Re: [PATCH 1/2] cfi: add CONFIG_CFI_ICALL_NORMALIZE_INTEGERS

[Peter Zijlstra]

On Tue, Jul 30, 2024 at 01:38:33PM +0200, Miguel Ojeda wrote:
> On Tue, Jul 30, 2024 at 11:40 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
> >
> > Introduce a Kconfig option for enabling the experimental option to
> > normalize integer types. This ensures that integer types of the same
> > size and signedness are considered compatible by the Control Flow
> > Integrity sanitizer.
> >
> > This option exists for compatibility with Rust, as C and Rust do not
> > have the same set of integer types. There are cases where C has two
> > different integer types of the same size and alignment, but Rust only
> > has one integer type of that size and alignment. When Rust calls into
> > C functions using such types in their signature, this results in CFI
> > failures.
> >
> > This patch introduces a dedicated option for this because it is
> > undesirable to have CONFIG_RUST affect CC_FLAGS in this way.
> 
> Is there any case where we would want CFI_ICALL_NORMALIZE_INTEGERS
> when Rust is not enabled, then? If not, is the idea here to make this
> an explicit extra question in the config before enabling Rust? Or why
> wouldn't it be done automatically?

I suspect CFI_ICALL_NORMALIZE_INTEGERS breaks ABI, then again, Linux
doesn't promise or preserve ABI except for the SYSCALL layer. So yeah,
meh.