Commit 7a35a5ca authored by Andrey Konovalov's avatar Andrey Konovalov Committed by Greg Kroah-Hartman

usb: raw-gadget: update documentation and Kconfig

Update Raw Gadget documentation and Kconfig. Make the description more
precise and clear, fix typos and grammar mistakes, and do other cleanups.
Signed-off-by: default avatarAndrey Konovalov <andreyknvl@google.com>
Link: https://lore.kernel.org/r/f4c650c94ae2b910e38819d51109cd5f0b251a2a.1611429174.git.andreyknvl@google.comSigned-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 4c1934bd
......@@ -2,83 +2,93 @@
USB Raw Gadget
==============
USB Raw Gadget is a kernel module that provides a userspace interface for
the USB Gadget subsystem. Essentially it allows to emulate USB devices
from userspace. Enabled with CONFIG_USB_RAW_GADGET. Raw Gadget is
currently a strictly debugging feature and shouldn't be used in
production, use GadgetFS instead.
USB Raw Gadget is a gadget driver that gives userspace low-level control over
the gadget's communication process.
Like any other gadget driver, Raw Gadget implements USB devices via the
USB gadget API. Unlike most gadget drivers, Raw Gadget does not implement
any concrete USB functions itself but requires userspace to do that.
Raw Gadget is currently a strictly debugging feature and should not be used
in production. Use GadgetFS instead.
Enabled with CONFIG_USB_RAW_GADGET.
Comparison to GadgetFS
~~~~~~~~~~~~~~~~~~~~~~
Raw Gadget is similar to GadgetFS, but provides a more low-level and
direct access to the USB Gadget layer for the userspace. The key
differences are:
Raw Gadget is similar to GadgetFS but provides more direct access to the
USB gadget layer for userspace. The key differences are:
1. Every USB request is passed to the userspace to get a response, while
1. Raw Gadget passes every USB request to userspace to get a response, while
GadgetFS responds to some USB requests internally based on the provided
descriptors. However note, that the UDC driver might respond to some
requests on its own and never forward them to the Gadget layer.
descriptors. Note that the UDC driver might respond to some requests on
its own and never forward them to the gadget layer.
2. GadgetFS performs some sanity checks on the provided USB descriptors,
while Raw Gadget allows you to provide arbitrary data as responses to
USB requests.
2. Raw Gadget allows providing arbitrary data as responses to USB requests,
while GadgetFS performs sanity checks on the provided USB descriptors.
This makes Raw Gadget suitable for fuzzing by providing malformed data as
responses to USB requests.
3. Raw Gadget provides a way to select a UDC device/driver to bind to,
while GadgetFS currently binds to the first available UDC.
while GadgetFS currently binds to the first available UDC. This allows
having multiple Raw Gadget instances bound to different UDCs.
4. Raw Gadget explicitly exposes information about endpoints addresses and
capabilities allowing a user to write UDC-agnostic gadgets.
capabilities. This allows the user to write UDC-agnostic gadgets.
5. Raw Gadget has ioctl-based interface instead of a filesystem-based one.
5. Raw Gadget has an ioctl-based interface instead of a filesystem-based
one.
Userspace interface
~~~~~~~~~~~~~~~~~~~
To create a Raw Gadget instance open /dev/raw-gadget. Multiple raw-gadget
instances (bound to different UDCs) can be used at the same time. The
interaction with the opened file happens through the ioctl() calls, see
comments in include/uapi/linux/usb/raw_gadget.h for details.
The user can interact with Raw Gadget by opening ``/dev/raw-gadget`` and
issuing ioctl calls; see the comments in include/uapi/linux/usb/raw_gadget.h
for details. Multiple Raw Gadget instances (bound to different UDCs) can be
used at the same time.
The typical usage of Raw Gadget looks like:
A typical usage scenario of Raw Gadget:
1. Open Raw Gadget instance via /dev/raw-gadget.
2. Initialize the instance via USB_RAW_IOCTL_INIT.
3. Launch the instance with USB_RAW_IOCTL_RUN.
4. In a loop issue USB_RAW_IOCTL_EVENT_FETCH calls to receive events from
Raw Gadget and react to those depending on what kind of USB device
needs to be emulated.
1. Create a Raw Gadget instance by opening ``/dev/raw-gadget``.
2. Initialize the instance via ``USB_RAW_IOCTL_INIT``.
3. Launch the instance with ``USB_RAW_IOCTL_RUN``.
4. In a loop issue ``USB_RAW_IOCTL_EVENT_FETCH`` to receive events from
Raw Gadget and react to those depending on what kind of USB gadget must
be implemented.
Note, that some UDC drivers have fixed addresses assigned to endpoints, and
therefore arbitrary endpoint addresses can't be used in the descriptors.
Nevertheles, Raw Gadget provides a UDC-agnostic way to write USB gadgets.
Once a USB_RAW_EVENT_CONNECT event is received via USB_RAW_IOCTL_EVENT_FETCH,
the USB_RAW_IOCTL_EPS_INFO ioctl can be used to find out information about
endpoints that the UDC driver has. Based on that information, the user must
chose UDC endpoints that will be used for the gadget being emulated, and
properly assign addresses in endpoint descriptors.
Note that some UDC drivers have fixed addresses assigned to endpoints, and
therefore arbitrary endpoint addresses cannot be used in the descriptors.
Nevertheless, Raw Gadget provides a UDC-agnostic way to write USB gadgets.
Once ``USB_RAW_EVENT_CONNECT`` is received via ``USB_RAW_IOCTL_EVENT_FETCH``,
``USB_RAW_IOCTL_EPS_INFO`` can be used to find out information about the
endpoints that the UDC driver has. Based on that, userspace must choose UDC
endpoints for the gadget and assign addresses in the endpoint descriptors
correspondingly.
You can find usage examples (along with a test suite) here:
Raw Gadget usage examples and a test suite:
https://github.com/xairy/raw-gadget
Internal details
~~~~~~~~~~~~~~~~
Currently every endpoint read/write ioctl submits a USB request and waits until
its completion. This is the desired mode for coverage-guided fuzzing (as we'd
like all USB request processing happen during the lifetime of a syscall),
and must be kept in the implementation. (This might be slow for real world
applications, thus the O_NONBLOCK improvement suggestion below.)
Every Raw Gadget endpoint read/write ioctl submits a USB request and waits
until its completion. This is done deliberately to assist with coverage-guided
fuzzing by having a single syscall fully process a single USB request. This
feature must be kept in the implementation.
Potential future improvements
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- Report more events (suspend, resume, etc.) through USB_RAW_IOCTL_EVENT_FETCH.
- Report more events (suspend, resume, etc.) through
``USB_RAW_IOCTL_EVENT_FETCH``.
- Support O_NONBLOCK I/O.
- Support ``O_NONBLOCK`` I/O. This would be another mode of operation, where
Raw Gadget would not wait until the completion of each USB request.
- Support USB 3 features (accept SS endpoint companion descriptor when
enabling endpoints; allow providing stream_id for bulk transfers).
enabling endpoints; allow providing ``stream_id`` for bulk transfers).
- Support ISO transfer features (expose frame_number for completed requests).
- Support ISO transfer features (expose ``frame_number`` for completed
requests).
......@@ -515,10 +515,15 @@ config USB_G_WEBCAM
config USB_RAW_GADGET
tristate "USB Raw Gadget"
help
USB Raw Gadget is a kernel module that provides a userspace interface
for the USB Gadget subsystem. Essentially it allows to emulate USB
devices from userspace. See Documentation/usb/raw-gadget.rst for
details.
USB Raw Gadget is a gadget driver that gives userspace low-level
control over the gadget's communication process.
Like any other gadget driver, Raw Gadget implements USB devices via
the USB gadget API. Unlike most gadget drivers, Raw Gadget does not
implement any concrete USB functions itself but requires userspace
to do that.
See Documentation/usb/raw-gadget.rst for details.
Say "y" to link the driver statically, or "m" to build a
dynamically linked module called "raw_gadget".
......
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