squashboot

Tiny, statically linked init for initramfs.

It searches for the first squashfs file in the initramfs root, attaches it to a loop device, mounts it as the new root, relocates the pseudo-filesystems into it, then execs /init, /sbin/init, or (if /init.sh and /sbin/getty are present) a getty running /init.sh.

Kernel requirements

The kernel must be built with the options listed in kernel.config. A config fragment is provided so you can merge it into an existing .config:

scripts/kconfig/merge_config.sh .config kernel.config

Config symbol	Purpose
CONFIG_BLK_DEV_LOOP	Attach the squashfs image file to a loop device
CONFIG_SQUASHFS	Mount the squashfs root image
CONFIG_SQUASHFS_ZLIB/LZ4/LZO/XZ/ZSTD	Decompression — enable the codec(s) your image uses
CONFIG_TMPFS	/tmp, /run, /dev/shm
CONFIG_DEVTMPFS + CONFIG_DEVTMPFS_MOUNT	Populate /dev before init runs
CONFIG_PROC_FS	/proc
CONFIG_SYSFS	/sys
CONFIG_DEVPTS_FS	/dev/pts
CONFIG_CGROUPS + CONFIG_CGROUP_V2	/sys/fs/cgroup
CONFIG_CONFIGFS_FS	/sys/kernel/config
CONFIG_PRINTK	/dev/kmsg early logging

Building

Requires CMake ≥ 3.22 and a C11 compiler. The binary is statically linked.

cmake -B build
cmake --build build
# binary: build/squashboot

Pre-built static binaries for amd64 and arm64 are available on the releases page.