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. 204Alternatively, build your kernel with the ``CONFIG_BOOT_CONFIG_FORCE`` 205Kconfig option selected. 206 207Embedding a Boot Config into Kernel 208----------------------------------- 209 210If you can not use initrd, you can also embed the bootconfig file in the 211kernel by Kconfig options. In this case, you need to recompile the kernel 212with the following configs:: 213 214 CONFIG_BOOT_CONFIG_EMBED=y 215 CONFIG_BOOT_CONFIG_EMBED_FILE="/PATH/TO/BOOTCONFIG/FILE" 216 217``CONFIG_BOOT_CONFIG_EMBED_FILE`` requires an absolute path or a relative 218path to the bootconfig file from source tree or object tree. 219The kernel will embed it as the default bootconfig. 220 221Just as when attaching the bootconfig to the initrd, you need ``bootconfig`` 222option on the kernel command line to enable the embedded bootconfig, or, 223alternatively, build your kernel with the ``CONFIG_BOOT_CONFIG_FORCE`` 224Kconfig option selected. 225 226Note that even if you set this option, you can override the embedded 227bootconfig by another bootconfig which attached to the initrd. 228 229Kernel parameters via Boot Config 230================================= 231 232In addition to the kernel command line, the boot config can be used for 233passing the kernel parameters. All the key-value pairs under ``kernel`` 234key will be passed to kernel cmdline directly. Moreover, the key-value 235pairs under ``init`` will be passed to init process via the cmdline. 236The parameters are concatenated with user-given kernel cmdline string 237as the following order, so that the command line parameter can override 238bootconfig parameters (this depends on how the subsystem handles parameters 239but in general, earlier parameter will be overwritten by later one.):: 240 241 [bootconfig params][cmdline params] -- [bootconfig init params][cmdline init params] 242 243Here is an example of the bootconfig file for kernel/init parameters.:: 244 245 kernel { 246 root = 01234567-89ab-cdef-0123-456789abcd 247 } 248 init { 249 splash 250 } 251 252This will be copied into the kernel cmdline string as the following:: 253 254 root="01234567-89ab-cdef-0123-456789abcd" -- splash 255 256If user gives some other command line like,:: 257 258 ro bootconfig -- quiet 259 260The final kernel cmdline will be the following:: 261 262 root="01234567-89ab-cdef-0123-456789abcd" ro bootconfig -- splash quiet 263 264 265Config File Limitation 266====================== 267 268Currently the maximum config size size is 32KB and the total key-words (not 269key-value entries) must be under 1024 nodes. 270Note: this is not the number of entries but nodes, an entry must consume 271more than 2 nodes (a key-word and a value). So theoretically, it will be 272up to 512 key-value pairs. If keys contains 3 words in average, it can 273contain 256 key-value pairs. In most cases, the number of config items 274will be under 100 entries and smaller than 8KB, so it would be enough. 275If the node number exceeds 1024, parser returns an error even if the file 276size is smaller than 32KB. (Note that this maximum size is not including 277the padding null characters.) 278Anyway, since bootconfig command verifies it when appending a boot config 279to initrd image, user can notice it before boot. 280 281 282Bootconfig APIs 283=============== 284 285User can query or loop on key-value pairs, also it is possible to find 286a root (prefix) key node and find key-values under that node. 287 288If you have a key string, you can query the value directly with the key 289using xbc_find_value(). If you want to know what keys exist in the boot 290config, you can use xbc_for_each_key_value() to iterate key-value pairs. 291Note that you need to use xbc_array_for_each_value() for accessing 292each array's value, e.g.:: 293 294 vnode = NULL; 295 xbc_find_value("key.word", &vnode); 296 if (vnode && xbc_node_is_array(vnode)) 297 xbc_array_for_each_value(vnode, value) { 298 printk("%s ", value); 299 } 300 301If you want to focus on keys which have a prefix string, you can use 302xbc_find_node() to find a node by the prefix string, and iterate 303keys under the prefix node with xbc_node_for_each_key_value(). 304 305But the most typical usage is to get the named value under prefix 306or get the named array under prefix as below:: 307 308 root = xbc_find_node("key.prefix"); 309 value = xbc_node_find_value(root, "option", &vnode); 310 ... 311 xbc_node_for_each_array_value(root, "array-option", value, anode) { 312 ... 313 } 314 315This accesses a value of "key.prefix.option" and an array of 316"key.prefix.array-option". 317 318Locking is not needed, since after initialization, the config becomes 319read-only. All data and keys must be copied if you need to modify it. 320 321 322Functions and structures 323======================== 324 325.. kernel-doc:: include/linux/bootconfig.h 326.. kernel-doc:: lib/bootconfig.c 327 328