linux-kernel - Re: Linux 3.3-rc4

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Date:	Mon, 20 Feb 2012 21:28:22 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ian Kent <raven@...maw.net>, David Miller <davem@...emloft.net>,
	thomas@...3r.de, linux-kernel@...r.kernel.org
Subject: Re: Linux 3.3-rc4

On 02/20/2012 09:06 PM, Linus Torvalds wrote:
> On Mon, Feb 20, 2012 at 8:52 PM, Ian Kent <raven@...maw.net> wrote:
>>
>> I think the patch attached to your original post needs a little work if
>> that is to be used. Correct me if I'm wrong but AFAICT there are more
>> architectures that use 8-byte alignment than just x86-64, such as alpha,
>> ia64 and ppc64 and I believe they may also be used in a compat mode.
> 
> The only issue is compat mode, and afaik, all other architectures
> except for x86-32 do __u64 with natural alignment.
> 
> So all 64-bit architectures use natural alignment, the only issue is
> the alignment of __u64 in 32-bit mode.
> 
> So it really is *not* about 8-byte alignment. Quite the reverse. It's
> about 4-byte alignment of 64-bit entities, and I suspect x86-32 is the
> only one that does that.
> 
> See "compat_u64", and notice how only in arch/x86/include/asm/compat.h
> do we have
> 
>   typedef u64 __attribute__((aligned(4))) compat_u64;
> 
> So it really is limited to only x86.
> 

m68k has alignment 2 for 32- and 64-bit quantities, so it's not just
x86; the only reason you don't see that one is because m68k doesn't have
a compat layer to worry about.

Holes are highly undesirable for another reason: they create security
holes where kernel information leaks out.

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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