LOADER(8) FreeBSD System Manager's Manual LOADER(8)
NAME
loader - kernel bootstrapping final stage
DESCRIPTION
The program called loader is the final stage of FreeBSD's kernel
bootstrapping process. On IA32 (i386) architectures, it is a BTX client.
It is linked statically to libstand(3) and usually located in the
directory /boot.
It provides a scripting language that can be used to automate tasks, do
pre-configuration or assist in recovery procedures. This scripting
language is roughly divided in two main components. The smaller one is a
set of commands designed for direct use by the casual user, called
"builtin commands" for historical reasons. The main drive behind these
commands is user-friendliness. The bigger component is an ANS Forth
compatible Forth interpreter based on FICL, by John Sadler.
During initialization, loader will probe for a console and set the
console variable, or set it to serial console ("comconsole") if the
previous boot stage used that. If multiple consoles are selected, they
will be listed separated by spaces. Then, devices are probed, currdev
and loaddev are set, and LINES is set to 24. Next, FICL is initialized,
the builtin words are added to its vocabulary, and /boot/boot.4th is
processed if it exists. No disk switching is possible while that file is
being read. The inner interpreter loader will use with FICL is then set
to interpret, which is FICL's default. After that, /boot/loader.rc is
processed if available. These files are processed through the include
command, which reads all of them into memory before processing them,
making disk changes possible.
At this point, if an autoboot has not been tried, and if autoboot_delay
is not set to "NO" (not case sensitive), then an autoboot will be tried.
If the system gets past this point, prompt will be set and loader will
engage interactive mode. Please note that historically even when
autoboot_delay is set to "0" user will be able to interrupt autoboot
process by pressing some key on the console while kernel and modules are
being loaded. In some cases such behaviour may be undesirable, to
prevent it set autoboot_delay to "-1", in this case loader will engage
interactive mode only if autoboot has failed.
BUILTIN COMMANDS
In loader, builtin commands take parameters from the command line.
Presently, the only way to call them from a script is by using evaluate
on a string. If an error condition occurs, an exception will be
generated, which can be intercepted using ANS Forth exception handling
words. If not intercepted, an error message will be displayed and the
interpreter's state will be reset, emptying the stack and restoring
interpreting mode.
The builtin commands available are:
autoboot [seconds [prompt]]
Proceeds to bootstrap the system after a number of seconds, if
not interrupted by the user. Displays a countdown prompt warning
the user the system is about to be booted, unless interrupted by
a key press. The kernel will be loaded first if necessary.
Defaults to 10 seconds.
bcachestat
Displays statistics about disk cache usage. For debugging only.
boot
boot kernelname [...]
boot -flag ...
Immediately proceeds to bootstrap the system, loading the kernel
if necessary. Any flags or arguments are passed to the kernel,
but they must precede the kernel name, if a kernel name is
provided.
WARNING: The behavior of this builtin is changed if loader.4th(8)
is loaded.
echo [-n] [<message>]
Displays text on the screen. A new line will be printed unless
-n is specified.
heap Displays memory usage statistics. For debugging purposes only.
help [topic [subtopic]]
Shows help messages read from /boot/loader.help. The special
topic index will list the topics available.
include file [file ...]
Process script files. Each file, in turn, is completely read
into memory, and then each of its lines is passed to the command
line interpreter. If any error is returned by the interpreter,
the include command aborts immediately, without reading any other
files, and returns an error itself (see ERRORS).
load [-t type] file ...
Loads a kernel, kernel loadable module (kld), disk image, or file
of opaque contents tagged as being of the type type. Kernel and
modules can be either in a.out or ELF format. Any arguments
passed after the name of the file to be loaded will be passed as
arguments to that file. Use the md_image type to make the kernel
create a file-backed md(4) disk. This is useful for booting from
a temporary rootfs. Currently, argument passing does not work
for the kernel.
load_geli [-n keyno] prov file
Loads a geli(8) encryption keyfile for the given provider name.
The key index can be specified via keyno or will default to zero.
ls [-l] [path]
Displays a listing of files in the directory path, or the root
directory if path is not specified. If -l is specified, file
sizes will be shown too.
lsdev [-v]
Lists all of the devices from which it may be possible to load
modules, as well as ZFS pools. If -v is specified, more details
are printed, including ZFS pool information in a format that
resembles zpool status output.
lsmod [-v]
Displays loaded modules. If -v is specified, more details are
shown.
lszfs filesystem
A ZFS extended command that can be used to explore the ZFS
filesystem hierarchy in a pool. Lists the immediate children of
the filesystem. The filesystem hierarchy is rooted at a
filesystem with the same name as the pool.
more file [file ...]
Display the files specified, with a pause at each LINES
displayed.
pnpscan [-v]
Scans for Plug-and-Play devices. This is not functional at
present.
read [-t seconds] [-p prompt] [variable]
Reads a line of input from the terminal, storing it in variable
if specified. A timeout can be specified with -t, though it will
be canceled at the first key pressed. A prompt may also be
displayed through the -p flag.
reboot Immediately reboots the system.
set variable
set variable=value
Set loader's environment variables.
show [variable]
Displays the specified variable's value, or all variables and
their values if variable is not specified.
unload Remove all modules from memory.
unset variable
Removes variable from the environment.
? Lists available commands.
BUILTIN ENVIRONMENT VARIABLES
The loader has actually two different kinds of `environment' variables.
There are ANS Forth's environmental queries, and a separate space of
environment variables used by builtins, which are not directly available
to Forth words. It is the latter type that this section covers.
Environment variables can be set and unset through the set and unset
builtins, and can have their values interactively examined through the
use of the show builtin. Their values can also be accessed as described
in BUILTIN PARSER.
Notice that these environment variables are not inherited by any shell
after the system has been booted.
A few variables are set automatically by loader. Others can affect the
behavior of either loader or the kernel at boot. Some options may
require a value, while others define behavior just by being set. Both
types of builtin variables are described below.
autoboot_delay
Number of seconds autoboot will wait before booting.
Configuration options are described in loader.conf(5).
boot_askname
Instructs the kernel to prompt the user for the name of the
root device when the kernel is booted.
boot_cdrom
Instructs the kernel to try to mount the root file system from
CD-ROM.
boot_ddb Instructs the kernel to start in the DDB debugger, rather than
proceeding to initialize when booted.
boot_dfltroot
Instructs the kernel to mount the statically compiled-in root
file system.
boot_gdb Selects gdb-remote mode for the kernel debugger by default.
boot_multicons
Enables multiple console support in the kernel early on boot.
In a running system, console configuration can be manipulated
by the conscontrol(8) utility.
boot_mute
All kernel console output is suppressed when console is muted.
In a running system, the state of console muting can be
manipulated by the conscontrol(8) utility.
boot_pause
During the device probe, pause after each line is printed.
boot_serial
Force the use of a serial console even when an internal console
is present.
boot_single
Prevents the kernel from initiating a multi-user startup;
instead, a single-user mode will be entered when the kernel has
finished device probing.
boot_verbose
Setting this variable causes extra debugging information to be
printed by the kernel during the boot phase.
bootfile List of semicolon-separated search path for bootable kernels.
The default is "kernel".
comconsole_speed
Defines the speed of the serial console (i386 and amd64 only).
If the previous boot stage indicated that a serial console is
in use then this variable is initialized to the current speed
of the console serial port. Otherwise it is set to 9600 unless
this was overridden using the BOOT_COMCONSOLE_SPEED variable
when loader was compiled. Changes to the comconsole_speed
variable take effect immediately.
comconsole_port
Defines the base i/o port used to access console UART (i386 and
amd64 only). If the variable is not set, its assumed value is
0x3F8, which corresponds to PC port COM1, unless overridden by
BOOT_COMCONSOLE_PORT variable during the compilation of loader.
Setting the comconsole_port variable automatically set
hw.uart.console environment variable to provide a hint to
kernel for location of the console. Loader console is changed
immediately after variable comconsole_port is set.
comconsole_pcidev
Defines the location of a PCI device of the 'simple
communication' class to be used as the serial console UART
(i386 and amd64 only). The syntax of the variable is
'bus:device:function[:bar]', where all members must be numeric,
with possible 0x prefix to indicate a hexadecimal value. The
bar member is optional and assumed to be 0x10 if omitted. The
bar must decode i/o space. Setting the variable
comconsole_pcidev automatically sets the variable
comconsole_port to the base of the selected bar, and hint
hw.uart.console. Loader console is changed immediately after
variable comconsole_pcidev is set.
console Defines the current console or consoles. Multiple consoles may
be specified. In that case, the first listed console will
become the default console for userland output (e.g. from
init(8)).
currdev Selects the default device to loader the kernel from. The
syntax is:
loader_device:
or
zfs:dataset:
Examples:
disk0p2:
zfs:zroot/ROOT/default:
dumpdev Sets the device for kernel dumps. This can be used to ensure
that a device is configured before the corresponding dumpdev
directive from rc.conf(5) has been processed, allowing kernel
panics that happen during the early stages of boot to be
captured.
init_chroot
See init(8).
init_exec
See init(8).
init_path
Sets the list of binaries which the kernel will try to run as
the initial process. The first matching binary is used. The
default list is "/sbin/init:/sbin/oinit:/sbin/init.bak:
/rescue/init".
init_script
See init(8).
init_shell
See init(8).
interpret
Has the value "OK" if the Forth's current state is
interpreting.
LINES Define the number of lines on the screen, to be used by the
pager.
module_path
Sets the list of directories which will be searched for modules
named in a load command or implicitly required by a dependency.
The default value for this variable is
"/boot/kernel;/boot/modules".
num_ide_disks
Sets the number of IDE disks as a workaround for some problems
in finding the root disk at boot. This has been deprecated in
favor of root_disk_unit.
prompt Value of loader's prompt. Defaults to "${interpret}". If
variable prompt is unset, the default prompt is `>'.
root_disk_unit
If the code which detects the disk unit number for the root
disk is confused, e.g. by a mix of SCSI and IDE disks, or IDE
disks with gaps in the sequence (e.g. no primary slave), the
unit number can be forced by setting this variable.
rootdev By default the value of currdev is used to set the root file
system when the kernel is booted. This can be overridden by
setting rootdev explicitly.
Other variables are used to override kernel tunable parameters. The
following tunables are available:
efi.rt.disabled
Disable UEFI runtime services in the kernel, if applicable.
Runtime services are only available and used if the kernel
is booted in a UEFI environment.
hw.physmem Limit the amount of physical memory the system will use.
By default the size is in bytes, but the k, K, m, M, g and
G suffixes are also accepted and indicate kilobytes,
megabytes and gigabytes respectively. An invalid suffix
will result in the variable being ignored by the kernel.
hw.pci.host_start_mem, hw.acpi.host_start_mem
When not otherwise constrained, this limits the memory
start address. The default is 0x80000000 and should be set
to at least size of the memory and not conflict with other
resources. Typically, only systems without PCI bridges
need to set this variable since PCI bridges typically
constrain the memory starting address (and the variable is
only used when bridges do not constrain this address).
hw.pci.enable_io_modes
Enable PCI resources which are left off by some BIOSes or
are not enabled correctly by the device driver. Tunable
value set to ON (1) by default, but this may cause problems
with some peripherals.
kern.maxusers
Set the size of a number of statically allocated system
tables; see tuning(7) for a description of how to select an
appropriate value for this tunable. When set, this tunable
replaces the value declared in the kernel compile-time
configuration file.
kern.ipc.nmbclusters
Set the number of mbuf clusters to be allocated. The value
cannot be set below the default determined when the kernel
was compiled.
kern.ipc.nsfbufs
Set the number of sendfile(2) buffers to be allocated.
Overrides NSFBUFS. Not all architectures use such buffers;
see sendfile(2) for details.
kern.maxswzone
Limits the amount of KVM to be used to hold swap metadata,
which directly governs the maximum amount of swap the
system can support, at the rate of approximately 200 MB of
swap space per 1 MB of metadata. This value is specified
in bytes of KVA space. If no value is provided, the system
allocates enough memory to handle an amount of swap that
corresponds to eight times the amount of physical memory
present in the system.
Note that swap metadata can be fragmented, which means that
the system can run out of space before it reaches the
theoretical limit. Therefore, care should be taken to not
configure more swap than approximately half of the
theoretical maximum.
Running out of space for swap metadata can leave the system
in an unrecoverable state. Therefore, you should only
change this parameter if you need to greatly extend the KVM
reservation for other resources such as the buffer cache or
kern.ipc.nmbclusters. Modifies kernel option
VM_SWZONE_SIZE_MAX.
kern.maxbcache
Limits the amount of KVM reserved for use by the buffer
cache, specified in bytes. The default maximum is 200MB on
i386, and 400MB on amd64. This parameter is used to
prevent the buffer cache from eating too much KVM in large-
memory machine configurations. Only mess around with this
parameter if you need to greatly extend the KVM reservation
for other resources such as the swap zone or
kern.ipc.nmbclusters. Note that the NBUF parameter will
override this limit. Modifies VM_BCACHE_SIZE_MAX.
kern.msgbufsize
Sets the size of the kernel message buffer. The default
limit of 96KB is usually sufficient unless large amounts of
trace data need to be collected between opportunities to
examine the buffer or dump it to a file. Overrides kernel
option MSGBUF_SIZE.
machdep.disable_mtrrs
Disable the use of i686 MTRRs (x86 only).
net.inet.tcp.tcbhashsize
Overrides the compile-time set value of TCBHASHSIZE or the
preset default of 512. Must be a power of 2.
twiddle_divisor
Throttles the output of the `twiddle' I/O progress
indicator displayed while loading the kernel and modules.
This is useful on slow serial consoles where the time spent
waiting for these characters to be written can add up to
many seconds. The default is 16; a value of 32 spins half
as fast, while a value of 8 spins twice as fast.
vm.kmem_size Sets the size of kernel memory (bytes). This overrides the
value determined when the kernel was compiled. Modifies
VM_KMEM_SIZE.
vm.kmem_size_min
vm.kmem_size_max
Sets the minimum and maximum (respectively) amount of
kernel memory that will be automatically allocated by the
kernel. These override the values determined when the
kernel was compiled. Modifies VM_KMEM_SIZE_MIN and
VM_KMEM_SIZE_MAX.
ZFS FEATURES
loader supports the following format for specifying ZFS filesystems which
can be used wherever loader(8) refers to a device specification:
zfs:pool/filesystem:
where pool/filesystem is a ZFS filesystem name as described in zfs(8).
If /etc/fstab does not have an entry for the root filesystem and
vfs.root.mountfrom is not set, but currdev refers to a ZFS filesystem,
then loader will instruct kernel to use that filesystem as the root
filesystem.
BUILTIN PARSER
When a builtin command is executed, the rest of the line is taken by it
as arguments, and it is processed by a special parser which is not used
for regular Forth commands.
This special parser applies the following rules to the parsed text:
1. All backslash characters are preprocessed.
• \b , \f , \r , \n and \t are processed as in C.
• \s is converted to a space.
• \v is converted to ASCII 11.
• \z is just skipped. Useful for things like "\0xf\z\0xf".
• \0xN and \0xNN are replaced by the hex N or NN.
• \NNN is replaced by the octal NNN ASCII character.
• \" , \' and \$ will escape these characters, preventing them
from receiving special treatment in Step 2, described below.
• \\ will be replaced with a single \ .
• In any other occurrence, backslash will just be removed.
2. Every string between non-escaped quotes or double-quotes will be
treated as a single word for the purposes of the remaining steps.
3. Replace any $VARIABLE or ${VARIABLE} with the value of the
environment variable VARIABLE.
4. Space-delimited arguments are passed to the called builtin command.
Spaces can also be escaped through the use of \\ .
An exception to this parsing rule exists, and is described in BUILTINS
AND FORTH.
BUILTINS AND FORTH
All builtin words are state-smart, immediate words. If interpreted, they
behave exactly as described previously. If they are compiled, though,
they extract their arguments from the stack instead of the command line.
If compiled, the builtin words expect to find, at execution time, the
following parameters on the stack:
addrN lenN ... addr2 len2 addr1 len1 N
where addrX lenX are strings which will compose the command line that
will be parsed into the builtin's arguments. Internally, these strings
are concatenated in from 1 to N, with a space put between each one.
If no arguments are passed, a 0 must be passed, even if the builtin
accepts no arguments.
While this behavior has benefits, it has its trade-offs. If the
execution token of a builtin is acquired (through ' or [']), and then
passed to catch or execute, the builtin behavior will depend on the
system state at the time catch or execute is processed! This is
particularly annoying for programs that want or need to handle
exceptions. In this case, the use of a proxy is recommended. For
example:
: (boot) boot;
FICL
FICL is a Forth interpreter written in C, in the form of a forth virtual
machine library that can be called by C functions and vice versa.
In loader, each line read interactively is then fed to FICL, which may
call loader back to execute the builtin words. The builtin include will
also feed FICL, one line at a time.
The words available to FICL can be classified into four groups. The ANS
Forth standard words, extra FICL words, extra FreeBSD words, and the
builtin commands; the latter were already described. The ANS Forth
standard words are listed in the STANDARDS section. The words falling in
the two other groups are described in the following subsections.
FICL EXTRA WORDS
.env
.ver
-roll
2constant
>name
body>
compare This is the STRING word set's compare.
compile-only
endif
forget-wid
parse-word
sliteral This is the STRING word set's sliteral.
wid-set-super
w@
w!
x.
empty
cell-
-rot
FREEBSD EXTRA WORDS
$ (--) Evaluates the remainder of the input buffer, after having
printed it first.
% (--) Evaluates the remainder of the input buffer under a catch
exception guard.
.# Works like . but without outputting a trailing space.
fclose (fd --)
Closes a file.
fkey (fd -- char)
Reads a single character from a file.
fload (fd --)
Processes a file fd.
fopen (addr len mode -- fd)
Opens a file. Returns a file descriptor, or -1 in case of
failure. The mode parameter selects whether the file is to be
opened for read access, write access, or both. The constants
O_RDONLY, O_WRONLY, and O_RDWR are defined in
/boot/support.4th, indicating read only, write only, and read-
write access, respectively.
fread (fd addr len -- len')
Tries to read len bytes from file fd into buffer addr. Returns
the actual number of bytes read, or -1 in case of error or end
of file.
heap? (-- cells)
Return the space remaining in the dictionary heap, in cells.
This is not related to the heap used by dynamic memory
allocation words.
inb (port -- char)
Reads a byte from a port.
key (-- char)
Reads a single character from the console.
key? (-- flag)
Returns true if there is a character available to be read from
the console.
ms (u --)
Waits u microseconds.
outb (port char --)
Writes a byte to a port.
seconds (-- u)
Returns the number of seconds since midnight.
tib> (-- addr len)
Returns the remainder of the input buffer as a string on the
stack.
trace! (flag --)
Activates or deactivates tracing. Does not work with catch.
FREEBSD DEFINED ENVIRONMENTAL QUERIES
arch-i386
TRUE if the architecture is IA32.
FreeBSD_version
FreeBSD version at compile time.
loader_version
loader version.
SECURITY
Access to the loader command line provides several ways of compromising
system security, including, but not limited to:
• Booting from removable storage, by setting the currdev or loaddev
variables
• Executing binary of choice, by setting the init_path or init_script
variables
• Overriding ACPI DSDT to inject arbitrary code into the ACPI subsystem
One can prevent unauthorized access to the loader command line by setting
the password, or setting autoboot_delay to -1. See loader.conf(5) for
details. In order for this to be effective, one should also configure
the firmware (BIOS or UEFI) to prevent booting from unauthorized devices.
FILES
/boot/loader loader itself.
/boot/boot.4th Additional FICL initialization.
/boot/defaults/loader.conf
/boot/loader.4th Extra builtin-like words.
/boot/loader.conf
/boot/loader.conf.local loader configuration files, as described
in loader.conf(5).
/boot/loader.rc loader bootstrapping script.
/boot/loader.help Loaded by help. Contains the help
messages.
/boot/support.4th loader.conf processing words.
/usr/share/examples/bootforth/ Assorted examples.
EXAMPLES
Boot in single user mode:
boot -s
Load the kernel, a splash screen, and then autoboot in five seconds.
Notice that a kernel must be loaded before any other load command is
attempted.
load kernel
load splash_bmp
load -t splash_image_data /boot/chuckrulez.bmp
autoboot 5
Set the disk unit of the root device to 2, and then boot. This would be
needed in a system with two IDE disks, with the second IDE disk hardwired
to ada2 instead of ada1.
set root_disk_unit=2
boot /boot/kernel/kernel
Set the default device used for loading a kernel from a ZFS filesystem:
set currdev=zfs:tank/ROOT/knowngood:
ERRORS
The following values are thrown by loader:
100 Any type of error in the processing of a builtin.
-1 Abort executed.
-2 Abort" executed.
-56 Quit executed.
-256 Out of interpreting text.
-257 Need more text to succeed -- will finish on next run.
-258 Bye executed.
-259 Unspecified error.
SEE ALSO
libstand(3), loader.conf(5), tuning(7), boot(8), btxld(8)
STANDARDS
For the purposes of ANS Forth compliance, loader is an ANS Forth System
with Environmental Restrictions, Providing .(, :noname, ?do, parse, pick,
roll, refill, to, value, \, false, true, <>, 0<>, compile, , erase, nip,
tuck and marker from the Core Extensions word set, Providing the
Exception Extensions word set, Providing the Locals Extensions word set,
Providing the Memory-Allocation Extensions word set, Providing .s, bye,
forget, see, words, [if], [else] and [then] from the Programming-Tools
extension word set, Providing the Search-Order extensions word set.
HISTORY
The loader first appeared in FreeBSD 3.1.
AUTHORS
The loader was written by Michael Smith <msmith@FreeBSD.org>.
FICL was written by John Sadler <john_sadler@alum.mit.edu>.
BUGS
The expect and accept words will read from the input buffer instead of
the console. The latter will be fixed, but the former will not.
FreeBSD 13.1-RELEASE-p6 October 2, 2020 FreeBSD 13.1-RELEASE-p6
man2web Home...