On Fri, Apr 19, 2019 at 4:39 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > On Fri, 19 Apr 2019, Greg Kroah-Hartman wrote: > > > > 3. Regarding that GadgetFS-like interface. > > > > > > 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. This interface is essentially > > > implemented in the following patch [3]. An example that emulates a USB > > > keyboard through this interface can be found here [4]. And the > > > syzkaller parts responsible for USB fuzzing are here [5], [6]. 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. > > > > > > The main differences between this interface (referred to as usb-fuzzer > > > for now) and GadgetFS are: > > > > > > 1) GadgetFS does some sanity checks on the provided USB descriptors, > > > which is something we don't want for fuzzing. We want the descriptors > > > to be as corrupted as they can. > > > > > > 2) GadgetFS handles some of the USB requests internally based on the > > > provided device descriptor, which is also something we don't want. For > > > example we may want to be able to provide differently corrupted > > > responses to the same request. > > > > > > 3) usb-fuzzer has ioctl-based interface instead of a filesystem-based > > > one. I wouldn't say it's that big of a deal, but it makes it somewhat > > > easier to incorporate into a fuzzer. > > > > > > 4) Somewhat related to the previous point: usb-fuzzer uses predictable > > > endpoint names across different UDCs. > > > > > > Right now each UDC driver defines endpoint names via EP_INFO() as it > > > pleases. And GadgetFS uses those names to create file entries for each > > > of the endpoints. As a result, endpoint names for different UDCs will > > > be different and it requires more work to write a portable userspace > > > gadget. The usb-fuzzer interface auto selects and assigns an endpoint > > > based on the required features like the transfer type. > > > > > > 5) GadgetFS binds to the first available UDC, usb-fuzzer provides a > > > way to select a UDC to bind to. > > > > > > Since the fuzzing happens in multiple processes each of which has its > > > own Dummy UDC assigned, we want to have control over which UDC we bind > > > to. This part is a bit confusing, but what I found out is that a UDC > > > is selected based on two different identifying names. I call the first > > > one "udc_device_name" and the second one "udc_driver_name". > > > "udc_device_name" has to be assigned to usb_gadget_driver->udc_name > > > when usb_gadget_probe_driver() is called, and "udc_driver_name" is > > > what we have to compare usb_gadget->name with inside of the > > > usb_gadget_driver->bind() callback. For example, Dummy UDC has > > > "dummy_udc" as its "udc_driver_name" and "dummy_udc.N" as its > > > "udc_device_name". At the same time the dwc2 driver that is used on > > > Raspberry Pi Zero, has "20980000.usb" as both "udc_driver_name" and > > > "udc_device_name". > > > > > > Overall, the usb-fuzzer interface implementation has a similar > > > structure to that of GadgetFS, but looks way simpler (although that > > > might be because I've missed to implement some functionality :). > > > > > > We'd like to get this upstreamed, but I'm not sure whether this should > > > be a separate interface (which we can rebrand as a raw usb gadget > > > interface or something like that) or we should try to make it a > > > special mode of GadgetFS. I like the former approach more, as GadgetFS > > > looks quite complicated from my point of view and making fundamental > > > changes to it doesn't seem like an easy task. This is where we'd like > > > to get your input. > > > > I'll leave the gadgetfs questions to others that know that api better > > than I do :) > > Assuming the usb-fuzzer implementation gets merged, I think it > definitely should be kept separate from gadgetfs. The differences in > functionality are too great; it doesn't make sense to combine them in > a single driver. > > Also, while it often is reasonable for people to build gadgetfs for > production use, usb-fuzzer is very much more of a developer/debugger > type of thing. OK, I'll clean up the patches and send them as an RFC. Thanks!
