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staging: comedi: disallow COMEDI_DEVCONFIG on non-board minors

commit 754ab5c upstream.

Comedi has two sorts of minor devices:
(a) normal board minor devices in the range 0 to
COMEDI_NUM_BOARD_MINORS-1 inclusive; and
(b) special subdevice minor devices in the range COMEDI_NUM_BOARD_MINORS
upwards that are used to open the same underlying comedi device as the
normal board minor devices, but with non-default read and write
subdevices for asynchronous commands.

The special subdevice minor devices get created when a board supporting
asynchronous commands is attached to a normal board minor device, and
destroyed when the board is detached from the normal board minor device.
One way to attach or detach a board is by using the COMEDI_DEVCONFIG
ioctl.  This should only be used on normal board minors as the special
subdevice minors are too ephemeral.  In particular, the change
introduced in commit 7d3135a ("staging:
comedi: prevent auto-unconfig of manually configured devices") breaks
horribly for special subdevice minor devices.

Since there's no legitimate use for the COMEDI_DEVCONFIG ioctl on a
special subdevice minor device node, disallow it and return -ENOTTY.

Signed-off-by: Ian Abbott <>
Signed-off-by: Greg Kroah-Hartman <>
Signed-off-by: Ben Hutchings <>
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1 parent 39b9d43 commit 6c7050330611973632ce7d73f0e914ed6f63f946 @ian-abbott ian-abbott committed with bwhacks Jan 28, 2013
Showing with 5 additions and 0 deletions.
  1. +5 −0 drivers/staging/comedi/comedi_fops.c
5 drivers/staging/comedi/comedi_fops.c
@@ -136,6 +136,11 @@ static long comedi_unlocked_ioctl(struct file *file, unsigned int cmd,
/* Device config is special, because it must work on
* an unconfigured device. */
if (cmd == COMEDI_DEVCONFIG) {
+ if (minor >= COMEDI_NUM_BOARD_MINORS) {
+ /* Device config not appropriate on non-board minors. */
+ rc = -ENOTTY;
+ goto done;
+ }
rc = do_devconfig_ioctl(dev,
(struct comedi_devconfig __user *)arg);
goto done;

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