linux-kernel - Re: [PATCH v2] fbmon: prevent division by zero in fb_videomode_from

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Date: Fri, 15 Mar 2024 09:44:08 +0100
From: Helge Deller <deller@....de>
To: Roman Smirnov <r.smirnov@....ru>, Daniel Vetter <daniel@...ll.ch>
Cc: Thomas Zimmermann <tzimmermann@...e.de>,
 Sergey Shtylyov <s.shtylyov@....ru>, Karina Yankevich <k.yankevich@....ru>,
 linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, lvc-project@...uxtesting.org
Subject: Re: [PATCH v2] fbmon: prevent division by zero in
 fb_videomode_from_videomode()

On 3/5/24 14:51, Roman Smirnov wrote:
> The expression htotal * vtotal can have a zero value on
> overflow.

I'm not sure if thos always results in zero in kernel on overflow.
Might be architecture-depended too, but let's assume it
can become zero, ....

> It is necessary to prevent division by zero like in
> fb_var_to_videomode().
>
> Found by Linux Verification Center (linuxtesting.org) with Svace.
>
> Signed-off-by: Roman Smirnov <r.smirnov@....ru>
> Reviewed-by: Sergey Shtylyov <s.shtylyov@....ru>
> ---
>   V1 -> V2: Replaced the code of the first version with a check.
>
>   drivers/video/fbdev/core/fbmon.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/video/fbdev/core/fbmon.c b/drivers/video/fbdev/core/fbmon.c
> index 79e5bfbdd34c..b137590386da 100644
> --- a/drivers/video/fbdev/core/fbmon.c
> +++ b/drivers/video/fbdev/core/fbmon.c
> @@ -1344,7 +1344,7 @@ int fb_videomode_from_videomode(const struct videomode *vm,
>   	vtotal = vm->vactive + vm->vfront_porch + vm->vback_porch +
>   		 vm->vsync_len;
>   	/* prevent division by zero */
> -	if (htotal && vtotal) {
> +	if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal)) {

why don't you then simply check for
	if .. ((htotal * vtotal) == 0) ...
instead?

Helge

>   		fbmode->refresh = vm->pixelclock / (htotal * vtotal);
>   	/* a mode must have htotal and vtotal != 0 or it is invalid */
>   	} else {