Touchpad Forensics

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In this post we collect some notes on our experience debugging a faulty touchpad on Hyeyoung's Lenovo X1 (deepersea), which we suspect is a hardware issue. Here are the steps we took to prove this conjecture before we ship the machine to Lenovo for repair.

  1. Make sure a virtual terminal (vt) is available. Using a vt will suspend the X window system and thus prevent it from interfering with our analysis. As of Ubuntu 17.04 (maybe earlier), vts are not available by default. To make a vt available, one must edit the file /etc/systemd/logind.conf, uncomment the line NAutoVTs=6, and restart the computer.

  2. Determine the input device handle assigned to the touchpad using the command cat /proc/bus/input/devices. There will be a lot of output, but we just need to find the following block:

    I: Bus=0011 Vendor=0002 Product=0007 Version=01b1
N: Name="SynPS/2 Synaptics TouchPad"
P: Phys=isa0060/serio1/input0
S: Sysfs=/devices/platform/i8042/serio1/input/input5
U: Uniq=
H: Handlers=mouse1 event5 
B: PROP=5
B: EV=b
B: KEY=e520 10000 0 0 0 0
B: ABS=660800011000003

    From the H field we determine that the device handle is event5.

  3. Install the evtest diagnostic tool: sudo apt install evtest.

  4. Switch to a virtual terminal using (e.g., Ctrl+Alt+F2, giving a mostly blank screen with a single login: prompt) and login using your username and password.

  5. Run the evtest tool by entering the following command:

    sudo evtest /dev/input/event5 > ~/evtest.log

    This will start touchpad event logging, so immediately start a very brief test of just a single feature of the touchpad and then (quickly) stop event logging with Ctrl+C.

  6. Inspect the resulting log file.

    Below is are excerpts from event log files collected from a normally functioning touchpad and a malfunctioning one.

    Working Touchpad

    Input driver version is 1.0.1
Input device ID: bus 0x11 vendor 0x2 product 0x7 version 0x1b1
Input device name: "SynPS/2 Synaptics TouchPad"
Supported events:
  Event type 0 (EV_SYN)
  Event type 1 (EV_KEY)
    Event code 272 (BTN_LEFT)
    Event code 325 (BTN_TOOL_FINGER)
    Event code 328 (BTN_TOOL_QUINTTAP)
    Event code 330 (BTN_TOUCH)
    Event code 333 (BTN_TOOL_DOUBLETAP)
    Event code 334 (BTN_TOOL_TRIPLETAP)
    Event code 335 (BTN_TOOL_QUADTAP)
  Event type 3 (EV_ABS)
    Event code 0 (ABS_X)
      Value   5550
      Min     1264
      Max     5678
      Resolution      46
    Event code 1 (ABS_Y)
      Value   1754
      Min     1132
      Max     4720
      Resolution      60
    Event code 24 (ABS_PRESSURE)
      Value      0
      Min        0
      Max      255
    Event code 28 (ABS_TOOL_WIDTH)
      Value      0
      Min        0
      Max       15
    Event code 47 (ABS_MT_SLOT)
      Value      0
      Min        0
      Max        1
    Event code 53 (ABS_MT_POSITION_X)
      Value      0
      Min     1264
      Max     5678
      Resolution      46
    Event code 54 (ABS_MT_POSITION_Y)
      Value      0
      Min     1132
      Max     4720
      Resolution      60
    Event code 57 (ABS_MT_TRACKING_ID)
      Value      0
      Min        0
      Max    65535
    Event code 58 (ABS_MT_PRESSURE)
      Value      0
      Min        0
      Max      255
Properties:
  Property type 0 (INPUT_PROP_POINTER)
  Property type 2 (INPUT_PROP_BUTTONPAD)
Testing ... (interrupt to exit)
Event: time 1636729221.197923, type 3 (EV_ABS), code 57 (ABS_MT_TRACKING_ID), value 125
Event: time 1636729221.197923, type 3 (EV_ABS), code 53 (ABS_MT_POSITION_X), value 1597
Event: time 1636729221.197923, type 3 (EV_ABS), code 54 (ABS_MT_POSITION_Y), value 4215
Event: time 1636729221.197923, type 3 (EV_ABS), code 58 (ABS_MT_PRESSURE), value 22
Event: time 1636729221.197923, type 1 (EV_KEY), code 330 (BTN_TOUCH), value 1
Event: time 1636729221.197923, type 3 (EV_ABS), code 0 (ABS_X), value 1597
Event: time 1636729221.197923, type 3 (EV_ABS), code 1 (ABS_Y), value 4215
Event: time 1636729221.197923, type 3 (EV_ABS), code 24 (ABS_PRESSURE), value 22
Event: time 1636729221.197923, type 1 (EV_KEY), code 325 (BTN_TOOL_FINGER), value 1
Event: time 1636729221.197923, -------------- SYN_REPORT ------------
Event: time 1636729221.222600, type 3 (EV_ABS), code 53 (ABS_MT_POSITION_X), value 1584
Event: time 1636729221.222600, type 3 (EV_ABS), code 54 (ABS_MT_POSITION_Y), value 4265
Event: time 1636729221.222600, type 3 (EV_ABS), code 58 (ABS_MT_PRESSURE), value 60
Event: time 1636729221.222600, type 3 (EV_ABS), code 0 (ABS_X), value 1584
Event: time 1636729221.222600, type 3 (EV_ABS), code 1 (ABS_Y), value 4265
Event: time 1636729221.222600, type 3 (EV_ABS), code 24 (ABS_PRESSURE), value 60
Event: time 1636729221.222600, -------------- SYN_REPORT ------------
Event: time 1636729221.234486, type 3 (EV_ABS), code 54 (ABS_MT_POSITION_Y), value 4269
Event: time 1636729221.234486, type 3 (EV_ABS), code 58 (ABS_MT_PRESSURE), value 64
Event: time 1636729221.234486, type 3 (EV_ABS), code 1 (ABS_Y), value 4269
Event: time 1636729221.234486, type 3 (EV_ABS), code 24 (ABS_PRESSURE), value 64
Event: time 1636729221.234486, -------------- SYN_REPORT ------------
Event: time 1636729221.246183, type 3 (EV_ABS), code 53 (ABS_MT_POSITION_X), value 1585
Event: time 1636729221.246183, type 3 (EV_ABS), code 54 (ABS_MT_POSITION_Y), value 4271
Event: time 1636729221.246183, type 3 (EV_ABS), code 58 (ABS_MT_PRESSURE), value 67
Event: time 1636729221.246183, type 3 (EV_ABS), code 0 (ABS_X), value 1585
Event: time 1636729221.246183, type 3 (EV_ABS), code 1 (ABS_Y), value 4271
Event: time 1636729221.246183, type 3 (EV_ABS), code 24 (ABS_PRESSURE), value 67
Event: time 1636729221.246183, -------------- SYN_REPORT ------------
Event: time 1636729221.258146, type 3 (EV_ABS), code 54 (ABS_MT_POSITION_Y), value 4275
Event: time 1636729221.258146, type 3 (EV_ABS), code 58 (ABS_MT_PRESSURE), value 69
Event: time 1636729221.258146, type 3 (EV_ABS), code 1 (ABS_Y), value 4275
Event: time 1636729221.258146, type 3 (EV_ABS), code 24 (ABS_PRESSURE), value 69

    Broken Touchpad