Lightweight rootfs for chroot using OverlayFS

setup_rootfs_overlay.sh

#!/bin/sh

# Since we want to create a exact sandbox of host,
# choose root to be the lower directory.

container_dir=$(mktemp -d --tmpdir=$HOME)
printf '%s ...\n' "Creating container directory: $container_dir"

original_username=$(whoami)

mkdir -p $container_dir/{work,upper,overlay_root}
upper_dir=$container_dir"/upper"
work_dir=$container_dir"/work"
merged=$container_dir"/overlay_root"
lower_dir="/"
printf '%s ...\n' "Making required directories for OverlayFS mounting"

sudo mount -t overlay overlay -o lowerdir=$lower_dir,upperdir=$upper_dir,workdir=$work_dir $merged
sudo mount proc "$merged/proc" -t proc -o nosuid,noexec,nodev &&
sudo mount sys "$merged/sys" -t sysfs -o nosuid,noexec,nodev,ro &&
sudo mount udev "$merged/dev" -t devtmpfs -o mode=0755,nosuid &&
sudo mount devpts "$merged/dev/pts" -t devpts -o mode=0620,gid=5,nosuid,noexec &&
sudo mount run "$merged/run" -t tmpfs -o nosuid,nodev,mode=0755 &&
sudo mount tmp "$merged/tmp" -t tmpfs -o mode=1777,strictatime,nodev,nosuid

sudo chroot $merged su $original_username