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 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.
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