Boot Configuration — The Linux Kernel 5.18.5-gnu-1 documentation

# 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

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)][#BOOTCONFIGn]

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.

## 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 concatinated 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

uint32_t xbc_calc_checksum(void *data, uint32_t size)

  Calculate checksum of bootconfig

Parameters

`void *data`

  Bootconfig data.

`uint32_t size`

  The size of the bootconfig data.

Description

Calculate the checksum value of the bootconfig data. The checksum will be used with the BOOTCONFIG_MAGIC and the size for embedding the bootconfig in the initrd image.

bool xbc_node_is_value(struct xbc_node *node)

  Test the node is a value node

Parameters

`struct xbc_node *node`

  An XBC node.

Description

Test the node is a value node and return true if a value node, false if not.

bool xbc_node_is_key(struct xbc_node *node)

  Test the node is a key node

Parameters

`struct xbc_node *node`

  An XBC node.

Description

Test the node is a key node and return true if a key node, false if not.

bool xbc_node_is_array(struct xbc_node *node)

  Test the node is an arraied value node

Parameters

`struct xbc_node *node`

  An XBC node.

Description

Test the node is an arraied value node.

bool xbc_node_is_leaf(struct xbc_node *node)

  Test the node is a leaf key node

Parameters

`struct xbc_node *node`

  An XBC node.

Description

Test the node is a leaf key node which is a key node and has a value node or no child. Returns true if it is a leaf node, or false if not. Note that the leaf node can have subkey nodes in addition to the value node.

const char *xbc_find_value(const char *key, struct xbc_node **vnode)

  Find a value which matches the key

Parameters

`const char *key`

  Search key

`struct xbc_node **vnode`

  A container pointer of XBC value node.

Description

Search a value whose key matches key from whole of XBC tree and return the value if found. Found value node is stored in *vnode. Note that this can return 0-length string and store NULL in *vnode for key-only (non-value) entry.

struct xbc_node *xbc_find_node(const char *key)

  Find a node which matches the key

Parameters

`const char *key`

  Search key

Description

Search a (key) node whose key matches key from whole of XBC tree and return the node if found. If not found, returns NULL.

struct xbc_node *xbc_node_get_subkey(struct xbc_node *node)

  Return the first subkey node if exists

Parameters

`struct xbc_node *node`

  Parent node

Description

Return the first subkey node of the node. If the node has no child or only value node, this will return NULL.

xbc_array_for_each_value

`xbc_array_for_each_value (anode, value)`

> Iterate value nodes on an array

Parameters

`anode`

  An XBC arraied value node

`value`

  A value

Description

Iterate array value nodes and values starts from anode. This is expected to be used with `xbc_find_value()` and `xbc_node_find_value()`, so that user can process each array entry node.

xbc_node_for_each_child

`xbc_node_for_each_child (parent, child)`

> Iterate child nodes

Parameters

`parent`

  An XBC node.

`child`

  Iterated XBC node.

Description

Iterate child nodes of parent. Each child nodes are stored to child. The child can be mixture of a value node and subkey nodes.

xbc_node_for_each_subkey

`xbc_node_for_each_subkey (parent, child)`

> Iterate child subkey nodes

Parameters

`parent`

  An XBC node.

`child`

  Iterated XBC node.

Description

Iterate subkey nodes of parent. Each child nodes are stored to child. The child is only the subkey node.

xbc_node_for_each_array_value

`xbc_node_for_each_array_value (node, key, anode, value)`

> Iterate array entries of geven key

Parameters

`node`

  An XBC node.

`key`

  A key string searched under node

`anode`

  Iterated XBC node of array entry.

`value`

  Iterated value of array entry.

Description

Iterate array entries of given key under node. Each array entry node is stored to anode and value. If the node doesn’t have key node, it does nothing. Note that even if the found key node has only one value (not array) this executes block once. However, if the found key node has no value (key-only node), this does nothing. So don’t use this for testing the key-value pair existence.

xbc_node_for_each_key_value

`xbc_node_for_each_key_value (node, knode, value)`

> Iterate key-value pairs under a node

Parameters

`node`

  An XBC node.

`knode`

  Iterated key node

`value`

  Iterated value string

Description

Iterate key-value pairs under node. Each key node and value string are stored in knode and value respectively.

xbc_for_each_key_value

`xbc_for_each_key_value (knode, value)`

> Iterate key-value pairs

Parameters

`knode`

  Iterated key node

`value`

  Iterated value string

Description

Iterate key-value pairs in whole XBC tree. Each key node and value string are stored in knode and value respectively.

int xbc_node_compose_key(struct xbc_node *node, char *buf, size_t size)

  Compose full key string of the XBC node

Parameters

`struct xbc_node *node`

  An XBC node.

`char *buf`

  A buffer to store the key.

`size_t size`

  The size of the buf.

Description

Compose the full-length key of the node into buf. Returns the total length of the key stored in buf. Or returns -EINVAL if node is NULL, and -ERANGE if the key depth is deeper than max depth.

int xbc_get_info(int *node_size, size_t *data_size)

  Get the information of loaded boot config

Parameters

`int *node_size`

  A pointer to store the number of nodes.

`size_t *data_size`

  A pointer to store the size of bootconfig data.

Description

Get the number of used nodes in node_size if it is not NULL, and the size of bootconfig data in data_size if it is not NULL. Return 0 if the boot config is initialized, or return -ENODEV.

struct xbc_node *xbc_root_node(void)

  Get the root node of extended boot config

Parameters

`void`

  no arguments

Description

Return the address of root node of extended boot config. If the extended boot config is not initiized, return NULL.

int xbc_node_index(struct xbc_node *node)

  Get the index of XBC node

Parameters

`struct xbc_node *node`

  A target node of getting index.

Description

Return the index number of node in XBC node list.

struct xbc_node *xbc_node_get_parent(struct xbc_node *node)

  Get the parent XBC node

Parameters

`struct xbc_node *node`

  An XBC node.

Description

Return the parent node of node. If the node is top node of the tree, return NULL.

struct xbc_node *xbc_node_get_child(struct xbc_node *node)

  Get the child XBC node

Parameters

`struct xbc_node *node`

  An XBC node.

Description

Return the first child node of node. If the node has no child, return NULL.

struct xbc_node *xbc_node_get_next(struct xbc_node *node)

  Get the next sibling XBC node

Parameters

`struct xbc_node *node`

  An XBC node.

Description

Return the NEXT sibling node of node. If the node has no next sibling, return NULL. Note that even if this returns NULL, it doesn’t mean node has no siblings. (You also has to check whether the parent’s child node is node or not.)

const char *xbc_node_get_data(struct xbc_node *node)

  Get the data of XBC node

Parameters

`struct xbc_node *node`

  An XBC node.

Description

Return the data (which is always a null terminated string) of node. If the node has invalid data, warn and return NULL.

struct xbc_node *xbc_node_find_subkey(struct xbc_node *parent, const char *key)

  Find a subkey node which matches given key

Parameters

`struct xbc_node *parent`

  An XBC node.

`const char *key`

  A key string.

Description

Search a key node under parent which matches key. The key can contain several words jointed with ‘.’. If parent is NULL, this searches the node from whole tree. Return NULL if no node is matched.

const char *xbc_node_find_value(struct xbc_node *parent, const char *key, struct xbc_node **vnode)

  Find a value node which matches given key

Parameters

`struct xbc_node *parent`

  An XBC node.

`const char *key`

  A key string.

`struct xbc_node **vnode`

  A container pointer of found XBC node.

Description

Search a value node under parent whose (parent) key node matches key, store it in *vnode, and returns the value string. The key can contain several words jointed with ‘.’. If parent is NULL, this searches the node from whole tree. Return the value string if a matched key found, return NULL if no node is matched. Note that this returns 0-length string and stores NULL in *vnode if the key has no value. And also it will return the value of the first entry if the value is an array.

int xbc_node_compose_key_after(struct xbc_node *root, struct xbc_node *node, char *buf, size_t size)

  Compose partial key string of the XBC node

Parameters

`struct xbc_node *root`

  Root XBC node

`struct xbc_node *node`

  Target XBC node.

`char *buf`

  A buffer to store the key.

`size_t size`

  The size of the buf.

Description

Compose the partial key of the node into buf, which is starting right after root (root is not included.) If root is NULL, this returns full key words of node. Returns the total length of the key stored in buf. Returns -EINVAL if node is NULL or root is not the ancestor of node or root is node, or returns -ERANGE if the key depth is deeper than max depth. This is expected to be used with `xbc_find_node()` to list up all (child) keys under given key.

struct xbc_node *xbc_node_find_next_leaf(struct xbc_node *root, struct xbc_node *node)

  Find the next leaf node under given node

Parameters

`struct xbc_node *root`

  An XBC root node

`struct xbc_node *node`

  An XBC node which starts from.

Description

Search the next leaf node (which means the terminal key node) of node under root node (including root node itself). Return the next node or NULL if next leaf node is not found.

const char *xbc_node_find_next_key_value(struct xbc_node *root, struct xbc_node **leaf)

  Find the next key-value pair nodes

Parameters

`struct xbc_node *root`

  An XBC root node

`struct xbc_node **leaf`

  A container pointer of XBC node which starts from.

Description

Search the next leaf node (which means the terminal key node) of *leaf under root node. Returns the value and update *leaf if next leaf node is found, or NULL if no next leaf node is found. Note that this returns 0-length string if the key has no value, or the value of the first entry if the value is an array.

void xbc_exit(void)

  Clean up all parsed bootconfig

Parameters

`void`

  no arguments

Description

This clears all data structures of parsed bootconfig on memory. If you need to reuse `xbc_init()` with new boot config, you can use this.

int xbc_init(const char *data, size_t size, const char **emsg, int *epos)

  Parse given XBC file and build XBC internal tree

Parameters

`const char *data`

  The boot config text original data

`size_t size`

  The size of data

`const char **emsg`

  A pointer of const char * to store the error message

`int *epos`

  A pointer of int to store the error position

Description

This parses the boot config text in data. size must be smaller than XBC_DATA_MAX. Return the number of stored nodes (>0) if succeeded, or -errno if there is any error. In error cases, emsg will be updated with an error message and epos will be updated with the error position which is the byte offset of buf. If the error is not a parser error, epos will be -1.