Re: usbfs, claiming entire usb devices

[Kay Sievers <kay.sievers@vrfy.org>]

On Thu, May 7, 2009 at 23:18, Alan Stern <stern@rowland.harvard.edu> wrote:
> On Thu, 7 May 2009, Kay Sievers wrote:
>> On Thu, May 7, 2009 at 21:55, Alan Stern <stern@rowland.harvard.edu> wrote:
>> > There is a proposal afoot to give user programs the ability to claim
>> > ownership of an entire USB device, rather than just individual
>> > interfaces.  In fact, we'd like processes to be able to own whatever
>> > device gets plugged into a particular port on a particular hub.
>> >
>> > The question is how the API should work.  A simple approach is to have
>> > a sysfs or usbfs file correspond to each port; when a process opens the
>> > file it would be granted ownership of any device plugged into that
>> > port.  Since the file is automatically closed when the process ends, we
>> > wouldn't have to worry about ownership never getting released.
>> >
>> > But there's a snag.  When a process goes to open the usbfs file for a
>> > device, the kernel needs to know whether or not the process owns that
>> > device.  In other words, we need to figure out whether or not the
>> > process has opened the corresponding port file.
>> >
>> > Is there a simple way to do this?  Is it reasonable to search through
>> > all the process's fd's, looking for one that matches a particular
>> > inode?
>> >
>> > Or would a completely different API approach be better?
>>
>> You have one file per device, and that file has normal unix file
>> permissions. Userspace can grant access to that file by ownership or
>> by adding an ACL. What else do we need?
>
> We need the ability to prevent the kernel from automatically
> configuring a device.  We need the ability to prevent kernel drivers
> from binding to a device before userspace programs get a chance.
>
>>  Why would the kernel care who
>> opened the file, when the one was able to get through the normal file
>> access check?
>
> Access checks can't be used, because programs want to stake their claim
> to the device (and its file) even before the device has been plugged
> in.  So there's no file and no ACL to set.

I see.

Can't userspace just unbind a possible driver, which is supported by
libusub? Other such use-cases do that, like the UPS userspace drivers,
which just unbind the device from a possible in-kernel driver to take
it over.

Or is that a specific requirement where things would go wrong when the
kernel binds to a device first?

Kay