Messages in this thread | | | From | Masahiro Yamada <> | Date | Thu, 1 Aug 2019 11:50:36 +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 4:04 AM Rikard Falkeborn <rikard.falkeborn@gmail.com> 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... > > include/linux/bits.h | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) > > diff --git a/include/linux/bits.h b/include/linux/bits.h > index 669d69441a62..73489579eef9 100644 > --- 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 > +#endif > + > #define GENMASK(h, l) \ > + (GENMASK_INPUT_CHECK(h, l) + \ > (((~UL(0)) - (UL(1) << (l)) + 1) & \ > - (~UL(0) >> (BITS_PER_LONG - 1 - (h)))) > + (~UL(0) >> (BITS_PER_LONG - 1 - (h))))) > > #define GENMASK_ULL(h, l) \ > + (GENMASK_INPUT_CHECK(h, l) + \ > (((~ULL(0)) - (ULL(1) << (l)) + 1) & \ > - (~ULL(0) >> (BITS_PER_LONG_LONG - 1 - (h)))) > + (~ULL(0) >> (BITS_PER_LONG_LONG - 1 - (h)))))
This is getting cluttered with so many parentheses.
One way of clean-up is to rename the current macros as follows:
GENMASK() -> __GENMASK() GENMASK_UL() -> __GENMASK_ULL()
Then,
#define GENMASK(h, l) (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l)) #define GENMASK_ULL(h, l) (GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(h, l))
-- Best Regards Masahiro Yamada
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