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

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Message-ID: <CAKv+Gu8hvxcc6Tiu0q7=HWqkEf4ZLfOvyoLGkp2mEgjz_1R9ew@mail.gmail.com>
Date:	Wed, 27 Jan 2016 20:39:23 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-alpha@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	linux-s390@...r.kernel.org, "x86@...nel.org" <x86@...nel.org>,
	"hpa@...or.com" <hpa@...or.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Richard Henderson <rth@...ddle.net>,
	Tony Luck <tony.luck@...el.com>,
	Will Deacon <will.deacon@....com>, Helge Deller <deller@....de>
Subject: Re: [PATCH v2 resend 6/6] arm64: switch to relative exception tables

On 27 January 2016 at 20:35, Andrew Morton <akpm@...ux-foundation.org> wrote:
> 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:

That may be true, but it extends an established pattern of #defines in
the various uaccess.h versions, i.e., ARCH_HAS_SORT_EXTABLE, and
ARCH_HAS_SEARCH_EXTABLE