.\" Copyright (c) 1999 Daniel C. Sobral
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.Dd September 29, 2021
.Dt LOADER_SIMP 8
.Os
.Sh NAME
.Nm loader_simp
.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 libstand 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.
.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.
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.
In the case of an error, an error message will be displayed and
the interpreter's state will be reset, emptying the stack and restoring
interpreting mode.
.Pp
The builtin commands available are:
.Pp
.Bl -tag -width Ds -compact
.It Ic autoboot Op Ar seconds Op Ar 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.
.Pp
.It Ic bcachestat
Displays statistics about disk cache usage.
For debugging only.
.Pp
.It Ic boot
.It Ic boot Ar kernelname Op Cm ...
.It Ic boot Fl flag Cm ...
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.
.Pp
.It Ic echo Xo
.Op Fl n
.Op Aq message
.Xc
Displays text on the screen.
A new line will be printed unless
.Fl n
is specified.
.Pp
.It Ic heap
Displays memory usage statistics.
For debugging purposes only.
.Pp
.It Ic help Op topic Op subtopic
Shows help messages read from
.Pa /boot/loader.help .
The special topic
.Em index
will list the topics available.
.Pp
.It Ic include Ar file Op Ar
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
.Sx ERRORS ) .
.Pp
.It Ic load Xo
.Op Fl t Ar type
.Ar file Cm ...
.Xc
Loads a kernel, kernel loadable module (kld), disk image,
or file of opaque contents tagged as being of the type
.Ar 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
.Li md_image
type to make the kernel create a file-backed
.Xr md 4
disk.
This is useful for booting from a temporary rootfs.
Currently, argument passing does not work for the kernel.
.Pp
.It Ic load_geli Xo
.Op Fl n Ar keyno
.Ar prov Ar file
.Xc
Loads a
.Xr geli 8
encryption keyfile for the given provider name.
The key index can be specified via
.Ar keyno
or will default to zero.
.Pp
.It Ic ls Xo
.Op Fl l
.Op Ar path
.Xc
Displays a listing of files in the directory
.Ar path ,
or the root directory if
.Ar path
is not specified.
If
.Fl l
is specified, file sizes will be shown too.
.Pp
.It Ic lsdev Op Fl v
Lists all of the devices from which it may be possible to load modules,
as well as ZFS pools.
If
.Fl v
is specified, more details are printed, including ZFS pool information
in a format that resembles
.Nm zpool Cm status
output.
.Pp
.It Ic lsmod Op Fl v
Displays loaded modules.
If
.Fl v
is specified, more details are shown.
.Pp
.It Ic lszfs Ar filesystem
A ZFS extended command that can be used to explore the ZFS filesystem
hierarchy in a pool.
Lists the immediate children of the
.Ar filesystem .
The filesystem hierarchy is rooted at a filesystem with the same name
as the pool.
.Pp
.It Ic more Ar file Op Ar
Display the files specified, with a pause at each
.Va LINES
displayed.
.Pp
.It Ic pnpscan Op Fl v
Scans for Plug-and-Play devices.
This is not functional at present.
.Pp
.It Ic read Xo
.Op Fl t Ar seconds
.Op Fl p Ar prompt
.Op Va variable
.Xc
Reads a line of input from the terminal, storing it in
.Va variable
if specified.
A timeout can be specified with
.Fl t ,
though it will be canceled at the first key pressed.
A prompt may also be displayed through the
.Fl p
flag.
.Pp
.It Ic reboot
Immediately reboots the system.
.Pp
.It Ic set Ar variable
.It Ic set Ar variable Ns = Ns Ar value
Set loader's environment variables.
.Pp
.It Ic show Op Va variable
Displays the specified variable's value, or all variables and their
values if
.Va variable
is not specified.
.Pp
.It Ic unload
Remove all modules from memory.
.Pp
.It Ic unset Va variable
Removes
.Va variable
from the environment.
.Pp
.It Ic \&?
Lists available commands.
.El
.Ss BUILTIN ENVIRONMENT VARIABLES
Environment variables can be set and unset through the
.Ic set
and
.Ic unset
builtins, and can have their values interactively examined through the
use of the
.Ic show
builtin.
Their values can also be accessed as described in
.Sx BUILTIN PARSER .
.Pp
Notice that these environment variables are not inherited by any shell
after the system has been booted.
.Pp
A few variables are set automatically by
.Nm .
Others can affect the behavior of either
.Nm
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.
.Bl -tag -width bootfile
.It Va autoboot_delay
Number of seconds
.Ic autoboot
will wait before booting.
Configuration options are described in
.Xr loader.conf 5 .
.It Va boot_askname
Instructs the kernel to prompt the user for the name of the root device
when the kernel is booted.
.It Va boot_cdrom
Instructs the kernel to try to mount the root file system from CD-ROM.
.It Va boot_ddb
Instructs the kernel to start in the DDB debugger, rather than
proceeding to initialize when booted.
.It Va boot_dfltroot
Instructs the kernel to mount the statically compiled-in root file system.
.It Va boot_gdb
Selects gdb-remote mode for the kernel debugger by default.
.It Va boot_multicons
Enables multiple console support in the kernel early on boot.
In a running system, console configuration can be manipulated
by the
.Xr conscontrol 8
utility.
.It Va 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
.Xr conscontrol 8
utility.
.It Va boot_pause
During the device probe, pause after each line is printed.
.It Va boot_serial
Force the use of a serial console even when an internal console
is present.
.It Va 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.
.It Va boot_verbose
Setting this variable causes extra debugging information to be printed
by the kernel during the boot phase.
.It Va bootfile
List of semicolon-separated search path for bootable kernels.
The default is
.Dq Li kernel .
.It Va 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
.Va BOOT_COMCONSOLE_SPEED
variable when
.Nm
was compiled.
Changes to the
.Va comconsole_speed
variable take effect immediately.
.It Va 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
.Va BOOT_COMCONSOLE_PORT
variable during the compilation of
.Nm .
Setting the
.Va comconsole_port
variable automatically set
.Va hw.uart.console
environment variable to provide a hint to kernel for location of the console.
Loader console is changed immediately after variable
.Va comconsole_port
is set.
.It Va 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
.Li 'bus:device:function[:bar]' ,
where all members must be numeric, with possible
.Li 0x
prefix to indicate a hexadecimal value.
The
.Va bar
member is optional and assumed to be 0x10 if omitted.
The bar must decode i/o space.
Setting the variable
.Va comconsole_pcidev
automatically sets the variable
.Va comconsole_port
to the base of the selected bar, and hint
.Va hw.uart.console .
Loader console is changed immediately after variable
.Va comconsole_pcidev
is set.
.It Va 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
.Xr init 8 ) .
.It Va currdev
Selects the default device to loader the kernel from.
The syntax is:
.Dl Ic loader_device:
or
.Dl Ic zfs:dataset:
Examples:
.Dl Ic disk0p2:
.Dl Ic zfs:zroot/ROOT/default:
.It Va dumpdev
Sets the device for kernel dumps.
This can be used to ensure that a device is configured before the corresponding
.Va dumpdev
directive from
.Xr rc.conf 5
has been processed, allowing kernel panics that happen during the early stages
of boot to be captured.
.It Va init_chroot
See
.Xr init 8 .
.It Va init_exec
See
.Xr init 8 .
.It Va 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
.Dq Li /sbin/init:/sbin/oinit:/sbin/init.bak:\:/rescue/init .
.It Va init_script
See
.Xr init 8 .
.It Va init_shell
See
.Xr init 8 .
.It Va interpret
Has the value
.Dq Li OK
if the Forth's current state is interpreting.
.It Va LINES
Define the number of lines on the screen, to be used by the pager.
.It Va 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
.Dq Li /boot/kernel;/boot/modules .
.It Va 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
.Va root_disk_unit .
.It Va prompt
Value of
.Nm Ns 's
prompt.
Defaults to
.Dq Li "${interpret}" .
If variable
.Va prompt
is unset, the default prompt is
.Ql > .
.It Va 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.
.It Va rootdev
By default the value of
.Va currdev
is used to set the root file system
when the kernel is booted.
This can be overridden by setting
.Va rootdev
explicitly.
.El
.Pp
Other variables are used to override kernel tunable parameters.
The following tunables are available:
.Bl -tag -width Va
.It Va 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.
.It Va hw.physmem
Limit the amount of physical memory the system will use.
By default the size is in bytes, but the
.Cm k , K , m , M , g
and
.Cm 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.
.It Va 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).
.It Va 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.
.It Va kern.maxusers
Set the size of a number of statically allocated system tables; see
.Xr 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.
.It Va 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.
.It Va kern.ipc.nsfbufs
Set the number of
.Xr sendfile 2
buffers to be allocated.
Overrides
.Dv NSFBUFS .
Not all architectures use such buffers; see
.Xr sendfile 2
for details.
.It Va 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.
.Pp
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.
.Pp
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
.Va kern.ipc.nmbclusters .
Modifies kernel option
.Dv VM_SWZONE_SIZE_MAX .
.It Va 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
.Va kern.ipc.nmbclusters .
Note that
the NBUF parameter will override this limit.
Modifies
.Dv VM_BCACHE_SIZE_MAX .
.It Va 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
.Dv MSGBUF_SIZE .
.It Va machdep.disable_mtrrs
Disable the use of i686 MTRRs (x86 only).
.It Va net.inet.tcp.tcbhashsize
Overrides the compile-time set value of
.Dv TCBHASHSIZE
or the preset default of 512.
Must be a power of 2.
.It Va twiddle_divisor
Throttles the output of the
.Sq 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.
.It Va vm.kmem_size
Sets the size of kernel memory (bytes).
This overrides the value determined when the kernel was compiled.
Modifies
.Dv VM_KMEM_SIZE .
.It Va vm.kmem_size_min
.It Va 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
.Dv VM_KMEM_SIZE_MIN
and
.Dv VM_KMEM_SIZE_MAX .
.El
.Ss ZFS FEATURES
.Nm
supports the following format for specifying ZFS filesystems which
can be used wherever
.Xr loader 8
refers to a device specification:
.Pp
.Ar zfs:pool/filesystem:
.Pp
where
.Pa pool/filesystem
is a ZFS filesystem name as described in
.Xr zfs 8 .
.Pp
If
.Pa /etc/fstab
does not have an entry for the root filesystem and
.Va vfs.root.mountfrom
is not set, but
.Va currdev
refers to a ZFS filesystem, then
.Nm
will instruct kernel to use that filesystem as the root filesystem.
.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.
.Pp
One can prevent unauthorized access
to the
.Nm
command line by booting unconditionally in
.Pa loader.rc .
In order for this to be effective, one should also configure the firmware
(BIOS or UEFI) to prevent booting from unauthorized devices.
.Sh FILES
.Bl -tag -width /boot/loader_simp -compact
.It Pa /boot/loader_simp
.Nm
itself.
.It Pa /boot/loader.rc
The script run by
.Nm
on startup.
.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 libstand 3 ,
.Xr loader.conf 5 ,
.Xr tuning 7 ,
.Xr boot 8 ,
.Xr btxld 8
.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 .