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#!/usr/bin/env python2.7
"""Docker From Scratch Workshop - Level 3: Switching from chroot to pivot_root.
Goal: Use pivot_root instead of chroot, and umount old_root.
Usage:
running:
rd.py run -i ubuntu /bin/sh
will:
- fork a new process in a new mount namespace with a new root
- make sure that you can't easily escape
"""
from __future__ import print_function
import linux
import tarfile
import uuid
import click
import os
import stat
import traceback
def _get_image_path(image_name, image_dir, image_suffix='tar'):
return os.path.join(image_dir, os.extsep.join([image_name, image_suffix]))
def _get_container_path(container_id, container_dir, *subdir_names):
return os.path.join(container_dir, container_id, *subdir_names)
def create_container_root(image_name, image_dir, container_id, container_dir):
image_path = _get_image_path(image_name, image_dir)
container_root = _get_container_path(container_id, container_dir, 'rootfs')
assert os.path.exists(image_path), "unable to locate image %s" % image_name
if not os.path.exists(container_root):
os.makedirs(container_root)
# TODO: uncomment (why?)
# linux.mount('tmpfs', container_root, 'tmpfs', 0, None)
with tarfile.open(image_path) as t:
# Fun fact: tar files may contain *nix devices! *facepalm*
members = [m for m in t.getmembers()
if m.type not in (tarfile.CHRTYPE, tarfile.BLKTYPE)]
t.extractall(container_root, members=members)
return container_root
@click.group()
def cli():
pass
def makedev(dev_path):
for i, dev in enumerate(['stdin', 'stdout', 'stderr']):
os.symlink('/proc/self/fd/%d' % i, os.path.join(dev_path, dev))
os.symlink('/proc/self/fd', os.path.join(dev_path, 'fd'))
# Add extra devices
DEVICES = {'null': (stat.S_IFCHR, 1, 3), 'zero': (stat.S_IFCHR, 1, 5),
'random': (stat.S_IFCHR, 1, 8), 'urandom': (stat.S_IFCHR, 1, 9),
'console': (stat.S_IFCHR, 136, 1), 'tty': (stat.S_IFCHR, 5, 0),
'full': (stat.S_IFCHR, 1, 7)}
for device, (dev_type, major, minor) in DEVICES.iteritems():
os.mknod(os.path.join(dev_path, device),
0o666 | dev_type, os.makedev(major, minor))
def contain(command, image_name, image_dir, container_id, container_dir):
try:
linux.unshare(linux.CLONE_NEWNS) # create a new mount namespace
except RuntimeError as e:
if getattr(e, 'args', '') == (1, 'Operation not permitted'):
print('Error: Use of CLONE_NEWNS with unshare(2) requires the '
'CAP_SYS_ADMIN capability (i.e. you probably want to retry '
'this with sudo)')
raise e
# TODO: we added MS_REC here. wanna guess why?
linux.mount(None, '/', None, linux.MS_PRIVATE | linux.MS_REC, None)
new_root = create_container_root(
image_name, image_dir, container_id, container_dir)
print('Created a new root fs for our container: {}'.format(new_root))
# Create mounts (/proc, /sys, /dev) under new_root
linux.mount('proc', os.path.join(new_root, 'proc'), 'proc', 0, '')
linux.mount('sysfs', os.path.join(new_root, 'sys'), 'sysfs', 0, '')
linux.mount('tmpfs', os.path.join(new_root, 'dev'), 'tmpfs',
linux.MS_NOSUID | linux.MS_STRICTATIME, 'mode=755')
# Add some basic devices
devpts_path = os.path.join(new_root, 'dev', 'pts')
if not os.path.exists(devpts_path):
os.makedirs(devpts_path)
linux.mount('devpts', devpts_path, 'devpts', 0, '')
makedev(os.path.join(new_root, 'dev'))
os.chroot(new_root) # TODO: replace with pivot_root
os.chdir('/')
# TODO: umount2 old root (HINT: see MNT_DETACH in man 2 umount)
os.execvp(command[0], command)
@cli.command(context_settings=dict(ignore_unknown_options=True,))
@click.option('--image-name', '-i', help='Image name', default='ubuntu')
@click.option('--image-dir', help='Images directory',
default='/workshop/images')
@click.option('--container-dir', help='Containers directory',
default='/workshop/containers')
@click.argument('Command', required=True, nargs=-1)
def run(image_name, image_dir, container_dir, command):
container_id = str(uuid.uuid4())
pid = os.fork()
if pid == 0:
# This is the child, we'll try to do some containment here
try:
contain(command, image_name, image_dir, container_id,
container_dir)
except Exception:
traceback.print_exc()
os._exit(1) # something went wrong in contain()
# This is the parent, pid contains the PID of the forked process
# wait for the forked child, fetch the exit status
_, status = os.waitpid(pid, 0)
print('{} exited with status {}'.format(pid, status))
if __name__ == '__main__':
cli()
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