linux-kernel - RE: [PATCH v11 11/12] clk: pwm: Assign u64 divisor to unsigned int before use

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Date:   Fri, 20 Mar 2020 18:42:39 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arnd Bergmann' <arnd@...db.de>,
        Guru Das Srinagesh <gurus@...eaurora.org>
CC:     Linux PWM List <linux-pwm@...r.kernel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Subbaraman Narayanamurthy <subbaram@...eaurora.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        linux-clk <linux-clk@...r.kernel.org>
Subject: RE: [PATCH v11 11/12] clk: pwm: Assign u64 divisor to unsigned int
 before use

From: Arnd Bergmann
> Sent: 20 March 2020 17:01
> On Fri, Mar 20, 2020 at 2:42 AM Guru Das Srinagesh <gurus@...eaurora.org> wrote:
> >
> > Since the PWM framework is switching struct pwm_args.period's datatype
> > to u64, prepare for this transition by assigning the 64-bit divisor to
> > an unsigned int variable to use as the divisor. This is being done
> > because the divisor is a 32-bit constant and the quotient will be zero
> > if the divisor exceeds 2^32.
> >
> > Cc: Michael Turquette <mturquette@...libre.com>
> > Cc: Stephen Boyd <sboyd@...nel.org>
> > Cc: linux-clk@...r.kernel.org
> > Cc: David Laight <David.Laight@...LAB.COM>
> >
> > Reported-by: kbuild test robot <lkp@...el.com>
> > Signed-off-by: Guru Das Srinagesh <gurus@...eaurora.org>
> > ---
> >  drivers/clk/clk-pwm.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/clk/clk-pwm.c b/drivers/clk/clk-pwm.c
> > index 87fe0b0e..c0b5da3 100644
> > --- a/drivers/clk/clk-pwm.c
> > +++ b/drivers/clk/clk-pwm.c
> > @@ -72,6 +72,7 @@ static int clk_pwm_probe(struct platform_device *pdev)
> >         struct pwm_device *pwm;
> >         struct pwm_args pargs;
> >         const char *clk_name;
> > +       unsigned int period;
> >         int ret;
> >
> >         clk_pwm = devm_kzalloc(&pdev->dev, sizeof(*clk_pwm), GFP_KERNEL);
> > @@ -88,8 +89,9 @@ static int clk_pwm_probe(struct platform_device *pdev)
> >                 return -EINVAL;
> >         }
> >
> > +       period = pargs.period;
> >         if (of_property_read_u32(node, "clock-frequency", &clk_pwm->fixed_rate))
> > -               clk_pwm->fixed_rate = NSEC_PER_SEC / pargs.period;
> > +               clk_pwm->fixed_rate = NSEC_PER_SEC / period;
> >
> >         if (pargs.period != NSEC_PER_SEC / clk_pwm->fixed_rate &&
> >             pargs.period != DIV_ROUND_UP(NSEC_PER_SEC, clk_pwm->fixed_rate)) {
> 
> Doesn't this one need a check for "pargs.period>UINT_MAX" or
> "pargs.period > NSEC_PER_SEC"?
> 
> It looks like truncating the 64-bit value to a 32-bit type can result in
> unexpected behavior.

I also suspect the last two lines ought to use the 32bit copy.
And there is a chance that the division will explode.

This whole series is failing to differentiate between the type of
the variables and the domain on the valid values.

	David

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