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

[Peter Collingbourne]

On Thu, Mar 25, 2021 at 4:28 PM Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:
>
> 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?

No, the assembly seems like the best way at the moment.

> > 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.

Maybe. The kernel's requirements seem quite specialized here though. A
normal C or C++ program has little need for the actual entry point, so
if you need it you are probably doing something low level enough to
require assembly anyway.

Peter