linux-kernel - Re: [PATCH v2 1/2] linux/bits.h: fix unsigned less than zero warnings

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Message-ID: <CAHp75VfY7fynQC9+ER2iPE9wEU2AwJvUesVi9mmwf-A-3NPoog@mail.gmail.com>
Date:   Mon, 8 Jun 2020 14:09:40 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Rikard Falkeborn <rikard.falkeborn@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Emil Velikov <emil.l.velikov@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kbuild test robot <lkp@...el.com>,
        Syed Nayyar Waris <syednwaris@...il.com>,
        William Breathitt Gray <vilhelm.gray@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>
Subject: Re: [PATCH v2 1/2] linux/bits.h: fix unsigned less than zero warnings

On Sun, Jun 7, 2020 at 11:34 PM Rikard Falkeborn
<rikard.falkeborn@...il.com> wrote:
>
> When calling the GENMASK and GENMASK_ULL macros with zero lower bit and
> an unsigned unknown high bit, some gcc versions warn due to the
> comparisons of the high and low bit in GENMASK_INPUT_CHECK.
>
> To silence the warnings, only perform the check if both inputs are
> known. This does not trigger any warnings, from the Wtype-limits help:
>
>         Warn if a comparison is always true or always false due to the
>         limited range of the data type, but do not warn for constant
>         expressions.
>
> As an example of the warning, kindly reported by the kbuild test robot:
>
> from drivers/mfd/atmel-smc.c:11:
> drivers/mfd/atmel-smc.c: In function 'atmel_smc_cs_encode_ncycles':
> include/linux/bits.h:26:28: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
> 26 |   __builtin_constant_p((l) > (h)), (l) > (h), 0)))
> |                            ^
> include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO'
> 16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> |                                                              ^
> include/linux/bits.h:39:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> 39 |  (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> |   ^~~~~~~~~~~~~~~~~~~
> >> drivers/mfd/atmel-smc.c:49:25: note: in expansion of macro 'GENMASK'
> 49 |  unsigned int lsbmask = GENMASK(msbpos - 1, 0);
> |                         ^~~~~~~

It's much better, than previous variant, thanks!
FWIW,
Reviewed-by: Andy Shevchenko <andy.shevchenko@...il.com>

> Fixes: 295bcca84916 ("linux/bits.h: add compile time sanity check of GENMASK inputs")
> Reported-by: kbuild test robot <lkp@...el.com>
> Reported-by: Emil Velikov <emil.l.velikov@...il.com>
> Reported-by: Syed Nayyar Waris <syednwaris@...il.com>
> Signed-off-by: Rikard Falkeborn <rikard.falkeborn@...il.com>
> ---
> v1->v2
> * Change to require both high and low bit to be constant expressions
>   instead of introducing somewhat arbitrary casts
>
>  include/linux/bits.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/bits.h b/include/linux/bits.h
> index 4671fbf28842..35ca3f5d11a0 100644
> --- a/include/linux/bits.h
> +++ b/include/linux/bits.h
> @@ -23,7 +23,8 @@
>  #include <linux/build_bug.h>
>  #define GENMASK_INPUT_CHECK(h, l) \
>         (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> -               __builtin_constant_p((l) > (h)), (l) > (h), 0)))
> +               __builtin_constant_p(l) && __builtin_constant_p(h), \
> +               (l) > (h), 0)))
>  #else
>  /*
>   * BUILD_BUG_ON_ZERO is not available in h files included from asm files,
> --
> 2.27.0
>


-- 
With Best Regards,
Andy Shevchenko