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"
| #!/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" |