1 .. SPDX-License-Identifier: GPL-2.0
9 :Author: Masami Hiramatsu <mhiramat@kernel.org>
14 The boot configuration expands the current kernel command line to support
15 additional key-value data when booting the kernel in an efficient way.
16 This allows administrators to pass a structured-Key config file.
21 The boot config syntax is a simple structured key-value. Each key consists
22 of dot-connected-words, and key and value are connected by ``=``. The value
23 has to be terminated by semi-colon (``;``) or newline (``\n``).
24 For array value, array entries are separated by comma (``,``). ::
26 KEY[.WORD[...]] = VALUE[, VALUE2[...]][;]
28 Unlike the kernel command line syntax, spaces are OK around the comma and ``=``.
30 Each key word must contain only alphabets, numbers, dash (``-``) or underscore
31 (``_``). And each value only contains printable characters or spaces except
32 for delimiters such as semi-colon (``;``), new-line (``\n``), comma (``,``),
33 hash (``#``) and closing brace (``}``).
35 If you want to use those delimiters in a value, you can use either double-
36 quotes (``"VALUE"``) or single-quotes (``'VALUE'``) to quote it. Note that
37 you can not escape these quotes.
39 There can be a key which doesn't have value or has an empty value. Those keys
40 are used for checking if the key exists or not (like a boolean).
45 The boot config file syntax allows user to merge partially same word keys
46 by brace. For example::
49 foo.bar.qux.quux = value2
51 These can be written also in::
58 Or more shorter, written as following::
60 foo.bar { baz = value1; qux.quux = value2 }
62 In both styles, same key words are automatically merged when parsing it
63 at boot time. So you can append similar trees or key-values.
68 It is prohibited that two or more values or arrays share a same-key.
72 foo = qux # !ERROR! we can not re-define same key
74 If you want to update the value, you must use the override operator
75 ``:=`` explicitly. For example::
80 then, the ``qux`` is assigned to ``foo`` key. This is useful for
81 overriding the default value by adding (partial) custom bootconfigs
82 without parsing the default bootconfig.
84 If you want to append the value to existing key as an array member,
85 you can use ``+=`` operator. For example::
90 In this case, the key ``foo`` has ``bar``, ``baz`` and ``qux``.
92 Moreover, sub-keys and a value can coexist under a parent key.
93 For example, following config is allowed.::
97 foo := value3 # This will update foo's value.
99 Note, since there is no syntax to put a raw value directly under a
100 structured key, you have to define it outside of the brace. For example::
110 Also, the order of the value node under a key is fixed. If there
111 are a value and subkeys, the value is always the first child node
112 of the key. Thus if user specifies subkeys first, e.g.::
117 In the program (and /proc/bootconfig), it will be shown as below::
125 The config syntax accepts shell-script style comments. The comments starting
126 with hash ("#") until newline ("\n") will be ignored.
131 foo = value # value is set to foo.
132 bar = 1, # 1st element
136 This is parsed as below::
141 Note that you can not put a comment between value and delimiter(``,`` or
142 ``;``). This means following config has a syntax error ::
151 /proc/bootconfig is a user-space interface of the boot config.
152 Unlike /proc/cmdline, this file shows the key-value style list.
153 Each key-value pair is shown in each line with following style::
155 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...]
158 Boot Kernel With a Boot Config
159 ==============================
161 There are two options to boot the kernel with bootconfig: attaching the
162 bootconfig to the initrd image or embedding it in the kernel itself.
164 Attaching a Boot Config to Initrd
165 ---------------------------------
167 Since the boot configuration file is loaded with initrd by default,
168 it will be added to the end of the initrd (initramfs) image file with
169 padding, size, checksum and 12-byte magic word as below.
171 [initrd][bootconfig][padding][size(le32)][checksum(le32)][#BOOTCONFIG\n]
173 The size and checksum fields are unsigned 32bit little endian value.
175 When the boot configuration is added to the initrd image, the total
176 file 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
178 file + padding bytes.
180 The Linux kernel decodes the last part of the initrd image in memory to
181 get the boot configuration data.
182 Because of this "piggyback" method, there is no need to change or
183 update the boot loader and the kernel image itself as long as the boot
184 loader passes the correct initrd file size. If by any chance, the boot
185 loader passes a longer size, the kernel fails to find the bootconfig data.
187 To do this operation, Linux kernel provides ``bootconfig`` command under
188 tools/bootconfig, which allows admin to apply or delete the config file
189 to/from initrd image. You can build it by the following command::
191 # make -C tools/bootconfig
193 To add your boot config file to initrd image, run bootconfig as below
194 (Old data is removed automatically if exists)::
196 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z
198 To remove the config from the image, you can use -d option as below::
200 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z
202 Then add "bootconfig" on the normal kernel command line to tell the
203 kernel to look for the bootconfig at the end of the initrd file.
205 Embedding a Boot Config into Kernel
206 -----------------------------------
208 If you can not use initrd, you can also embed the bootconfig file in the
209 kernel by Kconfig options. In this case, you need to recompile the kernel
210 with the following configs::
212 CONFIG_BOOT_CONFIG_EMBED=y
213 CONFIG_BOOT_CONFIG_EMBED_FILE="/PATH/TO/BOOTCONFIG/FILE"
215 ``CONFIG_BOOT_CONFIG_EMBED_FILE`` requires an absolute path or a relative
216 path to the bootconfig file from source tree or object tree.
217 The kernel will embed it as the default bootconfig.
219 Just as when attaching the bootconfig to the initrd, you need ``bootconfig``
220 option on the kernel command line to enable the embedded bootconfig.
222 Note that even if you set this option, you can override the embedded
223 bootconfig by another bootconfig which attached to the initrd.
225 Kernel parameters via Boot Config
226 =================================
228 In addition to the kernel command line, the boot config can be used for
229 passing the kernel parameters. All the key-value pairs under ``kernel``
230 key will be passed to kernel cmdline directly. Moreover, the key-value
231 pairs under ``init`` will be passed to init process via the cmdline.
232 The parameters are concatinated with user-given kernel cmdline string
233 as the following order, so that the command line parameter can override
234 bootconfig parameters (this depends on how the subsystem handles parameters
235 but in general, earlier parameter will be overwritten by later one.)::
237 [bootconfig params][cmdline params] -- [bootconfig init params][cmdline init params]
239 Here is an example of the bootconfig file for kernel/init parameters.::
242 root = 01234567-89ab-cdef-0123-456789abcd
248 This will be copied into the kernel cmdline string as the following::
250 root="01234567-89ab-cdef-0123-456789abcd" -- splash
252 If user gives some other command line like,::
254 ro bootconfig -- quiet
256 The final kernel cmdline will be the following::
258 root="01234567-89ab-cdef-0123-456789abcd" ro bootconfig -- splash quiet
261 Config File Limitation
262 ======================
264 Currently the maximum config size size is 32KB and the total key-words (not
265 key-value entries) must be under 1024 nodes.
266 Note: this is not the number of entries but nodes, an entry must consume
267 more than 2 nodes (a key-word and a value). So theoretically, it will be
268 up to 512 key-value pairs. If keys contains 3 words in average, it can
269 contain 256 key-value pairs. In most cases, the number of config items
270 will be under 100 entries and smaller than 8KB, so it would be enough.
271 If the node number exceeds 1024, parser returns an error even if the file
272 size is smaller than 32KB. (Note that this maximum size is not including
273 the padding null characters.)
274 Anyway, since bootconfig command verifies it when appending a boot config
275 to initrd image, user can notice it before boot.
281 User can query or loop on key-value pairs, also it is possible to find
282 a root (prefix) key node and find key-values under that node.
284 If you have a key string, you can query the value directly with the key
285 using xbc_find_value(). If you want to know what keys exist in the boot
286 config, you can use xbc_for_each_key_value() to iterate key-value pairs.
287 Note that you need to use xbc_array_for_each_value() for accessing
288 each array's value, e.g.::
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);
297 If you want to focus on keys which have a prefix string, you can use
298 xbc_find_node() to find a node by the prefix string, and iterate
299 keys under the prefix node with xbc_node_for_each_key_value().
301 But the most typical usage is to get the named value under prefix
302 or get the named array under prefix as below::
304 root = xbc_find_node("key.prefix");
305 value = xbc_node_find_value(root, "option", &vnode);
307 xbc_node_for_each_array_value(root, "array-option", value, anode) {
311 This accesses a value of "key.prefix.option" and an array of
312 "key.prefix.array-option".
314 Locking is not needed, since after initialization, the config becomes
315 read-only. All data and keys must be copied if you need to modify it.
318 Functions and structures
319 ========================
321 .. kernel-doc:: include/linux/bootconfig.h
322 .. kernel-doc:: lib/bootconfig.c