[RFC PATCH v2 07/21] cfi: Add type helper macros

[Sami Tolvanen]

On Sat, May 14, 2022 at 2:49 PM Kees Cook <keescook at chromium.org> wrote:
>
> On Fri, May 13, 2022 at 01:21:45PM -0700, Sami Tolvanen wrote:
> > With CONFIG_CFI_CLANG, assembly functions called indirectly
> > from C code must be annotated with type identifiers to pass CFI
> > checking. The compiler emits a __kcfi_typeid_<function> symbol for
> > each address-taken function declaration in C, which contains the
> > expected type identifier. Add typed versions of SYM_FUNC_START and
> > SYM_FUNC_START_ALIAS, which emit the type identifier before the
> > function.
> >
> > Signed-off-by: Sami Tolvanen <samitolvanen at google.com>
>
> And the reason to not make this change universally (i.e. directly in
> SYM_FUNC_START) is to minimize how many of these symbol annotations get
> emitted? (And to more directly indicate which asm is called indirectly?)

The reason not to add this to SYM_FUNC_START is that the compiler
doesn't emit the type symbols for all functions. It currently emits
them for all address-taken function declarations in each translation
unit. We could potentially further limit this by emitting them only
for function declarations with a specific attribute, for example, but
that's something we can optimize later.

> What happens if an asm function is called indirectly and it doesn't have
> this annotation?

It will fail the CFI check.

> (Is this case detectable at compile-time?)

It's not. I'll update the commit message in the next version to
clarify these points.

Sami