linux-kernel - Re: [PATCH][next] hwmon: corsair-psu: fix unintentional sign extension issue

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Date:   Thu, 5 Nov 2020 13:32:33 +0100
From:   Wilken Gottwalt <wilken.gottwalt@...teo.net>
To:     Colin King <colin.king@...onical.com>
Cc:     Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>,
        linux-hwmon@...r.kernel.org, kernel-janitors@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] hwmon: corsair-psu: fix unintentional sign
 extension issue

On Thu,  5 Nov 2020 11:50:19 +0000
Colin King <colin.king@...onical.com> wrote:

> From: Colin Ian King <colin.king@...onical.com>
> 
> The shifting of the u8 integer data[3] by 24 bits to the left will
> be promoted to a 32 bit signed int and then sign-extended to a
> long. In the event that the top bit of data[3] is set then all
> then all the upper 32 bits of a 64 bit long end up as also being
> set because of the sign-extension. Fix this by casting data[3] to
> a long before the shift.
> 
> Addresses-Coverity: ("Unintended sign extension")
> Fixes: ce15cd2cee8b ("hwmon: add Corsair PSU HID controller driver")
> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> ---
>  drivers/hwmon/corsair-psu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/hwmon/corsair-psu.c b/drivers/hwmon/corsair-psu.c
> index e92d0376e7ac..5d19a888231a 100644
> --- a/drivers/hwmon/corsair-psu.c
> +++ b/drivers/hwmon/corsair-psu.c
> @@ -241,7 +241,7 @@ static int corsairpsu_get_value(struct corsairpsu_data *priv, u8 cmd, u8
> rail, l
>  	 * the LINEAR11 conversion are the watts values which are about 1200 for the strongest
> psu
>  	 * supported (HX1200i)
>  	 */
> -	tmp = (data[3] << 24) + (data[2] << 16) + (data[1] << 8) + data[0];
> +	tmp = ((long)data[3] << 24) + (data[2] << 16) + (data[1] << 8) + data[0];
>  	switch (cmd) {
>  	case PSU_CMD_IN_VOLTS:
>  	case PSU_CMD_IN_AMPS:

Yeah, this could happen if the uptime value in the micro-controller gets bigger
than 68 years (in seconds), and it is the only value which actually uses more
than 2 bytes for the representation. So what about architectures which are 32 bit
wide and where a long has 32 bits? I guess this simple cast is not enough.

greetings,
Wilken