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

From: Shi, Yang
Date: Fri Nov 06 2015 - 12:23:47 EST

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:

bool "Compile the kernel with frame pointers"
depends on DEBUG_KERNEL && \
(CRIS || M68K || FRV || UML || \
AVR32 || SUPERH || BLACKFIN || MN10300 || METAG) || \
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

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



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