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Prepare, configure, and manage Firecracker microVMs in seconds!
virtualization
linux
microvm
firecracker
1#!/bin/sh
2# Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
3#
4# Licensed under the Apache License, Version 2.0 (the "License"). You may
5# not use this file except in compliance with the License. A copy of the
6# License is located at
7#
8# http://aws.amazon.com/apache2.0/
9#
10# or in the "license" file accompanying this file. This file is distributed
11# on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
12# express or implied. See the License for the specific language governing
13# permissions and limitations under the License.
14
15# Parameters:
16# 1. rw_root -- path where the read/write root is mounted
17# 2. work_dir -- path to the overlay workdir (must be on same filesystem as rw_root)
18# Overlay will be set up on /mnt, original root on /mnt/rom
19pivot() {
20 local rw_root work_dir
21 rw_root="$1"
22 work_dir="$2"
23 /bin/mount \
24 -o noatime,lowerdir=/,upperdir=${rw_root},workdir=${work_dir} \
25 -t overlay "overlayfs:${rw_root}" /mnt
26 pivot_root /mnt /mnt/rom
27}
28
29# Overlay is configured under /overlay
30# Global variable $overlay_root is expected to be set to either:
31# "ram", which configures a tmpfs as the rw overlay layer (this is
32# the default, if the variable is unset)
33# - or -
34# A block device name, relative to /dev, in which case it is assumed
35# to contain an ext4 filesystem suitable for use as a rw overlay
36# layer. e.g. "vdb"
37do_overlay() {
38 local overlay_dir="/overlay"
39 if [ "$overlay_root" = ram ] ||
40 [ -z "$overlay_root" ]; then
41 /bin/mount -t tmpfs -o noatime,mode=0755 tmpfs /overlay
42 else
43 /bin/mount -t ext4 "/dev/$overlay_root" /overlay
44 fi
45 mkdir -p /overlay/root /overlay/work
46 pivot /overlay/root /overlay/work
47}
48
49# If we're given an overlay, ensure that it really exists. Panic if not.
50if [ -n "$overlay_root" ] &&
51 [ "$overlay_root" != ram ] &&
52 [ ! -b "/dev/$overlay_root" ]; then
53 echo -n "FATAL: "
54 echo "Overlay root given as $overlay_root but /dev/$overlay_root does not exist"
55 exit 1
56fi
57
58do_overlay
59
60# invoke the actual system init program and proceed with the boot
61# process.
62exec /sbin/init $@