Re: [PATCH v6 0/5] Add ftrace direct call for arm64

[Mark Rutland]

On Wed, Apr 05, 2023 at 08:02:45PM +0200, Florent Revest wrote:
> This series adds ftrace direct call support to arm64.
> This makes BPF tracing programs (fentry/fexit/fmod_ret/lsm) work on arm64.
> 
> It is meant to be taken by the arm64 tree but it depends on the
> trace-direct-v6.3-rc3 tag of the linux-trace tree:
>   git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace.git
> That tag was created by Steven Rostedt so the arm64 tree can pull the prior work
> this depends on. [1]

Catalin, Will, are you happy to pick this via the arm64 tree, or for it to go
via the trace tree?

We'd been assuming the former, but it looks like there'll be a (simple) merge
conflict with the series adding FUNCTION_GRAPH_RETVAL:

  https://lore.kernel.org/lkml/cover.1680954589.git.pengdonglin@xxxxxxxxxxxxxx/

... as both series add some definitions to arm64's asm-offsets.c in the same
place, and all those additions need to be kept. Other than that, the two series
are independent.

IIUC Steve was hoping to take the FUNCTION_GRAPH_RETVAL series through the
trace tree, and if that's still the plan, maybe both should go that way?

Mark.

> Thanks to the ftrace refactoring under that tag, an ftrace_ops backing a ftrace
> direct call will only ever point to *one* direct call. This means we can look up
> the direct called trampoline address stored in the ops from the ftrace_caller
> trampoline in the case when the destination would be out of reach of a BL
> instruction at the ftrace callsite. This fixes limitations of previous attempts
> such as [2].
> 
> This series has been tested on arm64 with:
> 1- CONFIG_FTRACE_SELFTEST
> 2- samples/ftrace/*.ko (cf: patch 4)
> 3- tools/testing/selftests/bpf/test_progs (cf: patch 5)
> 
> Changes since v5 [3]:
> - Fixed saving the fourth argument of handle_mm_fault in both the x86 (patch 3)
>   and arm64 (as part of patch 4) "ftrace-direct-too" sample trampolines
> - Fixed the address of the traced function logged by some direct call samples
>   (ftrace-direct-multi and ftrace-direct-multi-modify) by moving lr into x0
> 
> 1: https://lore.kernel.org/all/ZB2Nl7fzpHoq5V20@FVFF77S0Q05N/
> 2: https://lore.kernel.org/all/20220913162732.163631-1-xukuohai@xxxxxxxxxxxxxxx/
> 3: https://lore.kernel.org/bpf/20230403113552.2857693-1-revest@xxxxxxxxxxxx/
> 
> Florent Revest (5):
>   arm64: ftrace: Add direct call support
>   arm64: ftrace: Simplify get_ftrace_plt
>   samples: ftrace: Save required argument registers in sample
>     trampolines
>   arm64: ftrace: Add direct call trampoline samples support
>   selftests/bpf: Update the tests deny list on aarch64
> 
>  arch/arm64/Kconfig                           |  6 ++
>  arch/arm64/include/asm/ftrace.h              | 22 +++++
>  arch/arm64/kernel/asm-offsets.c              |  6 ++
>  arch/arm64/kernel/entry-ftrace.S             | 90 ++++++++++++++++----
>  arch/arm64/kernel/ftrace.c                   | 46 +++++++---
>  samples/ftrace/ftrace-direct-modify.c        | 34 ++++++++
>  samples/ftrace/ftrace-direct-multi-modify.c  | 40 +++++++++
>  samples/ftrace/ftrace-direct-multi.c         | 24 ++++++
>  samples/ftrace/ftrace-direct-too.c           | 40 +++++++--
>  samples/ftrace/ftrace-direct.c               | 24 ++++++
>  tools/testing/selftests/bpf/DENYLIST.aarch64 | 82 ++----------------
>  11 files changed, 306 insertions(+), 108 deletions(-)
> 
> -- 
> 2.40.0.577.gac1e443424-goog
>