hidpp20 driver error handling in libratbag

[Haochen Tong]

Hi,

First I would like to say thanks for libratbag.

I'm trying to add support for MX Anywhere 2 but I get a crash by just
changing the device IDs in the MX Master 2 device file. The crash is
reported here[1].

While digging into the crash I am confused about the handling of return
values. In the hidpp20 driver there is a call chain like:

(1) hidpp20drv_init_feature
(2)   -> hidpp20drv_read_special_key_mouse
(3)     -> hidpp20_special_key_mouse_get_controls
(4)       -> hidpp20_special_keys_buttons_get_reporting
(5)         -> hidpp20_request_command
(6)           -> hidpp20_request_command_allow_error

In (1) it seems rc=0 means OK, and negative value shows an error.
In (2) rc is also used as num_controls when positive.
But in (6) the HID++ protocol error number, which is a positive integer
according to document[2], is returned, and it gets propagated all the
way up to (2), which then causes problem, because (2) expects rc to be a
valid num_controls when positive, not an HID++ protocol error. This
problem later causes the aforementioned crash, because
drv_data->controls is 0 while drv_data->num_controls is positive.

I'm thinking about fixing it but not really sure how to deal with both
protocol and system errors in a sane way.

In addition, in the function hidpp20_special_key_mouse_get_controls, if
get_info or get_reporting fails for any special key, all other keys are
ignored (free(c_list)). Is this resonable? On my mouse I always get an
"invalid argument" error when trying to get the reporting method of the
7th key.

Thanks in advance for any guidance.

[1] https://github.com/libratbag/libratbag/issues/558
[2] https://drive.google.com/file/d/0BxbRzx7vEV7eU3VfMnRuRXktZ3M/view

Best regards,

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