pychroot 0.9.15

a python library and cli tool that simplify chroot handling

pychroot

pychroot is a python library and cli tool that simplify chroot handling. Specifically, the library provides a Chroot context manager that enables more pythonic methods for running code in chroots while the pychroot utility works much like an extended chroot command in the terminal.

Usage

In its simplest form, the library can be used similar to the following:

from pychroot import Chroot

with Chroot('/path/to/chroot'):
    code that will be run
    inside the chroot

By default, this will bind mount the host’s /dev, /proc, and /sys filesystems into the chroot as well as the /etc/resolv.conf file (so DNS resolution works as expected in the chroot).

A simple chroot equivalent is also installed as pychroot. It can be used in a similar fashion to chroot; however, it also performs the bind mounts previously mentioned so the environment is usable. In addition, pychroot supports specifying custom bind mounts, for example:

pychroot -R /home/user ~/chroot

will recursively bind mount the user’s home directory at the same location inside the chroot directory in addition to the standard bind mounts. See pychroot’s help output for more options.

When running on a system with a recent kernel (Linux 3.8 and on) and user namespaces enabled pychroot can be run by a regular user. Note that currently pychroot just maps the current user to root in the chroot environment. This means that recursively chown-ing the chroot directory to the user running pychroot should essentially allow that user to act as root in the pychroot environment.

Implementation details

Namespaces are used by the context manager to isolate the chroot instance from the host system and to simplify the teardown phase for the environments. By default, new mount, UTS, IPC, and pid namespaces are used. In addition, if running as non-root, both user and network namespaces will be enabled as well so that the chrooting and mounting process will work without elevated permissions.

One quirk of note is that currently local variables are not propagated back from the chroot context to the main context due to the usage of separate processes running the contexts. This means that something similar to the following won’t work:

from pychroot import Chroot

with Chroot('/path/to/chroot'):
    answer = 42
print(answer)

In this case, a NameError exception will be raised unless the variable answer was previously defined. This will probably be fixed to some extent in a future release.

Requirements

Python versions 2.7, 3.3, 3.4 are supported. Note however, that pychroot is quite Linux specific due to the use of namespaces via the snakeoil library which also require proper kernel support. Specifically, the following kernel config options are required to be enabled for full namespace support:

CONFIG_NAMESPACES=y
CONFIG_UTS_NS=y
CONFIG_IPC_NS=y
CONFIG_USER_NS=y
CONFIG_PID_NS=y
CONFIG_NET_NS=y

Installing

In a virtualenv or elsewhere via pip:

pip install pychroot

From the repo manually:

python setup.py install

Bugs

Please create an issue in the issue tracker.

Tests

Tests are handled via pytest, run via:

py.test

which is also integrated into setup.py, run via:

python setup.py test

Also, tests for all supported python versions can be run together or individually via:

tox

Note that mock is required for testing with anything less than python 3.3.

Release Notes

pychroot 0.9.15 (2015-05-29)

• Fix new installs using pip.

pychroot 0.9.14 (2015-05-28)

• Move to generic scripts and docs framework used by pkgcore.

pychroot 0.9.13 (2015-12-13)

• Add –no-mounts option to disable the default mounts for the command line tool. This makes pychroot act similar to chroot.

• Make pychroot pip-installable without requiring wnakeoil to be manually installed first.

• Add lots of additional content to the pychroot utility man page.

pychroot 0.9.12 (2015-08-10)

• The main module was renamed from chroot to pychroot mostly for consistency to match the project name and cli tool installed alongside it.

• Add a man page for the pychroot cli tool.

• Add user namespace support so you can chroot as a regular user. Note that this also requires using a network namespace for which we only setup a loopback interface (if iproute2 is installed in the chroot) so external network access won’t work by default in this situation.

• Add an option to skip changing to the newroot directory after chrooting. This is similar to the option for chroot(1) but also allows skipping the directory change when the new root isn’t ‘/’. In other words, you can use a chroot environment against the host’s rootfs.

• Use $SHELL from the environment for the pychroot script to mirror chroot’s behavior.

• Move WithParentSkip, the main parent/child execution splitting context manager allowing this all to work, to snakeoil.contextlib and rename it SplitExec. It was moved in order to develop other context managers around it in snakeoil and elsewhere more easily.

• Fix additional argument parsing for the pychroot script. Now commands like:

  pychroot ~/chroot /bin/bash -c "ls -R /"

  will work as expected (i.e. how they normally work with chroot).

• Allow mount propagation from the host mount namespace to the chroot’s but not vice versa. Previously systems that set the rootfs mount as shared, e.g. running something like:

  mount --make-rshared /

  would leak mounts from the chroot mount namespace back into the host’s namespace. Now the chroot mount namespace is recursively slaved from the host’s so mount events will propagate down from host to chroot, but not back up from chroot to host.

• Add support for setting the chroot’s host and domain names for all versions of python. Previously we only supported setting the hostname for py33 and up. To set the domain name, pass an FQDN instead of a singular hostname. This also adds a “–hostname” option to the pychroot script that enables the same support for it.

pychroot 0.9.11 (2015-07-05)

• Fix pychroot script when no custom mountpoints as specified.

pychroot 0.9.10 (2015-04-09)

• Add support for custom bind mounts to the pychroot script. Now users are able to do things like:

  pychroot -R /home/user ~/chroot

  which will recursively bind mount their home directory into the chroot in addition to the standard set of bind mounts.

• Use “source[:dest]” as keys for mountpoints. This enables support for mounting the same source onto multiple destinations. For example, with pychroot it’s now possible to run:

  pychroot -B tmpfs:/dev/shm -B tmpfs:/tmp

pychroot 0.9.9 (2015-04-03)

• Install chroot(1) workalike as pychroot. This allows users to be lazier when doing basic chrooting since pychroot handles mounting and unmounting standard bind mounts automatically.