man mktirfs (8): tool for creating a tiny-initramfs image

SYNOPSIS

mktirfs -o outputfile [options] [kernel-version]

DESCRIPTION

The mktirfs script generates an initramfs image that contains the tiny-initramfs init binary. Invoking mktirfs is meant for expert users that want to have greater control over their initramfs image. Typically it would be called from update-tirfs.

Images created with this utility are designed to be very small and fast, and thus ideal for use cases where the amount of space available in the boot loader is very low. If no modules and no microcode updates are included in the image, it will be less than 16 kiB in size. This comes at the cost of runtime flexibility and supported features of the generated images. It is not meant to replace traditional initramfs implementations (such as dracut or initramfs-tools) in the generic case, but provide a lean alternative for common use cases.

Please read /usr/share/doc/tiny-initramfs-core/README.md for an overview of the use cases it was designed for and the features it supports.

OPTIONS

-o, --output=imagefile

Where to store the resulting initramfs image. This option must be specified.

-m, --auto-modules=yes/no

Whether to try to automatically detect which modules are required to mount the / and /usr file systems and include them in the resulting initramfs image.

Defaults to yes.

--include-modules=mod1,mod2,...

Which modules to directly add to the initramfs image, regardless of the setting for -m. Dependencies of the modules will be resolved by mktirfs. This option is useful if the required modules are not properly detected or one wants to create an image for a different computer.

By default this list is empty.

-M, --microcode=generic/yes/no

Whether to prepend CPU microcode updates (if installed) to the initramfs image. Microcode updates may be prepended to the initramfs, and the kernel will automatically load them at boot time. (This requires the CONFIG_MICROCODE_EARLY kernel option in conjunction with either CONFIG_MICROCODE_INTEL or CONFIG_MICROCODE_AMD compiled into the kernel.) The microcode updates can be found in the (non-free) packages intel-microcode and amd64-microcode packages.

If the generic setting is specified, all installed microcode updates will be added to the initramfs image; if yes is specified only the update for the processor on which mktirfs was run on will be included.

Microcode updates not being installed (or no microcode updates for the specific processor being available) is not considered to be an error and mktirfs will proceed regardless.

Defaults to yes. Is only supported on x86 processors and will be ignored otherwise.

-d, --debug=yes/no

If set to yes, this option will install a (larger) version of the init binary into the initramfs image that prints additional debug messages while booting, and waits 5 seconds after mounting the root file system before handing control over to the system's init process. This allows the administrator to debug boot problems with initramfs images.

Defaults to no.

kernel-version

The kernel version for which to generate the initramfs image. This is only relevant if modules are to be added (either automatically or manually) to the initramfs image. If no modules are included in the initramfs image, the resulting image may be used with any kernel that has the drivers required to mount the root filesystem built in.

By default the version returned by uname -r is used.

CONFIGURATION

Except for -o and the kernel version, defaults for all the options may be specified in a configuration file /etc/tiny-initramfs/tiny-initramfs.conf. This file does not exist by default, but an example version may be found in /usr/share/doc/tiny-initramfs-core/tiny-initramfs.conf.example.

Any settings that update-tirfs is to use when calling mktirfs should be set in that configuration file.

HOOKS

There are no hooks within the initramfs itself, the init binary is a very small statically linked C program.

However mktirfs supports hooks when it comes to creating the initramfs image. The overall logic of mktirfs is as follows:

step].: Determine the list of kernel module names (without dependencies) to be added to the initramfs.
step].: Run start hooks.
step].: Copy init executable, add directories, copy microcode.
step].: Run middle hooks.
step].: Determine dependencies of modules, add all module files.
step].: Create initramfs image.
step].: Run end hooks.
step].: Copy image to its final location.

mktirfs looks for hooks in the /usr/share/tiny-initramfs/hooks.type and /etc/tiny-initramfs/hooks.type directories, e.g. /etc/tiny-initramfs/hooks.start. The directory in /usr/share is the domain of other Debian packages, while the directory in /etc is designated for the administrator of the system. Hooks will be called in alphabetical order, and hooks in /etc will override hooks in /usr/share. run-parts(8) is used to determine what hooks should be considered.

Hooks are sourced, not executed, so calling exit in a hook aborts the creation of the initramfs image. It also allows the hooks to modify variables that will be interpreted later.

Note that mktirfs uses /bin/sh as the interpreter, which is dash(1) by default on Debian systems, so not all bash(1) functionality is available within hooks.

The following shell variables are available from within hooksL

VERSION: The kernel version for which the initramfs is built.
DEBUG: The value of the -d command line option.
AUTO_MODULES: The value of the -m command line option.
INCLUDE_MODULES: The value of the --include-modules command line option.
MICROCODE: The value of the -M command line option.
OUTPUT: The value of the -o command line option. This should not be used, as mktirfs should only replace it if image creation was successful.
image_name: The temporary file name of the initramfs image that is to be created. Will be empty except for end hooks.
initramfs_dir: The temporary directory that contains the contents of the initramfs image. Any directories and files placed here will be part of the final initramfs image.
early_dir: The temporary directory that contains the contents of the early part of the initramfs image. This is used for storing microcode updates for the CPU, but the kernel also supports other updates here, e.g. updates to ACPI tables. If this directory is non-empty after the middle hooks an uncompressed CPIO image of this directory will be prepended to the final initramfs image by mktirfs. If it is empty (no microcode, no other early updates), the initramfs image will just contain the compressed CPIO archive of the initramfs_dir directory.
modules_list: The name of the temporary file that contains the list of module names that will be added to the initramfs image (one line per module name). After the middle hooks all modules, together with their dependencies, will be added to the initramfs image, any module options defined in /etc/modprobe.d for those modules will also be included.

EXAMPLE

Create a new initramfs image for the current kernel:
        mktirfs -o /boot/initrd.img.tiny-$(uname -r)
Create a new initramfs image for a specific kernel:
        mktirfs -o /boot/initrd.img.tiny-4.2.0 4.2.0

FILES

/etc/tiny-initramfs/tiny-initramfs.conf: If present, values in there will be used as defaults for the command line arguments of mktirfs.
/usr/share/doc/tiny-initramfs-core/tiny-initramfs.conf.example: An example configuration file that may be copied to the above location.
/usr/share/tiny-initramfs/hooks.start
/usr/share/tiny-initramfs/hooks.middle
/usr/share/tiny-initramfs/hooks.end: If present, hooks created by other Debian packages will be sourced from these directories.
/etc/tiny-initramfs/hooks.start
/etc/tiny-initramfs/hooks.middle
/etc/tiny-initramfs/hooks.end: If present, hooks created by the system administrator will be sourced from thsee directories. Hooks will replace those with the same name as hooks in /usr/share.

AUTHORS

Christian Seiler <[email protected]>