linux-kernel - Re: [PATCH] lib/math/rational.c: Fix divide by zero

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Date:   Mon, 24 May 2021 16:38:13 -0700
From:   Daniel Latypov <dlatypov@...gle.com>
To:     Randy Dunlap <rdunlap@...radead.org>,
        David Gow <davidgow@...gle.com>,
        Brendan Higgins <brendanhiggins@...gle.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...el.com>,
        Trent Piepho <tpiepho@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        andy@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
        Oskar Schirmer <oskar@...ra.com>, Yiyuan Guo <yguoaz@...il.com>
Subject: Re: [PATCH] lib/math/rational.c: Fix divide by zero

On Mon, May 24, 2021 at 4:30 PM Randy Dunlap <rdunlap@...radead.org> wrote:
>
> On 5/24/21 3:56 PM, Daniel Latypov wrote:
> > On Mon, May 24, 2021 at 3:04 PM Randy Dunlap <rdunlap@...radead.org> wrote:
> >>
> >> On 5/24/21 9:55 AM, Daniel Latypov wrote:
> >>> diff --git a/lib/math/Kconfig b/lib/math/Kconfig
> >>> index f19bc9734fa7..20460b567493 100644
> >>> --- a/lib/math/Kconfig
> >>> +++ b/lib/math/Kconfig
> >>> @@ -15,3 +15,14 @@ config PRIME_NUMBERS
> >>>
> >>>  config RATIONAL
> >>>         bool
> >>> +
> >>> +config RATIONAL_KUNIT_TEST
> >>> +       tristate "KUnit test for rational number support" if !KUNIT_ALL_TESTS
> >>> +       # depends on KUNIT && RATIONAL  # this is how it should work, but
> >>> +       depends on KUNIT
> >>> +       select RATIONAL # I don't grok kconfig enough to know why this
> >>
> >> Only to set the symbol CONFIG_RATIONAL.
> >> Then when 'make' descends into the lib/math/ subdir and looks at its Makefile,
> >> it will decide to build the binary rational.o.
> >>
> >> obj-$(CONFIG_RATIONAL)          += rational.o
> >>
> >
> > Ack, I understand that much.
>
> Oh! Clearly I misunderstood the problem.
>
> I had to look thru 60 config files before I found one where CONFIG_RATIONAL
> was not set.
>
> And I'm still not sure, but I believe that it's because it has to be set
> by some other Kconfig entry doing a 'select' on it.
>
> Here are the kconfigs that select it (on i386, where I found it not set):
>
> - COMMON_CLK [=n] && !HAVE_LEGACY_CLK [=n]
> - SERIAL_8250_LPSS [=n] && TTY [=n] && HAS_IOMEM [=y] && SERIAL_8250 [=n] && PCI [=n] && (X86 [=y] || COMPILE_TEST [=y])
> - SERIAL_8250_MID [=n] && TTY [=n] && HAS_IOMEM [=y] && SERIAL_8250 [=n] && PCI [=n] && (X86 [=y] || COMPILE_TEST [=y])
> - SERIAL_IMX [=n] && TTY [=n] && HAS_IOMEM [=y] && (ARCH_MXC || COMPILE_TEST [=y])
> - VIDEO_V4L2 [=n] && MEDIA_SUPPORT [=n] && (I2C [=y] || I2C [=y]=n) && VIDEO_DEV [=n]
> - SND_SOC_ROCKCHIP_PDM [=n] && SOUND [=n] && !UML && SND [=n] && SND_SOC [=n] && CLKDEV_LOOKUP [=n] && SND_SOC_ROCKCHIP [=n]
> - COMMON_CLK_QCOM [=n] && COMMON_CLK [=n] && OF [=y] && (ARCH_QCOM || COMPILE_TEST [=y])
>
> but my test config has none of those enabled, so I cannot set RATIONAL.
>
> I guess the easiest solution is to have KUNIT or some sub-KUNIT test
> just select RATIONAL.

Yeah, the easiest thing would be to keep the `select RATIONAL` that I
showed in the example patch.

+David Gow +Brendan Higgins as they both particularly wanted to avoid
having any tests `select` their dependencies, however.

>
> > My confusion is why this doesn't work:
> >
> > $ ./tools/testing/kunit/kunit.py run --kunitconfig /dev/stdin <<EOF
> > CONFIG_KUNIT=y
> > CONFIG_RATIONAL=y
> > EOF
> > ...
> > ERROR:root:Provided Kconfig is not contained in validated .config.
> > Following fields found in kunitconfig, but not in .config:
> > CONFIG_RATIONAL=y
> >
> > What it's complaining about is that `make  ARCH=um olddefconfig` is
> > leaving CONFIG_RATIONAL=y unset.
> >
> > Stripping out kunit.py, it's this:
> >
> > $ echo -e 'CONFIG_KUNIT=y\nCONFIG_RATIONAL=y' > .kunit/.config
> > $ make ARCH=um olddefconfig O=.kunit
> > $ grep RATIONAL .kunit/.config
> >
> > I'm not versed in Kconfig enough to know why CONFIG_RATIONAL=y is
> > getting removed.
> >
> >>
> >>> is necessary
> >>> +       default KUNIT_ALL_TESTS
> >>> +       help
> >>> +               This builds unit tests for the rational number support.
> >>> +
> >>> +               If unsure, say N.
>
>
> --
> ~Randy
>