Messages in this thread | | | From | Masahiro Yamada <> | Date | Sat, 3 Aug 2019 12:03:13 +0900 | Subject | Re: [PATCH] linux/bits.h: Add compile time sanity check of GENMASK inputs |
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On Sat, Aug 3, 2019 at 3:19 AM Rikard Falkeborn <rikard.falkeborn@gmail.com> wrote: > > On Fri, Aug 02, 2019 at 12:25:06PM +0900, Masahiro Yamada wrote: > > On Fri, Aug 2, 2019 at 12:14 PM Joe Perches <joe@perches.com> wrote: > > > > > > On Fri, 2019-08-02 at 10:40 +0900, Masahiro Yamada wrote: > > > > 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? > > > > > > Consistency with the uses in what's now called __GENMASK > > > > Inconsistent with __GENMASK_ULL. > > Would you prefer to add GENMASK_ULL_INPUT_CHECK?
No.
> Or replace UL(0) with > 0 and then probably move the cast of BUILD_BUG_OR_ZERO (to avoid > GENMASK be of type size_t) to GENMASK and GENMASK_ULL?
No.
Your original code is absolutely fine.
C aligns the types to the wider one.
(unsigned long) + (int) -> (unsigned long) (unsigned long long) + (int) -> (unsigned long long)
Having GENMASK_INPUT_CHECK to return 'int' is OK. The resulted GENMASK(), GENMASK_ULL() still have unsigned long, unsigned long long, respectively.
BTW, v2 is already inconsistent. If you wanted GENMASK_INPUT_CHECK() to return 'unsigned long',, you would have to cast (low) > (high) as well:
(unsigned long)((low) > (high)), UL(0))))
This is totally redundant, and weird.
-- Best Regards Masahiro Yamada
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