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.Dd September 29, 2021
.Dt LOADER_4TH 8
.Os
.Sh NAME
.Nm loader_4th
.Nd kernel bootstrapping final stage
.Sh DESCRIPTION
The program called
.Nm
is the final stage of
.Fx Ns 's
kernel bootstrapping process.
On IA32 (i386) architectures, it is a
.Pa BTX
client.
It is linked statically to
.Xr libsa 3
and usually located in the directory
.Pa /boot .
.Pp
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
.Tn ANS
Forth compatible Forth interpreter based on FICL, by
.An John Sadler .
.Pp
During initialization,
.Nm
will probe for a console and set the
.Va console
variable, or set it to serial console
.Pq Dq Li comconsole
if the previous boot stage used that.
If multiple consoles are selected, they will be listed separated by spaces.
Then, devices are probed,
.Va currdev
and
.Va loaddev
are set, and
.Va LINES
is set to 24.
Next,
.Tn FICL
is initialized, the builtin words are added to its vocabulary, and
.Pa /boot/boot.4th
is processed if it exists.
No disk switching is possible while that file is being read.
The inner interpreter
.Nm
will use with
.Tn FICL
is then set to
.Ic interpret ,
which is
.Tn FICL Ns 's
default.
After that,
.Pa /boot/loader.rc
is processed if available.
These files are processed through the
.Ic include
command, which reads all of them into memory before processing them,
making disk changes possible.
.Pp
At this point, if an
.Ic autoboot
has not been tried, and if
.Va autoboot_delay
is not set to
.Dq Li NO
(not case sensitive), then an
.Ic autoboot
will be tried.
If the system gets past this point,
.Va prompt
will be set and
.Nm
will engage interactive mode.
Please note that historically even when
.Va autoboot_delay
is set to
.Dq Li 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
.Va autoboot_delay
to
.Dq Li -1 ,
in this case
.Nm
will engage interactive mode only if
.Ic autoboot
has failed.
.Sh BUILTIN COMMANDS
In
.Nm ,
builtin commands take parameters from the command line.
Presently,
the only way to call them from a script is by using
.Pa evaluate
on a string.
If an error condition occurs, an exception will be generated,
which can be intercepted using
.Tn 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
.Xr loader_simp 8
.Dq BUILTIN COMMANDS
section.
.Ss BUILTIN ENVIRONMENT VARIABLES
The environment variables common to all interpreters are described in the
.Xr loader_simp 8
.Dq BUILTIN ENVIRONMENT VARIABLES
section.
.Ss 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.
.Pp
This special parser applies the following rules to the parsed text:
.Bl -enum
.It
All backslash characters are preprocessed.
.Bl -bullet
.It
\eb , \ef , \er , \en and \et are processed as in C.
.It
\es is converted to a space.
.It
\ev is converted to
.Tn ASCII
11.
.It
\ez is just skipped.
Useful for things like
.Dq \e0xf\ez\e0xf .
.It
\e0xN and \e0xNN are replaced by the hex N or NN.
.It
\eNNN is replaced by the octal NNN
.Tn ASCII
character.
.It
\e" , \e' and \e$ will escape these characters, preventing them from
receiving special treatment in Step 2, described below.
.It
\e\e will be replaced with a single \e .
.It
In any other occurrence, backslash will just be removed.
.El
.It
Every string between non-escaped quotes or double-quotes will be treated
as a single word for the purposes of the remaining steps.
.It
Replace any
.Li $VARIABLE
or
.Li ${VARIABLE}
with the value of the environment variable
.Va VARIABLE .
.It
Space-delimited arguments are passed to the called builtin command.
Spaces can also be escaped through the use of \e\e .
.El
.Pp
An exception to this parsing rule exists, and is described in
.Sx BUILTINS AND FORTH .
.Ss 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.
.Pp
If compiled, the builtin words expect to find, at execution time, the
following parameters on the stack:
.D1 Ar addrN lenN ... addr2 len2 addr1 len1 N
where
.Ar 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.
.Pp
If no arguments are passed, a 0
.Em must
be passed, even if the builtin accepts no arguments.
.Pp
While this behavior has benefits, it has its trade-offs.
If the execution token of a builtin is acquired (through
.Ic '
or
.Ic ['] ) ,
and then passed to
.Ic catch
or
.Ic execute ,
the builtin behavior will depend on the system state
.Bf Em
at the time
.Ic catch
or
.Ic execute
is processed!
.Ef
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:
.Dl : (boot) boot ;
.Sh FICL
.Tn 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.
.Pp
In
.Nm ,
each line read interactively is then fed to
.Tn FICL ,
which may call
.Nm
back to execute the builtin words.
The builtin
.Ic include
will also feed
.Tn FICL ,
one line at a time.
.Pp
The words available to
.Tn FICL
can be classified into four groups.
The
.Tn ANS
Forth standard words, extra
.Tn FICL
words, extra
.Fx
words, and the builtin commands;
the latter were already described.
The
.Tn ANS
Forth standard words are listed in the
.Sx STANDARDS
section.
The words falling in the two other groups are described in the
following subsections.
.Ss FICL EXTRA WORDS
.Bl -tag -width wid-set-super
.It Ic .env
.It Ic .ver
.It Ic -roll
.It Ic 2constant
.It Ic >name
.It Ic body>
.It Ic compare
This is the STRING word set's
.Ic compare .
.It Ic compile-only
.It Ic endif
.It Ic forget-wid
.It Ic parse-word
.It Ic sliteral
This is the STRING word set's
.Ic sliteral .
.It Ic wid-set-super
.It Ic w@
.It Ic w!
.It Ic x.
.It Ic empty
.It Ic cell-
.It Ic -rot
.El
.Ss FREEBSD EXTRA WORDS
.Bl -tag -width XXXXXXXX
.It Ic \&$ Pq --
Evaluates the remainder of the input buffer, after having printed it first.
.It Ic \&% Pq --
Evaluates the remainder of the input buffer under a
.Ic catch
exception guard.
.It Ic .#
Works like
.Ic "."
but without outputting a trailing space.
.It Ic fclose Pq Ar fd --
Closes a file.
.It Ic fkey Pq Ar fd -- char
Reads a single character from a file.
.It Ic fload Pq Ar fd --
Processes a file
.Em fd .
.It Ic fopen Pq Ar addr len mode Li -- Ar fd
Opens a file.
Returns a file descriptor, or \-1 in case of failure.
The
.Ar mode
parameter selects whether the file is to be opened for read access, write
access, or both.
The constants
.Dv O_RDONLY , O_WRONLY ,
and
.Dv O_RDWR
are defined in
.Pa /boot/support.4th ,
indicating read only, write only, and read-write access, respectively.
.It Xo
.Ic fread
.Pq Ar fd addr len -- len'
.Xc
Tries to read
.Em len
bytes from file
.Em fd
into buffer
.Em addr .
Returns the actual number of bytes read, or -1 in case of error or end of
file.
.It Ic heap? Pq -- Ar cells
Return the space remaining in the dictionary heap, in cells.
This is not related to the heap used by dynamic memory allocation words.
.It Ic inb Pq Ar port -- char
Reads a byte from a port.
.It Ic key Pq -- Ar char
Reads a single character from the console.
.It Ic key? Pq -- Ar flag
Returns
.Ic true
if there is a character available to be read from the console.
.It Ic ms Pq Ar u --
Waits
.Em u
microseconds.
.It Ic outb Pq Ar port char --
Writes a byte to a port.
.It Ic seconds Pq -- Ar u
Returns the number of seconds since midnight.
.It Ic tib> Pq -- Ar addr len
Returns the remainder of the input buffer as a string on the stack.
.It Ic trace! Pq Ar flag --
Activates or deactivates tracing.
Does not work with
.Ic catch .
.El
.Ss FREEBSD DEFINED ENVIRONMENTAL QUERIES
.Bl -tag -width Ds
.It arch-i386
.Ic TRUE
if the architecture is IA32.
.It FreeBSD_version
.Fx
version at compile time.
.It loader_version
.Nm
version.
.El
.Sh SECURITY
Access to the
.Nm
command line provides several ways of compromising system security,
including, but not limited to:
.Pp
.Bl -bullet
.It
Booting from removable storage, by setting the
.Va currdev
or
.Va loaddev
variables
.It
Executing binary of choice, by setting the
.Va init_path
or
.Va init_script
variables
.It
Overriding ACPI DSDT to inject arbitrary code into the ACPI subsystem
.El
.Pp
One can prevent unauthorized access
to the
.Nm
command line by setting the
.Va password ,
or setting
.Va autoboot_delay
to -1.
See
.Xr 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.
.Sh MD
Memory disk (MD) can be used when the
.Nm
was compiled with
.Va MD_IMAGE_SIZE .
The size of the memory disk is determined by
.Va MD_IMAGE_SIZE .
If MD available, a file system can be embedded into the
.Nm
with
.Pa /sys/tools/embed_mfs.sh .
Then, MD will be probed and be set to
.Va currdev
during initialization.
.Pp
Currently, MD is only supported in
.Xr loader.efi 8 .
.Sh FILES
.Bl -tag -width /usr/share/examples/bootforth/ -compact
.It Pa /boot/loader
.Nm
itself.
.It Pa /boot/boot.4th
Additional
.Tn FICL
initialization.
.It Pa /boot/defaults/loader.conf
.It Pa /boot/loader.4th
Extra builtin-like words.
.It Pa /boot/loader.conf
.It Pa /boot/loader.conf.local
.Nm
configuration files, as described in
.Xr loader.conf 5 .
.It Pa /boot/loader.rc
.Nm
bootstrapping script.
.It Pa /boot/loader.help
Loaded by
.Ic help .
Contains the help messages.
.It Pa /boot/support.4th
.Pa loader.conf
processing words.
.It Pa /usr/share/examples/bootforth/
Assorted examples.
.El
.Sh EXAMPLES
Boot in single user mode:
.Pp
.Dl boot -s
.Pp
Load the kernel, a splash screen, and then autoboot in five seconds.
Notice that a kernel must be loaded before any other
.Ic load
command is attempted.
.Bd -literal -offset indent
load kernel
load splash_bmp
load -t splash_image_data /boot/chuckrulez.bmp
autoboot 5
.Ed
.Pp
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.
.Bd -literal -offset indent
set root_disk_unit=2
boot /boot/kernel/kernel
.Ed
.Pp
Set the default device used for loading a kernel from a ZFS filesystem:
.Bd -literal -offset indent
set currdev=zfs:tank/ROOT/knowngood:
.Ed
.Pp
.Sh ERRORS
The following values are thrown by
.Nm :
.Bl -tag -width XXXXX -offset indent
.It 100
Any type of error in the processing of a builtin.
.It -1
.Ic Abort
executed.
.It -2
.Ic Abort"
executed.
.It -56
.Ic Quit
executed.
.It -256
Out of interpreting text.
.It -257
Need more text to succeed -- will finish on next run.
.It -258
.Ic Bye
executed.
.It -259
Unspecified error.
.El
.Sh SEE ALSO
.Xr libsa 3 ,
.Xr loader.conf 5 ,
.Xr tuning 7 ,
.Xr boot 8 ,
.Xr btxld 8
.Sh STANDARDS
For the purposes of ANS Forth compliance, loader is an
.Bf Em
ANS Forth System with Environmental Restrictions, Providing
.Ef
.Bf Li
.No .( ,
.No :noname ,
.No ?do ,
parse, pick, roll, refill, to, value, \e, false, true,
.No <> ,
.No 0<> ,
compile\&, , erase, nip, tuck
.Ef
.Em and
.Li marker
.Bf Em
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
.Ef
.Bf Li
\&.s,
bye, forget, see, words,
\&[if],
\&[else]
.Ef
.Em and
.Li [then]
.Bf Em
from the Programming-Tools extension word set, Providing the
Search-Order extensions word set.
.Ef
.Sh HISTORY
The
.Nm
first appeared in
.Fx 3.1 .
.Sh AUTHORS
.An -nosplit
The
.Nm
was written by
.An Michael Smith Aq msmith@FreeBSD.org .
.Pp
.Tn FICL
was written by
.An John Sadler Aq john_sadler@alum.mit.edu .