linux-kernel - Re: [PATCH 4/5] mfd: sprd-sc27xx-spi: Fix divide by zero when allocating register offset/mask

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Date:   Mon, 29 Jun 2020 23:08:06 +0800
From:   Baolin Wang <baolin.wang7@...il.com>
To:     Johan Hovold <johan@...nel.org>
Cc:     Lee Jones <lee.jones@...aro.org>, Orson Zhai <orsonzhai@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Chunyan Zhang <zhang.lyra@...il.com>
Subject: Re: [PATCH 4/5] mfd: sprd-sc27xx-spi: Fix divide by zero when
 allocating register offset/mask

On Mon, Jun 29, 2020 at 10:43 PM Johan Hovold <johan@...nel.org> wrote:
>
> On Mon, Jun 29, 2020 at 10:35:06PM +0800, Baolin Wang wrote:
> > On Mon, Jun 29, 2020 at 10:01 PM Lee Jones <lee.jones@...aro.org> wrote:
> > >
> > > On Mon, 29 Jun 2020, Johan Hovold wrote:
> > >
> > > > On Mon, Jun 29, 2020 at 01:32:14PM +0100, Lee Jones wrote:
> > > > > Since ddata->irqs[] is already zeroed when allocated by devm_kcalloc() and
> > > > > dividing 0 by anything is still 0, there is no need to re-assign
> > > > > ddata->irqs[i].* values.  Instead, it should be safe to begin at 1.
> > > > >
> > > > > This fixes the following W=1 warning:
> > > > >
> > > > >  drivers/mfd/sprd-sc27xx-spi.c:255 sprd_pmic_probe() debug: sval_binop_unsigned: divide by zero
> > > > >
> > > > > Cc: Orson Zhai <orsonzhai@...il.com>
> > > > > Cc: Baolin Wang <baolin.wang7@...il.com>
> > > > > Cc: Chunyan Zhang <zhang.lyra@...il.com>
> > > > > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> > > > > ---
> > > > >  drivers/mfd/sprd-sc27xx-spi.c | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/drivers/mfd/sprd-sc27xx-spi.c b/drivers/mfd/sprd-sc27xx-spi.c
> > > > > index c305e941e435c..694a7d429ccff 100644
> > > > > --- a/drivers/mfd/sprd-sc27xx-spi.c
> > > > > +++ b/drivers/mfd/sprd-sc27xx-spi.c
> > > > > @@ -251,7 +251,7 @@ static int sprd_pmic_probe(struct spi_device *spi)
> > > > >             return -ENOMEM;
> > > > >
> > > > >     ddata->irq_chip.irqs = ddata->irqs;
> > > > > -   for (i = 0; i < pdata->num_irqs; i++) {
> > > > > +   for (i = 1; i < pdata->num_irqs; i++) {
> > > > >             ddata->irqs[i].reg_offset = i / pdata->num_irqs;
> > > > >             ddata->irqs[i].mask = BIT(i % pdata->num_irqs);
> > > > >     }
> > > >
> > > > This doesn't look right either.
> > > >
> > > > First, the loop is never executed if num_irqs is zero.
> > >
> > > The point of the patch is that 0 entries are never processed.
>
> So what's the problem? There's no division by zero here.
>
> And what compiler are you using, Lee? Seems broken.
>
> > > > Second, the current code looks bogus too as reg_offset is always set to
> > > > zero and mask to BIT(i)...
> >
> > Now the result is correct, since all PMIC irq mask bits are in one
> > register now, which means the reg_offset is always 0 can work well.
> > But I think the logics still can be improved if our PMIC irq numbers
> > are larger than 32 in future.
>
> The code is still bogus as pointed out above. Why do you bother to
> divide by num_irqs at all?

Right, no need to divide by num_irqs, can be simplified as below. Lee,
care to resend your patch with simplifying the code? Or you want me to
send a patch?
diff --git a/drivers/mfd/sprd-sc27xx-spi.c b/drivers/mfd/sprd-sc27xx-spi.c
index 33336cde4724..2ed5f3a4e79c 100644
--- a/drivers/mfd/sprd-sc27xx-spi.c
+++ b/drivers/mfd/sprd-sc27xx-spi.c
@@ -250,10 +250,8 @@ static int sprd_pmic_probe(struct spi_device *spi)
                return -ENOMEM;

        ddata->irq_chip.irqs = ddata->irqs;
-       for (i = 0; i < pdata->num_irqs; i++) {
-               ddata->irqs[i].reg_offset = i / pdata->num_irqs;
-               ddata->irqs[i].mask = BIT(i % pdata->num_irqs);
-       }
+       for (i = 0; i < pdata->num_irqs; i++)
+               ddata->irqs[i].mask = BIT(i);

-- 
Baolin Wang