usb: gadget: add raw-gadget interface

This patchset (currently a single patch) adds a new userspace interface for the USB Gadget subsystem called USB Raw Gadget. This is what is currently being used to enable coverage-guided USB fuzzing with syzkaller:

https://github.com/google/syzkaller/blob/master/docs/linux/external_fuzzing_usb.md

Initially I was using GadgetFS (together with the Dummy HCD/UDC module) to perform emulation of USB devices for fuzzing, but later switched to a custom written interface. The incentive to implement a different interface was to provide a somewhat raw and direct access to the USB Gadget layer for the userspace, where every USB request is passed to the userspace to get a response. See documentation for the list of differences between Raw Gadget and GadgetFS.

Currently Raw Gadget only supports blocking I/O mode, that synchronously waits for the result of each operation to allow collecting coverage per operation.

This patchset has been pushed to the public Linux kernel Gerrit instance:

https://linux-review.googlesource.com/c/linux/kernel/git/torvalds/linux/+/2144

Changes v5 -> v6:
- Prevent raw_process_ep_io() racing with raw_ioctl_ep_disable() by checking urb_queued flag in the latter.
- Use GFP_KERNEL instead of GFP_ATOMIC where possible.
- Reject opening raw-gadget with O_NONBLOCK to allow future extensions to support nonblocking IO.
- Reduce RAW_EVENT_QUEUE_SIZE to 16.

Changes v4 -> v5:
- Specified explicit usb_raw_event_type enum values for all entries.
- Dropped pointless locking in gadget_unbind().

Changes v3 -> v4:
- Print debug message when maxpacket check fails.
- Use module_misc_device() instead of module_init/exit().
- Reuse DRIVER_NAME macro in raw_device struct definition.
- Don't print WARNING in raw_release().
- Add comment that explains locking into raw_event_queue_fetch().
- Print a WARNING when event queue size is exceeded.
- Rename raw.c to raw_gadget.c.
- Mention module name in Kconfig.
- Reworked logging to use dev_err/dbg() instead of pr_err/debug().

Changes v2 -> v3:
- Updated device path in documentation.
- Changed usb_raw_init struct layout to make it the same for 32 bit compat mode.
- Added compat_ioctl to raw_fops.
- Changed raw_ioctl_init() to return EINVAL for invalid USB speeds, except for USB_SPEED_UNKNOWN, which defaults to USB_SPEED_HIGH.
- Reject endpoints with maxpacket = 0 in raw_ioctl_ep_enable().

Changes v1 -> v2:
- Moved raw.c to legacy/.
- Changed uapi header to use __u* types.
- Switched from debugfs entry to a misc device.
- Changed raw_dev from refcount to kref.
- Moved UDC_NAME_LENGTH_MAX to uapi headers.
- Used usb_endpoint_type() and usb_endpoint_dir_in/out() functions instead of open coding them.
- Added "WITH Linux-syscall-note" to SPDX id in the uapi header.
- Removed pr_err() if case dev_new() fails.
- Reduced the number of debugging messages.

Andrey Konovalov (1):
usb: gadget: add raw-gadget interface

 Documentation/usb/index.rst            |    1 +
 Documentation/usb/raw-gadget.rst       |   61 ++
 drivers/usb/gadget/legacy/Kconfig      |   11 +
 drivers/usb/gadget/legacy/Makefile     |    1 +
 drivers/usb/gadget/legacy/raw_gadget.c | 1078 ++++++++++++++++++++++++
 include/uapi/linux/usb/raw_gadget.h    |  167 ++++
 6 files changed, 1319 insertions(+)
 create mode 100644 Documentation/usb/raw-gadget.rst
 create mode 100644 drivers/usb/gadget/legacy/raw_gadget.c
 create mode 100644 include/uapi/linux/usb/raw_gadget.h

2.25.0.265.gbab2e86ba0-goog