TOMOYO Linux Cross Reference
Linux/Documentation/admin-guide/bootconfig.rst

   .. SPDX-License-Identifier: GPL-2.0
   
   .. _bootconfig:
   
   ==================
   Boot Configuration
   ==================
   
   :Author: Masami Hiramatsu <mhiramat@kernel.org>
  
  Overview
  ========
  
  The boot configuration expands the current kernel command line to support
  additional key-value data when booting the kernel in an efficient way.
  This allows administrators to pass a structured-Key config file.
  
  Config File Syntax
  ==================
  
  The boot config syntax is a simple structured key-value. Each key consists
  of dot-connected-words, and key and value are connected by ``=``. The value
  has to be terminated by semi-colon (``;``) or newline (``\n``).
  For array value, array entries are separated by comma (``,``). ::
  
    KEY[.WORD[...]] = VALUE[, VALUE2[...]][;]
  
  Unlike the kernel command line syntax, spaces are OK around the comma and ``=``.
  
  Each key word must contain only alphabets, numbers, dash (``-``) or underscore
  (``_``). And each value only contains printable characters or spaces except
  for delimiters such as semi-colon (``;``), new-line (``\n``), comma (``,``),
  hash (``#``) and closing brace (``}``).
  
  If you want to use those delimiters in a value, you can use either double-
  quotes (``"VALUE"``) or single-quotes (``'VALUE'``) to quote it. Note that
  you can not escape these quotes.
  
  There can be a key which doesn't have value or has an empty value. Those keys
  are used for checking if the key exists or not (like a boolean).
  
  Key-Value Syntax
  ----------------
  
  The boot config file syntax allows user to merge partially same word keys
  by brace. For example::
  
   foo.bar.baz = value1
   foo.bar.qux.quux = value2
  
  These can be written also in::
  
   foo.bar {
      baz = value1
      qux.quux = value2
   }
  
  Or more shorter, written as following::
  
   foo.bar { baz = value1; qux.quux = value2 }
  
  In both styles, same key words are automatically merged when parsing it
  at boot time. So you can append similar trees or key-values.
  
  Same-key Values
  ---------------
  
  It is prohibited that two or more values or arrays share a same-key.
  For example,::
  
   foo = bar, baz
   foo = qux  # !ERROR! we can not re-define same key
  
  If you want to update the value, you must use the override operator
  ``:=`` explicitly. For example::
  
   foo = bar, baz
   foo := qux
  
  then, the ``qux`` is assigned to ``foo`` key. This is useful for
  overriding the default value by adding (partial) custom bootconfigs
  without parsing the default bootconfig.
  
  If you want to append the value to existing key as an array member,
  you can use ``+=`` operator. For example::
  
   foo = bar, baz
   foo += qux
  
  In this case, the key ``foo`` has ``bar``, ``baz`` and ``qux``.
  
  Moreover, sub-keys and a value can coexist under a parent key.
  For example, following config is allowed.::
  
   foo = value1
   foo.bar = value2
   foo := value3 # This will update foo's value.
  
  Note, since there is no syntax to put a raw value directly under a
 structured key, you have to define it outside of the brace. For example::
 
  foo {
      bar = value1
      bar {
          baz = value2
          qux = value3
      }
  }
 
 Also, the order of the value node under a key is fixed. If there
 are a value and subkeys, the value is always the first child node
 of the key. Thus if user specifies subkeys first, e.g.::
 
  foo.bar = value1
  foo = value2
 
 In the program (and /proc/bootconfig), it will be shown as below::
 
  foo = value2
  foo.bar = value1
 
 Comments
 --------
 
 The config syntax accepts shell-script style comments. The comments starting
 with hash ("#") until newline ("\n") will be ignored.
 
 ::
 
  # comment line
  foo = value # value is set to foo.
  bar = 1, # 1st element
        2, # 2nd element
        3  # 3rd element
 
 This is parsed as below::
 
  foo = value
  bar = 1, 2, 3
 
 Note that you can not put a comment between value and delimiter(``,`` or
 ``;``). This means following config has a syntax error ::
 
  key = 1 # comment
        ,2
 
 
 /proc/bootconfig
 ================
 
 /proc/bootconfig is a user-space interface of the boot config.
 Unlike /proc/cmdline, this file shows the key-value style list.
 Each key-value pair is shown in each line with following style::
 
  KEY[.WORDS...] = "[VALUE]"[,"VALUE2"...]
 
 
 Boot Kernel With a Boot Config
 ==============================
 
 There are two options to boot the kernel with bootconfig: attaching the
 bootconfig to the initrd image or embedding it in the kernel itself.
 
 Attaching a Boot Config to Initrd
 ---------------------------------
 
 Since the boot configuration file is loaded with initrd by default,
 it will be added to the end of the initrd (initramfs) image file with
 padding, size, checksum and 12-byte magic word as below.
 
 [initrd][bootconfig][padding][size(le32)][checksum(le32)][#BOOTCONFIG\n]
 
 The size and checksum fields are unsigned 32bit little endian value.
 
 When the boot configuration is added to the initrd image, the total
 file size is aligned to 4 bytes. To fill the gap, null characters
 (``\0``) will be added. Thus the ``size`` is the length of the bootconfig
 file + padding bytes.
 
 The Linux kernel decodes the last part of the initrd image in memory to
 get the boot configuration data.
 Because of this "piggyback" method, there is no need to change or
 update the boot loader and the kernel image itself as long as the boot
 loader passes the correct initrd file size. If by any chance, the boot
 loader passes a longer size, the kernel fails to find the bootconfig data.
 
 To do this operation, Linux kernel provides ``bootconfig`` command under
 tools/bootconfig, which allows admin to apply or delete the config file
 to/from initrd image. You can build it by the following command::
 
  # make -C tools/bootconfig
 
 To add your boot config file to initrd image, run bootconfig as below
 (Old data is removed automatically if exists)::
 
  # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z
 
 To remove the config from the image, you can use -d option as below::
 
  # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z
 
 Then add "bootconfig" on the normal kernel command line to tell the
 kernel to look for the bootconfig at the end of the initrd file.
 Alternatively, build your kernel with the ``CONFIG_BOOT_CONFIG_FORCE``
 Kconfig option selected.
 
 Embedding a Boot Config into Kernel
 -----------------------------------
 
 If you can not use initrd, you can also embed the bootconfig file in the
 kernel by Kconfig options. In this case, you need to recompile the kernel
 with the following configs::
 
  CONFIG_BOOT_CONFIG_EMBED=y
  CONFIG_BOOT_CONFIG_EMBED_FILE="/PATH/TO/BOOTCONFIG/FILE"
 
 ``CONFIG_BOOT_CONFIG_EMBED_FILE`` requires an absolute path or a relative
 path to the bootconfig file from source tree or object tree.
 The kernel will embed it as the default bootconfig.
 
 Just as when attaching the bootconfig to the initrd, you need ``bootconfig``
 option on the kernel command line to enable the embedded bootconfig, or,
 alternatively, build your kernel with the ``CONFIG_BOOT_CONFIG_FORCE``
 Kconfig option selected.
 
 Note that even if you set this option, you can override the embedded
 bootconfig by another bootconfig which attached to the initrd.
 
 Kernel parameters via Boot Config
 =================================
 
 In addition to the kernel command line, the boot config can be used for
 passing the kernel parameters. All the key-value pairs under ``kernel``
 key will be passed to kernel cmdline directly. Moreover, the key-value
 pairs under ``init`` will be passed to init process via the cmdline.
 The parameters are concatenated with user-given kernel cmdline string
 as the following order, so that the command line parameter can override
 bootconfig parameters (this depends on how the subsystem handles parameters
 but in general, earlier parameter will be overwritten by later one.)::
 
  [bootconfig params][cmdline params] -- [bootconfig init params][cmdline init params]
 
 Here is an example of the bootconfig file for kernel/init parameters.::
 
  kernel {
    root = 01234567-89ab-cdef-0123-456789abcd
  }
  init {
   splash
  }
 
 This will be copied into the kernel cmdline string as the following::
 
  root="01234567-89ab-cdef-0123-456789abcd" -- splash
 
 If user gives some other command line like,::
 
  ro bootconfig -- quiet
 
 The final kernel cmdline will be the following::
 
  root="01234567-89ab-cdef-0123-456789abcd" ro bootconfig -- splash quiet
 
 
 Config File Limitation
 ======================
 
 Currently the maximum config size size is 32KB and the total key-words (not
 key-value entries) must be under 1024 nodes.
 Note: this is not the number of entries but nodes, an entry must consume
 more than 2 nodes (a key-word and a value). So theoretically, it will be
 up to 512 key-value pairs. If keys contains 3 words in average, it can
 contain 256 key-value pairs. In most cases, the number of config items
 will be under 100 entries and smaller than 8KB, so it would be enough.
 If the node number exceeds 1024, parser returns an error even if the file
 size is smaller than 32KB. (Note that this maximum size is not including
 the padding null characters.)
 Anyway, since bootconfig command verifies it when appending a boot config
 to initrd image, user can notice it before boot.
 
 
 Bootconfig APIs
 ===============
 
 User can query or loop on key-value pairs, also it is possible to find
 a root (prefix) key node and find key-values under that node.
 
 If you have a key string, you can query the value directly with the key
 using xbc_find_value(). If you want to know what keys exist in the boot
 config, you can use xbc_for_each_key_value() to iterate key-value pairs.
 Note that you need to use xbc_array_for_each_value() for accessing
 each array's value, e.g.::
 
  vnode = NULL;
  xbc_find_value("key.word", &vnode);
  if (vnode && xbc_node_is_array(vnode))
     xbc_array_for_each_value(vnode, value) {
       printk("%s ", value);
     }
 
 If you want to focus on keys which have a prefix string, you can use
 xbc_find_node() to find a node by the prefix string, and iterate
 keys under the prefix node with xbc_node_for_each_key_value().
 
 But the most typical usage is to get the named value under prefix
 or get the named array under prefix as below::
 
  root = xbc_find_node("key.prefix");
  value = xbc_node_find_value(root, "option", &vnode);
  ...
  xbc_node_for_each_array_value(root, "array-option", value, anode) {
     ...
  }
 
 This accesses a value of "key.prefix.option" and an array of
 "key.prefix.array-option".
 
 Locking is not needed, since after initialization, the config becomes
 read-only. All data and keys must be copied if you need to modify it.
 
 
 Functions and structures
 ========================
 
 .. kernel-doc:: include/linux/bootconfig.h
 .. kernel-doc:: lib/bootconfig.c
 

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