LOADER_4TH(8) FreeBSD System Manager's Manual LOADER_4TH(8)
NAME
loader_4th - kernel bootstrapping final stage
DESCRIPTION
The program called loader_4th is the final stage of FreeBSD's kernel
bootstrapping process. On IA32 (i386) architectures, it is a BTX client.
It is linked statically to libsa(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_4th 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_4th 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_4th
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_4th will
engage interactive mode only if autoboot has failed.
BUILTIN COMMANDS
In loader_4th, 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 commands are described in the loader_simp(8)
"BUILTIN COMMANDS" section.
BUILTIN ENVIRONMENT VARIABLES
The environment variables common to all interpreters are described in the
loader_simp(8) "BUILTIN ENVIRONMENT VARIABLES" section.
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.
o \b , \f , \r , \n and \t are processed as in C.
o \s is converted to a space.
o \v is converted to ASCII 11.
o \z is just skipped. Useful for things like "\0xf\z\0xf".
o \0xN and \0xNN are replaced by the hex N or NN.
o \NNN is replaced by the octal NNN ASCII character.
o \" , \' and \$ will escape these characters, preventing them
from receiving special treatment in Step 2, described below.
o \\ will be replaced with a single \ .
o 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_4th, each line read interactively is then fed to FICL, which
may call loader_4th 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_4th version.
SECURITY
Access to the loader_4th command line provides several ways of
compromising system security, including, but not limited to:
o Booting from removable storage, by setting the currdev or loaddev
variables
o Executing binary of choice, by setting the init_path or init_script
variables
o Overriding ACPI DSDT to inject arbitrary code into the ACPI subsystem
One can prevent unauthorized access to the loader_4th 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.
MD
Memory disk (MD) can be used when the loader_4th was compiled with
MD_IMAGE_SIZE. The size of the memory disk is determined by
MD_IMAGE_SIZE. If MD available, a file system can be embedded into the
loader_4th with /sys/tools/embed_mfs.sh. Then, MD will be probed and be
set to currdev during initialization.
Currently, MD is only supported in loader.efi(8).
FILES
/boot/loader loader_4th 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_4th configuration files, as
described in loader.conf(5).
/boot/loader.rc loader_4th 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_4th:
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
libsa(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_4th first appeared in FreeBSD 3.1.
AUTHORS
The loader_4th was written by Michael Smith <msmith@FreeBSD.org>.
FICL was written by John Sadler <john_sadler@alum.mit.edu>.
FreeBSD 15.1-STABLE-HBSD September 29, 2021 LOADER_4TH(8)