Re: [PATCH] USB: cdc-wdm: Call wake_up_all() when clearing WDM_IN_USE bit.

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

On 2020/05/30 5:41, Andrey Konovalov wrote:
> On Thu, May 28, 2020 at 10:58 PM Alan Stern <stern@rowland.harvard.edu> wrote:
>>
>> On Thu, May 28, 2020 at 09:51:35PM +0200, Andrey Konovalov wrote:
>>> On Thu, May 28, 2020 at 9:40 PM Alan Stern <stern@rowland.harvard.edu> wrote:
>>>>
>>>> On Thu, May 28, 2020 at 09:03:43PM +0200, Andrey Konovalov wrote:
>>>>
>>>>> Ah, so the problem is that when a process exits, it tries to close wdm
>>>>> fd first, which ends up calling wdm_flush(), which can't finish
>>>>> because the USB requests are not terminated before raw-gadget fd is
>>>>> closed, which is supposed to happen after wdm fd is closed. Is this
>>>>> correct? I wonder what will happen if a real device stays connected
>>>>> and ignores wdm requests.
>>>>>
>>>>> I don't understand though, how using wait_event_interruptible() will
>>>>> shadow anything here.
>>>>>
>>>>> Alan, Greg, is this acceptable behavior for a USB driver?
>>>>
>>>> I don't understand what the problem is.  Can you explain in more general
>>>> terms -- nothing specific to wdm or anything like that -- what you are
>>>> concerned about?  Is this something that could happen to any gadget
>>>> driver?  Or any USB class device driver?  Or does it only affect
>>>> usespace components of raw-gadget drivers?
>>>
>>> So, AFAIU, we have a driver whose flush() callback blocks on
>>> wait_event(), which can only terminate when either 1) the driver
>>> receives a particular USB response from the device or 2) the device
>>> disconnects.
>>
>> This sounds like a bug in the driver.  What would it do if someone had a
>> genuine (not emulated) but buggy USB device which didn't send the
>> desired response?  The only way to unblock the driver would be to unplug
>> the device!  That isn't acceptable behavior.
> 
> OK, that's what I thought.

I believe that this is not a bug in the driver but a problem of hardware
failure. Unless this is high-availability code which is designed for safely
failing over to other node, we don't need to care about hardware failure.

> 
>>
>>> For 1) the emulated device doesn't provide required
>>> responses. For 2) the problem is that the emulated via raw-gadget
>>> device disconnects when the process is killed (and raw-gadget fd is
>>> closed). But that process is the same process that is currently stuck
>>> on wait_event() in the flush callback(), and therefore unkillable.
>>
>> What would happen if you unload dummy-hcd at this point?  Or even just
>> do: echo 0 >/sys/bus/usb/devices/usbN/bConfigurationValue, where N is
>> the bus number of the dummy-hcd bus?
> 
> The device disconnects and flush() unblocks.
> 
>>> This can generally happen with any driver that goes into
>>> uninterruptible sleep within one of its code paths reachable from
>>> userspace that can only be unblocked by a particular behavior from the
>>> USB device. But I haven't seen any such drivers so far, wdm is the
>>> first.
>>
>> Drivers should never go into uninterruptible sleep states unless they
>> can guarantee that the duration will be bounded somehow (for example, by
>> a reasonable timeout).  Or that cutting the sleep state short would
>> cause the system to crash -- but that's not an issue here.
> 
> OK, thank you, Alan!
> 
> Tetsuo, could you clarify why you think that using
> wait_event_interruptible() is a bad fix here?
> 

If wait_event() in wdm_flush() were using timeout or interruptible, can the
wdm driver handle that case safely? Since cleanup(desc) from wdm_release()
might release "desc", wouldn't "not-waiting-for-completion" has a risk of
"use-after-free write"? Please add comment block why it is safe if it is
actually safe.