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

[Ard Biesheuvel]

On Thu, 3 Feb 2022 at 17:09, Nathan Chancellor <nathan at kernel.org> wrote:
>
> On Thu, Feb 03, 2022 at 10:16:57AM +0100, Arnd Bergmann wrote:
> > 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.
>
> For what it's worth, that check was eliminated in the same series as
> bumping the minimum version: e1ab4182ca11 ("Revert "ARM: 9070/1: Make
> UNWINDER_ARM depend on ld.bfd or ld.lld 11.0.0+"")
>

OK. This series is currently based on Russell's tree which does not
include v5.17-rc1 yet.