| Date: | 2013-07-22 |
|---|---|
| Version: | 3.1 |
| Manual section: | 1 |
proot [option] ... [command]
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 add their own features or use PRoot as a Linux process
instrumentation engine thanks to its extension mechanism. 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 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.
| -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 It is recommended to use the | |
| -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: | |
| -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
| |
| -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 | |
| -v value, --verbose=value | |
Set the level of debug information to value. The higher the integer value is, the more detailled 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. | |
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 | |
Set the kernel release and compatibility level to 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 syscalls that are available in the kernel release specified by string but that are missing in the current kernel. With some versions of the GNU C library, the following tweak is required if this fatal error still happens when this option is enabled: cd path/to/rootfs
sed --in-place=.bak s/Linux/PRoot/ \
lib*/ld-linux*.so.2 lib*/*/ld-linux*.so.2
| |
| -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
|
The following options are aliases for handy sets of options.
| -R path | Alias: Programs isolated in path, a guest rootfs, might still need to
access information about the host system, as it is illustrated in
the
|
| -B, -M | obsolete, use -R instead. |
| -Q command | obsolete, use -q and -R instead. |
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.
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
The bind mechanism permits to relocate files and directories. This is typically useful to cheat 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
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/ ? /lib/ld-linux.so.2 /bin/sh ? /lib/ld-linux.so.2 /usr/bin/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 /lib/ld-linux.so.2 /bin/sh pts/6 /lib/ld-linux.so.2 /usr/bin/ps -o tty,command
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 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
The latest release of PRoot is packaged on http://packages.proot.me and sources are hosted on http://github.proot.me. Also, highly compatible binaries are available on http://static.proot.me for a couple of architectures.
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.
- Slackware, Arch, Fedora for ARM:
- ftp://ftp.armedslack.org/slackwarearm/slackwarearm-devtools/minirootfs/roots/
- http://archlinuxarm.org/developers/downloads
- http://ftp.linux.org.uk/pub/linux/arm/fedora/rootfs/
- CentOS, Debian, Fedora, Scientific, Suse, Ubuntu, ALT, Arch, CERN, Gentoo, OpenSuse, Openwall, Slackware, SLES, and etc. for x86 and x86_64 CPUs:
- Gentoo for a lot of architectures:
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 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.
chroot(1), mount(8), binfmt_misc, ptrace(2), qemu(1), sb2(1), bindfs(1), fakeroot(1), fakechroot(1)
Visit http://proot.me for help, bug reports, suggestions, patchs, ... Copyright (C) 2013 STMicroelectronics, licensed under GPL v2 or later.
_____ _____ ___ | __ \ __ \_____ _____| |_ | __/ / _ \/ _ \ _| |__| |__|__\_____/\_____/\____|