linux-kernel - Re: [PATCH] iio: frequency: adf4371: Fix divide by zero exception bug

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Message-ID: <20200111110848.7c45a4f3@archlinux>
Date:   Sat, 11 Jan 2020 11:08:48 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Beniamin Bia <beniamin.bia@...log.com>
Cc:     <lars@...afoo.de>, <Michael.Hennerich@...log.com>,
        <pmeerw@...erw.net>, <linux-iio@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <biabeniamin@...look.com>,
        <knaack.h@....de>
Subject: Re: [PATCH] iio: frequency: adf4371: Fix divide by zero exception
 bug

On Tue, 7 Jan 2020 15:15:59 +0200
Beniamin Bia <beniamin.bia@...log.com> wrote:

> From: Michael Hennerich <michael.hennerich@...log.com>
> 
> During initialization adf4371_pll_fract_n_get_rate() is called on all
> output channels to determine if the device was setup. In this case
> mod2 is zero which can cause a divide by zero exception.
> Return before that can happen.
I'm confused by this description vs the code.

As far as I can see fract_n_get_rate is only called on a sysfs read of
the frequency. 

mod2 is set when fract_n_compute is called in the relevant set_freq calls.
This seems to occur on a sysfs set frequency call.

So the issue here is that a sysfs read before a write of the frequency
will cause a div zero?  If so, is there a sane set of initial values we
can put in mod2 and friends before exposing them via the device register?

If mod2==0 is a valid value and indicates for example that the channel
is turned off, then the description should make that clear.

Jonathan

> 
> Fixes: 7f699bd149134 ("iio: frequency: adf4371: Add support for ADF4371 PLL")
> Signed-off-by: Michael Hennerich <michael.hennerich@...log.com>
> Signed-off-by: Beniamin Bia <beniamin.bia@...log.com>
> ---
>  drivers/iio/frequency/adf4371.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/iio/frequency/adf4371.c b/drivers/iio/frequency/adf4371.c
> index e2a599b912e5..c21462238314 100644
> --- a/drivers/iio/frequency/adf4371.c
> +++ b/drivers/iio/frequency/adf4371.c
> @@ -191,6 +191,9 @@ static unsigned long long adf4371_pll_fract_n_get_rate(struct adf4371_state *st,
>  	unsigned long long val, tmp;
>  	unsigned int ref_div_sel;
>  
> +	if (st->mod2 == 0)
> +		return 0;
> +
>  	val = (((u64)st->integer * ADF4371_MODULUS1) + st->fract1) * st->fpfd;
>  	tmp = (u64)st->fract2 * st->fpfd;
>  	do_div(tmp, st->mod2);