From: Sergey Shtylyov <s.shtylyov@omp.ru>
To: Roman Smirnov <r.smirnov@omp.ru>, Helge Deller <deller@gmx.de>,
Daniel Vetter <daniel@ffwll.ch>
Cc: Thomas Zimmermann <tzimmermann@suse.de>,
Karina Yankevich <k.yankevich@omp.ru>,
"linux-fbdev@vger.kernel.org" <linux-fbdev@vger.kernel.org>,
"dri-devel@lists.freedesktop.org"
<dri-devel@lists.freedesktop.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"lvc-project@linuxtesting.org" <lvc-project@linuxtesting.org>
Subject: Re: [PATCH v2] fbmon: prevent division by zero in fb_videomode_from_videomode()
Date: Tue, 19 Mar 2024 11:22:49 +0300 [thread overview]
Message-ID: <e730b2cd-5f29-8685-7427-8a798066e111@omp.ru> (raw)
In-Reply-To: <57b3f6a6cc184c8ead51ecc50669b503@omp.ru>
On 3/19/24 11:12 AM, Roman Smirnov 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@omp.ru>
>>>>> Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.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...
>
> Okay, I'll prepare a third version with that change:
>
> if (htotal && vtotal && (htotal * vtotal))
I think the 1st 2 checks here are now redundant. Also, the inner
parens are not necessary...
> I think that will be enough.
More than enough. :-)
MBR, Sergey
prev parent reply other threads:[~2024-03-19 8:22 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-05 13:51 [PATCH v2] fbmon: prevent division by zero in fb_videomode_from_videomode() Roman Smirnov
2024-03-15 8:44 ` Helge Deller
2024-03-18 8:11 ` Roman Smirnov
2024-03-18 19:15 ` Helge Deller
2024-03-19 8:12 ` Roman Smirnov
2024-03-19 8:22 ` Sergey Shtylyov [this message]
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