From: Rikard Falkeborn <rikard.falkeborn@gmail.com>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Rikard Falkeborn <rikard.falkeborn@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
Arnd Bergmann <arnd@arndb.de>,
emil.l.velikov@gmail.com, Kees Cook <keescook@chromium.org>,
Linus Walleij <linus.walleij@linaro.org>,
Linux-Arch <linux-arch@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Syed Nayyar Waris <syednwaris@gmail.com>,
William Breathitt Gray <vilhelm.gray@gmail.com>,
Masahiro Yamada <yamada.masahiro@socionext.com>,
kbuild test robot <lkp@intel.com>
Subject: Re: [PATCH] linux/bits.h: fix unsigned less than zero warnings
Date: Fri, 5 Jun 2020 01:30:03 +0200 [thread overview]
Message-ID: <20200604233003.GA102768@rikard> (raw)
In-Reply-To: <CAHp75VcNVOF6jHZ7gtpqskg9rDgwt3MmtGZJJOXE-GwvXRPOhw@mail.gmail.com>
On Thu, Jun 04, 2020 at 09:41:29AM +0300, Andy Shevchenko wrote:
> On Thu, Jun 4, 2020 at 1:03 AM Rikard Falkeborn
> <rikard.falkeborn@gmail.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?
It you're entering 0x10000001 you're going to get a compiler warning
since it's going to be shifted out of range, when I wrote the check I
figured that should be enough, but perhaps I was wrong?
How about requiring both l and h bit to be constant? (that's
arguably the way I should have written in the first place). That's going
to remove the warnings for GENMASK(x, 0). It will not work as expected
when mixing negative and unsigned input, like GENMASK(-1, 0u) is not
going to fail the build while GENMASK(1u, -1) will. For GENMASK(-1, 0u),
you will get a compiler warning due to the shifts in GENMASK.
If we need to handle the negative inputs as well. I think I'd prefer to add
explicit checks for negative values over the casting. A macro for that
can probably be reused in other places as well.
>
> Please, add test cases.
Will do.
Rikard
>
> > Fixes: 295bcca84916 ("linux/bits.h: add compile time sanity check of GENMASK inputs")
> > Reported-by: kbuild test robot <lkp@intel.com>
> > Reported-by: Emil Velikov <emil.l.velikov@gmail.com>
> > Reported-by: Syed Nayyar Waris <syednwaris@gmail.com>
> > Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.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
next prev parent reply other threads:[~2020-06-04 23:30 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
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 [this message]
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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