Re: [PATCH] arm64: remove redundant FRAME_POINTER kconfig option

[Shi, Yang]

On 11/6/2015 8:25 AM, Will Deacon wrote:
> On Fri, Nov 06, 2015 at 04:21:09PM +0000, Catalin Marinas wrote:
> > On Fri, Nov 06, 2015 at 12:50:02PM +0000, Mark Rutland wrote:
> > > On Fri, Nov 06, 2015 at 12:30:09PM +0000, Will Deacon wrote:
> > > > On Wed, Nov 04, 2015 at 09:37:51AM -0800, Yang Shi wrote:
> > > > > FRAME_POINTER is defined in lib/Kconfig.debug, it is unnecessary to redefine
> > > > > it in arch/arm64/Kconfig.debug.
> > > >
> > > > It might be worth noting that this adds a dependency on DEBUG_KERNEL
> > > > for building with frame pointers. I'm ok with that (it appears to be
> > > > enabled in defconfig and follows the vast majority of other archs) but
> > > > it is a change in behaviour.
> > > >
> > > > With that:
> > > >
> > > >    Acked-by: Will Deacon <will.deacon@xxxxxxx>
> > >
> > > The code in arch/arm64/kernel/stacktrace.c assumes we have frame
> > > pointers regardless of FRAME_POINTER. Depending on what the compiler
> > > decides to use x29 for, we could get some weird fake unwinding and/or
> > > dodgy memory accesses.
> > >
> > > I think we should first audit the uses of frame pointers to ensure that
> > > they are guarded for !FRAME_POINTER.
> >
> > Or we just select FRAME_POINTER in the ARM64 Kconfig entry.
>
> Yang, did you see any benefit disabling frame pointers, or was this patch
> purely based on you spotting a duplicate Kconfig entry?

It just spots a duplicate Kconfig entry.

FRAME_POINTER is defined in both lib/Kconfig.debug and 
arch/arm64/Kconfig.debug.

The lib/Kconfig.debug one looks like:

config FRAME_POINTER
        bool "Compile the kernel with frame pointers"
        depends on DEBUG_KERNEL && \
                (CRIS || M68K || FRV || UML || \
                 AVR32 || SUPERH || BLACKFIN || MN10300 || METAG) || \
                ARCH_WANT_FRAME_POINTERS
        default y if (DEBUG_INFO && UML) || ARCH_WANT_FRAME_POINTERS
        help
          If you say Y here the resulting kernel image will be slightly
          larger and slower, but it gives very useful debugging information
          in case of kernel bugs. (precise oopses/stacktraces/warnings)

The common one just depends on DEBUG_KERNEL && ARCH_WANT_FRAME_POINTERS.

ARCH_WANT_FRAME_POINTERS is selected by ARM64 kconfig entry.

To answer Catalin's question about:

> However, the patch would allow one to
> disable FRAME_POINTERS (not sure it has any effect on the aarch64 gcc
> though).

No, it doesn't. Actually, FRAME_POINTER could be disabled regardless of 
the patch.

Thanks,
Yang

> Will

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