Re: [PATCH] drm/rockchip: cdn-dp: fix sign extension on an int multiply for a u64 result

[Guenter Roeck]

On Tue, Sep 15, 2020 at 9:20 AM Colin King <colin.king@xxxxxxxxxxxxx> wrote:
>
> From: Colin Ian King <colin.king@xxxxxxxxxxxxx>
>
> The variable bit_per_pix is a u8 and is promoted in the multiplication
> to an int type and then sign extended to a u64. If the result of the
> int multiplication is greater than 0x7fffffff then the upper 32 bits will
> be set to 1 as a result of the sign extension. Avoid this by casting
> tu_size_reg to u64 to avoid sign extension and also a potential overflow.
>
> Addresses-Coverity: ("Unintended sign extension")
> Fixes: 1a0f7ed3abe2 ("drm/rockchip: cdn-dp: add cdn DP support for rk3399")
> Signed-off-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>

Reviewed-by: Guenter Roeck <groeck@xxxxxxxxxxxx>

> ---
>  drivers/gpu/drm/rockchip/cdn-dp-reg.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/rockchip/cdn-dp-reg.c b/drivers/gpu/drm/rockchip/cdn-dp-reg.c
> index 9d2163ef4d6e..33fb4d05c506 100644
> --- a/drivers/gpu/drm/rockchip/cdn-dp-reg.c
> +++ b/drivers/gpu/drm/rockchip/cdn-dp-reg.c
> @@ -658,7 +658,7 @@ int cdn_dp_config_video(struct cdn_dp_device *dp)
>          */
>         do {
>                 tu_size_reg += 2;
> -               symbol = tu_size_reg * mode->clock * bit_per_pix;
> +               symbol = (u64)tu_size_reg * mode->clock * bit_per_pix;
>                 do_div(symbol, dp->max_lanes * link_rate * 8);
>                 rem = do_div(symbol, 1000);
>                 if (tu_size_reg > 64) {
> --
> 2.27.0
>