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8e3fee33af
This reverts commit 5e8ff010a1.
This broke all the URLs, we can't have that. (And actually, we probably don't
_want_ to make the change either. It's nicer to have all the pages in one
directory, so one doesn't have to figure out to which collection the page
belongs.)
71 lines
3.5 KiB
Markdown
71 lines
3.5 KiB
Markdown
---
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title: Initrd Interface
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category: Interfaces
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layout: default
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SPDX-License-Identifier: LGPL-2.1-or-later
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---
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# The initrd Interface of systemd
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The Linux initrd mechanism (short for "initial RAM disk", also known as
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"initramfs") refers to a small file system archive that is unpacked by the
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kernel and contains the first userspace code that runs. It typically finds and
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transitions into the actual root file system to use. systemd supports both
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initrd and initrd-less boots. If an initrd is used, it is a good idea to pass a
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few bits of runtime information from the initrd to systemd in order to avoid
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duplicate work and to provide performance data to the administrator. In this
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page we attempt to roughly describe the interfaces that exist between the
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initrd and systemd. These interfaces are currently used by
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[mkosi](https://github.com/systemd/mkosi)-generated initrds, dracut and the
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Arch Linux initrds.
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* The initrd should mount `/run/` as a tmpfs and pass it pre-mounted when
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jumping into the main system when executing systemd. The mount options should
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be `mode=0755,nodev,nosuid,strictatime`.
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* It's highly recommended that the initrd also mounts `/usr/` (if split off) as
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appropriate and passes it pre-mounted to the main system, to avoid the
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problems described in [Booting without /usr is
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Broken](https://www.freedesktop.org/wiki/Software/systemd/separate-usr-is-broken).
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* If the executable `/run/initramfs/shutdown` exists systemd will use it to
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jump back into the initrd on shutdown. `/run/initramfs/` should be a usable
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initrd environment to which systemd will pivot back and the `shutdown`
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executable in it should be able to detach all complex storage that for
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example was needed to mount the root file system. It's the job of the initrd
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to set up this directory and executable in the right way so that this works
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correctly. The shutdown binary is invoked with the shutdown verb as `argv[1]`,
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optionally followed (in `argv[2]`, `argv[3]`, … systemd's original command
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line options, for example `--log-level=` and similar.
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* Storage daemons run from the initrd should follow the guide on
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[systemd and Storage Daemons for the Root File System](ROOT_STORAGE_DAEMONS.md)
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to survive properly from the boot initrd all the way to the point where
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systemd jumps back into the initrd for shutdown.
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One last clarification: we use the term _initrd_ very generically here
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describing any kind of early boot file system, regardless whether that might be
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implemented as an actual ramdisk, ramfs or tmpfs. We recommend using _initrd_
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in this sense as a term that is unrelated to the actual backing technologies
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used.
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## Using systemd inside an initrd
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It is also possible and recommended to implement the initrd itself based on
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systemd. Here are a few terse notes:
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* Provide `/etc/initrd-release` in the initrd image. The idea is that it
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follows the same format as the usual `/etc/os-release` but describes the
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initrd implementation rather than the OS. systemd uses the existence of this
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file as a flag whether to run in initrd mode, or not.
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* When run in initrd mode, systemd and its components will read a couple of
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additional command line arguments, which are generally prefixed with `rd.`
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* To transition into the main system image invoke `systemctl switch-root`.
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* The switch-root operation will result in a killing spree of all running
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processes. Some processes might need to be excluded from that, see the guide
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on [systemd and Storage Daemons for the Root File System](ROOT_STORAGE_DAEMONS.md).
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