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