linux-kernel - Re: [PATCH] Introduce compat_u64 and compat

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Date:	Fri, 15 Jun 2007 13:00:36 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Andi Kleen <ak@...e.de>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Airlie <airlied@...ux.ie>, linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Introduce compat_u64 and compat_s64 types

On Fri, 2007-06-15 at 13:55 +0200, Andi Kleen wrote:
> On Friday 15 June 2007 11:31:37 Arnd Bergmann wrote:
> > One common problem with 32 bit system call and ioctl emulation
> > is the different alignment rules between i386 and 64 bit machines.
> > A number of drivers work around this by marking the compat
> > structures as 'attribute((packed))', which is not the right
> > solution because it breaks all the non-x86 architectures that
> > want to use the same compat code.
> 
> Why does it break them? It should just make them a little slower.
> 
> The network code requires unaligned accesses to work
> anyways so if your architecture doesn't support them it is already
> remotely crashable.

alignof(uint64_t) is 8 on just about every 32-bit architecture except
i386. Using __attribute__((packed)) for the 32-on-64 compat code is thus
wrong on every 64-bit architecture except x86_64 and ia64.

It's the _location_ which is wrong; the handling of unaligned loads is
irrelevant (and Linux actually supports a bunch of architecture on which
fixups are impossible now, btw).

-- 
dwmw2

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