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The "comedi_bond" driver allows a composite COMEDI device to be built up from the subdevices of other COMEDI devices, although it currently only supports digital I/O subdevices. Although it checks that it is not trying to bind to itself, it is possible to end up with a cycle of "comedi_bond" devices bound to each other. For example: 1. Configure /dev/comedi0 to use some COMEDI hardware device with digital I/O subdevices, but not a "comedi_bond" device. 2. Configure /dev/comedi1 as a "comedi_bond" device bound to /dev/comedi0. 3. Unconfigure /dev/comedi0 and reconfigure it as a "comedi_bond" device bound to /dev/comedi1. Now we have /dev/comedi0 and /dev/comedi1 bound in a cycle. When an operation is performed on the digital I/O subdevice of /dev/comedi0 for example, it will try and perform the operation on /dev/comedi1, which will try and perform the operation on /dev/comedi0. The task will end up deadlocked trying to lock /dev/comedi0's mutex which it has already locked. I discovered that possibility while investigating fix sysbot crash https://syzkaller.appspot.com/bug?extid=4a6138c17a47937dcea1 ("possible deadlock in comedi_do_insn"), but I think that report may be a false positive. To avoid that, replace the calls to `comedi_open()` and `comedi_close()` in "kcomedilib" with calls to `comedi_open_from()` and `comedi_close_from()`. These take an extra parameter that indicates the COMEDI minor device number from which the open or close is being performed. `comedi_open_from()` will refuse to open the device if doing so would result in a cycle. The cycle detection depends on the extra parameter having the correct value for this device and also for existing devices in the chain. Signed-off-by: Ian Abbott <abbotti@mev.co.uk> Link: https://patch.msgid.link/20251027153748.4569-3-abbotti@mev.co.uk Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.