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.

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 a 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 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:

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:

SEE ALSO

libsa(3), loader.conf(5), tuning(7), boot(8), btxld(8)

HISTORY

The loader_lua first appeared in FreeBSD 12.0.