xref: /freebsd/stand/man/loader_lua.8 (revision 11f49259c88ed4c110eefe89fa3862ef419b4150)

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.Dd September 29, 2021
.Dt LOADER_LUA 8
.Os
.Sh NAME
.Nm loader_lua
.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 the Lua interpreter.
.Pp
During initialization,
.Nm
probes for a console and sets the
.Va console
variable, or sets it to serial console
.Pq Dq Li comconsole
if the previous boot stage used that.
If multiple consoles are selected, they are listed separated by spaces.
Then, devices are probed,
.Va currdev
and
.Va loaddev
are set, and
.Va LINES
is set to 24.
Next, Lua is initialized, and
.Pa /boot/lua/loader.lua
is processed if it exists.
After that,
.Pa /boot/loader.conf
is processed if available.
.Pp
At this point, if an
.Ic autoboot
has not been attempted, and if
.Va autoboot_delay
is not set to
.Dq Li NO
(case insensitive), then an
.Ic autoboot
is attempted.
If the system gets past this point,
.Va prompt
is set and
.Nm
enters interactive mode.
Please note that, historically, even when
.Va autoboot_delay
is set to
.Dq Li 0 ,
the user can interrupt the autoboot process by pressing a key
on the console while the kernel and modules are being loaded.
To prevent this set
.Va autoboot_delay
to
.Dq Li -1 .
In this case
.Nm
enters 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 is generated,
which can be intercepted using Lua exception handling.
If not intercepted, an error message is displayed and
the interpreter's state is reset, emptying the stack and restoring
interpreting mode.
.Pp
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
as arguments, and it is processed by a special parser which
is not used for regular Lua commands.
.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 a 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 is probed and 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/defaults/loader.conf
.It Pa /boot/lua/loader.lua
Loader init
.It Pa /boot/loader.conf
.It Pa /boot/loader.conf.local
.Nm
configuration files, as described in
.Xr loader.conf 5 .
.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 HISTORY
The
.Nm
first appeared in
.Fx 12.0 .