Re: [PATCH] arm64: update compiler option for PAC

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Nov 16, 2021 at 10:11:47AM +0000, Will Deacon wrote:
> On Mon, Nov 15, 2021 at 11:18:08AM +0800, Kuan-Ying Lee wrote:
> > We pass -mbranch-protection=pac-ret+leaf to support PAC
> > when we use GCC 9 or later.
> > 
> > Before GCC 9, we pass -msign-return-address=all to support
> > PAC.
> > 
> > Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
> > ---
> >  Documentation/arm64/pointer-authentication.rst | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/Documentation/arm64/pointer-authentication.rst b/Documentation/arm64/pointer-authentication.rst
> > index f127666ea3a8..055d08b0d42d 100644
> > --- a/Documentation/arm64/pointer-authentication.rst
> > +++ b/Documentation/arm64/pointer-authentication.rst
> > @@ -54,8 +54,10 @@ virtual address size configured by the kernel. For example, with a
> >  virtual address size of 48, the PAC is 7 bits wide.
> >  
> >  Recent versions of GCC can compile code with APIAKey-based return
> > -address protection when passed the -msign-return-address option. This
> > -uses instructions in the HINT space (unless -march=armv8.3-a or higher
> > +address protection when passed compiler option as following.
> > +Pass -msign-return-address when we use GCC 7, 8.
> > +Pass -mbranch-protection when we use GCC 9 or later.
> > +This uses instructions in the HINT space (unless -march=armv8.3-a or higher
> >  is also passed), and such code can run on systems without the pointer
> >  authentication extension.
> 
> I think I'd be more inclined to delete this paragraph altogether. It doesn't
> really document anything to do with the kernel, and trying to document the
> behaviour of "recent" versions of GCC is futile.

Agreed. I think the only thing we might want to mention is that we rely
on the compiler to use the HINT space instructions (PACIASP and AUTIASP)
so that a kernel built with pointer authentication support works on HW
without the feature.

So replacing that paragraph with something like:

| When ARM64_PTR_AUTH_KERNEL is selected the kernel will be compiled
| with HINT space pointer authentication instructions protecting
| function returns. Kernels built with this option will work on
| hardware with or without pointer authentication support.

... would make sense to me.

Thanks,
Mark.