Re: [PATCH 0/8] Perf stack unwinding with pointer authentication

[James Clark <james.clark@arm.com>]

On 04/07/2022 15:53, Andrew Kilroy wrote:
> This patch series addresses issues that perf has when attempting to show
> userspace stacks in the presence of pointer authentication on arm64.
> 
> Depending on whether libunwind or libdw is used, perf incorrectly
> displays the userspace stack in 'perf report --stdio'.  With libunwind,
> only the leaf function is shown.
> 
>             |
>             ---0x200000004005bf
>                0x200000004005bf
>                my_leaf_function
> 
> With libdw, only the leaf function is shown even though there are
> callers in the application.
> 
>             |
>             ---my_leaf_function
> 
> 
> The reason perf cannot show the stack upon a perf report --stdio is
> because the unwinders are given instruction pointers which contain a
> pointer authentication code (PAC).  For the libraries to correctly
> unwind, they need to know which bits of the instruction pointer to turn
> off.
> 
> The kernel exposes the set of PAC bits via the NT_ARM_PAC_MASK regset.
> It is expected that this may vary per-task in future. The kernel also
> exposes which pointer authentication keys are enabled via the
> NT_ARM_PAC_ENABLED_KEYS regset, and this can change dynamically. These
> are per-task state which perf would need to sample.
> 
> It's not always feasible for perf to acquire these regsets via ptrace.
> When sampling system-wide or with inherited events this may require a
> large volume of ptrace requests, and by the time the perf tool processes
> a sample for a task, that task might already have terminated.
> 
> Instead, these patches allow this state to be sampled into the perf
> ringbuffer, where it can be consumed more easily by the perf tool.
> 
> The first patch changes the kernel to send the authentication PAC masks
> to userspace perf via the perf ring buffer.  This is published in the
> sample, using a new sample field PERF_SAMPLE_ARCH_1.
> 
> The subsequent patches are changes to userspace perf to
> 
> 1) request the PERF_SAMPLE_ARCH_1
> 2) supply the instruction mask to libunwind
> 3) ensure perf can cope with an older kernel that does not know about
>    the PERF_SAMPLE_ARCH_1 sample field.
> 4) checks if the version of libunwind has the capability to accept
>    an instruction mask from perf and if so enable the feature.
> 
> These changes depend on a change to libunwind, that is yet to be
> released, although the patch has been merged.
> 
>   https://github.com/libunwind/libunwind/pull/360
> 

For the whole set:

Reviewed-by: James Clark <james.clark@arm.com>

I checked that the new test passes on an AWS Graviton 3 instance and
with a build of mainline libunwind. I also checked that the PAC masks on
the samples look sensible.

The tests also still pass when run on N1SDP which doesn't have pointer
authentication.

> 
> Andrew Kilroy (6):
>   perf arm64: Send pointer auth masks to ring buffer
>   perf evsel: Do not request ptrauth sample field if not supported
>   perf tools: arm64: Read ptrauth data from kernel
>   perf libunwind: Feature check for libunwind ptrauth callback
>   perf libunwind: arm64 pointer authentication
>   perf tools: Print ptrauth struct in perf report
> 
> German Gomez (2):
>   perf test: Update arm64 tests to expect ptrauth masks
>   perf test arm64: Test unwinding with PACs on gcc & clang compilers
> 
>  arch/arm64/include/asm/arch_sample_data.h     |  38 ++++++
>  arch/arm64/kernel/Makefile                    |   2 +-
>  arch/arm64/kernel/arch_sample_data.c          |  37 ++++++
>  include/linux/perf_event.h                    |  24 ++++
>  include/uapi/linux/perf_event.h               |   5 +-
>  kernel/events/core.c                          |  35 ++++++
>  tools/build/Makefile.feature                  |   2 +
>  tools/build/feature/Makefile                  |   4 +
>  tools/build/feature/test-all.c                |   5 +
>  .../feature/test-libunwind-arm64-ptrauth.c    |  26 ++++
>  tools/include/uapi/linux/perf_event.h         |   5 +-
>  tools/perf/Makefile.config                    |  10 ++
>  tools/perf/Makefile.perf                      |   1 +
>  tools/perf/tests/Build                        |   1 +
>  tools/perf/tests/arm_unwind_pac.c             | 113 ++++++++++++++++++
>  tools/perf/tests/arm_unwind_pac.sh            |  57 +++++++++
>  tools/perf/tests/attr/README                  |   1 +
>  .../attr/test-record-graph-default-aarch64    |   3 +-
>  tools/perf/tests/attr/test-record-graph-dwarf |   1 +
>  .../attr/test-record-graph-dwarf-aarch64      |  13 ++
>  .../tests/attr/test-record-graph-fp-aarch64   |   3 +-
>  tools/perf/tests/builtin-test.c               |   1 +
>  tools/perf/tests/sample-parsing.c             |   2 +-
>  tools/perf/tests/tests.h                      |   1 +
>  tools/perf/util/event.h                       |   8 ++
>  tools/perf/util/evsel.c                       |  64 ++++++++++
>  tools/perf/util/evsel.h                       |   1 +
>  tools/perf/util/perf_event_attr_fprintf.c     |   2 +-
>  tools/perf/util/session.c                     |  15 +++
>  tools/perf/util/unwind-libunwind-local.c      |  12 ++
>  30 files changed, 485 insertions(+), 7 deletions(-)
>  create mode 100644 arch/arm64/include/asm/arch_sample_data.h
>  create mode 100644 arch/arm64/kernel/arch_sample_data.c
>  create mode 100644 tools/build/feature/test-libunwind-arm64-ptrauth.c
>  create mode 100644 tools/perf/tests/arm_unwind_pac.c
>  create mode 100755 tools/perf/tests/arm_unwind_pac.sh
>  create mode 100644 tools/perf/tests/attr/test-record-graph-dwarf-aarch64
>