Get kernel and initramfs from a disk image

This script, a response to the insecure and over-complex disk-image-get-kernel in OpenStack, shows how to use libguestfs to safely and easily get the kernel and initramfs from a disk image so you can boot it using an external kernel.

#!/usr/bin/python
# Get latest kernel & initramfs safely from a disk image.

# Note this will overwrite /tmp/kernel & /tmp/initramfs which you
# wouldn't want to do in production.

import sys
import guestfs

assert (len (sys.argv) == 2)
disk = sys.argv[1]

g = guestfs.GuestFS (python_return_dict=True)

# To enable tracing, uncomment the next line.
#g.trace (1)

# Attach the disk image read-only to libguestfs.
g.add_drive_opts (disk, readonly=1)

# Run the libguestfs back-end.
g.launch ()

# Ask libguestfs to inspect for operating systems.
roots = g.inspect_os ()
if len (roots) == 0:
    raise (Error ("no operating systems found"))
if len (roots) > 1:
    raise (Error ("dual/multi-boot images are not supported"))

root = roots[0]

# Mount up the disks, like guestfish -i.
#
# Sort keys by length, shortest first, so that we end up
# mounting the filesystems in the correct order.
mps = g.inspect_get_mountpoints (root)
def compare (a, b): return len(a) - len(b)
for device in sorted (mps.keys(), compare):
    try:
        g.mount_ro (mps[device], device)
    except RuntimeError as msg:
        print "%s (ignored)" % msg

# For debugging:
print "/boot directory of this guest:"
print (g.ll ("/boot"))

# Get all kernels & initramfses.
kernels = g.glob_expand ("/boot/vmlinuz-*")
initramfses = g.glob_expand ("/boot/initramfs-*")

# Old RHEL:
if len (initramfses) == 0:
    initramfses = g.glob_expand ("/boot/initrd-*")

# Debian/Ubuntu:
if len (initramfses) == 0:
    initramfses = g.glob_expand ("/boot/initrd.img-*")

if len (kernels) == 0:
    raise (Error ("no kernel found in this disk image"))
if len (initramfses) == 0:
    raise (Error ("no initramfs found in this disk image"))

# Sort by version so we get the latest.
from distutils.version import LooseVersion
kernels.sort (key=LooseVersion)
initramfses.sort (key=LooseVersion)

# Download the latest.
print ("downloading %s -> /tmp/kernel" % kernels[len (kernels)-1])
g.download (kernels[len (kernels)-1], "/tmp/kernel")
print ("downloading %s -> /tmp/initramfs" % initramfses[len (initramfses)-1])
g.download (initramfses[len (initramfses)-1], "/tmp/initramfs")

# Shutdown.
g.shutdown ()
g.close ()

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