PRoot

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.