Messages in this thread |  | | Date | Tue, 31 Mar 2020 15:36:38 -0700 | From | Nathan Chancellor <> | Subject | Re: [PATCH] compiler.h: fix error in BUILD_BUG_ON() reporting |
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On Tue, Mar 31, 2020 at 05:25:38PM -0500, Daniel Santos wrote: > 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
No, GCC 4.6 is the minimum required version and it is highly likely that the minimum version of GCC will be raised to 4.8 soon:
https://lore.kernel.org/lkml/20200123153341.19947-10-will@kernel.org/ https://git.kernel.org/peterz/queue/c/0e75b883b400ac4b1dafafe3cbd2e0a39b714232
Cheers, Nathan
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