Re: HID: wacom: check for wacom->shared before following the pointer

On Thu, Mar 19, 2015 at 9:06 AM, Benjamin Tissoires
[off-list ref] wrote:

Hi Dan,

On Mar 19 2015 or thereabouts, Dan Carpenter wrote:

quoted

Hello Benjamin Tissoires,

This is a semi-automatic email about new static checker warnings.

The patch e2c7d8877e5c: "HID: wacom: check for wacom->shared before
following the pointer" from Mar 5, 2015, leads to the following
Smatch complaint:

drivers/hid/wacom_wac.c:602 wacom_intuos_inout()
       error: we previously assumed 'wacom->shared' could be null (see line 584)

drivers/hid/wacom_wac.c
   583
   584                if (wacom->shared) {

In the original code we checked "if (features->quirks & WACOM_QUIRK_MULTI_INPUT)"
which is ensures that "wacom->shared" is non-NULL.

   585                        wacom->shared->stylus_in_proximity = true;
   586
   587                        if (wacom->shared->touch_down)
   588                                return 1;
   589                }
   590
   591                /* in Range while exiting */
   592                if (((data[1] & 0xfe) == 0x20) && wacom->reporting_data) {
   593                        input_report_key(input, BTN_TOUCH, 0);
   594                        input_report_abs(input, ABS_PRESSURE, 0);
   595                        input_report_abs(input, ABS_DISTANCE, wacom->features.distance_max);
   596                        return 2;
   597                }
   598
   599                /* Exit report */
   600                if ((data[1] & 0xfe) == 0x80) {
   601                        if (features->quirks & WACOM_QUIRK_MULTI_INPUT)
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
We still check for that here.  Smatch is confused.

   602                                wacom->shared->stylus_in_proximity = false;
                                ^^^^^^^^^^^^^
This is not a bug, but change the previous change to
"if (wacom->shared)" would make the code more consistent.

Yep, I agree. That's for these cases that I preferred having a test
against wacom->shared not null rather than (features->quirks &
WACOM_QUIRK_MULTI_INPUT).

quoted

   603                        wacom->reporting_data = false;
   604

[ snip ]

  1072  static int wacom_24hdt_irq(struct wacom_wac *wacom)
  1073  {
  1074          struct input_dev *input = wacom->input;
  1075          unsigned char *data = wacom->data;
  1076          int i;
  1077          int current_num_contacts = data[61];
  1078          int contacts_to_send = 0;
  1079          int num_contacts_left = 4; /* maximum contacts per packet */
  1080          int byte_per_packet = WACOM_BYTES_PER_24HDT_PACKET;
  1081          int y_offset = 2;
  1082          static int contact_with_no_pen_down_count = 0;
  1083
  1084          if (wacom->features.type == WACOM_27QHDT) {
  1085                  current_num_contacts = data[63];
  1086                  num_contacts_left = 10;
  1087                  byte_per_packet = WACOM_BYTES_PER_QHDTHID_PACKET;
  1088                  y_offset = 0;
  1089          }
  1090
  1091          /*
  1092           * First packet resets the counter since only the first
  1093           * packet in series will have non-zero current_num_contacts.
  1094           */
  1095          if (current_num_contacts) {
  1096                  wacom->num_contacts_left = current_num_contacts;
  1097                  contact_with_no_pen_down_count = 0;
  1098          }
  1099
  1100          contacts_to_send = min(num_contacts_left, wacom->num_contacts_left);
  1101
  1102          for (i = 0; i < contacts_to_send; i++) {
  1103                  int offset = (byte_per_packet * i) + 1;
  1104                  bool touch = (data[offset] & 0x1) && !wacom->shared->stylus_in_proximity;
                                                              ^^^^^^^^^^^^^
I assume this hardware is always quirky so this won't cause a NULL
deref?

Yes. 24hdt has the quirk WACOM_QUIRK_MULTI_INPUT, so wacom->shared can
not be null. I wonder what we could put in the code to make static
checkers happy...

I am working on a solution. Patches will be submitted soon.

Thank you Dan and Benjamin.

Ping