LOADER_LUA(8)           FreeBSD System Manager's Manual          LOADER_LUA(8)

NAME

     loader_lua - kernel bootstrapping final stage

DESCRIPTION

     The program called loader_lua 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 the Lua
     interpreter.

     During initialization, loader_lua probes for a console and sets the
     console variable, or sets it to serial console ("comconsole") if the
     previous boot stage used that.  If multiple consoles are selected, they
     are listed separated by spaces.  Then, devices are probed, currdev and
     loaddev are set, and LINES is set to 24.  Next, Lua is initialized, and
     /boot/lua/loader.lua is processed if it exists.  After that,
     /boot/loader.conf is processed if available.

     At this point, if an autoboot has not been attempted, and if
     autoboot_delay is not set to "NO" (case insensitive), then an autoboot is
     attempted.  If the system gets past this point, prompt is set and
     loader_lua enters interactive mode.  Please note that, historically, even
     when autoboot_delay is set to "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 autoboot_delay to "-1".  In this case
     loader_lua enters interactive mode only if autoboot has failed.

BUILTIN COMMANDS

     In loader_lua, 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 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.

     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 as
     arguments, and it is processed by a special parser which is not used for
     regular Lua commands.

SECURITY

     Access to the loader_lua 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 a 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_lua 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_lua 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_lua with /sys/tools/embed_mfs.sh.  Then, MD is probed and set to
     currdev during initialization.

     Currently, MD is only supported in loader.efi(8).

FILES

     /boot/loader                    loader_lua itself.
     /boot/defaults/loader.conf
     /boot/lua/loader.lua            Loader init
     /boot/loader.conf
     /boot/loader.conf.local         loader_lua configuration files, as
                                     described in loader.conf(5).

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_lua:

           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)

HISTORY

     The loader_lua first appeared in FreeBSD 12.0.

FreeBSD 15.1-STABLE-HBSD      September 29, 2021                 LOADER_LUA(8)