linux-kernel - Re: [PATCH v3 3/4] arm64: mm: support ARCH_MMAP_RND

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Message-ID: <20151127093249.GW3109@e104818-lin.cambridge.arm.com>
Date:	Fri, 27 Nov 2015 09:32:50 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Andrey Ryabinin <ryabinin.a.a@...il.com>
Cc:	Daniel Cashman <dcashman@...roid.com>,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	Will Deacon <will.deacon@....com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"H. Peter Anvin" <hpa@...or.com>, mingo <mingo@...nel.org>,
	aarcange@...hat.com, Russell King <linux@....linux.org.uk>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Jonathan Corbet <corbet@....net>, xypron.glpk@....de,
	"x86@...nel.org" <x86@...nel.org>, hecmargi@....es,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Borislav Petkov <bp@...e.de>, nnk@...gle.com,
	dzickus@...hat.com, Kees Cook <keescook@...omium.org>,
	jpoimboe@...hat.com, Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arm-kernel@...ts.infradead.org,
	LKML <linux-kernel@...r.kernel.org>, salyzyn@...roid.com,
	"Eric W. Biederman" <ebiederm@...ssion.com>, jeffv@...gle.com,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	dcashman@...gle.com
Subject: Re: [PATCH v3 3/4] arm64: mm: support ARCH_MMAP_RND_BITS.

On Fri, Nov 27, 2015 at 11:36:30AM +0300, Andrey Ryabinin wrote:
> 2015-11-25 23:39 GMT+03:00 Daniel Cashman <dcashman@...roid.com>:
> > On 11/25/2015 04:06 AM, Catalin Marinas wrote:
> >> For KASan, we ended up calculating KASAN_SHADOW_OFFSET in
> >> arch/arm64/Makefile. What would the formula be for the above
> >> ARCH_MMAP_RND_BITS_MAX?
> >
> > The general formula I used ended up being:
> > _max = floor(log(TASK_SIZE)) - log(PAGE_SIZE) - 3
> 
> For kasan, we calculate KASAN_SHADOW_OFFSET in Makefile, because we need to use
> that value in Makefiles.
> 
> For ARCH_MMAP_RND_COMPAT_BITS_MIN/MAX I don't see a reason why it has
> to be in Kconfig.
> Can't we just use your formula to #define ARCH_MMAP_RND_COMPAT_BITS_*
> in some arch header?

Because there is another option, ARCH_MMAP_RND_BITS depending on EXPERT
which uses the MIN/MAX range defined per architecture. Since it's an
expert feature, we could as well ignore the MIN/MAX in Kconfig and just
add BUILD_BUG_ON checks to the code. This way we could simply define
them in C files.

Alternatively, add arithmetics support to kbuild ;).

-- 
Catalin
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