Messages in this thread | | | Subject | Re: [PATCH] compiler.h: fix error in BUILD_BUG_ON() reporting | From | Daniel Santos <> | Date | Tue, 31 Mar 2020 17:25:38 -0500 |
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On 3/31/20 6:26 AM, Vegard Nossum wrote: > compiletime_assert() uses __LINE__ to create a unique function name. > This means that if you have more than one BUILD_BUG_ON() in the same > source line (which can happen if they appear e.g. in a macro), then > the error message from the compiler might output the wrong condition. > > For this source file: > > #include <linux/build_bug.h> > > #define macro() \ > BUILD_BUG_ON(1); \ > BUILD_BUG_ON(0); > > void foo() > { > macro(); > } > > gcc would output: > > ./include/linux/compiler.h:350:38: error: call to ‘__compiletime_assert_9’ declared with attribute error: BUILD_BUG_ON failed: 0 > _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__) > > However, it was not the BUILD_BUG_ON(0) that failed, so it should say 1 > instead of 0. With this patch, we use __COUNTER__ instead of __LINE__, so > each BUILD_BUG_ON() gets a different function name and the correct > condition is printed: > > ./include/linux/compiler.h:350:38: error: call to ‘__compiletime_assert_0’ declared with attribute error: BUILD_BUG_ON failed: 1 > _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__) > > Cc: Daniel Santos <daniel.santos@pobox.com> > Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk> > Cc: Masahiro Yamada <yamada.masahiro@socionext.com> > Cc: Ian Abbott <abbotti@mev.co.uk> > Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com> > --- > include/linux/compiler.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/include/linux/compiler.h b/include/linux/compiler.h > index 5e88e7e33abec..034b0a644efcc 100644 > --- a/include/linux/compiler.h > +++ b/include/linux/compiler.h > @@ -347,7 +347,7 @@ static inline void *offset_to_ptr(const int *off) > * compiler has support to do so. > */ > #define compiletime_assert(condition, msg) \ > - _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__) > + _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__) > > #define compiletime_assert_atomic_type(t) \ > compiletime_assert(__native_word(t), \
This will break builds using gcc 4.2 and earlier and the expectation was that you don't put two of them on the same line -- not helpful in macros where it all must be on the same line. Is gcc 4.2 still supported? If so, I recommend using another macro for the unique number that uses __COUNTER__ if available and __LINE__ otherwise. This was the decision for using __LINE__ when I wrote the original anyway.
Also note that this construct:
BUILD_BUG_ON_MSG(0, "I like chicken"); BUILD_BUG_ON_MSG(1, "I don't like chicken");
will incorrectly claim that I like chicken. This is because of how __attribute__((error)) works -- gcc will use the first declaration to define the error message.
I couple of years ago, I almost wrote a gcc extension to get rid of this whole mess and just __builtin_const_assert(cond, msg). Maybe I'll finish that this year.
Daniel
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