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 199Config File Limitation 200====================== 201 202Currently the maximum config size size is 32KB and the total key-words (not 203key-value entries) must be under 1024 nodes. 204Note: this is not the number of entries but nodes, an entry must consume 205more than 2 nodes (a key-word and a value). So theoretically, it will be 206up to 512 key-value pairs. If keys contains 3 words in average, it can 207contain 256 key-value pairs. In most cases, the number of config items 208will be under 100 entries and smaller than 8KB, so it would be enough. 209If the node number exceeds 1024, parser returns an error even if the file 210size is smaller than 32KB. (Note that this maximum size is not including 211the padding null characters.) 212Anyway, since bootconfig command verifies it when appending a boot config 213to initrd image, user can notice it before boot. 214 215 216Bootconfig APIs 217=============== 218 219User can query or loop on key-value pairs, also it is possible to find 220a root (prefix) key node and find key-values under that node. 221 222If you have a key string, you can query the value directly with the key 223using xbc_find_value(). If you want to know what keys exist in the boot 224config, you can use xbc_for_each_key_value() to iterate key-value pairs. 225Note that you need to use xbc_array_for_each_value() for accessing 226each array's value, e.g.:: 227 228 vnode = NULL; 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); 233 } 234 235If you want to focus on keys which have a prefix string, you can use 236xbc_find_node() to find a node by the prefix string, and iterate 237keys under the prefix node with xbc_node_for_each_key_value(). 238 239But the most typical usage is to get the named value under prefix 240or get the named array under prefix as below:: 241 242 root = xbc_find_node("key.prefix"); 243 value = xbc_node_find_value(root, "option", &vnode); 244 ... 245 xbc_node_for_each_array_value(root, "array-option", value, anode) { 246 ... 247 } 248 249This accesses a value of "key.prefix.option" and an array of 250"key.prefix.array-option". 251 252Locking is not needed, since after initialization, the config becomes 253read-only. All data and keys must be copied if you need to modify it. 254 255 256Functions and structures 257======================== 258 259.. kernel-doc:: include/linux/bootconfig.h 260.. kernel-doc:: lib/bootconfig.c 261 262