Invariants¶

These are the properties the design exists to hold. Code and tests across three gems cite them by number, so the numbers never change. Number 1 is retired.

1. A cell holds no application credentials, in its environment or on its filesystem.
2. The supervisor never evaluates image data, and a worker forked from it produces correct output.
3. Descriptor access modes are one-way: an input can't be written, and an output can't be read.
4. The cold side never opens a filesystem path that a hot side chose.
5. An operation can't exceed its cell's limits, whatever it declares.
6. A cell can't reach another cell's socket.
7. A worker can't read another request's memory, given kernel.yama.ptrace_scope >= 1 on the host and max_requests_per_worker: 1. Worker isolation covers files and the environment.
8. A tool subprocess sees only the environment that its operation wrote for it.

Tests cover the invariants where reading the code isn't enough: where the kernel enforces the property, where a break would be silent, or where a plausible refactor would remove it.

Invariant 2 is a property of a whole filesystem, so no single test asserts it. The tests check that the built image contains no application source tree, and bin/conformance boots the hardened container and checks its flags.