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

[James Clark]

On 27/10/2022 18:20, Peter Zijlstra wrote:
> On Tue, Oct 25, 2022 at 04:28:12PM +0100, James Clark wrote:
> 
>>> 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?
>>
>> You are probably right that it could be done that way. The reason that
>> we did it this way was to be consistent with ptrace feature [1] where it
>> is delivered to userspace on a per-process basis. And there is also a
>> prctl for the enabled key types [2] which can be changed dynamically.
>> Particularly for the last reason is why it was done per sample.
>>
>> Having said that, the enabled keys field is not used by perf, only the
>> mask is used, so I can drop the per sample data until enabled keys is
>> needed, which may be never.
>>
>> I'm going to assume that perf shouldn't use ptrace because of
>> permissions and conflicts with debuggers, so I could put the mask
>> somewhere like PERF_RECORD_FORK instead of per sample.
> 
> Yeah, or create an new RECORD type which you can also send around at
> prctl() time.
> 
> The only thing that's needed on top of that is exposing the mask
> somewhere in /proc for existing tasks; which is what perf also uses to
> syntesize RECORD_MMAP events on startup etc..
> 

Hmm ok, in that case I can just add the /proc interface for now because
the mask won't change and we can add the new record type at the point
it's needed in the future.

Thanks for the feedback.