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Simple Story

Whoever is holding the door stands outside the room, not inside it.

That sounds like a detail and it is the whole point. A pupil who has taken the room over can talk round anyone who is in there with them. They cannot talk round someone who arrived before they did and is holding the door from the other side.

What it is​

All three layers are wrappers: a process that configures its restriction and then executes the next layer, with the supervised build command innermost.

This is the property that distinguishes Linux-based security from the aspect-oriented programming (AOP) and architecture layers. Those run inside the Java Virtual Machine (JVM) they protect. A wrapper runs outside it, so the supervised process cannot switch the restriction off, however much control it gains over its own JVM.

The layers are composed by phobos.sh and each can be disabled independently with --no-timeout, --no-network and --no-filesystem.

Further reading​