Messages in this thread | | | Subject | Re: [PATCH v2 2/2] linux/bits.h: Add compile time sanity check of GENMASK inputs | From | Guenter Roeck <> | Date | Wed, 7 Aug 2019 09:52:33 -0700 |
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On 8/7/19 7:55 AM, Masahiro Yamada wrote: > 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. >
The same was done with the fallthrough warning in mainline, which still results in all "sh" builds failing there (and in -next, obviously). I don't understand the logic either, but maybe it is the new normal.
Guenter
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