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

[Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>]

On Tue, 2021-11-23 at 20:10 +0800, Mark Rutland wrote:
> 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.

Hi,

Thanks you both for suggestions.
I will rewrite this paragraph in v2.

Thanks,
Kuan-Ying Lee

> 
> Thanks,
> Mark.