From: Thomas Zimmermann <tzimmermann@suse.de>
To: Roman Smirnov <r.smirnov@omp.ru>, Daniel Vetter <daniel@ffwll.ch>,
Helge Deller <deller@gmx.de>
Cc: Sergey Shtylyov <s.shtylyov@omp.ru>,
Karina Yankevich <k.yankevich@omp.ru>,
linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org,
linux-kernel@vger.kernel.org, lvc-project@linuxtesting.org
Subject: Re: [PATCH] fbmon: prevent division by zero in fb_videomode_from_videomode()
Date: Tue, 5 Mar 2024 11:18:05 +0100 [thread overview]
Message-ID: <a4a77ad1-1f3c-45ca-9385-436a8bfe5bc3@suse.de> (raw)
In-Reply-To: <20240305082040.7445-1-r.smirnov@omp.ru>
Hi
Am 05.03.24 um 09:20 schrieb Roman Smirnov:
> 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@omp.ru>
> Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
> ---
> drivers/video/fbdev/core/fbmon.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/video/fbdev/core/fbmon.c b/drivers/video/fbdev/core/fbmon.c
> index 79e5bfbdd34c..bd98b138da6a 100644
> --- a/drivers/video/fbdev/core/fbmon.c
> +++ b/drivers/video/fbdev/core/fbmon.c
> @@ -1311,7 +1311,7 @@ int fb_get_mode(int flags, u32 val, struct fb_var_screeninfo *var, struct fb_inf
> int fb_videomode_from_videomode(const struct videomode *vm,
> struct fb_videomode *fbmode)
> {
> - unsigned int htotal, vtotal;
> + unsigned int htotal, vtotal, hfreq;
>
> fbmode->xres = vm->hactive;
> fbmode->left_margin = vm->hback_porch;
> @@ -1345,7 +1345,8 @@ int fb_videomode_from_videomode(const struct videomode *vm,
> vm->vsync_len;
> /* prevent division by zero */
> if (htotal && vtotal) {
> - fbmode->refresh = vm->pixelclock / (htotal * vtotal);
> + hfreq = vm->pixelclock / htotal;
> + fbmode->refresh = hfreq / vtotal;
I think this can change the end result because of integer rounding on
the intermediate result. Maybe use
if (htotal && vtotal && (vm->pixelclock / htotal >= vtotal))
for the test. That rules out overflowing multiplication and sets refresh
to 0 in such cases.
Best regards
Thomas
> /* a mode must have htotal and vtotal != 0 or it is invalid */
> } else {
> fbmode->refresh = 0;
--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)
prev parent reply other threads:[~2024-03-05 10:18 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-05 8:20 [PATCH] fbmon: prevent division by zero in fb_videomode_from_videomode() Roman Smirnov
2024-03-05 10:18 ` Thomas Zimmermann [this message]
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