[clang] [llvm] [X86] Enhance kCFI type IDs with a 3-bit arity indicator. (PR #117121)

[Scott Constable via cfe-commits]

scottconstable wrote:

I also do not want to lose sight of one of the other obvious advantages that was mentioned in the PR description:

> One additional benefit of this patch is that it can benefit other CFI approaches that build on kCFI, such as FineIBT. For example, this proposed enhancement to FineIBT must be able to infer (at kernel init time) which registers are live at an indirect call target: https://lkml.org/lkml/2024/9/27/982. If the arity bits are available in the kCFI type ID, then this information is trivial to infer.

To elaborate, we are concurrently working on a Linux kernel patch to enhance FineIBT (which is a KCFI-like solution that utilizes x86 Indirect Branch Tracking). The goal is to extend FineIBT to poison live argument registers if a hash check fails after a branch mis-prediction. This enhancement can help to mitigate a variety of Spectre attacks in the Linux kernel.

https://github.com/llvm/llvm-project/pull/117121