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
/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)][#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.
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¶
-
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(bool early)¶
Clean up all parsed bootconfig
Parameters
bool early
Set true if this is called before budy system is initialized.
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.
Comments¶
The config syntax accepts shell-script style comments. The comments starting with hash (“#”) until newline (”n”) will be ignored.
This is parsed as below:
Note that you can not put a comment between value and delimiter(
,
or;
). This means following config has a syntax error