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SubjectRe: [PATCH v3 3/3] linux/bits.h: Add compile time sanity check of GENMASK inputs
Hi Geert,

On Tue, Oct 8, 2019 at 4:23 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> Hi Rikard,
>
> On Sun, Aug 11, 2019 at 8:52 PM Rikard Falkeborn
> <rikard.falkeborn@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.falkeborn@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.
>
> <noreply@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 -- geert@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

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