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

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Message-ID: <CABVgOSnB2_WVtedhE+EPMM9v+ePOp2KMFze84oxGS9Pgf31HxA@mail.gmail.com>
Date:   Tue, 25 May 2021 08:42:08 +0800
From:   David Gow <davidgow@...gle.com>
To:     Daniel Latypov <dlatypov@...gle.com>
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        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 Tue, May 25, 2021 at 7:38 AM Daniel Latypov <dlatypov@...gle.com> wrote:
>
> 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.
>

This came from a thread[1], and one of the causes behind it was not
wanting to have KUNIT_ALL_TESTS enable things like filesystems and
drivers which wouldn't otherwise be built.

Personally, I think that RATIONAL is probably an okay thing to select
here: it's not as heavyweight as drivers/filesystems/etc, and our
general guidance here is "avoid select where sensible to do so", not
"don't use it under any circumstances".

The other option would be to have a separate config entry which just
selected RATIONAL, but even I think that's probably uglier, however
nice it is for guaranteeing flexibility.

[1]: https://lore.kernel.org/linux-ext4/52959e99-4105-3de9-730c-c46894b82bdd@infradead.org/T/#t

> >
> > > 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
> >