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

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Date:   Thu, 1 Aug 2019 11:50:36 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Rikard Falkeborn <rikard.falkeborn@...il.com>
Cc:     Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Berg <johannes@...solutions.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] linux/bits.h: Add compile time sanity check of GENMASK inputs

On Thu, Aug 1, 2019 at 4:04 AM Rikard Falkeborn
<rikard.falkeborn@...il.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 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@...il.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...
>
>  include/linux/bits.h | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/bits.h b/include/linux/bits.h
> index 669d69441a62..73489579eef9 100644
> --- 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
> +#endif
> +
>  #define GENMASK(h, l) \
> +       (GENMASK_INPUT_CHECK(h, l) + \
>         (((~UL(0)) - (UL(1) << (l)) + 1) & \
> -        (~UL(0) >> (BITS_PER_LONG - 1 - (h))))
> +        (~UL(0) >> (BITS_PER_LONG - 1 - (h)))))
>
>  #define GENMASK_ULL(h, l) \
> +       (GENMASK_INPUT_CHECK(h, l) + \
>         (((~ULL(0)) - (ULL(1) << (l)) + 1) & \
> -        (~ULL(0) >> (BITS_PER_LONG_LONG - 1 - (h))))
> +        (~ULL(0) >> (BITS_PER_LONG_LONG - 1 - (h)))))


This is getting cluttered with so many parentheses.

One way of clean-up is to rename the current macros as follows:

   GENMASK()    ->  __GENMASK()
   GENMASK_UL() ->  __GENMASK_ULL()

Then,

#define GENMASK(h, l)       (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
#define GENMASK_ULL(h, l)   (GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(h, l))




-- 
Best Regards
Masahiro Yamada