85 lines
3.4 KiB
ReStructuredText
85 lines
3.4 KiB
ReStructuredText
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USB Raw Gadget
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==============
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USB Raw Gadget is a kernel module that provides a userspace interface for
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the USB Gadget subsystem. Essentially it allows to emulate USB devices
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from userspace. Enabled with CONFIG_USB_RAW_GADGET. Raw Gadget is
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currently a strictly debugging feature and shouldn't be used in
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production, use GadgetFS instead.
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Comparison to GadgetFS
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~~~~~~~~~~~~~~~~~~~~~~
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Raw Gadget is similar to GadgetFS, but provides a more low-level and
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direct access to the USB Gadget layer for the userspace. The key
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differences are:
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1. Every USB request is passed to the 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. However note, that the UDC driver might respond to some
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requests on its own and never forward them to the Gadget layer.
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2. GadgetFS performs some sanity checks on the provided USB descriptors,
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while Raw Gadget allows you to provide arbitrary data as responses to
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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.
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4. Raw Gadget explicitly exposes information about endpoints addresses and
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capabilities allowing a user to write UDC-agnostic gadgets.
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5. Raw Gadget has ioctl-based interface instead of a filesystem-based one.
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Userspace interface
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~~~~~~~~~~~~~~~~~~~
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To create a Raw Gadget instance open /dev/raw-gadget. Multiple raw-gadget
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instances (bound to different UDCs) can be used at the same time. The
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interaction with the opened file happens through the ioctl() calls, see
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comments in include/uapi/linux/usb/raw_gadget.h for details.
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The typical usage of Raw Gadget looks like:
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1. Open Raw Gadget instance via /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 calls to receive events from
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Raw Gadget and react to those depending on what kind of USB device
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needs to be emulated.
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Note, that some UDC drivers have fixed addresses assigned to endpoints, and
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therefore arbitrary endpoint addresses can't be used in the descriptors.
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Nevertheles, Raw Gadget provides a UDC-agnostic way to write USB gadgets.
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Once a USB_RAW_EVENT_CONNECT event is received via USB_RAW_IOCTL_EVENT_FETCH,
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the USB_RAW_IOCTL_EPS_INFO ioctl can be used to find out information about
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endpoints that the UDC driver has. Based on that information, the user must
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chose UDC endpoints that will be used for the gadget being emulated, and
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properly assign addresses in endpoint descriptors.
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You can find usage examples (along with a test suite) here:
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https://github.com/xairy/raw-gadget
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Internal details
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~~~~~~~~~~~~~~~~
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Currently every endpoint read/write ioctl submits a USB request and waits until
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its completion. This is the desired mode for coverage-guided fuzzing (as we'd
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like all USB request processing happen during the lifetime of a syscall),
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and must be kept in the implementation. (This might be slow for real world
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applications, thus the O_NONBLOCK improvement suggestion below.)
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Potential future improvements
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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- Report more events (suspend, resume, etc.) through USB_RAW_IOCTL_EVENT_FETCH.
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- Support O_NONBLOCK I/O.
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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 requests).
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