linux-kernel - Re: [PATCH v7 1/4] bitops: Introduce the the for_each_set

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Date:   Sun, 31 May 2020 14:00:45 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Syed Nayyar Waris <syednwaris@...il.com>,
        Rikard Falkeborn <rikard.falkeborn@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Kees Cook <keescook@...omium.org>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        William Breathitt Gray <vilhelm.gray@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 1/4] bitops: Introduce the the for_each_set_clump macro

On Sun, May 31, 2020 at 4:11 AM Syed Nayyar Waris <syednwaris@...il.com> wrote:
> On Sat, May 30, 2020 at 2:50 PM Andy Shevchenko
> <andy.shevchenko@...il.com> wrote:
> > On Sat, May 30, 2020 at 11:45 AM Syed Nayyar Waris <syednwaris@...il.com> wrote:
> > > On Sat, May 30, 2020 at 3:49 AM Andy Shevchenko
> > > <andy.shevchenko@...il.com> wrote:

...

> #if (l) == 0
> #define GENMASK_INPUT_CHECK(h, l)  0
> #elif
> #define GENMASK_INPUT_CHECK(h, l) \
>         (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
>                 __builtin_constant_p((l) > (h)), (l) > (h), 0)))
> #endif
>
> I have verified that this works. Basically this just avoids the sanity
> check when the 'lower' bound 'l' is zero. Let me know if it looks
> fine.

Unfortunately, it's not enough. We need to take care about the following cases
1) h or l negative;
2) h == 0, if l == 0, I dunno what is this. it's basically either 0 or warning;
3) l == 0;
4) h and l > 0.

Now, on top of that (since it's a macro) we have to keep in mind that
h and l can be signed and / or unsigned types.
And macro shall work for all 4 cases (by type signedess).

> Regarding min, max macro that you suggested I am also looking further into it.

Since this has been introduced in v5.7 and not only your code is
affected by this I think we need to ping original author either to fix
or revert.

So, I Cc'ed to the author and reviewers, because they probably know
better why that had been done in the first place and breaking existing
code.

-- 
With Best Regards,
Andy Shevchenko