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

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Date:	Fri, 15 Jun 2007 06:11:33 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Arnd Bergmann <arnd@...db.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 Fri, Jun 15, 2007 at 11:31:37AM +0200, 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.
> 
> Hopefully, this patch improves the situation, it introduces two
> new types, compat_u64 and compat_s64. These are defined on all
> architectures to have the same size and alignment as the 32 bit
> version of u64 and s64.

You're relying on compat_[us]64 being only used in structures which are
already packed.  If someone uses them in a non-packed struct, they won't
decrease the alignment.  I think it would be more effective to specify
it as:

__attribute__((aligned(4), packed))

The other problem is that if someone defines a struct like this:

struct foo {
	short bar;
	compat_s64 baz;
} __attribute__((packed))

it'll have different definitions on x86 and ia64.

So I think we should be aiming for the ((aligned, packed)) definition and
remove the __attribute__((packed)) from the struct definitions.  What do
you think?
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