boot a tarball or oci image ref

README.md

boot a tarball or oci imag ref

Get yourself a kernel

# kernel from vmlinuz on ubuntu
kdist=/boot/$(uname -r)
# https://bugs.launchpad.net/ubuntu/+source/linux/+bug/759725
sudo cat "$kdist" > kernel

Now convert a oci or local tarball to kernel flavor cpio. this script adds a very basic 'init'

fakeroot ./create-initramfs my-initramfs base.tar

boot it

qemu-system-x86_64 -enable-kvm -m 2048 -nographic \
    -kernel ./kernel -initrd ./my-initramfs \
    -append "console=ttyS0 this is my cmdline"

create-initramfs

#!/bin/sh
TEMP_D=""

Usage() {
    cat <<EOF
${0##*/} out.cpio tar-or-ociref [init]
EOF
}

winit() {
    cat <<"EOF"
#!/bin/sh
# vi: ts=4 expandtab
msg() {
    local d="" written=false
    for d in /dev/ttyS0 /dev/tty1; do
        [ -c "$d" ] || continue
        echo "$@" >"$d" 2>&1 && written=true;
    done
    [ "$written" = "true" ] && return 0
    echo "$@"
}
panic() {
    msg "FATAL:" "$@"
}

msg "==== MINI INITRAMFS INIT ===="
mkdir -p /proc /dev /tmp /sys
mount -t devtmpfs /dev /dev ||
    panic "failed mount devtmpfs"
mount -t proc /proc /proc ||
    panic "failed mount proc"
mount -t sysfs /sys /sys ||
    panic "failed mount /sys"

echo "6 4 1 7" >/proc/sys/kernel/printk

read cmdline < /proc/cmdline ||
    panic "failed to read cmdline"

msg "KERNEL COMMAND LINE:" "$cmdline"

echo o >/proc/sysrq-trigger

read || panic "Read returned $?"
panic "Read returned success; What to do now?"
exit 1
EOF
}

fail() { echo "$@" 1>&2; exit 1; }
cleanup() { [ "$TEMP_D" = "" ] || rm -Rf "$TEMP_D"; }
stderr() { echo "$@" 1>&2; }

[ "$1" = "-h" -o "$1" = "--help" ] && { Usage ; exit 1; }
[ $# -gt 1 -a $# -lt 4 ] || fail "got $# expect 2 or 3"


trap cleanup EXIT
out=$1
input=$2
init=${3:-builtin}


[ "$(id -u)" = "0" ] || fail "have to be root. or use fakeroot"

TEMP_D=$(mktemp -d "${TMPDIR:-/tmp}/${0##*/}.XXXXXX") || fail "failed mktemp -d"

initf=""
if [ "$init" = "builtin" ]; then
    initf="${TEMP_D}/init"
    winit > "$initf"
elif [ -n "$init" ]; then
    [ -f "$initf" ] || fail "init '$initf' not a file"
    initf="$init"
fi

case "$input" in
    cgr.dev/*) 
        tar="${TEMP_D}/tmp.tar"
        stderr "downloading $input to $tar"
        crane export "$input" "$tar" || fail "crane export failed"
        ;;
    *)
        [ -f "$input" ] || fail "$input: didn't look like a tarball"
        tar="$input"
        ;;
esac


rootd="${TEMP_D}/rootfs"
flist="${TEMP_D}/files"
mkdir "$rootd"
tar -C "$rootd" -xf "$tar" || fail "failed etract $tar"

if [ -n "$init" ]; then
    cp "$initf" "$rootd/init" &&
        chmod 755 "$rootd/init" || fail "failed to write $init"
fi

( cd "$rootd" && find . > "$flist" ) || fail "failed find files"

( cd "$rootd" && cpio --format=newc -o ) < "$flist" |
    gzip --to-stdout > "$out" ||
	fail "failed to create update cpio"

stderr "wrote $out"