EMPTY_CONTEXT is a bit hazardous

[Mark-Simulacrum]

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Problem:

EMPTY_CONTEXT is easy to misuse as an empty blob of the right size put on the stack. However, it's actually a region initialized at s2n startup and shouldn't be written to by most code. See #4230 (comment).

Solution:

A description of the possible solution in terms of S2N architecture. Highlight and explain any potentially controversial design decisions taken.

No solution described just yet.

• Does this change what S2N sends over the wire? If yes, explain.
• Does this change any public APIs? If yes, explain.
• Which versions of TLS will this impact?

Requirements / Acceptance Criteria:

What must a solution address in order to solve the problem? How do we know the solution is complete?

• RFC links: Links to relevant RFC(s)
• Related Issues: Link any relevant issues
• Will the Usage Guide or other documentation need to be updated?
• Testing: How will this change be tested? Call out new integration tests, functional tests, or particularly interesting/important unit tests.
  • Will this change trigger SAW changes? Changes to the state machine, the s2n_handshake_io code that controls state transitions, the DRBG, or the corking/uncorking logic could trigger SAW failures.
  • Should this change be fuzz tested? Will it handle untrusted input? Create a separate issue to track the fuzzing work.

Out of scope:

Is there anything the solution will intentionally NOT address?