linux-kernel - Re: [PATCH v2 resend 6/6] arm64: switch to relative exception tables

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Message-Id: <20160127113548.d4f2f2a7029b3b89f0e596da@linux-foundation.org>
Date:	Wed, 27 Jan 2016 11:35:48 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
	linux-s390@...r.kernel.org, x86@...nel.org, hpa@...or.com,
	heiko.carstens@...ibm.com, rth@...ddle.net, tony.luck@...el.com,
	will.deacon@....com, deller@....de
Subject: Re: [PATCH v2 resend 6/6] arm64: switch to relative exception
 tables

On Wed, 27 Jan 2016 11:55:23 +0100 Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:

> Instead of using absolute addresses for both the exception location
> and the fixup, use offsets relative to the exception table entry values.
> Not only does this cut the size of the exception table in half, it is
> also a prerequisite for KASLR, since absolute exception table entries
> are subject to dynamic relocation, which is incompatible with the sorting
> of the exception table that occurs at build time.
> 
> This patch also introduces the _ASM_EXTABLE preprocessor macro (which
> exists on x86 as well) and its _asm_extable assembly counterpart, as
> shorthands to emit exception table entries.

checkpatch speaketh truth:

ERROR: #define of 'ARCH_HAS_RELATIVE_EXTABLE' is wrong - use Kconfig variables or standard guards instead
#113: FILE: arch/arm64/include/asm/uaccess.h:56: