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From: Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH] Introduce compat_u64 and compat_s64 types
Date: Fri, 15 Jun 2007 14:03:53 +0200
References: <200706150159.l5F1xNgM000459@hera.kernel.org> <200706151131.38429.arnd@arndb.de> <200706151355.57464.ak@suse.de>
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To: Andi Kleen <ak@suse.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>, David Woodhouse <dwmw2@infradead.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Dave Airlie <airlied@linux.ie>, linux-arch@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>
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On Friday 15 June 2007, 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.
> 

It doesn't break in all cases, but quite often, you have
something like:

struct foo {
	__u32 a;
	__u64 b;
};

If you define a

struct compat_foo {
	__u32 a;
	__u64 b;
} __attribute__((packed));

That is broken on all non-x86 architectures, because it removes the
padding that is inserted on the respective 32 bit platforms, while

struct compat_foo {
	__u32 a;
	compat_u64 b;
};

Is a correct definition on all architectures. It also produces
somewhat better code if the architecture does not support unaligned
data access, but that is just an unintended side-effect.

	Arnd <><