From: Arnd Bergmann <arnd@arndb.de>
To: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH]: Make ioctl.h compatible with userland
Date: Tue, 12 Aug 2008 17:12:18 +0200
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Cc: Michael Abbott <michael@araneidae.co.uk>, linux-kernel@vger.kernel.org
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[PATCH] Make _IOC_TYPECHECK use BUILD_BUG_ON_ZERO

This converts _IOC_TYPECHECK from a link error to a compile-time
error using BUILD_BUG_ON_ZERO. This makes it possible to use
the standard _IOC macros in user space even with non-optizing
compilers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>

---
On Wednesday 30 July 2008, Andrew Morton wrote:
> We could also replace that open-coded assertion with the shiny new
> BUILD_BUG_ON(), which would a) be cleaner and b) fix the problem which
> you describe. ?I expect that would be quite safe, but obviously doesn't
> have all the testing which the above patch has, so shrug.

There is one significant difference: using BUILD_BUG_ON_ZERO will
break user space code that uses broken ioctl number definitions
like _IOC('x', 1, sizeof(int)) that were fixed up in the kernel
but not in external copies of the definitions.
I'm undecided whether such breakage would be a good or a bad thing.

--- a/include/asm-generic/ioctl.h
+++ b/include/asm-generic/ioctl.h
@@ -69,11 +69,9 @@
 	 ((size) << _IOC_SIZESHIFT))
 
 /* provoke compile error for invalid uses of size argument */
-extern unsigned int __invalid_size_argument_for_IOC;
-#define _IOC_TYPECHECK(t) \
-	((sizeof(t) == sizeof(t[1]) && \
-	  sizeof(t) < (1 << _IOC_SIZEBITS)) ? \
-	  sizeof(t) : __invalid_size_argument_for_IOC)
+#define _IOC_TYPECHECK(t) (sizeof(t) + \
+	BUILD_BUG_ON_ZERO(sizeof(t) != sizeof(t[1]) || \
+			  sizeof(t) >= (1 << _IOC_SIZEBITS)))
 
 /* used to create numbers */
 #define _IO(type,nr)		_IOC(_IOC_NONE,(type),(nr),0)
--
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