linux-kernel - Re: [PATCH] drm/bridge: nwl-dsi: Avoid potential multiplication overflow on 32-bit

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Date:   Tue, 12 Jan 2021 06:37:45 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Geert Uytterhoeven <geert+renesas@...der.be>
Cc:     Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/bridge: nwl-dsi: Avoid potential multiplication
 overflow on 32-bit

Hi Geert,

Thank you for the patch.

On Mon, Jan 11, 2021 at 01:57:02PM +0100, Geert Uytterhoeven wrote:
> As nwl_dsi.lanes is u32, and NSEC_PER_SEC is 1000000000L, the second
> multiplication in
> 
>     dsi->lanes * 8 * NSEC_PER_SEC
> 
> will overflow on a 32-bit platform.  Fix this by making the constant
> unsigned long long, forcing 64-bit arithmetic.
> 
> While iMX8 is arm64, this driver is currently used on 64-bit platforms
> only, where long is 64-bit, so this cannot happen.  But the issue may
> start to happen when the driver is reused for a 32-bit SoC, or when code
> is copied for a new driver.
> 
> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
> ---
> Compile-tested only.
> ---
>  drivers/gpu/drm/bridge/nwl-dsi.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/bridge/nwl-dsi.c b/drivers/gpu/drm/bridge/nwl-dsi.c
> index 66b67402f1acd57d..a8da3081efdcc84e 100644
> --- a/drivers/gpu/drm/bridge/nwl-dsi.c
> +++ b/drivers/gpu/drm/bridge/nwl-dsi.c
> @@ -195,7 +195,7 @@ static u32 ps2bc(struct nwl_dsi *dsi, unsigned long long ps)
>  	u32 bpp = mipi_dsi_pixel_format_to_bpp(dsi->format);
>  
>  	return DIV64_U64_ROUND_UP(ps * dsi->mode.clock * bpp,
> -				  dsi->lanes * 8 * NSEC_PER_SEC);
> +				  dsi->lanes * 8ULL * NSEC_PER_SEC);

I wonder if we could get rid of a whole class of bugs by turning
NSEC_PER_SEC into a ULL, but I suppose there are valid cases where a
32-bit integer is enough.

Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>

How did you come across this by the way ?

>  }
>  
>  /*

-- 
Regards,

Laurent Pinchart