Indirect branch tracking

[jcs]

OpenBSD now enforces indirect branch tracking (BTI on arm64, IBT on x86_64) which breaks Crystal on OpenBSD going forward.

I'm not sure whether this would all be handled by llvm and Crystal's compiler just needs to turn it on (in clang it's -fcf-protection=branch) or if there is assembly in Crystal that needs to be modified with the new instructions.

[HertzDevil]

It says that indirect branch tracking is already on by default unless there is an opt-out linker flag, so do we actually need to do anything?

[jcs]

Yes, the opt-out is temporary.

[GeopJr]

OpenBSD enforces it now and Crystal has been marked as broken

[joshturge]

I'd like to help get Crystal supported on OpenBSD, what work needs to be done in order to get IBT support? I'm not sure where to start.

[wecing]

@joshturge maybe check out QBE's implementation

Note that it was reverted due to tooling reasons, but I am trying to do a roll-forward since the blocker seems gone.

UPDATE: or, call linker with -z,nobtcfi (see https://github.com/openbsd/ports/commit/71f21f5bf7fed6ec8faa947b5cda278558c0c8de)