Re: [PATCH] counter: stm32-timer-cnt: fix ceiling write max value

[William Breathitt Gray <vilhelm.gray@gmail.com>]

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On Tue, Mar 02, 2021 at 03:43:55PM +0100, Fabrice Gasnier wrote:
> The ceiling value isn't checked before writing it into registers. The user
> could write a value higher than the counter resolution (e.g. 16 or 32 bits
> indicated by max_arr). This makes most significant bits to be truncated.
> Fix it by checking the max_arr to report a range error [1] to the user.
> 
> Fixes: ad29937e206f ("counter: Add STM32 Timer quadrature encoder")
> 
> [1] https://lkml.org/lkml/2021/2/12/358
> 
> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>

Acked-by: William Breathitt Gray <vilhelm.gray@gmail.com>

Side question: if priv->ceiling is tracking the current ceiling
configuration, would it make sense to change stm32_count_ceiling_read()
to print the value of priv->ceiling instead of doing a regmap_read()
call?

> ---
>  drivers/counter/stm32-timer-cnt.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/counter/stm32-timer-cnt.c b/drivers/counter/stm32-timer-cnt.c
> index ef2a974..2cf0c05 100644
> --- a/drivers/counter/stm32-timer-cnt.c
> +++ b/drivers/counter/stm32-timer-cnt.c
> @@ -32,6 +32,7 @@ struct stm32_timer_cnt {
>  	struct regmap *regmap;
>  	struct clk *clk;
>  	u32 ceiling;
> +	u32 max_arr;
>  	bool enabled;
>  	struct stm32_timer_regs bak;
>  };
> @@ -185,6 +186,9 @@ static ssize_t stm32_count_ceiling_write(struct counter_device *counter,
>  	if (ret)
>  		return ret;
>  
> +	if (ceiling > priv->max_arr)
> +		return -ERANGE;
> +
>  	/* TIMx_ARR register shouldn't be buffered (ARPE=0) */
>  	regmap_update_bits(priv->regmap, TIM_CR1, TIM_CR1_ARPE, 0);
>  	regmap_write(priv->regmap, TIM_ARR, ceiling);
> @@ -360,6 +364,7 @@ static int stm32_timer_cnt_probe(struct platform_device *pdev)
>  	priv->regmap = ddata->regmap;
>  	priv->clk = ddata->clk;
>  	priv->ceiling = ddata->max_arr;
> +	priv->max_arr = ddata->max_arr;
>  
>  	priv->counter.name = dev_name(dev);
>  	priv->counter.parent = dev;
> -- 
> 2.7.4
> 

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