https://github.com/angt/slashinit

Minimal PID 1 for initramfs
https://github.com/angt/slashinit

c init initramfs linux pid1

Last synced: 3 months ago
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Minimal PID 1 for initramfs

• Host: GitHub
• URL: https://github.com/angt/slashinit
• Owner: angt
• License: bsd-2-clause
• Created: 2020-02-23T18:39:19.000Z (about 6 years ago)
• Default Branch: master
• Last Pushed: 2025-01-16T20:59:54.000Z (about 1 year ago)
• Last Synced: 2025-01-16T21:48:04.754Z (about 1 year ago)
• Topics: c, init, initramfs, linux, pid1
• Language: C
• Homepage:
• Size: 19.5 KB
• Stars: 15
• Watchers: 3
• Forks: 1
• Open Issues: 1
• Metadata Files:
  • Readme: README.md
  • License: LICENSE

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README

          
# `/init`

Minimal PID 1 for initramfs.

This is used for diskless servers with only two stages: `/etc/boot` and `/etc/reboot`.

For all spawned processes:

- inputs from `stdin` are dropped.

- outputs from `stdout` are redirected to `/dev/console`.

- logs from `stderr` are redirected to `/dev/kmsg`.

## Build & Install

The binary must be installed as `/init` and nothing else.

    $ make install DESTDIR=

## Update & reboot

When your system is running with `/init` you don't need any extra tool to do an update.

Just put the new kernel at `/kernel` and `/init` will automatically kexec it after the `/etc/reboot` stage.

If you want to update your system in the usual way (pxe, efi,...) you need a cold reboot:

    $ touch /reboot

    $ reboot

With `reboot` a script that stops your supervisor. For example with `runit`:

    $ cat /bin/reboot

    #!/bin/sh

    exec pkill -HUP runsvdir

## Configuration

Only two files are needed, for example:

    $ cat /etc/boot

    #!/bin/sh

    # your personal stuff..

    exec runsvdir -P /run/service

and

    $ cat /etc/reboot

    #!/bin/sh

    # your personal stuff..

    # you can still use kexec-tools for old kernels or unsupported arch.

    kexec -l /kernel --reuse-cmdline && kexec -e

## File system

`/init` will create a minimal file system that should look like this if all goes well:

    # cat /proc/mounts

    none / rootfs rw,nosuid 0 0

    none /proc proc rw,nosuid,nodev,noexec,relatime 0 0

    none /sys sysfs rw,nosuid,nodev,noexec,relatime 0 0

    none /sys/fs/cgroup cgroup2 rw,nosuid,nodev,noexec,relatime 0 0

    none /dev devtmpfs rw,nosuid,noexec 0 0

    devpts /dev/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0

Note that `/init` mounts `cgroup2` as `/sys/fs/cgroup` and not the old `cgroup`.

## Kernel configuration

A minimal kernel configuration to enable all features of `/init` would look like this:

    CONFIG_BLK_DEV_INITRD=y

    CONFIG_INITRAMFS_SOURCE=  # root directory with /init

    CONFIG_BINFMT_ELF=y

    CONFIG_BINFMT_SCRIPT=y

    CONFIG_DEVTMPFS=y

    CONFIG_PROC_FS=y

    CONFIG_SYSFS=y

Since all logs go into the kernel, it's a good idea to increase the size of the log buffer:

    CONFIG_LOG_BUF_SHIFT=24

Required for updates:

    CONFIG_RELOCATABLE=y

    CONFIG_CRYPTO=y

    CONFIG_CRYPTO_SHA256=y

    CONFIG_KEXEC_FILE=y

Not mandatory but useful to see what's going on :)

    CONFIG_PRINTK=y

    CONFIG_TTY=y

    CONFIG_SERIAL_8250=y

    CONFIG_SERIAL_8250_CONSOLE=y

---

For feature requests and bug reports,

please create an [issue](https://github.com/angt/slashinit/issues).