Messages in this thread | | | Subject | Re: [PATCH v2 2/2] linux/bits.h: Add compile time sanity check of GENMASK inputs | From | Joe Perches <> | Date | Thu, 01 Aug 2019 16:14:12 -0700 |
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On Fri, 2019-08-02 at 01:03 +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_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. > > Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com> > --- > Changes in v2: > - Add comment about why inputs are not checked when used in asm file > - Use UL(0) instead of 0 > - Extract mask creation in a separate macro to improve readability > - Use high and low instead of h and l (part of this was extracted to a > separate patch) > - Updated commit message > > Joe Perches sent a series to fix the existing misuses of GENMASK() that > needs to be merged before this to avoid build failures. Currently, 6 of > the patches are not in Linus tree, and 2 are not in linux-next.
Thanks Rikard.
It wouldn't surprise me if this change finds more misuses as the compiler will perform substitutions on #define values where the search I did was just for decimal uses.
For instance, this new macro should build error on:
#define FOO 5 #define BAR 6
GENMASK(FOO, BAR)
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