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Date:   Thu, 20 Oct 2022 19:00:56 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     James Clark <james.clark@arm.com>
Cc:     linux-perf-users@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        broonie@kernel.org, acme@kernel.org,
        Andrew Kilroy <andrew.kilroy@arm.com>,
        Vince Weaver <vincent.weaver@maine.edu>,
        Mark Rutland <mark.rutland@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH v2 1/1] perf arm64: Send pointer auth masks to ring buffer
Message-ID: <Y1F+yAWy4VVaDb+5@hirez.programming.kicks-ass.net>
References: <20221020101921.1219533-1-james.clark@arm.com>
 <20221020101921.1219533-2-james.clark@arm.com>
 <Y1F8DUWaWhlh/phL@hirez.programming.kicks-ass.net>
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On Thu, Oct 20, 2022 at 06:49:17PM +0200, Peter Zijlstra wrote:
> On Thu, Oct 20, 2022 at 11:19:20AM +0100, James Clark wrote:
> > From: Andrew Kilroy <andrew.kilroy@arm.com>
> > 
> > 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

Mark got me: https://events.static.linuxfound.org/sites/events/files/slides/slides_23.pdf

> 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?