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

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Message-ID: <CAHp75VcNVOF6jHZ7gtpqskg9rDgwt3MmtGZJJOXE-GwvXRPOhw@mail.gmail.com>
Date:   Thu, 4 Jun 2020 09:41:29 +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.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>,
        Syed Nayyar Waris <syednwaris@...il.com>,
        William Breathitt Gray <vilhelm.gray@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        kbuild test robot <lkp@...el.com>
Subject: Re: [PATCH] linux/bits.h: fix unsigned less than zero warnings

On Thu, Jun 4, 2020 at 1:03 AM 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, cast the inputs to int before doing the
> comparisons. The only valid inputs to GENMASK() and GENMASK_ULL() are
> are 0 to 31 or 63. Anything outside this is undefined due to the shifts
> in GENMASK()/GENMASK_ULL(). Therefore, casting the inputs to int do not
> change the values for valid known inputs. For unknown values, the check
> does not change anything since it's a compile-time check only.
>
> 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);
> |                         ^~~~~~~
>

Thank you for the patch!

I think there is still a possibility to improve (as I mentioned there
are test cases that are absent right now).
What if we will have unsigned long value 0x100000001? Would it be 1
after casting?

Maybe cast to (long) or (long long) more appropriate?

Please, add test cases.

> 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>
> ---
>  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..293d1ee71a48 100644
> --- a/include/linux/bits.h
> +++ b/include/linux/bits.h
> @@ -21,9 +21,10 @@
>  #if !defined(__ASSEMBLY__) && \
>         (!defined(CONFIG_CC_IS_GCC) || CONFIG_GCC_VERSION >= 49000)
>  #include <linux/build_bug.h>
> +/* Avoid Wtype-limits warnings by casting the inputs to int */
>  #define GENMASK_INPUT_CHECK(h, l) \
>         (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> -               __builtin_constant_p((l) > (h)), (l) > (h), 0)))
> +               __builtin_constant_p((int)(l) > (int)(h)), (int)(l) > (int)(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