linux-kernel - Re: [PATCH v13 2/2] leds: Add driver for Qualcomm LPG

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Date:   Wed, 2 Mar 2022 16:03:15 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Pavel Machek <pavel@....cz>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, Lee Jones <lee.jones@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Jonathan Corbet <corbet@....net>, linux-leds@...r.kernel.org,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        linux-pwm <linux-pwm@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Luca Weiss <luca@...tu.xyz>
Subject: Re: [PATCH v13 2/2] leds: Add driver for Qualcomm LPG

Hi,

On Fri, Feb 18, 2022 at 10:29 AM Bjorn Andersson
<bjorn.andersson@...aro.org> wrote:
>
> +static void lpg_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
> +                             struct pwm_state *state)
> +{
> +       struct lpg *lpg = container_of(chip, struct lpg, pwm);
> +       struct lpg_channel *chan = &lpg->channels[pwm->hwpwm];
> +       unsigned int pre_div;
> +       unsigned int refclk;
> +       unsigned int val;
> +       unsigned int m;
> +       u16 pwm_value;
> +       int ret;
> +
> +       ret = regmap_read(lpg->map, chan->base + LPG_SIZE_CLK_REG, &val);
> +       if (ret)
> +               return;
> +
> +       refclk = lpg_clk_rates[(val & PWM_CLK_SELECT_MASK) - 1];

I don't know why I didn't notice it before (maybe I was accidentally
not building with KASAN?), but in my recent boots I'm getting a KASAN
error pointing at the line above.

Sure enough, the above looks a bit on the unsafe side. If (val & 0x3)
is 0 then the "-1" will not be so wonderful. I put some printouts and,
indeed, it's not so great.

[    7.201635] DOUG: val is 0x00000004

Amazingly my `refclk` ends up as 0 and I guess somehow this doesn't
cause a divide by 0.


-Doug