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 161Since the boot configuration file is loaded with initrd, it will be added 162to the end of the initrd (initramfs) image file with padding, size, 163checksum and 12-byte magic word as below. 164 165[initrd][bootconfig][padding][size(le32)][checksum(le32)][#BOOTCONFIG\n] 166 167The size and checksum fields are unsigned 32bit little endian value. 168 169When the boot configuration is added to the initrd image, the total 170file 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 172file + padding bytes. 173 174The Linux kernel decodes the last part of the initrd image in memory to 175get the boot configuration data. 176Because of this "piggyback" method, there is no need to change or 177update the boot loader and the kernel image itself as long as the boot 178loader passes the correct initrd file size. If by any chance, the boot 179loader passes a longer size, the kernel fails to find the bootconfig data. 180 181To do this operation, Linux kernel provides ``bootconfig`` command under 182tools/bootconfig, which allows admin to apply or delete the config file 183to/from initrd image. You can build it by the following command:: 184 185 # make -C tools/bootconfig 186 187To add your boot config file to initrd image, run bootconfig as below 188(Old data is removed automatically if exists):: 189 190 # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z 191 192To remove the config from the image, you can use -d option as below:: 193 194 # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z 195 196Then add "bootconfig" on the normal kernel command line to tell the 197kernel to look for the bootconfig at the end of the initrd file. 198 199 200Kernel parameters via Boot Config 201================================= 202 203In addition to the kernel command line, the boot config can be used for 204passing the kernel parameters. All the key-value pairs under ``kernel`` 205key will be passed to kernel cmdline directly. Moreover, the key-value 206pairs under ``init`` will be passed to init process via the cmdline. 207The parameters are concatinated with user-given kernel cmdline string 208as the following order, so that the command line parameter can override 209bootconfig parameters (this depends on how the subsystem handles parameters 210but in general, earlier parameter will be overwritten by later one.):: 211 212 [bootconfig params][cmdline params] -- [bootconfig init params][cmdline init params] 213 214Here is an example of the bootconfig file for kernel/init parameters.:: 215 216 kernel { 217 root = 01234567-89ab-cdef-0123-456789abcd 218 } 219 init { 220 splash 221 } 222 223This will be copied into the kernel cmdline string as the following:: 224 225 root="01234567-89ab-cdef-0123-456789abcd" -- splash 226 227If user gives some other command line like,:: 228 229 ro bootconfig -- quiet 230 231The final kernel cmdline will be the following:: 232 233 root="01234567-89ab-cdef-0123-456789abcd" ro bootconfig -- splash quiet 234 235 236Config File Limitation 237====================== 238 239Currently the maximum config size size is 32KB and the total key-words (not 240key-value entries) must be under 1024 nodes. 241Note: this is not the number of entries but nodes, an entry must consume 242more than 2 nodes (a key-word and a value). So theoretically, it will be 243up to 512 key-value pairs. If keys contains 3 words in average, it can 244contain 256 key-value pairs. In most cases, the number of config items 245will be under 100 entries and smaller than 8KB, so it would be enough. 246If the node number exceeds 1024, parser returns an error even if the file 247size is smaller than 32KB. (Note that this maximum size is not including 248the padding null characters.) 249Anyway, since bootconfig command verifies it when appending a boot config 250to initrd image, user can notice it before boot. 251 252 253Bootconfig APIs 254=============== 255 256User can query or loop on key-value pairs, also it is possible to find 257a root (prefix) key node and find key-values under that node. 258 259If you have a key string, you can query the value directly with the key 260using xbc_find_value(). If you want to know what keys exist in the boot 261config, you can use xbc_for_each_key_value() to iterate key-value pairs. 262Note that you need to use xbc_array_for_each_value() for accessing 263each array's value, e.g.:: 264 265 vnode = NULL; 266 xbc_find_value("key.word", &vnode); 267 if (vnode && xbc_node_is_array(vnode)) 268 xbc_array_for_each_value(vnode, value) { 269 printk("%s ", value); 270 } 271 272If you want to focus on keys which have a prefix string, you can use 273xbc_find_node() to find a node by the prefix string, and iterate 274keys under the prefix node with xbc_node_for_each_key_value(). 275 276But the most typical usage is to get the named value under prefix 277or get the named array under prefix as below:: 278 279 root = xbc_find_node("key.prefix"); 280 value = xbc_node_find_value(root, "option", &vnode); 281 ... 282 xbc_node_for_each_array_value(root, "array-option", value, anode) { 283 ... 284 } 285 286This accesses a value of "key.prefix.option" and an array of 287"key.prefix.array-option". 288 289Locking is not needed, since after initialization, the config becomes 290read-only. All data and keys must be copied if you need to modify it. 291 292 293Functions and structures 294======================== 295 296.. kernel-doc:: include/linux/bootconfig.h 297.. kernel-doc:: lib/bootconfig.c 298 299