Hi Rikard, On Sun, Aug 11, 2019 at 8:52 PM Rikard Falkeborn <rikard.falkeb...@gmail.com> 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 > 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(). > > __builtin_constant_p does not evaluate is argument, it only checks if it > is a constant or not at compile time, and __builtin_choose_expr does not > evaluate the expression that is not chosen. Therefore, GENMASK(x++, 0) > does only evaluate x++ once. > > Commit 95b980d62d52 ("linux/bits.h: make BIT(), GENMASK(), and friends > available in assembly") made the macros in linux/bits.h available in > assembly. Since BUILD_BUG_OR_ZERO() is not asm compatible, disable the > checks if the file is included in an asm file. > > Signed-off-by: Rikard Falkeborn <rikard.falkeb...@gmail.com> > --- > Changes in v3: > - Changed back to shorter macro argument names > - Remove casts and use 0 instead of UL(0) in GENMASK_INPUT_CHECK(), > since all results in GENMASK_INPUT_CHECK() are now ints. Update > commit message to reflect that. > > 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, 5 of > the patches are not in Linus tree, and 2 are not in linux-next. There is > also a patch pending by Nathan Chancellor that also needs to be merged > before this patch is merged to avoid build failures. > > include/linux/bits.h | 21 +++++++++++++++++++-- > 1 file changed, 19 insertions(+), 2 deletions(-) > > diff --git a/include/linux/bits.h b/include/linux/bits.h > index 669d69441a62..4ba0fb609239 100644 > --- a/include/linux/bits.h > +++ b/include/linux/bits.h > @@ -18,12 +18,29 @@ > * position @h. For example > * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000. > */ > -#define GENMASK(h, l) \ > +#ifndef __ASSEMBLY__ > +#include <linux/build_bug.h> > +#define GENMASK_INPUT_CHECK(h, l) \ > + (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \ > + __builtin_constant_p((l) > (h)), (l) > (h), 0))) > +#else > +/* > + * BUILD_BUG_ON_ZERO is not available in h files included from asm files, > + * disable the input check if that is the case. > + */ > +#define GENMASK_INPUT_CHECK(h, l) 0 > +#endif > + > +#define __GENMASK(h, l) \ > (((~UL(0)) - (UL(1) << (l)) + 1) & \ > (~UL(0) >> (BITS_PER_LONG - 1 - (h)))) > +#define GENMASK(h, l) \ > + (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l)) > > -#define GENMASK_ULL(h, l) \ > +#define __GENMASK_ULL(h, l) \ > (((~ULL(0)) - (ULL(1) << (l)) + 1) & \ > (~ULL(0) >> (BITS_PER_LONG_LONG - 1 - (h)))) > +#define GENMASK_ULL(h, l) \ > + (GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(h, l)) > > #endif /* __LINUX_BITS_H */
This is now commit 0fd35cd30a2fece1 ("linux/bits.h: add compile time sanity check of GENMASK inputs") in next-20191008. <nore...@ellerman.id.au> reported the following failure in sun3_defconfig, which I managed to reproduce with gcc-4.6.3: lib/842/842_compress.c: In function '__split_add_bits': lib/842/842_compress.c:164:25: error: first argument to '__builtin_choose_expr' not a constant lib/842/842_compress.c:164:25: error: bit-field '<anonymous>' width not an integer constant scripts/Makefile.build:265: recipe for target 'lib/842/842_compress.o' failed __split_add_bits() calls GENMASK_ULL() with a non-constant. However __split_add_bits() itself is called with constants only. Apparently gcc fails to inline __split_add_bits(). Adding inline or always_inline doesn't help. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds