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virtualization
linux
microvm
firecracker
feat: add overlay initialization script for boot process
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crates/firecracker-prepare/src/scripts/overlay-init.sh
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crates/firecracker-prepare/src/scripts/overlay-init.sh
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#!/bin/sh
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# Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
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#
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# Licensed under the Apache License, Version 2.0 (the "License"). You may
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# not use this file except in compliance with the License. A copy of the
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# License is located at
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#
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# http://aws.amazon.com/apache2.0/
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#
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# or in the "license" file accompanying this file. This file is distributed
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# on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
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# express or implied. See the License for the specific language governing
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# permissions and limitations under the License.
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# Parameters:
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# 1. rw_root -- path where the read/write root is mounted
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# 2. work_dir -- path to the overlay workdir (must be on same filesystem as rw_root)
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# Overlay will be set up on /mnt, original root on /mnt/rom
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pivot() {
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local rw_root work_dir
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rw_root="$1"
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work_dir="$2"
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/bin/mount \
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-o noatime,lowerdir=/,upperdir=${rw_root},workdir=${work_dir} \
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-t overlay "overlayfs:${rw_root}" /mnt
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pivot_root /mnt /mnt/rom
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}
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# Overlay is configured under /overlay
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# Global variable $overlay_root is expected to be set to either:
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# "ram", which configures a tmpfs as the rw overlay layer (this is
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# the default, if the variable is unset)
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# - or -
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# A block device name, relative to /dev, in which case it is assumed
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# to contain an ext4 filesystem suitable for use as a rw overlay
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# layer. e.g. "vdb"
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do_overlay() {
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local overlay_dir="/overlay"
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if [ "$overlay_root" = ram ] ||
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[ -z "$overlay_root" ]; then
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/bin/mount -t tmpfs -o noatime,mode=0755 tmpfs /overlay
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else
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/bin/mount -t ext4 "/dev/$overlay_root" /overlay
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fi
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mkdir -p /overlay/root /overlay/work
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pivot /overlay/root /overlay/work
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}
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# If we're given an overlay, ensure that it really exists. Panic if not.
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if [ -n "$overlay_root" ] &&
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[ "$overlay_root" != ram ] &&
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[ ! -b "/dev/$overlay_root" ]; then
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echo -n "FATAL: "
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echo "Overlay root given as $overlay_root but /dev/$overlay_root does not exist"
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exit 1
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fi
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do_overlay
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# invoke the actual system init program and proceed with the boot
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# process.
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exec /sbin/init $@