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

[Joe Perches]

On Fri, 2019-08-02 at 10:40 +0900, Masahiro Yamada wrote:
> 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?

Consistency with the uses in what's now called __GENMASK