[llvm] [X86][KCFI] Do not emit a type prefix for nocf_check functions (PR #158133)

[Sami Tolvanen via llvm-commits]

samitolvanen wrote:

> I see this is only testing the IR level, but not the front-end.

Correct, this doesn't change the behavior for `nocf_check` in Clang.

> Do you want `-fsanitize=kcfi` to work with `nocf_check` regardless of `-fcf-protection`? I think it should -- there may actually be some value in being able to remove preambles for a function regardless of `-fcf-protection`. (This is the logic I'm implementing in the GCC KCFI.)

That sounds reasonable to me. Out of curiosity, `nocf_check` is currently limited to X86 only in Clang. Does your GCC KCFI implementation support `nocf_check` on other architectures too?

> That way we won't get these kinds of errors if someone tries to use `nocf_check`:
> 
> ```
> kcfi-runtime.c:17:16: warning: 'nocf_check' attribute ignored; use -fcf-protection to enable the
>       attribute [-Wignored-attributes]
>    17 | __attribute__((nocf_check))
>       |                ^
> ```

I think this isn't actually a problem in the kernel though, as `nocf_check` is only used with `CONFIG_X86_KERNEL_IBT`.

https://github.com/llvm/llvm-project/pull/158133