Re: [PATCH v5 12/18] arm64: implement function_nocfi

[Sami Tolvanen <samitolvanen@google.com>]

On Tue, Apr 6, 2021 at 4:37 AM Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Thu, Apr 01, 2021 at 04:32:10PM -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 function_nocfi() macro, which returns the actual function address
> > instead.
> >
> > Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> > Reviewed-by: Kees Cook <keescook@chromium.org>
>
> I think that it's unfortunate that we have to drop to assembly here, but
> given this is infrequent I agree it's not the end of the world, so:
>
> Acked-by: Mark Rutland <mark.rutland@arm.com>
>
> > ---
> >  arch/arm64/include/asm/memory.h | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> > index 0aabc3be9a75..b55410afd3d1 100644
> > --- a/arch/arm64/include/asm/memory.h
> > +++ b/arch/arm64/include/asm/memory.h
> > @@ -321,6 +321,21 @@ static inline void *phys_to_virt(phys_addr_t x)
> >  #define virt_to_pfn(x)               __phys_to_pfn(__virt_to_phys((unsigned long)(x)))
> >  #define sym_to_pfn(x)                __phys_to_pfn(__pa_symbol(x))
> >
> > +#ifdef CONFIG_CFI_CLANG
> > +/*
> > + * With CONFIG_CFI_CLANG, the compiler replaces function address
> > + * references with the address of the function's CFI jump table
> > + * entry. The function_nocfi macro always returns the address of the
> > + * actual function instead.
> > + */
> > +#define function_nocfi(x) ({                                         \
> > +     void *addr;                                                     \
> > +     asm("adrp %0, " __stringify(x) "\n\t"                           \
> > +         "add  %0, %0, :lo12:" __stringify(x) : "=r" (addr));        \
>
> If it's not too painful, could we please move the asm constrain onto its
> own line? That makes it slightly easier to read, and aligns with what
> we've (mostly) done elsewhere in arm64.

Sure, I'll change this in the next version.

> Not a big deal either way, and the ack stands regardless.
>
> Thanks,
> Mark.

Sami