[PATCH v3 08/13] ARM: ftrace: enable the graph tracer with the EABI unwinder

[Arnd Bergmann]

On Thu, Feb 3, 2022 at 9:21 AM Ard Biesheuvel <ardb at kernel.org> wrote:
>
> Enable the function graph tracer in combination with the EABI unwinder,
> so that Thumb2 builds or Clang ARM builds can make use of it.
>
> This involves using the unwinder to locate the return address of an
> instrumented function on the stack, so that it can be overridden and
> made to refer to the ftrace handling routines that need to be called at
> function return.
>
> Given that for these builds, it is not guaranteed that the value of the
> link register is stored on the stack, fall back to the stack slot that
> will be used by the ftrace exit code to restore LR in the instrumented
> function's execution context.
>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> Reviewed-by: Steven Rostedt (Google) <rostedt at goodmis.org>

> diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
> index 07055a503022..cc7523f44be4 100644
> --- a/arch/arm/Kconfig.debug
> +++ b/arch/arm/Kconfig.debug
> @@ -65,7 +65,7 @@ config UNWINDER_FRAME_POINTER
>
>  config UNWINDER_ARM
>         bool "ARM EABI stack unwinder"
> -       depends on AEABI && !FUNCTION_GRAPH_TRACER
> +       depends on AEABI
>         # https://github.com/ClangBuiltLinux/linux/issues/732
>         depends on !LD_IS_LLD || LLD_VERSION >= 110000
>         select ARM_UNWIND

What are the remaining tradeoffs between the two unwinders
on EABI kernels? Since we require llvm-11 as the minimum
toolchain now, we can probably drop the LLD_VERSION
check here as well, so UNWINDER_ARM should finally work
for any EABI kernel.

According to the help text, the main downside is a small size
increase that I've measured at under 1% in multi_v7_defconfig,
however that seems to be far outweighed by allowing thumb2
kernels that reduce the size by 15% in the same config.

Are there any other reasons for keeping the frame pointer
unwinder as a normal option for ARMv7?

        Arnd