameters via Boot Config
=================================

In addition to the kernel command line, the boot config can be used for
passing the kernel parameters. All the key-value pairs under ``kernel``
key will be passed to kernel cmdline directly. Moreover, the key-value
pairs under ``init`` will be passed to init process via the cmdline.
The parameters are concatinated with user-given kernel cmdline string
as the following order, so that the command line parameter can override
bootconfig parameters (this depends on how the subsystem handles parameters
but in general, earlier parameter will be overwritten by later one.)::

 [bootconfig params][cmdline params] -- [bootconfig init params][cmdline init params]

Here is an example of the bootconfig file for kernel/init parameters.::

 kernel {
   root = 01234567-89ab-cdef-0123-456789abcd
 }
 init {
  splash
 }

This will be copied into the kernel cmdline string as the following::

 root="01234567-89ab-cdef-0123-456789abcd" -- splash

If user gives some other command line like,::

 ro bootconfig -- quiet

The final kernel cmdline will be the following::

 root="01234567-89ab-cdef-0123-456789abcd" ro bootconfig -- splash quiet


Config File Limitation
======================

Currently the maximum config size size is 32KB and the total key-words (not
key-value entries) must be under 1024 nodes.
Note: this is not the number of entries but nodes, an entry must consume
more than 2 nodes (a key-word and a value). So theoretically, it will be
up to 512 key-value pairs. If keys contains 3 words in average, it can
contain 256 key-value pairs. In most cases, the number of config items
will be under 100 entries and smaller than 8KB, so it would be enough.
If the node number exceeds 1024, parser returns an error even if the file
size is smaller than 32KB. (Note that this maximum size is not including
the padding null characters.)
Anyway, since bootconfig command verifies it when appending a boot config
to initrd image, user can notice it before boot.


Bootconfig APIs
===============

User can query or loop on key-value pairs, also it is possible to find
a root (prefix) key node and find key-values under that node.

If you have a key string, you can query the value directly with the key
using xbc_find_value(). If you want to know what keys exist in the boot
config, you can use xbc_for_each_key_value() to iterate key-value pairs.
Note that you need to use xbc_array_for_each_value() for accessing
each array's value, e.g.::

 vnode = NULL;
 xbc_find_value("key.word", &vnode);
 if (vnode && xbc_node_is_array(vnode))
    xbc_array_for_each_value(vnode, value) {
      printk("%s ", value);
    }

If you want to focus on keys which have a prefix string, you can use
xbc_find_node() to find a node by the prefix string, and iterate
keys under the prefix node with xbc_node_for_each_key_value().

But the most typical usage is to get the named value under prefix
or get the named array under prefix as below::

 root = xbc_find_node("key.prefix");
 value = xbc_node_find_value(root, "option", &vnode);
 ...
 xbc_node_for_each_array_value(root, "array-option", value, anode) {
    ...
 }

This accesses a value of "key.prefix.option" and an array of
"key.prefix.array-option".

Locking is not needed, since after initialization, the config becomes
read-only. All data and keys must be copied if you need to modify it.


Functions and structures
========================

.. kernel-doc:: include/linux/bootconfig.h
.. kernel-doc:: lib/bootconfig.c