Re: [PATCH v3 12/17] arm64: implement __va_function

[Sami Tolvanen]

On Thu, Mar 25, 2021 at 3:38 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Tue, Mar 23, 2021 at 01:39:41PM -0700, Sami Tolvanen wrote:
> > With CONFIG_CFI_CLANG, the compiler replaces function addresses in
> > instrumented C code with jump table addresses. This change implements
> > the __va_function() macro, which returns the actual function address
> > instead.
> >
> > Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>
> Is there really no attribute or builtin that can be used to do this
> without assembly?

I don't think the compiler currently offers anything that could help
us here. Peter, can you think of another way to avoid the function
address to jump table address conversion with
-fno-sanitize-cfi-canonical-jump-tables?

> IIUC from other patches the symbol tables will contain the "real"
> non-cfi entry points (unless we explciitly asked to make the jump table
> address canonical), so AFAICT here the compiler should have all the
> necessary information to generate either the CFI or non-CFI entry point
> addresses, even if it doesn't expose an interface for that today.
>
> It'd be a lot nicer if we could get the compiler to do this for us.

I agree, that would be quite useful in the kernel.

Sami