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

Date: 2012-04-24
Version: 1.8.4
Manual section:1


proot [option] ... /path/to/guest/rootfs [program [arg] ...]


PRoot is a user-space implementation of chroot, mount --bind, and binfmt_misc. This means that users don't need any privilege or setup to do things like: using an arbitrary directory as the new root file-system or making files accessible somewhere else in the file-system hierarchy or executing programs built for another CPU architecture transparently through QEMU user-mode. 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.


The command-line interface is composed of three ordered parts: PRoot's options (optional), path to the guest rootfs (mandatory), and the command to launch (/bin/sh by default).

This section describes the options supported by PRoot, that is, the first part of its command-line interface.

Regular options

-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*. Such bindings are said "asymmetric".

-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.

This option is automatically enabled by the -Q option.

-w path, --pwd=path, --cwd=path

Set the initial working directory to path, default is /.

Some programs expect to be launched from a given directory but do not perform any chdir by themselves, the most common example is ./configure scripts. This option avoids the need for running a shell and then entering the directory manually.

See the -W option.

-u, --allow-unknown-syscalls

Allow the execution of unknown syscalls.

PRoot has to know the semantics of a syscall to translate its arguments. This is why any syscall that PRoot isn't aware of is blocked. This option disables this default behavior.

-k string, --kernel-release=string

Force syscall uname to report string as kernel release.

Technically the GNU C library relies on syscalls provided by the kernel that's why it performs a sanity check at each program start-up to verify whether the current kernel is known to be compatible. If users are running a GNU C library that expects a kernel more recent than the one used on their computers, they will get the error "FATAL: kernel too old". This option allows users to cheat this sanity check by faking the release number of the current kernel. This option should be used with care since it does not improve the compatibility.

-0, --root-id

Force some syscalls to behave as if executed by "root".

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.

-v, --verbose Increase the level of debug information.
-V, --version, --about
 Print version, copyright, license and contact, then exit.
-h, --help, --usage
 Print the version and the command-line usage, then exit.

Alias options

The following options are aliases for handy sets of options.

-B, -M

Alias: -b for each path of a recommended list

There are a couple of bindings that are needed for most guest programs to behave correctly regarding the configuration part of the host computer which is not specific to the host Linux distribution, such as: user/group information, network setup, run-time information, users' files, ... This highly recommended option enables the following bindings:

  • /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/
  • $HOME
-Q command

Alias: -q *command* -B

This option is highly recommended when using QEMU user-mode; it enables all the recommended bindings.


Alias: -b . -w .

Make the current working directory accessible in the guest rootfs and then use it as the initial working directory. This option is typically useful to launch ./configure scripts directly, for instance.

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.


PRoot reads links in /proc/<pid>/fd/ to support openat(2)-like syscalls made by the guest programs.


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 /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 /mnt/slackware-8.0/

$ cat /etc/motd
Welcome to Slackware Linux 8.0

chroot + mount --bind equivalent

The option -b, short for --bind, makes 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 /mnt/slackware-8.0/

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

works better with:

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

$ ps -o tty,command
?        -bash
?        proot -b /proc /mnt/slackware-8.0/
?        /lib/ /bin/sh
?        /lib/ /usr/bin/ps -o tty,command

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

proot -B /mnt/slackware-8.0/

$ ps -o tty,command
pts/6    -bash
pts/6    proot -B /mnt/slackware-8.0/
pts/6    /lib/ /bin/sh
pts/6    /lib/ /usr/bin/ps -o tty,command

mount --bind equivalent

The bind mechanism can also be used to relocate files and directories in the host rootfs by using / as the path to the guest rootfs and by specifying an alternate binding location (separated by :). It is typically useful to cheat programs that perform access to hard-coded locations, like some installation scripts:

proot -b /tmp/alternate_usr:/usr /

$ cd to/sources
$ make install
install -m 755 prog "/usr/bin"
[...] # prog was installed in "/tmp/alternate_usr/bin" actually

It can also be used to substitute system configuration files, for example the DNS setting:

proot -b ./alternate_resolv.conf:/etc/resolv.conf / resolveip

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 or -Q (this latter implies -B):

proot -Q qemu-arm /mnt/armslack-12.2/

$ cat /etc/motd
Welcome to ARMedSlack Linux 12.2

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

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

PRoot allows 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 -Q qemu-arm /mnt/armslack-12.2/

$ 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 -Q qemu-arm /mnt/armslack-12.2/

$ 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 -Q qemu-arm -b /usr/bin/make /mnt/armslack-12.2/

$ which 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



The latest release of PRoot is packaged on and sources are hosted on


Here follows a couple of URLs where some rootfs archives can be freely downloaded. Note that the errors reported by tar when extracting these archives can be safely ignored. Obviously these files are not required when PRoot is used as a mount --bind equivalent only, that is, when the path to the guest rootfs is /.

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 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)


Visit for help, bug reports, suggestions, patchs, ... Copyright (C) 2012 STMicroelectronics, licensed under GPL v2 or later.

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