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:

quoted

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 <redacted>
---
 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.

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