Re: [PATCH v2 2/2] linux/bits.h: Add compile time sanity check of GENMASK inputs

[Guenter Roeck]

On 8/7/19 7:55 AM, Masahiro Yamada wrote:
> On Wed, Aug 7, 2019 at 11:27 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> > On Fri, Aug 02, 2019 at 01:03:58AM +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_ON_ZERO() must be
> > > used instead of BUILD_BUG_ON(), and __is_constexpr() must be used instead
> > > of __builtin_constant_p().
> > >
> > > If successful, BUILD_BUG_OR_ZERO() returns 0 of type size_t. To avoid
> > > problems with implicit conversions, cast the result of BUILD_BUG_OR_ZERO
> > > to unsigned long.
> > >
> > > Since both BUILD_BUG_ON_ZERO() and __is_constexpr() only uses sizeof()
> > > on the arguments passed to them, neither of them evaluate the expression
> > > unless it is a VLA. Therefore, GENMASK(1, x++) still behaves as
> > > expected.
> > >
> > > 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.
> >
> > Who is going to fix the fallout ? For example, arm64:defconfig no longer
> > compiles with this patch applied.
> >
> > It seems to me that the benefit of catching misuses of GENMASK is much
> > less than the fallout from no longer compiling kernels, since those
> > kernels won't get any test coverage at all anymore.
>
>
> We cannot apply this until we fix all errors.
>
> I do not understand why Andrew picked up this so soon.

The same was done with the fallthrough warning in mainline, which still results
in all "sh" builds failing there (and in -next, obviously). I don't understand
the logic either, but maybe it is the new normal.

Guenter