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 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.
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!*.
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.
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.
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.
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.
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.
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.
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:
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:
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
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 '22.214.171.124 google.com' > /etc/hosts $ resolveip google.com IP address of google.com is 126.96.36.199 $ echo '188.8.131.52 google.com' > /etc/hosts $ resolveip google.com IP address of google.com is 184.108.40.206
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
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
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...
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
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 /
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