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
92Moreover, sub-keys and a value can coexist under a parent key.
93For example, following config is allowed.::
94
95 foo = value1
96 foo.bar = value2
97 foo := value3 # This will update foo's value.
98
99Note, since there is no syntax to put a raw value directly under a
100structured key, you have to define it outside of the brace. For example::
101
102 foo {
103     bar = value1
104     bar {
105         baz = value2
106         qux = value3
107     }
108 }
109
110Also, the order of the value node under a key is fixed. If there
111are a value and subkeys, the value is always the first child node
112of the key. Thus if user specifies subkeys first, e.g.::
113
114 foo.bar = value1
115 foo = value2
116
117In the program (and /proc/bootconfig), it will be shown as below::
118
119 foo = value2
120 foo.bar = value1
121
122Comments
123--------
124
125The config syntax accepts shell-script style comments. The comments starting
126with hash ("#") until newline ("\n") will be ignored.
127
128::
129
130 # comment line
131 foo = value # value is set to foo.
132 bar = 1, # 1st element
133       2, # 2nd element
134       3  # 3rd element
135
136This is parsed as below::
137
138 foo = value
139 bar = 1, 2, 3
140
141Note that you can not put a comment between value and delimiter(``,`` or
142``;``). This means following config has a syntax error ::
143
144 key = 1 # comment
145       ,2
146
147
148/proc/bootconfig
149================
150
151/proc/bootconfig is a user-space interface of the boot config.
152Unlike /proc/cmdline, this file shows the key-value style list.
153Each key-value pair is shown in each line with following style::
154
155 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...]
156
157
158Boot Kernel With a Boot Config
159==============================
160
161There are two options to boot the kernel with bootconfig: attaching the
162bootconfig to the initrd image or embedding it in the kernel itself.
163
164Attaching a Boot Config to Initrd
165---------------------------------
166
167Since the boot configuration file is loaded with initrd by default,
168it will be added to the end of the initrd (initramfs) image file with
169padding, size, checksum and 12-byte magic word as below.
170
171[initrd][bootconfig][padding][size(le32)][checksum(le32)][#BOOTCONFIG\n]
172
173The size and checksum fields are unsigned 32bit little endian value.
174
175When the boot configuration is added to the initrd image, the total
176file size is aligned to 4 bytes. To fill the gap, null characters
177(``\0``) will be added. Thus the ``size`` is the length of the bootconfig
178file + padding bytes.
179
180The Linux kernel decodes the last part of the initrd image in memory to
181get the boot configuration data.
182Because of this "piggyback" method, there is no need to change or
183update the boot loader and the kernel image itself as long as the boot
184loader passes the correct initrd file size. If by any chance, the boot
185loader passes a longer size, the kernel fails to find the bootconfig data.
186
187To do this operation, Linux kernel provides ``bootconfig`` command under
188tools/bootconfig, which allows admin to apply or delete the config file
189to/from initrd image. You can build it by the following command::
190
191 # make -C tools/bootconfig
192
193To add your boot config file to initrd image, run bootconfig as below
194(Old data is removed automatically if exists)::
195
196 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z
197
198To remove the config from the image, you can use -d option as below::
199
200 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z
201
202Then add "bootconfig" on the normal kernel command line to tell the
203kernel to look for the bootconfig at the end of the initrd file.
204
205Embedding a Boot Config into Kernel
206-----------------------------------
207
208If you can not use initrd, you can also embed the bootconfig file in the
209kernel by Kconfig options. In this case, you need to recompile the kernel
210with the following configs::
211
212 CONFIG_BOOT_CONFIG_EMBED=y
213 CONFIG_BOOT_CONFIG_EMBED_FILE="/PATH/TO/BOOTCONFIG/FILE"
214
215``CONFIG_BOOT_CONFIG_EMBED_FILE`` requires an absolute path or a relative
216path to the bootconfig file from source tree or object tree.
217The kernel will embed it as the default bootconfig.
218
219Just as when attaching the bootconfig to the initrd, you need ``bootconfig``
220option on the kernel command line to enable the embedded bootconfig.
221
222Note that even if you set this option, you can override the embedded
223bootconfig by another bootconfig which attached to the initrd.
224
225Kernel parameters via Boot Config
226=================================
227
228In addition to the kernel command line, the boot config can be used for
229passing the kernel parameters. All the key-value pairs under ``kernel``
230key will be passed to kernel cmdline directly. Moreover, the key-value
231pairs under ``init`` will be passed to init process via the cmdline.
232The parameters are concatinated with user-given kernel cmdline string
233as the following order, so that the command line parameter can override
234bootconfig parameters (this depends on how the subsystem handles parameters
235but in general, earlier parameter will be overwritten by later one.)::
236
237 [bootconfig params][cmdline params] -- [bootconfig init params][cmdline init params]
238
239Here is an example of the bootconfig file for kernel/init parameters.::
240
241 kernel {
242   root = 01234567-89ab-cdef-0123-456789abcd
243 }
244 init {
245  splash
246 }
247
248This will be copied into the kernel cmdline string as the following::
249
250 root="01234567-89ab-cdef-0123-456789abcd" -- splash
251
252If user gives some other command line like,::
253
254 ro bootconfig -- quiet
255
256The final kernel cmdline will be the following::
257
258 root="01234567-89ab-cdef-0123-456789abcd" ro bootconfig -- splash quiet
259
260
261Config File Limitation
262======================
263
264Currently the maximum config size size is 32KB and the total key-words (not
265key-value entries) must be under 1024 nodes.
266Note: this is not the number of entries but nodes, an entry must consume
267more than 2 nodes (a key-word and a value). So theoretically, it will be
268up to 512 key-value pairs. If keys contains 3 words in average, it can
269contain 256 key-value pairs. In most cases, the number of config items
270will be under 100 entries and smaller than 8KB, so it would be enough.
271If the node number exceeds 1024, parser returns an error even if the file
272size is smaller than 32KB. (Note that this maximum size is not including
273the padding null characters.)
274Anyway, since bootconfig command verifies it when appending a boot config
275to initrd image, user can notice it before boot.
276
277
278Bootconfig APIs
279===============
280
281User can query or loop on key-value pairs, also it is possible to find
282a root (prefix) key node and find key-values under that node.
283
284If you have a key string, you can query the value directly with the key
285using xbc_find_value(). If you want to know what keys exist in the boot
286config, you can use xbc_for_each_key_value() to iterate key-value pairs.
287Note that you need to use xbc_array_for_each_value() for accessing
288each array's value, e.g.::
289
290 vnode = NULL;
291 xbc_find_value("key.word", &vnode);
292 if (vnode && xbc_node_is_array(vnode))
293    xbc_array_for_each_value(vnode, value) {
294      printk("%s ", value);
295    }
296
297If you want to focus on keys which have a prefix string, you can use
298xbc_find_node() to find a node by the prefix string, and iterate
299keys under the prefix node with xbc_node_for_each_key_value().
300
301But the most typical usage is to get the named value under prefix
302or get the named array under prefix as below::
303
304 root = xbc_find_node("key.prefix");
305 value = xbc_node_find_value(root, "option", &vnode);
306 ...
307 xbc_node_for_each_array_value(root, "array-option", value, anode) {
308    ...
309 }
310
311This accesses a value of "key.prefix.option" and an array of
312"key.prefix.array-option".
313
314Locking is not needed, since after initialization, the config becomes
315read-only. All data and keys must be copied if you need to modify it.
316
317
318Functions and structures
319========================
320
321.. kernel-doc:: include/linux/bootconfig.h
322.. kernel-doc:: lib/bootconfig.c
323
324