[PATH 0/4] usbip: make vhci_hcd.* objects independent of vhci_hcd.0

[Salvador Fandino]

Let me start by explaining the problem that have motivated me to write
this patches:

I work on the QVD, a virtual desktop platform for Linux. This software
runs Linux desktops (i.e. XFCE, KDE) and their applications inside LXC
containers, and makes then available through the network to remote
users.

Supporting USB devices is a common feature customers have been
requesting us for a long time (in order to use, for instance, remote
signature pads, bar-code scanners, fingerprint readers, etc.). So, we
have been working on that feature using the USB/IP layer on the
kernel.

Connecting and disconnecting devices and transferring data works
seamless for the devices listed above. But we also want to make the
usbip operations private to the container where they are run.  For
instance, it is unacceptable for our product, that one user could list
the devices connected by other users or access them.

We can control how can access every device using cgroups once those
are attached, but the usbip layer is not providing any mechanism for
controlling who can attach, detach or list the devices.

So, we got the idea that in order to enforce that remote usbip devices
are only visible inside the container where they were imported, we
could conveniently mount-bind inside every container just one of the
vhci_hcd directories below /sys/devices/platform. So that it is as if
every container had a vhci_hcd just for itself (and also, we restrict
access to the matching USB ports in cgroups).

Unfortunately, all the vhci_hcd.* devices are controlled through
attributes in vhci_hcd.0 making our approach fail and so... well, that
is what this patch series changes. It makes every vhci_hcd device
controllable through attributes inside its own sysfs directory.

The first patch, does that in the kernel, and the second and third
patches change user space, adapting the libusbip and the usbip tools
code respectively.

Then there is a fourth patch, that allows to create much more USB
hubs per machine. It was limited to 64 and we are running thousands of
containers (every one requiring a hub) per host.

These changes are not completely backward compatible. In the sysfs
side, besides moving around the attribute files, now the port numbers
go from 0 to CONFIG_USBIP_VHCI_HC_PORTS * 2 - 1 and are reused for
every vhci_hcd device. I could have maintained the absolute numeration
but I think reusing the numbers is a better and simpler approach.

I have considered that until very recently, support for vhci_hcd
devices above the .0 was broken in the uspip tools (see
1ac7c8a78be85f84b019d3d2742d1a9f07255cc5 "usbip: fix usbip attach to
find a port that matches the requested speed") and that has been the
place where I have set the bar for backward compatibility: usage of
the tools must remain unchanged for accessing "vhci_hcd.0", but may
require changes otherwise. The same is true for the library functions
which now provides new functions for selecting the target vhci_hcd
device. The old functions now always target "vhci_hcd.0".

So, for instance, now, "usbip port" by default only shows "vhci_hcd.0"
ports but "usbip port -a" shows all of them, and, for instance, "usbip
port -i 4", shows the ports in "vhci_hcd.4".

Cheers.