1==============
2USB Raw Gadget
3==============
4
5USB Raw Gadget is a gadget driver that gives userspace low-level control over
6the gadget's communication process.
7
8Like any other gadget driver, Raw Gadget implements USB devices via the
9USB gadget API. Unlike most gadget drivers, Raw Gadget does not implement
10any concrete USB functions itself but requires userspace to do that.
11
12Raw Gadget is currently a strictly debugging feature and should not be used
13in production. Use GadgetFS instead.
14
15Enabled with CONFIG_USB_RAW_GADGET.
16
17Comparison to GadgetFS
18~~~~~~~~~~~~~~~~~~~~~~
19
20Raw Gadget is similar to GadgetFS but provides more direct access to the
21USB gadget layer for userspace. The key differences are:
22
231. Raw Gadget passes every USB request to userspace to get a response, while
24   GadgetFS responds to some USB requests internally based on the provided
25   descriptors. Note that the UDC driver might respond to some requests on
26   its own and never forward them to the gadget layer.
27
282. Raw Gadget allows providing arbitrary data as responses to USB requests,
29   while GadgetFS performs sanity checks on the provided USB descriptors.
30   This makes Raw Gadget suitable for fuzzing by providing malformed data as
31   responses to USB requests.
32
333. Raw Gadget provides a way to select a UDC device/driver to bind to,
34   while GadgetFS currently binds to the first available UDC. This allows
35   having multiple Raw Gadget instances bound to different UDCs.
36
374. Raw Gadget explicitly exposes information about endpoints addresses and
38   capabilities. This allows the user to write UDC-agnostic gadgets.
39
405. Raw Gadget has an ioctl-based interface instead of a filesystem-based
41   one.
42
43Userspace interface
44~~~~~~~~~~~~~~~~~~~
45
46The user can interact with Raw Gadget by opening ``/dev/raw-gadget`` and
47issuing ioctl calls; see the comments in include/uapi/linux/usb/raw_gadget.h
48for details. Multiple Raw Gadget instances (bound to different UDCs) can be
49used at the same time.
50
51A typical usage scenario of Raw Gadget:
52
531. Create a Raw Gadget instance by opening ``/dev/raw-gadget``.
542. Initialize the instance via ``USB_RAW_IOCTL_INIT``.
553. Launch the instance with ``USB_RAW_IOCTL_RUN``.
564. In a loop issue ``USB_RAW_IOCTL_EVENT_FETCH`` to receive events from
57   Raw Gadget and react to those depending on what kind of USB gadget must
58   be implemented.
59
60Note that some UDC drivers have fixed addresses assigned to endpoints, and
61therefore arbitrary endpoint addresses cannot be used in the descriptors.
62Nevertheless, Raw Gadget provides a UDC-agnostic way to write USB gadgets.
63Once ``USB_RAW_EVENT_CONNECT`` is received via ``USB_RAW_IOCTL_EVENT_FETCH``,
64``USB_RAW_IOCTL_EPS_INFO`` can be used to find out information about the
65endpoints that the UDC driver has. Based on that, userspace must choose UDC
66endpoints for the gadget and assign addresses in the endpoint descriptors
67correspondingly.
68
69Raw Gadget usage examples and a test suite:
70
71https://github.com/xairy/raw-gadget
72
73Internal details
74~~~~~~~~~~~~~~~~
75
76Every Raw Gadget endpoint read/write ioctl submits a USB request and waits
77until its completion. This is done deliberately to assist with coverage-guided
78fuzzing by having a single syscall fully process a single USB request. This
79feature must be kept in the implementation.
80
81Potential future improvements
82~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
83
84- Report more events (suspend, resume, etc.) through
85  ``USB_RAW_IOCTL_EVENT_FETCH``.
86
87- Support ``O_NONBLOCK`` I/O. This would be another mode of operation, where
88  Raw Gadget would not wait until the completion of each USB request.
89
90- Support USB 3 features (accept SS endpoint companion descriptor when
91  enabling endpoints; allow providing ``stream_id`` for bulk transfers).
92
93- Support ISO transfer features (expose ``frame_number`` for completed
94  requests).
95