linux-kernel - Re: [PATCH] divide by 3*sizeof(u32) when computing array

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Message-ID: <20210713063053.qqttzxlopvpnadj3@pengutronix.de>
Date:   Tue, 13 Jul 2021 08:30:53 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Salah Triki <salah.triki@...il.com>
Cc:     fabrice.gasnier@...s.st.com, thierry.reding@...il.com,
        lee.jones@...aro.org, mcoquelin.stm32@...il.com,
        alexandre.torgue@...s.st.com, linux-pwm@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] divide by 3*sizeof(u32) when computing array_size

Hello Salah,

On Tue, Jul 13, 2021 at 12:19:10AM +0100, Salah Triki wrote:
> Divide by 3*sizeof(u32) when computing array_size, since stm32_breakinput
> has 3 fields of type u32.
> 
> Signed-off-by: Salah Triki <salah.triki@...il.com>
> ---
>  drivers/pwm/pwm-stm32.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pwm/pwm-stm32.c b/drivers/pwm/pwm-stm32.c
> index 794ca5b02968..fb21bc2b2dd6 100644
> --- a/drivers/pwm/pwm-stm32.c
> +++ b/drivers/pwm/pwm-stm32.c
> @@ -544,7 +544,7 @@ static int stm32_pwm_probe_breakinputs(struct stm32_pwm *priv,
>  		return -EINVAL;
>  
>  	priv->num_breakinputs = nb;
> -	array_size = nb * sizeof(struct stm32_breakinput) / sizeof(u32);
> +	array_size = nb * sizeof(struct stm32_breakinput) / (3 * sizeof(u32));
>  	ret = of_property_read_u32_array(np, "st,breakinput",
>  					 (u32 *)priv->breakinputs, array_size);
>  	if (ret)

I agree with Philipp here; this looks strange and needs a better
description.

Looking a bit more in details:

 - priv->breakinputs has type struct stm32_breakinput[MAX_BREAKINPUT]
 - nb is in [0 .. MAX_BREAKINPUT]
 - sizeof(struct stm32_breakinput) == 3 * sizeof(u32)
 - of_property_read_u32_array reads $array_size u32 quantities

so to read $nb members of type stm32_breakinput array_size must be a
multiple of 3 which isn't given any more after your patch. This makes me
believe your suggested change to be wrong.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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