PRoot — chroot, mount --bind, and binfmt_misc without privilege/setup

# PRoot

- Description
- Examples
- Downloads
- Support
- News+

## Description

PRoot is a user-space implementation of `chroot`, `mount --bind`, and `binfmt_misc`. This means that users don't need any privileges or setup to do things like using an arbitrary directory as the new root filesystem, making files accessible somewhere else in the filesystem hierarchy, or executing programs built for another CPU architecture transparently through QEMU user-mode. Also, developers can use PRoot as a generic Linux process instrumentation engine thanks to its extension mechanism, see CARE for an example. Technically PRoot relies on `ptrace`, an unprivileged system-call available in every Linux kernel.

The new root file-system, a.k.a guest rootfs, typically contains a Linux distribution. By default PRoot confines the execution of programs to the guest rootfs only, however users can use the built-in mount/bind mechanism to access files and directories from the actual root file-system, a.k.a host rootfs, just as if they were part of the guest rootfs.

When the guest Linux distribution is made for a CPU architecture incompatible with the host one, PRoot uses the CPU emulator QEMU user-mode to execute transparently guest programs. It's a convenient way to develop, to build, and to validate any guest Linux packages seamlessly on users' computer, just as if they were in a native guest environment. That way all of the cross-compilation issues are avoided.

PRoot can also mix the execution of host programs and the execution of guest programs emulated by QEMU user-mode. This is useful to use host equivalents of programs that are missing from the guest rootfs and to speed up build-time by using cross-compilation tools or CPU-independent programs, like interpreters.

It is worth noting that the guest kernel is never involved, regardless of whether QEMU user-mode is used or not. Technically, when guest programs perform access to system resources, PRoot translates their requests before sending them to the host kernel. This means that guest programs can use host resources (devices, network, ...) just as if they were "normal" host programs.

## Examples

In the following examples the directories `/mnt/slackware-8.0` and `/mnt/armslack-12.2/` contain a Linux distribution respectively made for x86 CPUs and ARM CPUs.

### `chroot` equivalent

To execute a command inside a given Linux distribution, just give `proot` the path to the guest rootfs followed by the desired command. The example below executes the program `cat` to print the content of a file:

    proot -r /mnt/slackware-8.0/ cat /etc/motd

    Welcome to Slackware Linux 8.0

The default command is `/bin/sh` when none is specified. Thus the shortest way to confine an interactive shell and all its sub-programs is:

    proot -r /mnt/slackware-8.0/

    $ cat /etc/motd
    Welcome to Slackware Linux 8.0

### `mount --bind` equivalent

The bind mechanism enables one to relocate files and directories. This is typically useful to trick programs that perform access to hard-coded locations, like some installation scripts:

    proot -b /tmp/alternate_opt:/opt

    $ cd to/sources
    $ make install
    [...]
    install -m 755 prog "/opt/bin"
    [...] # prog is installed in "/tmp/alternate_opt/bin" actually

As shown in this example, it is possible to bind over files not even owned by the user. This can be used to overlay system configuration files, for instance the DNS setting:

    ls -l /etc/hosts
    -rw-r--r-- 1 root root 675 Mar  4  2011 /etc/hosts

    proot -b ~/alternate_hosts:/etc/hosts

    $ echo '1.2.3.4 google.com' > /etc/hosts
    $ resolveip google.com
    IP address of google.com is 1.2.3.4
    $ echo '5.6.7.8 google.com' > /etc/hosts
    $ resolveip google.com
    IP address of google.com is 5.6.7.8

Another example: on most Linux distributions `/bin/sh` is a symbolic link to `/bin/bash`, whereas it points to `/bin/dash` on Debian and Ubuntu. As a consequence a `#!/bin/sh` script tested with Bash might not work with Dash. In this case, the binding mechanism of PRoot can be used to set non-disruptively `/bin/bash` as the default `/bin/sh` on these two Linux distributions:

    proot -b /bin/bash:/bin/sh [...]

Because `/bin/sh` is initially a symbolic link to `/bin/dash`, the content of `/bin/bash` is actually bound over this latter:

    proot -b /bin/bash:/bin/sh

    $ md5sum /bin/sh
    089ed56cd74e63f461bef0fdfc2d159a  /bin/sh
    $ md5sum /bin/bash
    089ed56cd74e63f461bef0fdfc2d159a  /bin/bash
    $ md5sum /bin/dash
    089ed56cd74e63f461bef0fdfc2d159a  /bin/dash

In most cases this shouldn't be a problem, but it is still possible to strictly bind `/bin/bash` over `/bin/sh` -- without dereferencing it -- by specifying the `!` character at the end:

    proot -b '/bin/bash:/bin/sh!'

    $ md5sum /bin/sh
    089ed56cd74e63f461bef0fdfc2d159a  /bin/sh
    $ md5sum /bin/bash
    089ed56cd74e63f461bef0fdfc2d159a  /bin/bash
    $ md5sum /bin/dash
    c229085928dc19e8d9bd29fe88268504  /bin/dash

### `chroot` + `mount --bind` equivalent

The two features above can be combined to make any file from the host rootfs accessible in the confined environment just as if it were initially part of the guest rootfs. It is sometimes required to run programs that rely on some specific files:

    proot -r /mnt/slackware-8.0/

    $ ps -o tty,command
    Error, do this: mount -t proc none /proc

works better with:

    proot -r /mnt/slackware-8.0/ -b /proc

    $ ps -o tty,command
    TT       COMMAND
    ?        bash
    ?        proot -b /proc /mnt/slackware-8.0/
    ?        sh
    ?        ps -o tty,command

Actually there's a bunch of such specific files, that's why PRoot provides the option `-R` to bind automatically a pre-defined list of recommended paths:

    proot -R /mnt/slackware-8.0/

    $ ps -o tty,command
    TT       COMMAND
    pts/6    bash
    pts/6    proot -R /mnt/slackware-8.0/
    pts/6    sh
    pts/6    ps -o tty,command

### `chroot` + `mount --bind` + `su` equivalent

Some programs will not work correctly if they are not run by the "root" user, this is typically the case with package managers. PRoot can fake the root identity and its privileges when the `-0` (zero) option is specified:

    proot -r /mnt/slackware-8.0/ -0

    # id
    uid=0(root) gid=0(root) [...]

    # mkdir /tmp/foo
    # chmod a-rwx /tmp/foo
    # echo 'I bypass file-system permissions.' > /tmp/foo/bar
    # cat /tmp/foo/bar
    I bypass file-system permissions.

This option is typically required to create or install packages into the guest rootfs. Note it is not recommended to use the `-R` option when installing packages since they may try to update bound system files, like `/etc/group`. Instead, it is recommended to use the `-S` option. This latter enables the `-0` option and binds only paths that are known to not be updated by packages:

    proot -S /mnt/slackware-8.0/

    # installpkg perl.tgz
    Installing package perl...

### `chroot` + `mount --bind` + `binfmt_misc` equivalent

PRoot uses QEMU user-mode to execute programs built for a CPU architecture incompatible with the host one. From users' point-of-view, guest programs handled by QEMU user-mode are executed transparently, that is, just like host programs. To enable this feature users just have to specify which instance of QEMU user-mode they want to use with the option `-q`:

    proot -R /mnt/armslack-12.2/ -q qemu-arm

    $ cat /etc/motd
    Welcome to ARMedSlack Linux 12.2

The parameter of the `-q` option is actually a whole QEMU user-mode command, for instance to enable its GDB server on port 1234:

    proot -R /mnt/armslack-12.2/ -q "qemu-arm -g 1234" emacs

PRoot allows one to mix transparently the emulated execution of guest programs and the native execution of host programs in the same file-system namespace. It's typically useful to extend the list of available programs and to speed up build-time significantly. This mixed-execution feature is enabled by default when using QEMU user-mode, and the content of the host rootfs is made accessible through `/host-rootfs`:

    proot -R /mnt/armslack-12.2/ -q qemu-arm

    $ file /bin/echo
    [...] ELF 32-bit LSB executable, ARM [...]
    $ /bin/echo 'Hello world!'
    Hello world!

    $ file /host-rootfs/bin/echo
    [...] ELF 64-bit LSB executable, x86-64 [...]
    $ /host-rootfs/bin/echo 'Hello mixed world!'
    Hello mixed world!

Since both host and guest programs use the guest rootfs as `/`, users may want to deactivate explicitly cross-filesystem support found in most GNU cross-compilation tools. For example with GCC configured to cross-compile to the ARM target:

    proot -R /mnt/armslack-12.2/ -q qemu-arm

    $ export CC=/host-rootfs/opt/cross-tools/arm-linux/bin/gcc
    $ export CFLAGS="--sysroot=/"   # could be optional indeed
    $ ./configure; make

As with regular files, a host instance of a program can be bound over its guest instance. Here is an example where the guest binary of `make` is overlaid by the host one:

    proot -R /mnt/armslack-12.2/ -q qemu-arm -b /usr/bin/make

    $ which make
    /usr/bin/make
    $ make --version # overlaid
    GNU Make 3.82
    Built for x86_64-slackware-linux-gnu

It's worth mentioning that even when mixing the native execution of host programs and the emulated execution of guest programs, they still believe they are running in a native guest environment. As a demonstration, here is a partial output of a typical `./configure` script:

    checking whether the C compiler is a cross-compiler... no

## Downloads

### PRoot

The latest release of PRoot is packaged on http://packages.proot.me and sources are hosted on http://github.proot.me. It is also available as highly compatible static binaries:

- for x86_64: http://static.proot.me/proot-x86_64

- for x86: http://static.proot.me/proot-x86

- for ARM: http://static.proot.me/proot-arm

- other architectures: on demand.

### Rootfs

Here follows a couple of URLs where some rootfs archives can be freely downloaded. Note that `mknod` errors reported by `tar` when extracting these archives can be safely ignored since special files are typically bound (see `-R` option for details).

- http://download.openvz.org/template/precreated/

- https://images.linuxcontainers.org/images/

- http://distfiles.gentoo.org/releases/

- http://cdimage.ubuntu.com/ubuntu-core/releases/

- http://archlinuxarm.org/developers/downloads

Technically such rootfs archive can be created by running the following command on the expected Linux distribution:

    tar --one-file-system --create --gzip --file my_rootfs.tar.gz /

### QEMU user-mode

QEMU user-mode is required only if the guest rootfs was made for a CPU architecture incompatible with the host one, for instance when using a ARM rootfs on a x86_64 computer. This package can be installed either from http://qemu.proot.me or from the host package manager under the name of "qemu-user" on most Linux distro. In case one would like to build QEMU user-mode from sources, the `--enable-linux-user` option has to be specified to the `./configure` script.

## Support

Feel free to send your questions, bug reports, suggestions, and patchs to the mailing-list or to the forum, but please be sure that your answer isn't in the user manual first.

Also, Rémi Duraffort has written interesting articles on how to use a foreign Debian rootfs with PRoot in order to build and test VLC on this guest Linux distribution.