Messages in this thread | | | Date | Tue, 30 Oct 2012 17:19:33 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH v4 6/9] compiler.h, bug.h: Prevent double error messages with BUILD_BUG{,_ON} |
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On Sun, Oct 28, 2012 at 03:57:12PM -0500, danielfsantos@att.net wrote: > Prior to the introduction of __attribute__((error("msg"))) in gcc 4.3, > creating compile-time errors required a little trickery. > BUILD_BUG{,_ON} uses this attribute when available to generate > compile-time errors, but also uses the negative-sized array trick for > older compilers, resulting in two error messages in some cases. The > reason it's "some" cases is that as of gcc 4.4, the negative-sized array > will not create an error in some situations, like inline functions. > > This patch replaces the negative-sized array code with the new > __compiletime_error_fallback() macro which expands to the same thing > unless the the error attribute is available, in which case it expands to > do{}while(0), resulting in exactly one compile-time error on all > versions of gcc. > > Signed-off-by: Daniel Santos <daniel.santos@pobox.com> > --- > include/linux/bug.h | 4 ++-- > include/linux/compiler.h | 7 +++++++ > 2 files changed, 9 insertions(+), 2 deletions(-) > > diff --git a/include/linux/bug.h b/include/linux/bug.h > index 03259d7..da03dc1 100644 > --- a/include/linux/bug.h > +++ b/include/linux/bug.h > @@ -57,13 +57,13 @@ struct pt_regs; > * track down. > */ > #ifndef __OPTIMIZE__ > -#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)])) > +#define BUILD_BUG_ON(condition) __compiletime_error_fallback(condition) > #else > #define BUILD_BUG_ON(condition) \ > do { \ > extern void __build_bug_on_failed(void) \ > __compiletime_error("BUILD_BUG_ON failed"); \ > - ((void)sizeof(char[1 - 2*!!(condition)])); \ > + __compiletime_error_fallback(condition); \ > if (condition) \ > __build_bug_on_failed(); \
If we're defining a fallback, shouldn't it come second? I.e.:
if (condition) __build_bug_on_failed(); __compiletime_error_fallback(condition);
Also, the error message from __build_bug_on_failed is much more informative:
arch/x86/kernel/cpu/amd.c: In function ‘early_init_amd’: arch/x86/kernel/cpu/amd.c:486:2: error: call to ‘__build_bug_on_failed’ declared with attribute error: BUILD_BUG_ON failed make[1]: *** [arch/x86/kernel/cpu/amd.o] Error 1 make[1]: *** Waiting for unfinished jobs.... make: *** [arch/x86/kernel/cpu/] Error 2
than
arch/x86/kernel/cpu/amd.c: In function ‘early_init_amd’: arch/x86/kernel/cpu/amd.c:486:2: error: size of unnamed array is negative make[1]: *** [arch/x86/kernel/cpu/amd.o] Error 1 make[1]: *** Waiting for unfinished jobs.... make: *** [arch/x86/kernel/cpu/] Error 2
Finally, you need to do:
bool __cond = !!(condition);
and use __cond so that condition doesn't get evaluated multiple times (possibly with side effects).
Thanks.
-- Regards/Gruss, Boris. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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