linux-kernel - Re: [PATCH 16/16] fix handling of integer constant expressions

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Message-Id: <200706242007.10585.arnd@arndb.de>
Date:	Sun, 24 Jun 2007 20:07:10 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Al Viro <viro@....linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-sparse@...r.kernel.org
Subject: Re: [PATCH 16/16] fix handling of integer constant expressions

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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