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

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Date:   Tue, 15 Sep 2020 10:29:39 -0700
From:   Guenter Roeck <groeck@...gle.com>
To:     Colin King <colin.king@...onical.com>
Cc:     Sandy Huang <hjc@...k-chips.com>,
        Heiko Stübner <heiko@...ech.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Chris Zhong <zyw@...k-chips.com>,
        Guenter Roeck <groeck@...omium.org>,
        Sean Paul <seanpaul@...omium.org>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>,
        "moderated list:ARM PORT" <linux-arm-kernel@...ts.infradead.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        kernel-janitors@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/rockchip: cdn-dp: fix sign extension on an int
 multiply for a u64 result

On Tue, Sep 15, 2020 at 9:20 AM Colin King <colin.king@...onical.com> wrote:
>
> From: Colin Ian King <colin.king@...onical.com>
>
> 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@...onical.com>

Reviewed-by: Guenter Roeck <groeck@...omium.org>

> ---
>  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
>