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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: Andrey Konovalov <andreyknvl@google.com> Link: https://lore.kernel.org/r/f4c650c94ae2b910e38819d51109cd5f0b251a2a.1611429174.git.andreyknvl@google.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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==============
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USB Raw Gadget
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==============
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USB Raw Gadget is a gadget driver that gives userspace low-level control over
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the gadget's communication process.
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Like any other gadget driver, Raw Gadget implements USB devices via the
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USB gadget API. Unlike most gadget drivers, Raw Gadget does not implement
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any concrete USB functions itself but requires userspace to do that.
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Raw Gadget is currently a strictly debugging feature and should not be used
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in production. Use GadgetFS instead.
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Enabled with CONFIG_USB_RAW_GADGET.
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Comparison to GadgetFS
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~~~~~~~~~~~~~~~~~~~~~~
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Raw Gadget is similar to GadgetFS but provides more direct access to the
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USB gadget layer for userspace. The key differences are:
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1. Raw Gadget passes every USB request to userspace to get a response, while
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GadgetFS responds to some USB requests internally based on the provided
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descriptors. Note that the UDC driver might respond to some requests on
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its own and never forward them to the gadget layer.
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2. Raw Gadget allows providing arbitrary data as responses to USB requests,
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while GadgetFS performs sanity checks on the provided USB descriptors.
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This makes Raw Gadget suitable for fuzzing by providing malformed data as
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responses to USB requests.
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3. Raw Gadget provides a way to select a UDC device/driver to bind to,
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while GadgetFS currently binds to the first available UDC. This allows
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having multiple Raw Gadget instances bound to different UDCs.
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4. Raw Gadget explicitly exposes information about endpoints addresses and
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capabilities. This allows the user to write UDC-agnostic gadgets.
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5. Raw Gadget has an ioctl-based interface instead of a filesystem-based
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one.
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Userspace interface
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~~~~~~~~~~~~~~~~~~~
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The user can interact with Raw Gadget by opening ``/dev/raw-gadget`` and
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issuing ioctl calls; see the comments in include/uapi/linux/usb/raw_gadget.h
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for details. Multiple Raw Gadget instances (bound to different UDCs) can be
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used at the same time.
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A typical usage scenario of Raw Gadget:
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1. Create a Raw Gadget instance by opening ``/dev/raw-gadget``.
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2. Initialize the instance via ``USB_RAW_IOCTL_INIT``.
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3. Launch the instance with ``USB_RAW_IOCTL_RUN``.
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4. In a loop issue ``USB_RAW_IOCTL_EVENT_FETCH`` to receive events from
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Raw Gadget and react to those depending on what kind of USB gadget must
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be implemented.
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Note that some UDC drivers have fixed addresses assigned to endpoints, and
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therefore arbitrary endpoint addresses cannot be used in the descriptors.
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Nevertheless, Raw Gadget provides a UDC-agnostic way to write USB gadgets.
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Once ``USB_RAW_EVENT_CONNECT`` is received via ``USB_RAW_IOCTL_EVENT_FETCH``,
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``USB_RAW_IOCTL_EPS_INFO`` can be used to find out information about the
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endpoints that the UDC driver has. Based on that, userspace must choose UDC
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endpoints for the gadget and assign addresses in the endpoint descriptors
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correspondingly.
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Raw Gadget usage examples and a test suite:
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https://github.com/xairy/raw-gadget
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Internal details
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~~~~~~~~~~~~~~~~
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Every Raw Gadget endpoint read/write ioctl submits a USB request and waits
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until its completion. This is done deliberately to assist with coverage-guided
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fuzzing by having a single syscall fully process a single USB request. This
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feature must be kept in the implementation.
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Potential future improvements
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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- Report more events (suspend, resume, etc.) through
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``USB_RAW_IOCTL_EVENT_FETCH``.
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- Support ``O_NONBLOCK`` I/O. This would be another mode of operation, where
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Raw Gadget would not wait until the completion of each USB request.
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- Support USB 3 features (accept SS endpoint companion descriptor when
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enabling endpoints; allow providing ``stream_id`` for bulk transfers).
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- Support ISO transfer features (expose ``frame_number`` for completed
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requests).
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