Re: A potential race

[Pavel Andrianov <andrianov@ispras.ru>]

01.07.2016 19:53, Hans Verkuil пишет:
> On 07/01/2016 04:39 PM, Pavel Andrianov wrote:
>>   Hi!
>>
>> There is a potential race condition between usbvision_v4l2_close and usbvision_disconnect. The possible scenario may be the following. usbvision_disconnect starts execution, assigns usbvision->remove_pending = 1, and is interrupted
>> (rescheduled) after mutex_unlock. After that usbvision_v4l2_close is executed, decrease usbvision->user-- , checks usbvision->remove_pending, executes usbvision_release and finishes. Then usbvision_disconnect continues its execution. It checks
>> usbversion->user (it is already 0) and also execute usbvision_release. Thus, release is executed twice. The same situation may
>> occur if usbvision_v4l2_close is interrupted by usbvision_disconnect. Moreover, the same problem is in usbvision_radio_close. In all these cases the check before call usbvision_release under mutex_lock protection does not solve the problem, because  there may occur an open() after the check and the race takes place again. The question is: why the usbvision_release
>> is called from close() (usbvision_v4l2_close and usbvision_radio_close)? Usually release functions are called from
>> disconnect.
> Please don't use html mail, mailinglists will silently reject this.
>
> The usbvision driver is old and unloved and known to be very bad code. It needs a huge amount of work to make all this work correctly.
>
> I don't see anyone picking this up...
>
> Regards,
>
> 	Hans
If you know the driver, could you, please, explain me, why 
usbvision_release is called from close functions (usbvision_v4l2_close 
and usbvision_radio_close) and not only from disconnect? Thanks!

-- 
Pavel Andrianov
Linux Verification Center, ISPRAS
web: http://linuxtesting.org
e-mail: andrianov@ispras.ru