Messages in this thread | | | Date | Tue, 13 Nov 2012 16:24:15 -0600 | From | Daniel Santos <> | Subject | Re: [PATCH v5 7/9] bug.h: Make BUILD_BUG_ON generate compile-time error |
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Borislav,
Please note that this patch has changed slightly since you Acked it. I have moved the location of the negative-size array code to the end of the macro. I don't think this really matters honestly, but I figured this was the best place to put that change.
Daniel
On 11/13/2012 04:13 PM, danielfsantos@att.net wrote: > Negative sized arrays wont create a compile-time error in some cases > starting with gcc 4.4 (e.g., inlined functions), but gcc 4.3 introduced > the error function attribute that will. This patch modifies > BUILD_BUG_ON to behave like BUILD_BUG already does, using the error > function attribute so that you don't have to build the entire kernel to > discover that you have a problem, and then enjoy trying to track it down > from a link-time error. > > Also, we are only including asm/bug.h and then expecting that > linux/compiler.h will eventually be included to define __linktime_error > (used in BUILD_BUG_ON). This patch includes it directly for clarity and > to avoid the possibility of changes in <arch>/*/include/asm/bug.h being > changed or not including linux/compiler.h for some reason. > > Signed-off-by: Daniel Santos <daniel.santos@pobox.com> > Acked-by: Borislav Petkov <bp@alien8.de> > --- > include/linux/bug.h | 32 +++++++++++++++++++------------- > 1 files changed, 19 insertions(+), 13 deletions(-) > > diff --git a/include/linux/bug.h b/include/linux/bug.h > index ccd44ce..dd4f506 100644 > --- a/include/linux/bug.h > +++ b/include/linux/bug.h > @@ -2,6 +2,7 @@ > #define _LINUX_BUG_H > > #include <asm/bug.h> > +#include <linux/compiler.h> > > enum bug_trap_type { > BUG_TRAP_TYPE_NONE = 0, > @@ -42,25 +43,30 @@ struct pt_regs; > * @condition: the condition which the compiler should know is false. > * > * If you have some code which relies on certain constants being equal, or > - * other compile-time-evaluated condition, you should use BUILD_BUG_ON to > + * some other compile-time-evaluated condition, you should use BUILD_BUG_ON to > * detect if someone changes it. > * > - * The implementation uses gcc's reluctance to create a negative array, but > - * gcc (as of 4.4) only emits that error for obvious cases (eg. not arguments > - * to inline functions). So as a fallback we use the optimizer; if it can't > - * prove the condition is false, it will cause a link error on the undefined > - * "__build_bug_on_failed". This error message can be harder to track down > - * though, hence the two different methods. > + * The implementation uses gcc's reluctance to create a negative array, but gcc > + * (as of 4.4) only emits that error for obvious cases (e.g. not arguments to > + * inline functions). Luckily, in 4.3 they added the "error" function > + * attribute just for this type of case. Thus, we use a negative sized array > + * (should always create an error on gcc versions older than 4.4) and then call > + * an undefined function with the error attribute (should always create an > + * error on gcc 4.3 and later). If for some reason, neither creates a > + * compile-time error, we'll still have a link-time error, which is harder to > + * track down. > */ > #ifndef __OPTIMIZE__ > #define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)])) > #else > -extern int __build_bug_on_failed; > -#define BUILD_BUG_ON(condition) \ > - do { \ > - bool __cond = !!(condition); \ > - ((void)sizeof(char[1 - 2*!!(__cond)])); \ > - if (__cond) __build_bug_on_failed = 1; \ > +#define BUILD_BUG_ON(condition) \ > + do { \ > + bool __cond = !!(condition); \ > + extern void __build_bug_on_failed(void) \ > + __compiletime_error("BUILD_BUG_ON failed"); \ > + if (__cond) \ > + __build_bug_on_failed(); \ > + ((void)sizeof(char[1 - 2*!!(__cond)])); \ > } while(0) > #endif >
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