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LOADER_SIMP(8) FreeBSD System Manager's Manual LOADER_SIMP(8)
NAME loader_simp - kernel bootstrapping final stage
DESCRIPTION The program called loader_simp 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.
During initialization, loader_simp 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. After that, /boot/loader.rc is processed if available. These files are processed through the include command, which reads all of them into memory before processing them, making disk changes possible.
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_simp 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_simp will engage interactive mode only if autoboot has failed.
BUILTIN COMMANDS In loader_simp, 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. 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.
The builtin commands available are:
autoboot [seconds [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.
bcachestat Displays statistics about disk cache usage. For debugging only.
boot boot kernelname [...] boot -flag ... -n is specified.
heap Displays memory usage statistics. For debugging purposes only.
help [topic [subtopic]] Shows help messages read from /boot/loader.help. The special topic index will list the topics available.
include file [file ...] 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 ERRORS).
load [-t type] file ... Loads a kernel, kernel loadable module (kld), disk image, or file of opaque contents tagged as being of the type 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 md_image type to make the kernel create a file-backed md(4) disk. This is useful for booting from a temporary rootfs. Currently, argument passing does not work for the kernel.
load_geli [-n keyno] prov file Loads a geli(8) encryption keyfile for the given provider name. The key index can be specified via keyno or will default to zero.
ls [-l] [path] Displays a listing of files in the directory path, or the root directory if path is not specified. If -l is specified, file sizes will be shown too.
lsdev [-v] Lists all of the devices from which it may be possible to load modules, as well as ZFS pools. If -v is specified, more details are printed, including ZFS pool information in a format that resembles zpool status output.
lsmod [-v] Displays loaded modules. If -v is specified, more details are shown.
lszfs filesystem A ZFS extended command that can be used to explore the ZFS filesystem hierarchy in a pool. Lists the immediate children of the filesystem. The filesystem hierarchy is rooted at a filesystem with the same name as the pool.
more file [file ...] Display the files specified, with a pause at each LINES displayed.
pnpscan [-v] Scans for Plug-and-Play devices. This is not functional at present.
read [-t seconds] [-p prompt] [variable] set variable set variable=value Set loader's environment variables.
show [variable] Displays the specified variable's value, or all variables and their values if variable is not specified.
unload Remove all modules from memory.
unset variable Removes variable from the environment.
? Lists available commands.
BUILTIN ENVIRONMENT VARIABLES Environment variables can be set and unset through the set and unset builtins, and can have their values interactively examined through the use of the show builtin. Their values can also be accessed as described in BUILTIN PARSER.
Notice that these environment variables are not inherited by any shell after the system has been booted.
A few variables are set automatically by loader_simp. Others can affect the behavior of either loader_simp 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.
autoboot_delay Number of seconds autoboot will wait before booting. Configuration options are described in loader.conf(5).
boot_askname Instructs the kernel to prompt the user for the name of the root device when the kernel is booted.
boot_cdrom Instructs the kernel to try to mount the root file system from CD-ROM.
boot_ddb Instructs the kernel to start in the DDB debugger, rather than proceeding to initialize when booted.
boot_dfltroot Instructs the kernel to mount the statically compiled-in root file system.
boot_gdb Selects gdb-remote mode for the kernel debugger by default.
boot_multicons Enables multiple console support in the kernel early on boot. In a running system, console configuration can be manipulated by the conscontrol(8) utility.
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 conscontrol(8) utility.
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.
boot_verbose Setting this variable causes extra debugging information to be printed by the kernel during the boot phase.
bootfile List of semicolon-separated search path for bootable kernels. The default is "kernel".
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 115200 unless this was overridden using the BOOT_COMCONSOLE_SPEED variable when loader_simp was compiled. Changes to the comconsole_speed variable take effect immediately.
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 BOOT_COMCONSOLE_PORT variable during the compilation of loader_simp. Setting the comconsole_port variable automatically set hw.uart.console environment variable to provide a hint to kernel for location of the console. Loader console is changed immediately after variable comconsole_port is set.
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 'bus:device:function[:bar]', where all members must be numeric, with possible 0x prefix to indicate a hexadecimal value. The bar member is optional and assumed to be 0x10 if omitted. The bar must decode i/o space. Setting the variable comconsole_pcidev automatically sets the variable comconsole_port to the base of the selected bar, and hint hw.uart.console. Loader console is changed immediately after variable comconsole_pcidev is set.
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 init(8)).
currdev Selects the default device to loader the kernel from. The syntax is: loader_device: or zfs:dataset: Examples: disk0p2: zfs:zroot/ROOT/default:
init_chroot See init(8).
init_exec See init(8).
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 "/sbin/init:/sbin/oinit:/sbin/init.bak: /rescue/init".
init_script See init(8).
init_shell See init(8).
interpret Has the value "OK" if the Forth's current state is interpreting.
LINES Define the number of lines on the screen, to be used by the pager.
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 "/boot/kernel;/boot/modules".
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 root_disk_unit.
prompt Value of loader_simp's prompt. Defaults to "${interpret}". If variable prompt is unset, the default prompt is `>'.
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.
rootdev By default the value of currdev is used to set the root file system when the kernel is booted. This can be overridden by setting rootdev explicitly.
Other variables are used to override kernel tunable parameters. The following tunables are available:
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.
hw.physmem Limit the amount of physical memory the system will use. By default the size is in bytes, but the k, K, m, M, g and 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).
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.
kern.maxusers Set the size of a number of statically allocated system tables; see 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.
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.
kern.ipc.nsfbufs Set the number of sendfile(2) buffers to be allocated. Overrides NSFBUFS. Not all architectures use such buffers; see sendfile(2) for details.
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.
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.
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 kern.ipc.nmbclusters. Modifies kernel option VM_SWZONE_SIZE_MAX.
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 trace data need to be collected between opportunities to examine the buffer or dump it to a file. Overrides kernel option MSGBUF_SIZE.
machdep.disable_mtrrs Disable the use of i686 MTRRs (x86 only).
net.inet.tcp.tcbhashsize Overrides the compile-time set value of TCBHASHSIZE or the preset default of 512. Must be a power of 2.
twiddle_divisor Throttles the output of the `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.
vm.kmem_size Sets the size of kernel memory (bytes). This overrides the value determined when the kernel was compiled. Modifies VM_KMEM_SIZE.
vm.kmem_size_min
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 VM_KMEM_SIZE_MIN and VM_KMEM_SIZE_MAX.
ZFS FEATURES loader_simp supports the following format for specifying ZFS filesystems which can be used wherever loader(8) refers to a device specification:
zfs:pool/filesystem:
where pool/filesystem is a ZFS filesystem name as described in zfs(8).
If /etc/fstab does not have an entry for the root filesystem and vfs.root.mountfrom is not set, but currdev refers to a ZFS filesystem, then loader_simp will instruct kernel to use that filesystem as the root filesystem.
SECURITY Access to the loader_simp command line provides several ways of compromising system security, including, but not limited to:
o Booting from removable storage.
One can prevent unauthorized access to the loader_simp command line by booting unconditionally in loader.rc. In order for this to be effective, one should also configure the firmware (BIOS or UEFI) to prevent booting from unauthorized devices.
FILES /boot/loader_simp loader_simp itself. /boot/loader.rc The script run by loader_simp on startup. 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_simp:
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_simp first appeared in FreeBSD 3.1.
AUTHORS The loader_simp was written by Michael Smith <msmith@FreeBSD.org>.
FreeBSD 14.0-RELEASE-p11 September 29, 2021 FreeBSD 14.0-RELEASE-p11