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 libstand(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 interpter.

During initialization, loader_lua 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, 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 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_lua 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_lua will engage 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 will be generated, which can be intercepted using Lua exception handling. 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 Lua commands.

SECURITY
Access to the loader_lua command line provides several ways of compromising system security, including, but not limited to:

•   Booting from removable storage, by setting the currdev or loaddev variables

•   Executing binary of choice, by setting the init_path or init_script variables

•   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 will be probed and be 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
libstand(3), loader.conf(5), tuning(7), boot(8), btxld(8)

HISTORY
The loader_lua first appeared in FreeBSD 12.0.