Messages in this thread | | | Subject | Re: [PATCH] linux/bits.h: Add compile time sanity check of GENMASK inputs | From | Joe Perches <> | Date | Wed, 31 Jul 2019 12:27:38 -0700 |
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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)
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?
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