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Date:   Fri, 24 Jul 2020 14:34:57 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Oscar Carter <oscar.carter@gmx.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Kees Cook <keescook@chromium.org>,
        linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com,
        Jann Horn <jannh@google.com>
Subject: Re: [PATCH v2 2/2] kernel/trace: Remove function callback casts
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On Fri, 24 Jul 2020 19:55:00 +0200
Oscar Carter <oscar.carter@gmx.com> wrote:

> > Which one of the above is this patch set for?  
> 
> This patch is the result of a warning obtained with the following:
> 
> make allmodconfig ARCH=powerpc
> make ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnu- -j4
> 
> And with the -Wcast-function-type enabled in the top level makefile.

Looking into powerpc I found this:

arch/powerpc/include/asm/ftrace.h:

  #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
  #define ARCH_SUPPORTS_FTRACE_OPS 1
  #endif


arch/powerpc/Kconfig:

	select HAVE_DYNAMIC_FTRACE_WITH_REGS	if MPROFILE_KERNEL
[..]

  config MPROFILE_KERNEL
	depends on PPC64 && CPU_LITTLE_ENDIAN && FUNCTION_TRACER
	def_bool $(success,$(srctree)/arch/powerpc/tools/gcc-check-mprofile-kernel.sh $(CC) -I$(srctree)/include -D__KERNEL__)

So, it looks like you need to be 64bit PowerPC, Little Endian, and gcc
needs to support -mprofile.

Otherwise, it falls back to the old way that does the type casting.

If you are really concerned about this, I would recommend adding
support to the architecture you care about, and then this will no
longer be an issue.

The funny part is, you can still add support for ftrace_ops, without
adding support for DYNAMIC_FTRACE_WITH_REGS, if you only care about not
having to do that typecast.

My NAK still stands. I wont let an intrusive patch be added to the
ftrace core code to deal with an unsupported feature in an architecture.

I would be will to add that linker trick to remove the warning. Or we
just use that warning as incentive to get architecture developers to
implement this feature ;-)

-- Steve