Re: [RFC PATCH 09/21] arm64: Add CFI error handling

[Sami Tolvanen]

On Thu, May 5, 2022 at 8:45 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> It would be a bit nicer if we could encode the register index into the BRK
> immediate, i.e. allocate a range of 32 immediates (or 31 given BLR XZR is
> nonsensical), and have:
>
>         BRK #CFI_BRK_IMM + n
>
> ... where `n` is the Xn index.
>
> That way the kernel doesn't need to know the specific code sequence and
> wouldn't have to decode the instruction to find the relevant register -- we
> could determine that from the ESR alone. That would also avoid tying the
> compiler into a specific code sequence, and would allow that to change.
>
> Since the BRK immediate is 16 bits, we have enough space to also encode the
> index of the wB register, which would allow the kernel's BRK handler to recover
> and log the expected type value and the the value at the target of the branch
> (that latter we can recover from xN, so we don't need wA to be encoded into the
> immediate).

Sure, sounds like a good idea.

> ... does the compiler side of that sound possible?

Yes, this should be doable. I'll take a look and change this in the
next version.

Sami