do-chroot

#!/bin/bash
# (c) 2020 Leif Sawyer
# License: GPL 3.0 (see https://github.com/akhepcat/)
# Permanent home:  https://github.com/akhepcat/Miscellaneous/
# Direct download: https://raw.githubusercontent.com/akhepcat/Miscellaneous/master/do-chroot
# 
ROOT=${1}
trap cleanup SIGINT SIGTERM SIGKILL SIGQUIT SIGABRT SIGSTOP SIGSEGV

cleanup() {
	echo "bailing out..."
	[[ ${MOUNTED} -eq 1 ]] && do_umounts
	exit 0
}

resolv_conf() {
	if [ -L "${ROOT:-undefined_rootfs}/etc/resolv.conf" ]
	then
		if [ -n "$(ls -l ${ROOT:-undefined_rootfs}/etc/resolv.conf 2>&1 | egrep -i 'run/.*(resolvconf|networkmanager)')" ]
		then
			# it's a symbolic link to a dynamic resolv file, so we can overwrite it with impunity
			# first get the actual location
			LRC=${ROOT:-undefined_rootfs}$(ls -l ${ROOT:-undefined_rootfs}/etc/resolv.conf | sed 's/.*-> //g;')
			# check for /var/run -> /run, since that breaks chrooting (use relative links, dists!)
			VRR=$(ls -dl ${ROOT:-undefined_rootfs}/var/run | sed 's/.*-> //g;')
			if [ -L "${ROOT:-undefined_rootfs}/var/run" -a "${VRR}" = "/run" ]
			then
				# remove the 'var' part
				LRC=${LRC//\/var/}
			fi
			# touch it so that we know the full path exists
			mkdir -p "${LRC//resolv.conf/}"
			touch -a "${LRC}"
			cat /etc/resolv.conf > "${LRC}"
		else
			echo "resolv.conf may need manually updating for full network connectivity"
		fi
	else
		# it's a real file
		echo "resolv.conf may need manually updating for full network connectivity"
	fi
}

do_mounts() {
	# grab all the mountpoints, and submountpoints...
	[[ ! -e ${ROOT:-undefined_rootfs}/dev/tty0 ]] && ${DEBUG:+echo} mount --rbind /dev ${ROOT:-undefined_rootfs}/dev
	[[ ! -e ${ROOT:-undefined_rootfs}/dev/pts/ptmx ]] && ${DEBUG:+echo} mount --rbind /dev/pts ${ROOT:-undefined_rootfs}/dev/pts
	[[ ! -e ${ROOT:-undefined_rootfs}/proc/version ]] && ${DEBUG:+echo} mount --rbind /proc ${ROOT:-undefined_rootfs}/proc
	# don't rbind /sys, because we don't want the cgroups!
	[[ ! -e ${ROOT:-undefined_rootfs}/sys/devices/cpu ]] && ${DEBUG:+echo} mount --bind /sys ${ROOT:-undefined_rootfs}/sys

	MOUNTED=1
}

do_umounts() {
	if [ ${MOUNTED:-0} -eq 1 -a -n "${ROOT:-undefined_rootfs}" ]
	then
		${DEBUG:+echo} umount --force --lazy \
			${ROOT:-undefined_rootfs}/sys \
			${ROOT:-undefined_rootfs}/proc \
			${ROOT:-undefined_rootfs}/dev/pts \
			${ROOT:-undefined_rootfs}/dev

		echo "Sometimes umounting kills the permissions on the directory /dev/shm, and umounts both /dev/pts and /proc/sys/fs/binfmt_misc"
		echo "Attempting to detect and fix"
		if [ "$(stat --format='%a' /dev/shm)" != "1777" ]
		then
			chmod 1777 /dev/shm
		fi
		[[ ! -e /dev/pts/ptmx ]] && mount -t devpts none /dev/pts
		[[ ! -e /proc/sys/fs/binfmt_misc/status ]] && mount -t binfmt_misc none /proc/sys/fs/binfmt_misc
	else
		echo "not mounted, or ${ROOT:-chrootfs} not accessible"
	fi

}

check_emu() {
	HOST=$(uname -m)
	if [ "${HOST}" != "${HOST##*arm*}" ]
	then
		HOST=arm
	elif [ "${HOST}" != "${HOST##*x86_64*}" -o "${HOST}" != "${HOST##*x86-64*}" ]
	then
		HOST=x86_64
	fi
	if [ -z "$(ls /proc/sys/fs/binfmt_misc/qemu-*  2>/dev/null)" ]
	then
		echo "qemu binfmt_misc support not registered. You may have problems with foreign archs"
		HOST=""
	fi
	# this should only find one, but compatibility libs will show up first, so only use the last found
	LDLIB=$(find ${ROOT:-undefined_rootfs}/lib ${ROOT:-undefined_rootfs}/lib64 -maxdepth 1 -type f -iname 'ld-uclibc*' -o -iname 'ld-linux*' 2>/dev/null | tail -1)
	TEMU=$(file -L "${LDLIB}" )

	# we can see the library, so we can figure out the emulation
	if [ -z "${TEMU##*x86_64*}" -o -z "${TEMU##*x86-64*}" -o -z "${TEMU##*32-bit*80386*}" ]
	then
		# Intel 32-bit and 64-bit are handled by aarch64
		EMU=aarch64
	elif [ -n "$(ls -l ${ROOT:-undefined_rootfs}/lib/ld-linux* 2>/dev/null | grep armeb )" ]
	then
		EMU=armeb
	elif [ -n "$(ls -l ${ROOT:-undefined_rootfs}/lib/ld-linux* 2>/dev/null | grep arm-)" ]
	then
		EMU=arm
	elif [ -z "${TEMU##*32*ARM*}" ]
	then
		EMU=armeb
	elif [ -z "${TEMU##*64*ARM*}" ]
	then
		EMU=aarch64
	fi

	if [ -z "${HOST}" -a -z "${EMU}" ]
	then
		echo "Can't reliably determine host/chroot architecture. Aborting."
		cleanup
	fi
	
	if [ -n "${EMU}" -a ! -r "/proc/sys/fs/binfmt_misc/qemu-${EMU}" ]
	then
		EMU=""
		if [ "${EMU}" = "${HOST}" ]
		then
			echo "attempting native chroot without binfmt_misc support"
		else
			echo "no qemu emulator for architecture ${EMU}"
			cleanup
		fi
	fi
}

get_shell() {

	if [ -r ${ROOT:-undefined_rootfs}/bin/bash ];
	then
		SHELL=/bin/bash
	elif [ -r ${ROOT:-undefined_rootfs}/sbin/bash ];
	then
		SHELL=/sbin/bash
	elif [ -r ${ROOT:-undefined_rootfs}/sbin/ash ];
	then
		SHELL=/sbin/ash
	elif [ -r ${ROOT:-undefined_rootfs}/bin/ash ];
	then
		SHELL=/bin/ash
	elif [ -r ${ROOT:-undefined_rootfs}/sbin/sh ];
	then
		SHELL=/sbin/sh
	elif [ -r ${ROOT:-undefined_rootfs}/sbin/sh ];
	then
		SHELL=/sbin/sh
	else
		#HOPE!
		SHELL=sh
	fi
}

# Does it look like a mounted filesystem?
if [ -n "${ROOT}" ]
then
    ROOT=${ROOT%/}	# strip off a trailing '/'
    if [ -d "${ROOT:-undefined_rootfs}/etc" ]
    then
	check_emu

	do_mounts

	resolv_conf

	[[ -r "${ROOT:-undefined_rootfs}/etc/hostname" ]] && export debian_chroot=$(cat ${ROOT:-undefined_rootfs}/etc/hostname)
	if [ -n "${EMU}" -a ! -r "${ROOT:-undefined_rootfs}/usr/bin/qemu-${EMU}-static" ]
	then
		cp /usr/bin/qemu-${EMU}-static ${ROOT:-undefined_rootfs}/usr/bin/ 2>/dev/null
		if [ $? -ne 0 ];
		then
			echo "can't cp emulation binary; check filesystem"
			cleanup
		fi
	fi

	get_shell

	chroot ${ROOT:-undefined_rootfs} ${SHELL}

	echo "clean to umount ${ROOT:-undefined_rootfs}"
    else
	echo "Can't chroot to dir ${ROOT:-undefined_rootfs} - is it a valid filesystem?"
    fi
else
    echo "chroot to where?"
fi

cleanup