Messages in this thread | | | From | Masahiro Yamada <> | Date | Wed, 7 Aug 2019 23:55:34 +0900 | Subject | Re: [PATCH v2 2/2] linux/bits.h: Add compile time sanity check of GENMASK inputs |
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On Wed, Aug 7, 2019 at 11:27 PM Guenter Roeck <linux@roeck-us.net> wrote: > > On Fri, Aug 02, 2019 at 01:03:58AM +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_ON_ZERO() must be > > used instead of BUILD_BUG_ON(), and __is_constexpr() must be used instead > > of __builtin_constant_p(). > > > > If successful, BUILD_BUG_OR_ZERO() returns 0 of type size_t. To avoid > > problems with implicit conversions, cast the result of BUILD_BUG_OR_ZERO > > to unsigned long. > > > > Since both BUILD_BUG_ON_ZERO() and __is_constexpr() only uses sizeof() > > on the arguments passed to them, neither of them evaluate the expression > > unless it is a VLA. Therefore, GENMASK(1, x++) still behaves as > > expected. > > > > 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. > > > > Who is going to fix the fallout ? For example, arm64:defconfig no longer > compiles with this patch applied. > > It seems to me that the benefit of catching misuses of GENMASK is much > less than the fallout from no longer compiling kernels, since those > kernels won't get any test coverage at all anymore.
We cannot apply this until we fix all errors.
I do not understand why Andrew picked up this so soon.
-- Best Regards Masahiro Yamada
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