linux-kernel - Re: [PATCH] arm64: remove redundant FRAME

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Message-ID: <20151106162538.GU6087@arm.com>
Date:	Fri, 6 Nov 2015 16:25:38 +0000
From:	Will Deacon <will.deacon@....com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Mark Rutland <mark.rutland@....com>,
	Yang Shi <yang.shi@...aro.org>, linaro-kernel@...ts.linaro.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: remove redundant FRAME_POINTER kconfig option

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@....com>
> > 
> > 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?

Will
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