Provided by: proot_5.1.0-1vivid1ubuntu1_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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