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@xxxxxxxxxxxx> > > --- > > 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.
