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    SubjectRe: [PATCH] linux/bits.h: Add compile time sanity check of GENMASK inputs
    On Sat, Aug 3, 2019 at 3:19 AM Rikard Falkeborn
    <rikard.falkeborn@gmail.com> wrote:
    >
    > On Fri, Aug 02, 2019 at 12:25:06PM +0900, Masahiro Yamada wrote:
    > > On Fri, Aug 2, 2019 at 12:14 PM Joe Perches <joe@perches.com> wrote:
    > > >
    > > > On Fri, 2019-08-02 at 10:40 +0900, Masahiro Yamada wrote:
    > > > > On Thu, Aug 1, 2019 at 4:27 AM Joe Perches <joe@perches.com> 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@gmail.com>
    > > > > > > ---
    > > > > > > 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
    > >
    > > Inconsistent with __GENMASK_ULL.
    >
    > Would you prefer to add GENMASK_ULL_INPUT_CHECK?

    No.

    > Or replace UL(0) with
    > 0 and then probably move the cast of BUILD_BUG_OR_ZERO (to avoid
    > GENMASK be of type size_t) to GENMASK and GENMASK_ULL?

    No.



    Your original code is absolutely fine.


    C aligns the types to the wider one.


    (unsigned long) + (int) -> (unsigned long)
    (unsigned long long) + (int) -> (unsigned long long)


    Having GENMASK_INPUT_CHECK to return 'int' is OK.
    The resulted GENMASK(), GENMASK_ULL() still
    have unsigned long, unsigned long long, respectively.




    BTW, v2 is already inconsistent.
    If you wanted GENMASK_INPUT_CHECK() to return 'unsigned long',,
    you would have to cast (low) > (high) as well:

    (unsigned long)((low) > (high)), UL(0))))

    This is totally redundant, and weird.





    --
    Best Regards
    Masahiro Yamada

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