Re: [PATCH v2 1/1] perf arm64: Send pointer auth masks to ring buffer

[Peter Zijlstra]

On Thu, Oct 20, 2022 at 11:19:20AM +0100, James Clark wrote:
> From: Andrew Kilroy <andrew.kilroy@xxxxxxx>
> 
> Perf report cannot produce callgraphs using dwarf on arm64 where pointer
> authentication is enabled.  This is because libunwind and libdw cannot
> unmangle instruction pointers that have a pointer authentication code
> (PAC) embedded in them.
> 
> libunwind and libdw need to be given an instruction mask which they can
> use to arrive at the correct return address that does not contain the
> PAC.
> 
> The bits in the return address that contain the PAC can differ by
> process, so this patch adds a new sample field PERF_SAMPLE_ARCH_1
> to allow the kernel to send the masks up to userspace perf.
> 
> This field can be used in a architecture specific fashion, but on
> arm64, it contains the ptrauth mask information. The event will
> currently fail to open on architectures other than arm64 if
> PERF_SAMPLE_ARCH_1 is set. It will also fail to open on arm64 if
> CONFIG_ARM64_PTR_AUTH isn't set, as the data would always be zeros.

A little more information please; wth is pointer authentication? Are we
going to be having the same thing with x86 LAM where only a subset of
the available bits have meaning to the hardware?

Why do we want the same mask repeated over and over with each sample;
should this not be part of the address space (side-band) data?