linux-kernel - Re: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions

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Date:	Sun, 17 Aug 2008 19:33:19 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 
> 
> On Sun, 17 Aug 2008, Ingo Molnar wrote:
> > +/*
> > + * Force a compilation error if condition is true [array index becomes
> > + * negative], and a linker error if condition is not constant [non-defined
> > + * variable is used as an array index]:
> > + *
> > + * ( The linker trick relies on gcc optimizing out a multiplication with
> > + *   constant zero - which should be reasonable enough. )
> > + */
> > +extern unsigned int __BUILD_BUG_ON_non_constant;
> > +
> > +#define BUILD_BUG_ON(condition)					\
> > +do {								\
> > +	(void)sizeof(char[1 - 2*!!(condition)]);		\
> > +	if (!__builtin_constant_p(condition))			\
> > +		__BUILD_BUG_ON_non_constant++;			\
> > +} while (0)
> 
> Gag me now.
> 
> Why not just do
> 
>  #define __BBO(c)                sizeof(const char[1 - 2*!!(c)])
>  #define __BBONC(c)              __BBO(!__builtin_constant_p(c))
>  #define BUILD_BUG_ON_ZERO(c)    (__BBO(c) - __BBONC(c))
>  #define BUILD_BUG_ON(c)         (void)BUILD_BUG_ON_ZERO(c)
> 
> and be done with it?

yeah, i first tried a few variants of that (compile-time warnings are 
much better than link time warnings), but none worked when i tested 
various failure modes.

try the patch below - it only gives this error during build:

 kernel/sched.c: In function ‘test':
 kernel/sched.c:9193: error: size of array ‘type name' is negative
 make[1]: *** [kernel/sched.o] Error 1

it doesnt notice the error on the next line. I suspect this is because 
__builtin_constant_p() is special (or broken). I tried it with gcc 4.2.3 
and 4.3.0.

	Ingo

Index: linux/kernel/sched.c
===================================================================
--- linux.orig/kernel/sched.c
+++ linux/kernel/sched.c
@@ -9181,3 +9181,16 @@ struct cgroup_subsys cpuacct_subsys = {
 	.subsys_id = cpuacct_subsys_id,
 };
 #endif	/* CONFIG_CGROUP_CPUACCT */
+
+#define __BBO(c)                sizeof(const char[1 - 2*!!(c)])
+#define __BBONC(c)              __BBO(!__builtin_constant_p(c))
+#define BUILD_BUG_ON_ZERO2(c)    (__BBO(c) - __BBONC(c))
+#define BUILD_BUG_ON2(c)         (void)BUILD_BUG_ON_ZERO(c)
+
+void test(void)
+{
+	BUILD_BUG_ON2(0);
+	BUILD_BUG_ON2(1);
+	BUILD_BUG_ON2(panic_timeout);
+}
+
--
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