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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 10:05:51 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 10:05:51 +0000 |
commit | 5d1646d90e1f2cceb9f0828f4b28318cd0ec7744 (patch) | |
tree | a94efe259b9009378be6d90eb30d2b019d95c194 /Documentation/admin-guide/bootconfig.rst | |
parent | Initial commit. (diff) | |
download | linux-upstream/5.10.209.tar.xz linux-upstream/5.10.209.zip |
Adding upstream version 5.10.209.upstream/5.10.209upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to '')
-rw-r--r-- | Documentation/admin-guide/bootconfig.rst | 239 |
1 files changed, 239 insertions, 0 deletions
diff --git a/Documentation/admin-guide/bootconfig.rst b/Documentation/admin-guide/bootconfig.rst new file mode 100644 index 000000000..9b90efcc3 --- /dev/null +++ b/Documentation/admin-guide/bootconfig.rst @@ -0,0 +1,239 @@ +.. 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``. + +However, a sub-key and a value can not co-exist under a parent key. +For example, following config is NOT allowed.:: + + foo = value1 + foo.bar = value2 # !ERROR! subkey "bar" and value "value1" can NOT co-exist + foo.bar := value2 # !ERROR! even with the override operator, this is NOT allowed. + + +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 +============================== + +Since the boot configuration file is loaded with initrd, 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 feils 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. + +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 + |