Initial commit

init.sh

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+#!/bin/sh
+# nnd-init
+# Adapted from mkinitfs of Alpine Linux Project
+
+sysroot=/sysroot
+init=/sbin/init
+init_args=''
+
+# Set-up basic utilities
+/bin/busybox mkdir -p /usr/bin /usr/sbin /proc /sys /dev /tmp $sysroot
+/bin/busybox --install -s
+export PATH=/usr/bin:/bin:/usr/sbin:/sbin
+
+# Prepare /dev/null to void unwanted error messages if none are pre-provided by the kernel
+[ -c /dev/null ] || mknod -m 666 /dev/null c 1 3
+
+# Mount essential filesystems
+mount -t devtmpfs -o exec,nosuid,mode=0755,size=2M devtmpfs /dev 2>/dev/null \
+	|| mount -t tmpfs -o exec,nosuid,mode=0755,size=2M tmpfs /dev
+
+# Prepare kmsg for earlyprintk support
+[ -c /dev/kmsg ] || mknod -m 660 /dev/kmsg c 1 11
+
+# Prepare pseudo-terminal for any early ttys
+[ -c /dev/ptmx ] || mknod -m 666 /dev/ptmx c 5 2
+[ -d /dev/pts ] || mkdir -m 755 /dev/pts
+mount -t devpts -o gid=5,mode=0620,noexec,nosuid devpts /dev/pts
+
+# Set-up rest of filesystems
+# These should ideally be carried over in the true root via mount moving
+mount -t sysfs -o noexec,nosuid,nodev sysfs /sys
+mount -t proc -o noexec,nosuid,nodev,hidepid=2 proc /proc
+mount -t tmpfs -o nosuid,nodev none /run
+
+# Provided mounts at this point:
+# /dev
+# /dev/pts
+# /sys
+# /proc
+# /run
+
+exec sh

notes.md

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+After basic bootstrapping, the source for the rootfs needs to be found, via hardware coldplugging and if wanted, a device manager to extend what the kernel provides by default.
+
+This is provided by nlplug-findfs on Alpine Linux.
+
+Once the block is present it can be mounted, but care must be taken so that the filesystem is either autodetected or provided to the mount call.
+
+Once sysroot is mounted, all special filesystems that have been so far mounted and optionally everything else must be moved over to new root.
+
+switch_root claims to move /dev, /proc, /sys and /run but this is probably only true for the coreutils switch_root. Care should be taken that these mounts are moved manually ahead of time to ensure presence.
+
+switch_root must be exec'd to overtake PID 1 and continue the init in the true rootfs.

switch

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+# NOTE: switch_root MAY move the mounts itself and the busybox implementation doesn't, so we move them ourselves
+
+# Usual case
+cat /proc/mounts | while read DEV DIR TYPE OPTS ; do
+	if [ "$DIR" != "/" -a "$DIR" != "/sysroot" -a -d "$DIR" ]; then
+		mkdir -p /sysroot/$DIR
+		mount -o move $DIR /sysroot/$DIR
+	fi
+done
+sync
+exec switch_root /sysroot /sbin/init
+
+# Template
+cat /proc/mounts | while read DEV DIR TYPE OPTS ; do
+	if [ "$DIR" != "/" -a "$DIR" != "$sysroot" -a -d "$DIR" ]; then
+		mkdir -p $sysroot/$DIR
+		mount -o move $DIR $sysroot/$DIR
+	fi
+done
+sync
+exec switch_root "$sysroot" "$init" "$init_args"