Re: [PATCH] Input: Do not add SYN_REPORT in between a single packet data

[Aniroop Mathur]

On Wed, Apr 6, 2016 at 11:08 PM, Dmitry Torokhov
<dmitry.torokhov@xxxxxxxxx> wrote:
> On Wed, Apr 06, 2016 at 08:26:39PM +0530, Aniroop Mathur wrote:
>> On Sat, Apr 2, 2016 at 10:31 PM, Aniroop Mathur
>> <aniroop.mathur@xxxxxxxxx> wrote:
>> > Hello Mr. Torokhov,
>> >
>> > First of all, Thank you for your reply.
>> >
>> > On Sat, Apr 2, 2016 at 3:21 AM, Dmitry Torokhov
>> > <dmitry.torokhov@xxxxxxxxx> wrote:
>> >> On Fri, Mar 11, 2016 at 12:26:57AM +0530, Aniroop Mathur wrote:
>> >>> Hi Henrik,
>> >>>
>> >>> On Thu, Mar 10, 2016 at 7:15 PM, Henrik Rydberg <rydberg@xxxxxxxxxxx> wrote:
>> >>> > Hi Dmitry,
>> >>> >
>> >>> >>> diff --git a/drivers/input/input.c b/drivers/input/input.c
>> >>> >>> index 8806059..262ef77 100644
>> >>> >>> --- a/drivers/input/input.c
>> >>> >>> +++ b/drivers/input/input.c
>> >>> >>> @@ -401,8 +401,7 @@ static void input_handle_event(struct input_dev *dev,
>> >>> >>>                 if (dev->num_vals >= 2)
>> >>> >>>                         input_pass_values(dev, dev->vals, dev->num_vals);
>> >>> >>>                 dev->num_vals = 0;
>> >>> >>> -       } else if (dev->num_vals >= dev->max_vals - 2) {
>> >>> >>> -               dev->vals[dev->num_vals++] = input_value_sync;
>> >>> >>> +       } else if (dev->num_vals >= dev->max_vals - 1) {
>> >>> >>>                 input_pass_values(dev, dev->vals, dev->num_vals);
>> >>> >>>                 dev->num_vals = 0;
>> >>> >>>         }
>> >>> >>
>> >>> >> This makes sense to me. Henrik?
>> >>> >
>> >>> > I went through the commits that made these changes, and I cannot see any strong
>> >>> > reason to keep it. However, this code path only triggers if no SYN events are
>> >>> > seen, as in a driver that fails to emit them and consequently fills up the
>> >>> > buffer. In other words, this change would only affect a device that is already,
>> >>> > to some degree, broken.
>> >>> >
>> >>> > So, the question to Aniroop is: do you see this problem in practise, and in that
>> >>> > case, for what driver?
>> >>> >
>> >>>
>> >>> Nope. So far I have not dealt with any such driver.
>> >>> I made this change because it is breaking protocol of SYN_REPORT event code.
>> >>>
>> >>> Further from the code, I could deduce that max_vals is just an estimation of
>> >>> packet_size and it does not guarantee that packet_size is same as max_vals.
>> >>> So real packet_size can be more than max_vals value and hence we could not
>> >>> insert SYN_REPORT until packet ends really.
>> >>> Further, if we consider that there exists a driver or will exist in future
>> >>> which sets capability of x event code according to which max_value comes out to
>> >>> y and the real packet size is z i.e. driver wants to send same event codes
>> >>> again in the same packet, so input event reader would be expecting SYN_REPORT
>> >>> after z events but due to current code SYN_REPORT will get inserted
>> >>> automatically after y events, which is a wrong behaviour.
>> >>
>> >> Well, I think I agree with Aniroop that even if driver is to a degree
>> >> broken we should not be inserting random SYN_REPORT events into the
>> >> stream. I wonder if we should not add WARN_ONCE() there to highlight
>> >> potential problems with the way we estimate the number of events.
>> >>
>> >> However I think there is an issue with the patch. If we happen to pass
>> >> values just before the final SYN_REPORT sent by the driver then we reset
>> >> dev->num_vals to 0 and will essentially suppress the final SYN_REPORT
>> >> event, which is not good either.
>> >>
>> >
>> > Yes, right!
>> >
>> > I think it can be fixed by sending the rest of events but not the last event
>> > in case number of events becomes greater than max_vals. The last event will be
>> > saved to be sent in next set of events. This way immediate SYN_REPORT will not
>> > be suppressed and duplicate SYN_REPORT event will not be sent as well.
>> >
>> > Change:
>> > @@ -401,8 +401,7 @@ static void input_handle_event(struct input_dev *dev,
>> >                 if (dev->num_vals >= 2)
>> >                         input_pass_values(dev, dev->vals, dev->num_vals);
>> >                 dev->num_vals = 0;
>> > -       } else if (dev->num_vals >= dev->max_vals - 2) {
>> > -               dev->vals[dev->num_vals++] = input_value_sync;
>> > -               input_pass_values(dev, dev->vals, dev->num_vals);
>> > -               dev->num_vals = 0;
>> > +       } else if (dev->num_vals == dev->max_vals) {
>> > +                input_pass_values(dev, dev->vals, dev->num_vals - 1);
>> > +                dev->num_vals = 0;
>> > +                dev->vals[dev->num_vals++] = dev->vals[dev->max_vals - 1];
>> >         }
>> >
>> > So, does the above patch looks good now?
>> >
>
> No, consider what will happen if you need to switch slot when your queue
> is at dev->max_vals - 1. With your patch you will end up with out of
> bounds write.
>

Sorry, I know very less about MT protocol, so could not catch this.
I have worked only on normal input device drivers.
input_abs_set_val(dev, ABS_MT_SLOT, mt->slot); --> I guess I missed this? :(

I have modified condition to handle it, so does it look fine now?

diff --git a/drivers/input/input.c b/drivers/input/input.c
index 8806059..799941c 100644
--- a/drivers/input/input.c
+++ b/drivers/input/input.c
@@ -401,12 +401,11 @@ static void input_handle_event(struct input_dev *dev,
                if (dev->num_vals >= 2)
                        input_pass_values(dev, dev->vals, dev->num_vals);
                dev->num_vals = 0;
-       } else if (dev->num_vals >= dev->max_vals - 2) {
-               dev->vals[dev->num_vals++] = input_value_sync;
-               input_pass_values(dev, dev->vals, dev->num_vals);
+       } else if (dev->num_vals >= dev->max_vals - 1) {
+               input_pass_values(dev, dev->vals, dev->num_vals - 1);
                dev->num_vals = 0;
+               dev->vals[dev->num_vals++] = dev->vals[dev->num_vals - 1];
        }
-
 }

>>
>>
>> Hello Mr. Torokhov,
>>
>> Could you please update about this?
>> It would be appreciating if you could help out to conclude it quickly.  Thanks!
>
> I am not sure what the urgency is. It is more of a theoretical problem
> ans so far the proposed solutions were actually introducing more
> problems than they were solving.
>
> I am sorry, bit this particular topic is not a priority for me.
>

There is no hurry at all. :-) As you know request is made a long time ago,
so I am only very curious to complete it.

>>
>>
>> > And may be about WARN_ONCE, do you mean to add something like below in above
>> > code?
>> > WARN_ONCE(1, "Packet did not complete yet but generally expected to be
>> > completed before generation of %d events.\n", dev->max_vals);
>> >
>> >
>> > Thanks,
>> > Aniroop Mathur
>> >
>> >> Thanks.
>> >>
>> >> --
>> >> Dmitry
>
> Thanks.
>
> --
> Dmitry