/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=<PATH>

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=<PATH>  # root directory with <PATH>/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

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For feature requests and bug reports, please create an issue.