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 23string has to be terminated by the following delimiters described below. 24 25Each key word must contain only alphabets, numbers, dash (``-``) or underscore 26(``_``). And each value only contains printable characters or spaces except 27for delimiters such as semi-colon (``;``), new-line (``\n``), comma (``,``), 28hash (``#``) and closing brace (``}``). 29 30If the ``=`` is followed by whitespace up to one of these delimiters, the 31key is assigned an empty value. 32 33For arrays, the array values are comma (``,``) separated, and comments and 34line breaks with newline (``\n``) are allowed between array values for 35readability. Thus the first entry of the array must be on the same line as 36the key.:: 37 38 KEY[.WORD[...]] = VALUE[, VALUE2[...]][;] 39 40Unlike the kernel command line syntax, white spaces (including tabs) are 41ignored around the comma and ``=``. 42 43If you want to use those delimiters in a value, you can use either double- 44quotes (``"VALUE"``) or single-quotes (``'VALUE'``) to quote it. Note that 45you can not escape these quotes. 46 47There can be a key which doesn't have value or has an empty value. Those keys 48are used for checking if the key exists or not (like a boolean). 49 50Key-Value Syntax 51---------------- 52 53The boot config file syntax allows user to merge partially same word keys 54by brace. For example:: 55 56 foo.bar.baz = value1 57 foo.bar.qux.quux = value2 58 59These can be written also in:: 60 61 foo.bar { 62 baz = value1 63 qux.quux = value2 64 } 65 66Or more shorter, written as following:: 67 68 foo.bar { baz = value1; qux.quux = value2 } 69 70In both styles, same key words are automatically merged when parsing it 71at boot time. So you can append similar trees or key-values. 72 73Same-key Values 74--------------- 75 76It is prohibited that two or more values or arrays share a same-key. 77For example,:: 78 79 foo = bar, baz 80 foo = qux # !ERROR! we can not re-define same key 81 82If you want to update the value, you must use the override operator 83``:=`` explicitly. For example:: 84 85 foo = bar, baz 86 foo := qux 87 88then, the ``qux`` is assigned to ``foo`` key. This is useful for 89overriding the default value by adding (partial) custom bootconfigs 90without parsing the default bootconfig. 91 92If you want to append the value to existing key as an array member, 93you can use ``+=`` operator. For example:: 94 95 foo = bar, baz 96 foo += qux 97 98In this case, the key ``foo`` has ``bar``, ``baz`` and ``qux``. 99 100Moreover, sub-keys and a value can coexist under a parent key. 101For example, following config is allowed.:: 102 103 foo = value1 104 foo.bar = value2 105 foo := value3 # This will update foo's value. 106 107Note, since there is no syntax to put a raw value directly under a 108structured key, you have to define it outside of the brace. For example:: 109 110 foo { 111 bar = value1 112 bar { 113 baz = value2 114 qux = value3 115 } 116 } 117 118Also, the order of the value node under a key is fixed. If there 119are a value and subkeys, the value is always the first child node 120of the key. Thus if user specifies subkeys first, e.g.:: 121 122 foo.bar = value1 123 foo = value2 124 125In the program (and /proc/bootconfig), it will be shown as below:: 126 127 foo = value2 128 foo.bar = value1 129 130Comments 131-------- 132 133The config syntax accepts shell-script style comments. The comments starting 134with hash ("#") until newline ("\n") will be ignored. 135 136:: 137 138 # comment line 139 foo = value # value is set to foo. 140 bar = 1, # 1st element 141 2, # 2nd element 142 3 # 3rd element 143 144This is parsed as below:: 145 146 foo = value 147 bar = 1, 2, 3 148 149Note that you can NOT put a comment or a newline between value and delimiter 150(``,`` or ``;``). This means following config has a syntax error :: 151 152 key = 1 # comment 153 ,2 154 155 156/proc/bootconfig 157================ 158 159/proc/bootconfig is a user-space interface of the boot config. 160Unlike /proc/cmdline, this file shows the key-value style list. 161Each key-value pair is shown in each line with following style:: 162 163 KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...] 164 165 166Boot Kernel With a Boot Config 167============================== 168 169There are two options to boot the kernel with bootconfig: attaching the 170bootconfig to the initrd image or embedding it in the kernel itself. 171 172Attaching a Boot Config to Initrd 173--------------------------------- 174 175Since the boot configuration file is loaded with initrd by default, 176it will be added to the end of the initrd (initramfs) image file with 177padding, size, checksum and 12-byte magic word as below. 178 179[initrd][bootconfig][padding][size(le32)][checksum(le32)][#BOOTCONFIG\n] 180 181The size and checksum fields are unsigned 32bit little endian value. 182 183When the boot configuration is added to the initrd image, the total 184file size is aligned to 4 bytes. To fill the gap, null characters 185(``\0``) will be added. Thus the ``size`` is the length of the bootconfig 186file + padding bytes. 187 188The Linux kernel decodes the last part of the initrd image in memory to 189get the boot configuration data. 190Because of this "piggyback" method, there is no need to change or 191update the boot loader and the kernel image itself as long as the boot 192loader passes the correct initrd file size. If by any chance, the boot 193loader passes a longer size, the kernel fails to find the bootconfig data. 194 195To do this operation, Linux kernel provides ``bootconfig`` command under 196tools/bootconfig, which allows admin to apply or delete the config file 197to/from initrd image. You can build it by the following command:: 198 199 # make -C tools/bootconfig 200 201To add your boot config file to initrd image, run bootconfig as below 202(Old data is removed automatically if exists):: 203 204 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z 205 206To remove the config from the image, you can use -d option as below:: 207 208 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z 209 210Then add "bootconfig" on the normal kernel command line to tell the 211kernel to look for the bootconfig at the end of the initrd file. 212Alternatively, build your kernel with the ``CONFIG_BOOT_CONFIG_FORCE`` 213Kconfig option selected. 214 215Embedding a Boot Config into Kernel 216----------------------------------- 217 218If you can not use initrd, you can also embed the bootconfig file in the 219kernel by Kconfig options. In this case, you need to recompile the kernel 220with the following configs:: 221 222 CONFIG_BOOT_CONFIG_EMBED=y 223 CONFIG_BOOT_CONFIG_EMBED_FILE="/PATH/TO/BOOTCONFIG/FILE" 224 225``CONFIG_BOOT_CONFIG_EMBED_FILE`` requires an absolute path or a relative 226path to the bootconfig file from source tree or object tree. 227The kernel will embed it as the default bootconfig. 228 229Just as when attaching the bootconfig to the initrd, you need ``bootconfig`` 230option on the kernel command line to enable the embedded bootconfig, or, 231alternatively, build your kernel with the ``CONFIG_BOOT_CONFIG_FORCE`` 232Kconfig option selected. 233 234Note that even if you set this option, you can override the embedded 235bootconfig by another bootconfig which attached to the initrd. 236 237Kernel parameters via Boot Config 238================================= 239 240In addition to the kernel command line, the boot config can be used for 241passing the kernel parameters. All the key-value pairs under ``kernel`` 242key will be passed to kernel cmdline directly. Moreover, the key-value 243pairs under ``init`` will be passed to init process via the cmdline. 244The parameters are concatenated with user-given kernel cmdline string 245as the following order, so that the command line parameter can override 246bootconfig parameters (this depends on how the subsystem handles parameters 247but in general, earlier parameter will be overwritten by later one.):: 248 249 [bootconfig params][cmdline params] -- [bootconfig init params][cmdline init params] 250 251Here is an example of the bootconfig file for kernel/init parameters.:: 252 253 kernel { 254 root = 01234567-89ab-cdef-0123-456789abcd 255 } 256 init { 257 splash 258 } 259 260This will be copied into the kernel cmdline string as the following:: 261 262 root="01234567-89ab-cdef-0123-456789abcd" -- splash 263 264If user gives some other command line like,:: 265 266 ro bootconfig -- quiet 267 268The final kernel cmdline will be the following:: 269 270 root="01234567-89ab-cdef-0123-456789abcd" ro bootconfig -- splash quiet 271 272 273Config File Limitation 274====================== 275 276Currently the maximum config size is 32KB and the total key-words (not 277key-value entries) must be under 1024 nodes. 278Note: this is not the number of entries but nodes, an entry must consume 279more than 2 nodes (a key-word and a value). So theoretically, it will be 280up to 512 key-value pairs. If keys contains 3 words in average, it can 281contain 256 key-value pairs. In most cases, the number of config items 282will be under 100 entries and smaller than 8KB, so it would be enough. 283If the node number exceeds 1024, parser returns an error even if the file 284size is smaller than 32KB. (Note that this maximum size is not including 285the padding null characters.) 286Anyway, since bootconfig command verifies it when appending a boot config 287to initrd image, user can notice it before boot. 288 289 290Bootconfig APIs 291=============== 292 293User can query or loop on key-value pairs, also it is possible to find 294a root (prefix) key node and find key-values under that node. 295 296If you have a key string, you can query the value directly with the key 297using xbc_find_value(). If you want to know what keys exist in the boot 298config, you can use xbc_for_each_key_value() to iterate key-value pairs. 299Note that you need to use xbc_array_for_each_value() for accessing 300each array's value, e.g.:: 301 302 vnode = NULL; 303 xbc_find_value("key.word", &vnode); 304 if (vnode && xbc_node_is_array(vnode)) 305 xbc_array_for_each_value(vnode, value) { 306 printk("%s ", value); 307 } 308 309If you want to focus on keys which have a prefix string, you can use 310xbc_find_node() to find a node by the prefix string, and iterate 311keys under the prefix node with xbc_node_for_each_key_value(). 312 313But the most typical usage is to get the named value under prefix 314or get the named array under prefix as below:: 315 316 root = xbc_find_node("key.prefix"); 317 value = xbc_node_find_value(root, "option", &vnode); 318 ... 319 xbc_node_for_each_array_value(root, "array-option", value, anode) { 320 ... 321 } 322 323This accesses a value of "key.prefix.option" and an array of 324"key.prefix.array-option". 325 326Locking is not needed, since after initialization, the config becomes 327read-only. All data and keys must be copied if you need to modify it. 328 329 330Functions and structures 331======================== 332 333.. kernel-doc:: include/linux/bootconfig.h 334.. kernel-doc:: lib/bootconfig.c 335 336