Provided by: proot_5.1.0-1.2_amd64 
NAME
PRoot - chroot, mount --bind, and binfmt_misc without privilege/setup
SYNOPSIS
proot [option] ... [command]
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.
OPTIONS
The command-line interface is composed of two parts: first PRoot's options (optional), then the command
to launch (/bin/sh if not specified). This section describes the options supported by PRoot, that is,
the first part of its command-line interface.
Regular options
-r path, --rootfs=path
Use path as the new guest root file-system, default is /.
The specified path typically contains a Linux distribution where all new programs will be
confined. The default rootfs is / when none is specified, this makes sense when the bind
mechanism is used to relocate host files and directories, see the -b option and the Examples
section for details.
It is recommended to use the -R or -S options instead.
-b path, --bind=path, -m path, --mount=path
Make the content of path accessible in the guest rootfs.
This option makes any file or directory of the host rootfs accessible in the confined environment
just as if it were part of the guest rootfs. By default the host path is bound to the same path
in the guest rootfs but users can specify any other location with the syntax: -b
*host_path*:*guest_location*. If the guest location is a symbolic link, it is dereferenced to
ensure the new content is accessible through all the symbolic links that point to the overlaid
content. In most cases this default behavior shouldn't be a problem, although it is possible to
explicitly not dereference the guest location by appending it the ! character: -b
*host_path*:*guest_location!*.
-q command, --qemu=command
Execute guest programs through QEMU as specified by command.
Each time a guest program is going to be executed, PRoot inserts the QEMU user-mode command in
front of the initial request. That way, guest programs actually run on a virtual guest CPU
emulated by QEMU user-mode. The native execution of host programs is still effective and the
whole host rootfs is bound to /host-rootfs in the guest environment.
-w path, --pwd=path, --cwd=path
Set the initial working directory to path.
Some programs expect to be launched from a given directory but do not perform any chdir by
themselves. This option avoids the need for running a shell and then entering the directory
manually.
-v value, --verbose=value
Set the level of debug information to value.
The higher the integer value is, the more detailed debug information is printed to the standard
error stream. A negative value makes PRoot quiet except on fatal errors.
-V, --version, --about
Print version, copyright, license and contact, then exit.
-h, --help, --usage
Print the version and the command-line usage, then exit.
Extension options
The following options enable built-in extensions. Technically developers can add their own features to
PRoot or use it as a Linux process instrumentation engine thanks to its extension mechanism, see the
sources for further details.
-k string, --kernel-release=string
Make current kernel appear as kernel release string.
If a program is run on a kernel older than the one expected by its GNU C library, the following
error is reported: "FATAL: kernel too old". To be able to run such programs, PRoot can emulate
some of the features that are available in the kernel release specified by string but that are
missing in the current kernel.
-0, --root-id
Make current user appear as "root" and fake its privileges.
Some programs will refuse to work if they are not run with "root" privileges, even if there is no
technical reason for that. This is typically the case with package managers. This option allows
users to bypass this kind of limitation by faking the user/group identity, and by faking the
success of some operations like changing the ownership of files, changing the root directory to /,
... Note that this option is quite limited compared to fakeroot.
-i string, --change-id=string
Make current user and group appear as string "uid:gid".
This option makes the current user and group appear as uid and gid. Likewise, files actually
owned by the current user and group appear as if they were owned by uid and gid instead. Note
that the -0 option is the same as -i 0:0.
Alias options
The following options are aliases for handy sets of options.
-R path
Alias: -r *path* + a couple of recommended -b.
Programs isolated in path, a guest rootfs, might still need to access information about the host
system, as it is illustrated in the Examples section of the manual. These host information are
typically: user/group definition, network setup, run-time information, users' files, ... On all
Linux distributions, they all lie in a couple of host files and directories that are automatically
bound by this option:
• /etc/host.conf
• /etc/hosts
• /etc/hosts.equiv
• /etc/mtab
• /etc/netgroup
• /etc/networks
• /etc/passwd
• /etc/group
• /etc/nsswitch.conf
• /etc/resolv.conf
• /etc/localtime
• /dev/
• /sys/
• /proc/
• /tmp/
• /run/
• /var/run/dbus/system_bus_socket
• $HOME
• path
-S path
Alias: -0 -r *path* + a couple of recommended -b.
This option is useful to safely create and install packages into the guest rootfs. It is similar
to the -R option expect it enables the -0 option and binds only the following minimal set of paths
to avoid unexpected changes on host files:
• /etc/host.conf
• /etc/hosts
• /etc/nsswitch.conf
• /etc/resolv.conf
• /dev/
• /sys/
• /proc/
• /tmp/
• /run/shm
• $HOME
• path
EXIT STATUS
If an internal error occurs, proot returns a non-zero exit status, otherwise it returns the exit status
of the last terminated program. When an error has occurred, the only way to know if it comes from the
last terminated program or from proot itself is to have a look at the error message.
FILES
PRoot reads links in /proc/<pid>/fd/ to support openat(2)-like syscalls made by the guest 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/
? -
? 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 -
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.
SEE ALSO
chroot(1), mount(8), binfmt_misc, ptrace(2), qemu(1), sb2(1), bindfs(1), fakeroot(1), fakechroot(1)
COLOPHON
Visit http://proot.me for help, bug reports, suggestions, patches, ... Copyright (C) 2014
STMicroelectronics, licensed under GPL v2 or later.
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