Re: [PATCH] linux/bits.h: Add compile time sanity check of GENMASK inputs

[Masahiro Yamada]

On Thu, Aug 1, 2019 at 4:27 AM Joe Perches <joe@xxxxxxxxxxx> wrote:
>
> On Wed, 2019-07-31 at 21:03 +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_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@xxxxxxxxx>
> > ---
> > 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...
> []
> > diff --git 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
>
> A few things:
>
> o Reading the final code is a bit confusing.
>   Perhaps add a comment description saying it's not checked
>   in asm .h uses.
>
> o Maybe use:
>   #define GENMASK_INPUT_CHECK(h, l) UL(0)

Why?


> o The compiler error message when the arguments are in the
>   wrong order isn't obvious.  Is there some way to improve
>   the compiler error output, maybe by using BUILD_BUG_ON_MSG
>   or some other mechanism?
>
>


-- 
Best Regards
Masahiro Yamada