1.. SPDX-License-Identifier: GPL-2.0
2
3.. _bootconfig:
4
5==================
6Boot Configuration
7==================
8
9:Author: Masami Hiramatsu <mhiramat@kernel.org>
10
11Overview
12========
13
14The boot configuration expands the current kernel command line to support
15additional key-value data when booting the kernel in an efficient way.
16This allows administrators to pass a structured-Key config file.
17
18Config File Syntax
19==================
20
21The boot config syntax is a simple structured key-value. Each key consists
22of dot-connected-words, and key and value are connected by ``=``. The value
23has to be terminated by semi-colon (``;``) or newline (``\n``).
24For array value, array entries are separated by comma (``,``). ::
25
26  KEY[.WORD[...]] = VALUE[, VALUE2[...]][;]
27
28Unlike the kernel command line syntax, spaces are OK around the comma and ``=``.
29
30Each key word must contain only alphabets, numbers, dash (``-``) or underscore
31(``_``). And each value only contains printable characters or spaces except
32for delimiters such as semi-colon (``;``), new-line (``\n``), comma (``,``),
33hash (``#``) and closing brace (``}``).
34
35If you want to use those delimiters in a value, you can use either double-
36quotes (``"VALUE"``) or single-quotes (``'VALUE'``) to quote it. Note that
37you can not escape these quotes.
38
39There can be a key which doesn't have value or has an empty value. Those keys
40are used for checking if the key exists or not (like a boolean).
41
42Key-Value Syntax
43----------------
44
45The boot config file syntax allows user to merge partially same word keys
46by brace. For example::
47
48 foo.bar.baz = value1
49 foo.bar.qux.quux = value2
50
51These can be written also in::
52
53 foo.bar {
54    baz = value1
55    qux.quux = value2
56 }
57
58Or more shorter, written as following::
59
60 foo.bar { baz = value1; qux.quux = value2 }
61
62In both styles, same key words are automatically merged when parsing it
63at boot time. So you can append similar trees or key-values.
64
65Same-key Values
66---------------
67
68It is prohibited that two or more values or arrays share a same-key.
69For example,::
70
71 foo = bar, baz
72 foo = qux  # !ERROR! we can not re-define same key
73
74If you want to update the value, you must use the override operator
75``:=`` explicitly. For example::
76
77 foo = bar, baz
78 foo := qux
79
80then, the ``qux`` is assigned to ``foo`` key. This is useful for
81overriding the default value by adding (partial) custom bootconfigs
82without parsing the default bootconfig.
83
84If you want to append the value to existing key as an array member,
85you can use ``+=`` operator. For example::
86
87 foo = bar, baz
88 foo += qux
89
90In this case, the key ``foo`` has ``bar``, ``baz`` and ``qux``.
91
92However, a sub-key and a value can not co-exist under a parent key.
93For example, following config is NOT allowed.::
94
95 foo = value1
96 foo.bar = value2 # !ERROR! subkey "bar" and value "value1" can NOT co-exist
97 foo.bar := value2 # !ERROR! even with the override operator, this is NOT allowed.
98
99
100Comments
101--------
102
103The config syntax accepts shell-script style comments. The comments starting
104with hash ("#") until newline ("\n") will be ignored.
105
106::
107
108 # comment line
109 foo = value # value is set to foo.
110 bar = 1, # 1st element
111       2, # 2nd element
112       3  # 3rd element
113
114This is parsed as below::
115
116 foo = value
117 bar = 1, 2, 3
118
119Note that you can not put a comment between value and delimiter(``,`` or
120``;``). This means following config has a syntax error ::
121
122 key = 1 # comment
123       ,2
124
125
126/proc/bootconfig
127================
128
129/proc/bootconfig is a user-space interface of the boot config.
130Unlike /proc/cmdline, this file shows the key-value style list.
131Each key-value pair is shown in each line with following style::
132
133 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...]
134
135
136Boot Kernel With a Boot Config
137==============================
138
139Since the boot configuration file is loaded with initrd, it will be added
140to the end of the initrd (initramfs) image file with padding, size,
141checksum and 12-byte magic word as below.
142
143[initrd][bootconfig][padding][size(le32)][checksum(le32)][#BOOTCONFIG\n]
144
145The size and checksum fields are unsigned 32bit little endian value.
146
147When the boot configuration is added to the initrd image, the total
148file size is aligned to 4 bytes. To fill the gap, null characters
149(``\0``) will be added. Thus the ``size`` is the length of the bootconfig
150file + padding bytes.
151
152The Linux kernel decodes the last part of the initrd image in memory to
153get the boot configuration data.
154Because of this "piggyback" method, there is no need to change or
155update the boot loader and the kernel image itself as long as the boot
156loader passes the correct initrd file size. If by any chance, the boot
157loader passes a longer size, the kernel fails to find the bootconfig data.
158
159To do this operation, Linux kernel provides "bootconfig" command under
160tools/bootconfig, which allows admin to apply or delete the config file
161to/from initrd image. You can build it by the following command::
162
163 # make -C tools/bootconfig
164
165To add your boot config file to initrd image, run bootconfig as below
166(Old data is removed automatically if exists)::
167
168 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z
169
170To remove the config from the image, you can use -d option as below::
171
172 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z
173
174Then add "bootconfig" on the normal kernel command line to tell the
175kernel to look for the bootconfig at the end of the initrd file.
176
177Config File Limitation
178======================
179
180Currently the maximum config size size is 32KB and the total key-words (not
181key-value entries) must be under 1024 nodes.
182Note: this is not the number of entries but nodes, an entry must consume
183more than 2 nodes (a key-word and a value). So theoretically, it will be
184up to 512 key-value pairs. If keys contains 3 words in average, it can
185contain 256 key-value pairs. In most cases, the number of config items
186will be under 100 entries and smaller than 8KB, so it would be enough.
187If the node number exceeds 1024, parser returns an error even if the file
188size is smaller than 32KB. (Note that this maximum size is not including
189the padding null characters.)
190Anyway, since bootconfig command verifies it when appending a boot config
191to initrd image, user can notice it before boot.
192
193
194Bootconfig APIs
195===============
196
197User can query or loop on key-value pairs, also it is possible to find
198a root (prefix) key node and find key-values under that node.
199
200If you have a key string, you can query the value directly with the key
201using xbc_find_value(). If you want to know what keys exist in the boot
202config, you can use xbc_for_each_key_value() to iterate key-value pairs.
203Note that you need to use xbc_array_for_each_value() for accessing
204each array's value, e.g.::
205
206 vnode = NULL;
207 xbc_find_value("key.word", &vnode);
208 if (vnode && xbc_node_is_array(vnode))
209    xbc_array_for_each_value(vnode, value) {
210      printk("%s ", value);
211    }
212
213If you want to focus on keys which have a prefix string, you can use
214xbc_find_node() to find a node by the prefix string, and iterate
215keys under the prefix node with xbc_node_for_each_key_value().
216
217But the most typical usage is to get the named value under prefix
218or get the named array under prefix as below::
219
220 root = xbc_find_node("key.prefix");
221 value = xbc_node_find_value(root, "option", &vnode);
222 ...
223 xbc_node_for_each_array_value(root, "array-option", value, anode) {
224    ...
225 }
226
227This accesses a value of "key.prefix.option" and an array of
228"key.prefix.array-option".
229
230Locking is not needed, since after initialization, the config becomes
231read-only. All data and keys must be copied if you need to modify it.
232
233
234Functions and structures
235========================
236
237.. kernel-doc:: include/linux/bootconfig.h
238.. kernel-doc:: lib/bootconfig.c
239
240