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#!/usr/bin/env python2.7
"""Docker From Scratch Workshop - Level 2: Adding mount namespace.
Goal: Separate our mount table from the other processes.
Usage:
running:
rd.py run -i ubuntu /bin/sh
will:
- fork a new chrooted process in a new mount namespace
"""
from __future__ import print_function
import linux
import tarfile
import uuid
import click
import os
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)
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 contain(command, image_name, image_dir, container_id, container_dir):
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))
# TODO: time to say goodbye to the old mount namespace,
# see "man 2 unshare" to get some help
# HINT 1: there is no os.unshare(), time to use the linux module we made
# just for you!
# HINT 2: the linux module includes both functions and constants!
# e.g. linux.CLONE_NEWNS
# TODO: remember shared subtrees?
# (https://www.kernel.org/doc/Documentation/filesystems/sharedsubtree.txt)
# Make / a private mount to avoid littering our host mount table.
# 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, '')
for i, dev in enumerate(['stdin', 'stdout', 'stderr']):
os.symlink('/proc/self/fd/%d' % i, os.path.join(new_root, 'dev', dev))
# TODO: add more devices (e.g. null, zero, random, urandom) using os.mknod.
os.chroot(new_root)
os.chdir('/')
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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