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Alan Stern
4a2a8a2cce
The usbfs code doesn't provide sufficient mutual exclusion among open, release, and remove. Release vs. remove is okay because they both acquire the device lock, but open is not exclusive with either one. All three routines modify the udev->filelist linked list, so they must not run concurrently. Apparently someone gave this a minimum amount of thought in the past by explicitly acquiring the BKL at the start of the usbdev_open routine. Oddly enough, there's a comment pointing out that locking is unnecessary because chrdev_open already has acquired the BKL. But this ignores the point that the files in /proc/bus/usb/* are not char device files; they are regular files and so they don't get any special locking. Furthermore it's necessary to acquire the same lock in the release and remove routines, which the code does not do. Yet another problem arises because the same file_operations structure is accessible through both the /proc/bus/usb/* and /dev/usb/usbdev* file nodes. Even when one of them has been removed, it's still possible for userspace to open the other. So simple locking around the individual remove routines is insufficient; we need to lock the entire usb_notify_remove_device notifier chain. Rather than rely on the BKL, this patch (as723) introduces a new private mutex for the purpose. Holding the BKL while invoking a notifier chain doesn't seem like a good idea. Signed-off-by: Alan Stern <stern@rowland.harvard.edu> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
To understand all the Linux-USB framework, you'll use these resources:
* This source code. This is necessarily an evolving work, and
includes kerneldoc that should help you get a current overview.
("make pdfdocs", and then look at "usb.pdf" for host side and
"gadget.pdf" for peripheral side.) Also, Documentation/usb has
more information.
* The USB 2.0 specification (from www.usb.org), with supplements
such as those for USB OTG and the various device classes.
The USB specification has a good overview chapter, and USB
peripherals conform to the widely known "Chapter 9".
* Chip specifications for USB controllers. Examples include
host controllers (on PCs, servers, and more); peripheral
controllers (in devices with Linux firmware, like printers or
cell phones); and hard-wired peripherals like Ethernet adapters.
* Specifications for other protocols implemented by USB peripheral
functions. Some are vendor-specific; others are vendor-neutral
but just standardized outside of the www.usb.org team.
Here is a list of what each subdirectory here is, and what is contained in
them.
core/ - This is for the core USB host code, including the
usbfs files and the hub class driver ("khubd").
host/ - This is for USB host controller drivers. This
includes UHCI, OHCI, EHCI, and others that might
be used with more specialized "embedded" systems.
gadget/ - This is for USB peripheral controller drivers and
the various gadget drivers which talk to them.
Individual USB driver directories. A new driver should be added to the
first subdirectory in the list below that it fits into.
image/ - This is for still image drivers, like scanners or
digital cameras.
input/ - This is for any driver that uses the input subsystem,
like keyboard, mice, touchscreens, tablets, etc.
media/ - This is for multimedia drivers, like video cameras,
radios, and any other drivers that talk to the v4l
subsystem.
net/ - This is for network drivers.
serial/ - This is for USB to serial drivers.
storage/ - This is for USB mass-storage drivers.
class/ - This is for all USB device drivers that do not fit
into any of the above categories, and work for a range
of USB Class specified devices.
misc/ - This is for all USB device drivers that do not fit
into any of the above categories.