linux-kernel - Re: [PATCH v3] fbmon: prevent division by zero in fb_videomode_from

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Date: Tue, 19 Mar 2024 13:22:57 +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 v3] fbmon: prevent division by zero in
 fb_videomode_from_videomode()

On 3/19/24 09:13, Roman Smirnov wrote:
> The expression htotal * vtotal can have a zero value on
> overflow. 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.
>   V2 -> V3: Replaced the code of the second version with a zero 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..ca946919d962 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 && (htotal * vtotal)) {
>   		fbmode->refresh = vm->pixelclock / (htotal * vtotal);

I modified your patch like this:
..
-       if (htotal && vtotal) {
-               fbmode->refresh = vm->pixelclock / (htotal * vtotal);
+       total = htotal * vtotal;
+       if (total) {
+               fbmode->refresh = vm->pixelclock / total;

and added it to the fbdev git tree.
https://git.kernel.org/pub/scm/linux/kernel/git/deller/linux-fbdev.git/commit/?h=for-next

Thanks,
Helge