Hi Geert, On Tue, Oct 8, 2019 at 4:23 PM Geert Uytterhoeven <ge...@linux-m68k.org> wrote: > > 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:
Oh dear. I was able to reproduce this for gcc 4.7 or 4.8, but I did not see any problem for gcc 4.9+ Perhaps, is this due to broken __builtin_choose_expr or __builtin_constant_p for old compilers? > > 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 -- Best Regards Masahiro Yamada