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

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Date:   Mon, 1 Jun 2020 11:33:30 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Rikard Falkeborn <rikard.falkeborn@...il.com>
Cc:     Emil Velikov <emil.l.velikov@...il.com>,
        Syed Nayyar Waris <syednwaris@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Kees Cook <keescook@...omium.org>,
        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 Mon, Jun 01, 2020 at 12:37:16AM +0200, Rikard Falkeborn wrote:
> + Emil who was working on a patch for this
> 
> On Sun, May 31, 2020 at 02:00:45PM +0300, Andy Shevchenko wrote:
> > 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:
> > 
> > ...
> > 
> Sorry about that, it seems it's only triggered by gcc-9, that's why I
> missed it.

I guess every compiler (more or less recent) will warn here.
(Sorry, there is a cut in the thread, the problem is with comparison unsigned
 type(s) to 0).

> > > #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.
> 
> I don't understand how you mean this? You can't use l before you have
> defined GENMASK_INPUT_CHECK to take l as input? Am I missing something?
> 
> How about the following (with an added comment about why the casts are
> necessary):
> 
> diff --git a/include/linux/bits.h b/include/linux/bits.h
> index 4671fbf28842..5fdb9909fbff 100644
> --- a/include/linux/bits.h
> +++ b/include/linux/bits.h
> @@ -23,7 +23,7 @@
>  #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((int)(l) > (int)(h)), (int)(l) > (int)(h), 0)))
>  #else
>  /*
>   * BUILD_BUG_ON_ZERO is not available in h files included from asm files,
> 
> I can send a proper patch if this is ok.
> > 
> > Unfortunately, it's not enough. We need to take care about the following cases
> 
> The __GENMASK macro is only valid for values of h and l between 0 and 63
> (or 31, if unsigned long is 32 bits). Negative values or values >=
> sizeof(unsigned long) (or unsigned long long for GENMASK_ULL) result in
> compiler warnings (-Wshift-count-negative or -Wshift-count-overflow). So
> when I wrote the GENMASK_INPUT_CHECK macro, the intention was to catch
> cases where l and h were swapped and let the existing compiler warnings
> catch negative or too large values.

GENAMSK sometimes is used with non-constant arguments that's why your check
made a regression.

What I described below are the cases to consider w/o what should we do. What
you answered is the same what I implied. So, we are on the same page here.

> > 1) h or l negative;
> 
> Any of these cases will trigger a compiler warning (h negative triggers 
> Wshift-count-overflow, l negative triggers Wshift-count-negative).
> 
> > 2) h == 0, if l == 0, I dunno what is this. it's basically either 0 or warning;
> 
> h == l == 0 is a complicated way of saying 1 (or BIT(0)). l negative
> triggers compiler warning.

Oh, yes GENMASK(h, l), when h==l==0 should be equivalent to BIT(0) with no
warning given.

> > 3) l == 0;
> 
> if h is negative, compiler warning (see 1). If h == 0, see 2. If h is
> positive, there is no error in GENMASK_INPUT_CHECK.
> 
> > 4) h and l > 0.
> 
> The comparisson works as intended.

> > 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).
> 
> If we cast to int, we don't need to worry about the signedness. If
> someone enters a value that can't be cast to int, there will still
> be a compiler warning about shift out of range.

If the argument unsigned long long will it be the warning (it should not)?

> > > 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.

Please, when you do something there, add a test case to test_bitops.c.

-- 
With Best Regards,
Andy Shevchenko