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

Andrii Nakryiko
2025-01-24T17:59:50Z

On Tue, Jan 21, 2025 at 6:32 PM Josh Poimboeuf <jpoimboe@kernel.org> 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 <jpoimboe@kernel.org>
> ---
> 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)
> …