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

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Message-Id: <1181911628.25228.483.camel@pmac.infradead.org>
Date:	Fri, 15 Jun 2007 13:47:08 +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 14:40 +0200, Andi Kleen wrote:
> > That is broken on all non-x86 architectures, 

> It cannot be broken, it just might be somewhat slower 

No, Andi. It's broken.

We're speaking of a 32-bit ioctl compat routine. I would say it's more
than 'convention' that the structure used by the compat_ioctl routine
actually matches the 'real' structure as it gets laid out on the 32-bit
architecture.

The 'real' structure as used by the 32-bit userspace does not have the
'packed' attribute. Thus the u64 member of the structure is aligned to 8
bytes on _all_ relevant 32-bit architectures except for i386.

By adding 'packed' in the compat_ioctl routine, you cause it to expect a
structure which does not match what userspace is using, for all non-x86
architectures. That's kind of not very compatible. I call that 'broken'.

-- 
dwmw2

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