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

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Message-ID: <b5d52c4c-534f-4897-ab96-351f09bee89a@gmx.de>
Date: Mon, 18 Mar 2024 20:15:55 +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" <linux-fbdev@...r.kernel.org>,
 "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "lvc-project@...uxtesting.org" <lvc-project@...uxtesting.org>
Subject: Re: [PATCH v2] fbmon: prevent division by zero in
 fb_videomode_from_videomode()

On 3/18/24 09:11, Roman Smirnov wrote:
> On Fri, 15 Mar 2024 09:44:08 +0100 Helge Deller wrote:
>> 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 those 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
>
> Thomas Zimmermann from the previous discussion said:
>
> On Tue, 5 Mar 2024 11:18:05 +0100 Thomas Zimmerman wrote:
>> 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.
>
> This prevents overflow, which is also a problematic case.

I don't like adding another division here and I doubt we have
a problem with possible overflow.
So, I suggest to keep it simple, something like:
	...
	total = htotal * vtotal;
	if (total)
		fbmode->refresh = vm->pixelclock / total;
	else...
Helge