Re: [Libusb-devel] Patch: libusb/os/linux_usbfs.c: Distingush between usbfs and other kernel mode drivers

[Alan S. <st...@ro...>]

On Thu, 7 Oct 2010, Michael Plante wrote:

> Xiaofan Chen wrote:
> >> I mean it would be good to extend the following two API
> >> or new APIs.
> >>
> >> 1) libusb_kernel_driver_active() can have options to return
> >> the driver name, just like the 0.1 API usb_get_driver_np().
> >> Or we can have new API for this purpose. I think extending
> >> this API is probably better.
> >>
> >> 2) libusb_attach_kernel_driver() can be extended to take the driver
> >> name as a parameter so that user can specify the
> >> driver they want the kernel to attach.
> 
> It's not clear if you're suggesting in #2 something that changes an IOCTL or
> modifies an existing one, as opposed to a user-mode change only.
> 
> If you are suggesting a kernel change, I would suggest an additional one.
> libusb_detach_kernel_driver() should pass the name of a driver never to
> detach, namely "usbfs".  I believe the ability to do that would probably
> eliminate the race I mentioned.  The underlying IOCTL would atomically
> decide if the string matches the driver, and, if not, it would detach it,
> else it would fail and do nothing.  We would then pass "usbfs" as the name,
> such that usbfs is never detached.
> 
> Of course, I've not seen any comments on this race, so maybe I'm imagining
> it?

A related topic has been discussed on a kernel mailing list (this was
some time ago and I don't have a pointer to the emails).  The feeling
was that cooperating programs should have their own techniques for
avoiding these races and the kernel shouldn't get involved.  Such a
technique could be implemented easily enough within libusb (some sort
of exclusive open, perhaps).

Alan Stern