Messages in this thread | | | From | Masahiro Yamada <> | Date | Fri, 2 Aug 2019 10:40:38 +0900 | Subject | Re: [PATCH] linux/bits.h: Add compile time sanity check of GENMASK inputs |
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On Thu, Aug 1, 2019 at 4:27 AM Joe Perches <joe@perches.com> wrote: > > On Wed, 2019-07-31 at 21:03 +0200, Rikard Falkeborn wrote: > > GENMASK() and GENMASK_ULL() are supposed to be called with the high bit > > as the first argument and the low bit as the second argument. Mixing > > them will return a mask with zero bits set. > > > > Recent commits show getting this wrong is not uncommon, see e.g. > > commit aa4c0c9091b0 ("net: stmmac: Fix misuses of GENMASK macro") and > > commit 9bdd7bb3a844 ("clocksource/drivers/npcm: Fix misuse of GENMASK > > macro"). > > > > To prevent such mistakes from appearing again, add compile time sanity > > checking to the arguments of GENMASK() and GENMASK_ULL(). If both the > > arguments are known at compile time, and the low bit is higher than the > > high bit, break the build to detect the mistake immediately. > > > > Since GENMASK() is used in declarations, BUILD_BUG_OR_ZERO() must be > > used instead of BUILD_BUG_ON(), and __is_constexpr() must be used instead > > of __builtin_constant_p(). > > > > Commit 95b980d62d52 ("linux/bits.h: make BIT(), GENMASK(), and friends > > available in assembly") made the macros in linux/bits.h available in > > assembly. Since neither BUILD_BUG_OR_ZERO() or __is_constexpr() are asm > > compatible, disable the checks if the file is included in an asm file. > > > > Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com> > > --- > > Joe Perches sent a series to fix the existing misuses of GENMASK() that > > needs to be merged before this to avoid build failures. Currently, 7 of > > the patches were not in Linus tree, and 2 were not in linux-next. > > > > Also, there's currently no asm users of bits.h, but since it was made > > asm-compatible just two weeks ago it would be a shame to break it right > > away... > [] > > diff --git a/include/linux/bits.h b/include/linux/bits.h > [] > > @@ -18,12 +18,22 @@ > > * position @h. For example > > * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000. > > */ > > +#ifndef __ASSEMBLY__ > > +#include <linux/build_bug.h> > > +#define GENMASK_INPUT_CHECK(h, l) BUILD_BUG_ON_ZERO(__builtin_choose_expr( \ > > + __is_constexpr(h) && __is_constexpr(l), (l) > (h), 0)) > > +#else > > +#define GENMASK_INPUT_CHECK(h, l) 0 > > A few things: > > o Reading the final code is a bit confusing. > Perhaps add a comment description saying it's not checked > in asm .h uses. > > o Maybe use: > #define GENMASK_INPUT_CHECK(h, l) UL(0)
Why?
> o The compiler error message when the arguments are in the > wrong order isn't obvious. Is there some way to improve > the compiler error output, maybe by using BUILD_BUG_ON_MSG > or some other mechanism? > >
-- Best Regards Masahiro Yamada
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