linux-kernel - Re: [PATCH 1/2] media: v4l2-core: Shifting signed 32-bit value by 31 bits error

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Message-ID: <6b4654b1-7cd5-8fea-8c08-472ade8f3ebb@xs4all.nl>
Date:   Tue, 11 Jun 2019 22:50:16 +0200
From:   Hans Verkuil <hverkuil-cisco@...all.nl>
To:     Shuah Khan <skhan@...uxfoundation.org>, mchehab@...nel.org,
        sakari.ailus@...ux.intel.com,
        niklas.soderlund+renesas@...natech.se, ezequiel@...labora.com,
        paul.kocialkowski@...tlin.com
Cc:     Randy Dunlap <rdunlap@...radead.org>, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] media: v4l2-core: Shifting signed 32-bit value by 31
 bits error

On 6/11/19 9:42 PM, Shuah Khan wrote:
> On 6/6/19 12:33 AM, Hans Verkuil wrote:
>> On 6/6/19 5:22 AM, Randy Dunlap wrote:
>>> On 6/5/19 2:53 PM, Shuah Khan wrote:
>>>> Fix the following cppcheck error:
>>>>
>>>> Checking drivers/media/v4l2-core/v4l2-ioctl.c ...
>>>> [drivers/media/v4l2-core/v4l2-ioctl.c:1370]: (error) Shifting signed 32-bit value by 31 bits is undefined behaviour
>>>>
>>>> Signed-off-by: Shuah Khan <skhan@...uxfoundation.org>
>>>> ---
>>>>   drivers/media/v4l2-core/v4l2-ioctl.c | 2 +-
>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/media/v4l2-core/v4l2-ioctl.c b/drivers/media/v4l2-core/v4l2-ioctl.c
>>>> index 6859bdac86fe..333e387bafeb 100644
>>>> --- a/drivers/media/v4l2-core/v4l2-ioctl.c
>>>> +++ b/drivers/media/v4l2-core/v4l2-ioctl.c
>>>> @@ -1364,7 +1364,7 @@ static void v4l_fill_fmtdesc(struct v4l2_fmtdesc *fmt)
>>>>   					(char)((fmt->pixelformat >> 8) & 0x7f),
>>>>   					(char)((fmt->pixelformat >> 16) & 0x7f),
>>>>   					(char)((fmt->pixelformat >> 24) & 0x7f),
>>>> -					(fmt->pixelformat & (1 << 31)) ? "-BE" : "");
>>>> +					(fmt->pixelformat & BIT(31)) ? "-BE" : "");
>>>>   			break;
>>>>   		}
>>>>   	}
>>>>
>>>
>>> If this builds, I guess #define BIT(x) got pulled in indirectly
>>> since bits.h nor bitops.h is currently #included in that source file.
>>>
> 
> It does build. You are right that I should have included bitops.h
> 
>>> Documentation/process/submit-checklist.rst rule #1 says:
>>> 1) If you use a facility then #include the file that defines/declares
>>>     that facility.  Don't depend on other header files pulling in ones
>>>     that you use.
>>>
>>> Please add #include <linux/bits or bitops.h>
>>>
>>
>> I'm not sure about this patch. '1 << 31' is used all over in the kernel,
>> including in public headers (e.g. media.h, videodev2.h).
>>
>> It seems arbitrary to change it only here, but not anywhere else.
>>
> 
> Right. We have several places in the kernel that do that.
> 
>> In this particular example for the fourcc handling I would prefer to just
>> use '1U << 31', both in v4l2-ioctl.c and videodev2.h.
>>
> 
> If you would like to take the patch, I can send v2 fixing it using
> 1U << 31 - This is simpler since it doesn't nee additional includes.

I would like to have this cleaned up in the public media APIs. Those can be
used by other compilers as well and it makes sense to me not to have
undefined behavior in those headers.

> 
>> A separate patch doing the same for MEDIA_ENT_ID_FLAG_NEXT in media.h would
>> probably be a good idea either: that way the public API at least will do
>> the right thing.
>>
> 
> I should have explained it better. I wanted to start with one or two
> places first to see if it is worth our time to fix these:
> 
> The full kernel cppcheck log for "Shifting signed 32-bit value by 31 
> bits is undefined behaviour" can be found at:
> 
> https://drive.google.com/file/d/19Xu7UqBGJ7BpzxEp92ZQYb6F8UPrk3z3/view

I don't think it makes sense to fix this for drivers. If gcc would do this
wrong, we'd have noticed it ages ago.

But I think it makes sense to fix this in public headers.

Regards,

	Hans