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From: Arnd Bergmann <arnd@arndb.de>
To: Al Viro <viro@ftp.linux.org.uk>
Subject: Re: [PATCH 16/16] fix handling of integer constant expressions
Date: Sun, 24 Jun 2007 20:07:10 +0200
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On Sunday 24 June 2007, Al Viro wrote:
> but the latter... ?Probably ought to be
> #define _IOC_TYPECHECK(t) \
> ? ? ? ? (sizeof(t) + BUILD_BUG_ON_ZERO(sizeof(t) == sizeof(t[1]) && \
> ? ? ? ? ? sizeof(t) < (1 << _IOC_SIZEBITS)))
> 
> Objections? ?The only reason that doesn't break gcc to hell and back is
> that gcc has unfixed bugs in that area. ?It certainly is not a valid C
> or even a remotely sane one.

Yes, looks good. I originally came up with _IOC_TYPECHECK before we had
the generic BUILD_BUG_ON(). 

One minor issue though:
While BUILD_BUG_ON and a few other macros in linux/kernel.h are currently
exported to user space, I would think that they should really be hidden
in #ifdef __KERNEL__, which means that we also need something like

#ifdef __KERNEL__
#define _IOC_TYPECHECK(t) \
        (sizeof(t) + BUILD_BUG_ON_ZERO(sizeof(t) == sizeof(t[1]) && \
          sizeof(t) < (1 << _IOC_SIZEBITS)))
#else
#define _IOC_TYPECHECK(t) sizeof(t)
#endif

	Arnd <><
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