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List:       linux-hwmon
Subject:    Re: [PATCH 1/2] hwmon: pwm-fan: Ensure that calculation doesn't discard big period values
From:       Guenter Roeck <linux () roeck-us ! net>
Date:       2020-12-30 16:20:42
Message-ID: 20201230162042.GA102327 () roeck-us ! net
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On Tue, Dec 15, 2020 at 10:20:30AM +0100, Uwe Kleine-König wrote:
> With MAX_PWM being defined to 255 the code
> 
> 	unsigned long period;
> 	...
> 	period = ctx->pwm->args.period;
> 	state.duty_cycle = DIV_ROUND_UP(pwm * (period - 1), MAX_PWM);
> 
> calculates a too small value for duty_cycle if the configured period is
> big (either by discarding the 64 bit value ctx->pwm->args.period or by
> overflowing the multiplication). As this results in a too slow fan and
> so maybe an overheating machine better be safe than sorry and error out
> in .probe.
> 
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

Applied.

Thanks,
Guenter

> ---
>  drivers/hwmon/pwm-fan.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> 
> base-commit: 2c85ebc57b3e1817b6ce1a6b703928e113a90442
> 
> diff --git a/drivers/hwmon/pwm-fan.c b/drivers/hwmon/pwm-fan.c
> index 1f63807c0399..ec171f2b684a 100644
> --- a/drivers/hwmon/pwm-fan.c
> +++ b/drivers/hwmon/pwm-fan.c
> @@ -324,8 +324,18 @@ static int pwm_fan_probe(struct platform_device *pdev)
>  
>  	ctx->pwm_value = MAX_PWM;
>  
> -	/* Set duty cycle to maximum allowed and enable PWM output */
>  	pwm_init_state(ctx->pwm, &state);
> +	/*
> +	 * __set_pwm assumes that MAX_PWM * (period - 1) fits into an unsigned
> +	 * long. Check this here to prevent the fan running at a too low
> +	 * frequency.
> +	 */
> +	if (state.period > ULONG_MAX / MAX_PWM + 1) {
> +		dev_err(dev, "Configured period too big\n");
> +		return -EINVAL;
> +	}
> +
> +	/* Set duty cycle to maximum allowed and enable PWM output */
>  	state.duty_cycle = ctx->pwm->args.period - 1;
>  	state.enabled = true;
>  
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