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

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Date:   Mon, 1 Jun 2020 00:37:16 +0200
From:   Rikard Falkeborn <rikard.falkeborn@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Emil Velikov <emil.l.velikov@...il.com>
Cc:     Syed Nayyar Waris <syednwaris@...il.com>,
        Rikard Falkeborn <rikard.falkeborn@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Kees Cook <keescook@...omium.org>,
        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

+ 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.

> > #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.

> 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.

> 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.

Rikard

> > 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