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 Since the boot configuration file is loaded with initrd, it will be added
162 to the end of the initrd (initramfs) image file with padding, size,
163 checksum and 12-byte magic word as below.
165 [initrd][bootconfig][padding][size(le32)][checksum(le32)][#BOOTCONFIG\n]
167 The size and checksum fields are unsigned 32bit little endian value.
169 When the boot configuration is added to the initrd image, the total
170 file size is aligned to 4 bytes. To fill the gap, null characters
171 (``\0``) will be added. Thus the ``size`` is the length of the bootconfig
172 file + padding bytes.
174 The Linux kernel decodes the last part of the initrd image in memory to
175 get the boot configuration data.
176 Because of this "piggyback" method, there is no need to change or
177 update the boot loader and the kernel image itself as long as the boot
178 loader passes the correct initrd file size. If by any chance, the boot
179 loader passes a longer size, the kernel fails to find the bootconfig data.
181 To do this operation, Linux kernel provides "bootconfig" command under
182 tools/bootconfig, which allows admin to apply or delete the config file
183 to/from initrd image. You can build it by the following command::
185 # make -C tools/bootconfig
187 To add your boot config file to initrd image, run bootconfig as below
188 (Old data is removed automatically if exists)::
190 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z
192 To remove the config from the image, you can use -d option as below::
194 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z
196 Then add "bootconfig" on the normal kernel command line to tell the
197 kernel to look for the bootconfig at the end of the initrd file.
199 Config File Limitation
200 ======================
202 Currently the maximum config size size is 32KB and the total key-words (not
203 key-value entries) must be under 1024 nodes.
204 Note: this is not the number of entries but nodes, an entry must consume
205 more than 2 nodes (a key-word and a value). So theoretically, it will be
206 up to 512 key-value pairs. If keys contains 3 words in average, it can
207 contain 256 key-value pairs. In most cases, the number of config items
208 will be under 100 entries and smaller than 8KB, so it would be enough.
209 If the node number exceeds 1024, parser returns an error even if the file
210 size is smaller than 32KB. (Note that this maximum size is not including
211 the padding null characters.)
212 Anyway, since bootconfig command verifies it when appending a boot config
213 to initrd image, user can notice it before boot.
219 User can query or loop on key-value pairs, also it is possible to find
220 a root (prefix) key node and find key-values under that node.
222 If you have a key string, you can query the value directly with the key
223 using xbc_find_value(). If you want to know what keys exist in the boot
224 config, you can use xbc_for_each_key_value() to iterate key-value pairs.
225 Note that you need to use xbc_array_for_each_value() for accessing
226 each array's value, e.g.::
229 xbc_find_value("key.word", &vnode);
230 if (vnode && xbc_node_is_array(vnode))
231 xbc_array_for_each_value(vnode, value) {
232 printk("%s ", value);
235 If you want to focus on keys which have a prefix string, you can use
236 xbc_find_node() to find a node by the prefix string, and iterate
237 keys under the prefix node with xbc_node_for_each_key_value().
239 But the most typical usage is to get the named value under prefix
240 or get the named array under prefix as below::
242 root = xbc_find_node("key.prefix");
243 value = xbc_node_find_value(root, "option", &vnode);
245 xbc_node_for_each_array_value(root, "array-option", value, anode) {
249 This accesses a value of "key.prefix.option" and an array of
250 "key.prefix.array-option".
252 Locking is not needed, since after initialization, the config becomes
253 read-only. All data and keys must be copied if you need to modify it.
256 Functions and structures
257 ========================
259 .. kernel-doc:: include/linux/bootconfig.h
260 .. kernel-doc:: lib/bootconfig.c