[RFC/RFT] CFI: Add support for gcc CFI in aarch64

[Dan Li]

Hi Kees,

On 01/06, Kees Cook wrote:
> On Mon, Dec 19, 2022 at 05:32:04AM -0800, Dan Li wrote:
> > Hi Peter,
> Hi!
> 
> First of all, thank you thank you for working on this in GCC. This will
> make a big difference for folks that don't have the option to build with
> Clang to gain CFI coverage.
> 
> As for the implementation details, the core issue is really that this
> type of CFI is specifically designed for the Linux kernel, and it took a
> rather long time to figure out all the specifics needed (down to the
> byte counts and instruction layouts). GCC's version will ultimately need
> to exactly match the Clang output, or Linux is unlikely to support it.
> 
> We're already on our second CFI -- the original Clang CFI was just too
> clunky for long-term use in Linux, so unless we're going to improve on
> the latest Clang KCFI implementation in some way, it's better to stick
> to exactly byte-for-byte identical results. The KCFI support in Linux
> depends on the arm64 and x86_64 runtimes for catching the traps, and the
> post-processing done (on x86_64) with objtool that prepares the kernel
> for IBT use, and converts to the optional FineIBT CFI mechanism. With
> all those moving parts, there needs to be a very compelling reason to
> have GCC KCFI implementation differ from Clang's.
> 
> Hopefully that context helps a little. I'm excited to try out future
> versions!

Thanks for the context, it makes sense and helped me a lot. :)

In the next version I'll make the gcc implementation consistent with clang.

Thanks,
Dan.

> 
> -Kees
> 
> -- 
> Kees Cook