Messages in this thread Patch in this message | | | From | Rikard Falkeborn <> | Subject | [PATCH] linux/bits.h: Add compile time sanity check of GENMASK inputs | Date | Wed, 31 Jul 2019 21:03:09 +0200 |
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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))))) #endif /* __LINUX_BITS_H */ -- 2.22.0
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