From: Andy Shevchenko <andy.shevchenko@gmail.com>
To: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Cc: Emil Velikov <emil.l.velikov@gmail.com>,
Syed Nayyar Waris <syednwaris@gmail.com>,
Masahiro Yamada <yamada.masahiro@socionext.com>,
Kees Cook <keescook@chromium.org>,
Linus Walleij <linus.walleij@linaro.org>,
Andrew Morton <akpm@linux-foundation.org>,
William Breathitt Gray <vilhelm.gray@gmail.com>,
Arnd Bergmann <arnd@arndb.de>,
Linux-Arch <linux-arch@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v7 1/4] bitops: Introduce the the for_each_set_clump macro
Date: Mon, 1 Jun 2020 11:33:30 +0300 [thread overview]
Message-ID: <20200601083330.GB1634618@smile.fi.intel.com> (raw)
In-Reply-To: <20200531223716.GA20752@rikard>
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@gmail.com> wrote:
> > > On Sat, May 30, 2020 at 2:50 PM Andy Shevchenko
> > > <andy.shevchenko@gmail.com> wrote:
> > > > On Sat, May 30, 2020 at 11:45 AM Syed Nayyar Waris <syednwaris@gmail.com> wrote:
> > > > > On Sat, May 30, 2020 at 3:49 AM Andy Shevchenko
> > > > > <andy.shevchenko@gmail.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
next prev parent reply other threads:[~2020-06-01 8:33 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-24 5:00 [PATCH v7 0/4] Introduce the for_each_set_clump macro Syed Nayyar Waris
2020-05-24 5:01 ` [PATCH v7 1/4] bitops: Introduce the " Syed Nayyar Waris
2020-05-24 14:44 ` kbuild test robot
2020-05-29 18:08 ` Syed Nayyar Waris
2020-05-29 18:38 ` Andy Shevchenko
2020-05-29 20:02 ` Syed Nayyar Waris
2020-05-29 21:31 ` William Breathitt Gray
2020-05-29 21:53 ` Syed Nayyar Waris
2020-05-29 21:42 ` Andy Shevchenko
2020-05-29 22:07 ` Andy Shevchenko
2020-05-29 22:11 ` Syed Nayyar Waris
2020-05-29 22:19 ` Andy Shevchenko
2020-05-30 8:45 ` Syed Nayyar Waris
2020-05-30 9:20 ` Andy Shevchenko
2020-05-31 1:11 ` Syed Nayyar Waris
2020-05-31 11:00 ` Andy Shevchenko
2020-05-31 22:37 ` Rikard Falkeborn
2020-06-01 0:31 ` Syed Nayyar Waris
2020-06-01 8:33 ` Andy Shevchenko [this message]
2020-06-02 19:01 ` Rikard Falkeborn
2020-06-03 8:49 ` Andy Shevchenko
2020-06-03 21:53 ` Rikard Falkeborn
2020-06-03 21:58 ` Andy Shevchenko
2020-06-03 21:59 ` Andy Shevchenko
2020-06-03 22:02 ` [PATCH] linux/bits.h: fix unsigned less than zero warnings Rikard Falkeborn
2020-06-04 6:41 ` Andy Shevchenko
2020-06-04 16:49 ` Joe Perches
2020-06-04 23:30 ` Rikard Falkeborn
2020-06-07 20:34 ` [PATCH v2 1/2] " Rikard Falkeborn
2020-06-07 20:34 ` [PATCH v2 2/2] bits: Add tests of GENMASK Rikard Falkeborn
2020-06-08 7:33 ` Geert Uytterhoeven
2020-06-08 18:42 ` Rikard Falkeborn
2020-06-08 22:18 ` [PATCH v3 1/2] linux/bits.h: fix unsigned less than zero warnings Rikard Falkeborn
2020-06-08 22:18 ` [PATCH v3 2/2] bits: Add tests of GENMASK Rikard Falkeborn
2020-06-09 14:11 ` Andy Shevchenko
2020-06-21 4:36 ` Andrew Morton
2020-06-21 5:42 ` [PATCH v4 1/2] linux/bits.h: fix unsigned less than zero warnings Rikard Falkeborn
2020-06-21 5:42 ` [PATCH v4 2/2] bits: Add tests of GENMASK Rikard Falkeborn
2021-04-22 19:40 ` Nico Pache
2021-04-22 21:30 ` Nico Pache
2020-07-09 12:30 ` [PATCH v4 1/2] linux/bits.h: fix unsigned less than zero warnings Herbert Xu
2020-07-09 18:13 ` Linus Torvalds
2020-07-10 6:38 ` Herbert Xu
2020-06-15 19:52 ` [PATCH v3 " Emil Velikov
2020-06-08 8:08 ` [PATCH v2 2/2] bits: Add tests of GENMASK Andy Shevchenko
2020-06-08 8:08 ` Andy Shevchenko
2020-06-08 18:44 ` Rikard Falkeborn
2020-06-08 11:09 ` [PATCH v2 1/2] linux/bits.h: fix unsigned less than zero warnings Andy Shevchenko
2020-05-24 5:04 ` [PATCH v7 2/4] lib/test_bitmap.c: Add for_each_set_clump test cases Syed Nayyar Waris
2020-05-30 19:20 ` kbuild test robot
2020-06-04 20:42 ` Syed Nayyar Waris
2020-06-05 12:24 ` Andy Shevchenko
2020-06-06 23:15 ` Syed Nayyar Waris
2020-06-08 13:18 ` Andy Shevchenko
2020-06-10 5:38 ` Rong Chen
2020-05-24 5:05 ` [PATCH v7 3/4] gpio: thunderx: Utilize for_each_set_clump macro Syed Nayyar Waris
2020-05-24 5:06 ` [PATCH v7 4/4] gpio: xilinx: " Syed Nayyar Waris
2020-05-25 9:36 ` [PATCH v7 0/4] Introduce the " Bartosz Golaszewski
2020-06-15 12:46 ` Syed Nayyar Waris
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