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

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Message-Id: <200706151403.53855.arnd@arndb.de>
Date:	Fri, 15 Jun 2007 14:03:53 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Andi Kleen <ak@...e.de>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Woodhouse <dwmw2@...radead.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 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 <><
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