Re: [PATCH v4 12/39] unwind_user: Add frame pointer support

On Tue, Jan 21, 2025 at 6:32 PM Josh Poimboeuf <[email protected]> wrote:
>
> Add optional support for user space frame pointer unwinding.  If
> supported, the arch needs to enable CONFIG_HAVE_UNWIND_USER_FP and
> define ARCH_INIT_USER_FP_FRAME.
>
> By encoding the frame offsets in struct unwind_user_frame, much of this
> code can also be reused for future unwinder implementations like sframe.
>
> Signed-off-by: Josh Poimboeuf <[email protected]>
> ---
>  arch/Kconfig                      |  4 +++
>  include/asm-generic/unwind_user.h |  9 ++++++
>  include/linux/unwind_user_types.h |  1 +
>  kernel/unwind/user.c              | 49 +++++++++++++++++++++++++++++--
>  4 files changed, 60 insertions(+), 3 deletions(-)
>  create mode 100644 include/asm-generic/unwind_user.h
>

Do you plan to reuse this logic for stack unwinding done by perf
subsystem in perf_callchain_user()? See is_uprobe_at_func_entry()
parts and also fixup_uretprobe_trampoline_entries() for some of the
quirks that have to be taken into account when doing frame
pointer-based unwinding. It would be great not to lose those in this
new reimplementation.

Not sure what's the best way to avoid duplicating the logic, but I
thought I'd bring that up.

> diff --git a/arch/Kconfig b/arch/Kconfig
> index c6fa2b3ecbc6..cf996cbb8142 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -438,6 +438,10 @@ config HAVE_HARDLOCKUP_DETECTOR_ARCH
>  config UNWIND_USER
>         bool
>
> +config HAVE_UNWIND_USER_FP
> +       bool
> +       select UNWIND_USER
> +
>  config AS_SFRAME
>         def_bool $(as-instr,.cfi_sections .sframe\n.cfi_startproc\n.cfi_endproc)
>

[...]