Scratch Sensor Board needs access to TTYUSB*

[C. Scott Ananian]

On Fri, Sep 12, 2008 at 9:29 AM,  <pgf at laptop.org> wrote:
> c. scott ananian wrote:
>  > On Thu, Sep 11, 2008 at 5:50 PM, John Maloney <jmaloney at media.mit.edu> wrote:
>  > > To use the Scratch Sensor Board or Pico Sensor Board
>  > > (http://scratch.wik.is/Support/Sensor_Boards), you must add a file to the
>  > > folder:
>  > >
>  > > /etc/udev/rules.d
>  > >
>  > > This file should contain the single line:
>  > >
>  > > KERNEL=="ttyUSB*", MODE="0666"
>  > >
>  > > Adding this file allows Scratch to read and write data to the sensor board.
>  > > You will need to make yourself root using the "su" command in order to add a
>  > > file to that folder.
>  >
>  > Isn't the traditional thing to have the tty* devices owned by the
>  > serial group (uucp or dialout group if you're a real old-timer) and
>  > then set certain users to be members of that group?  It seems like you
>  > really want scratch's UID to be a member of the 'serial' group.
>  > Michael?
>
> but this would only solve the problem for scratch, and not for
> other activities and legacy programs that want to use USB serial.
> you're suggesting that every activity that might need access to
> a serial port be modifide?
>
> (frankly, the restriction on device access to specific groups only
> makes sense when those devices are "important" in some way.
> since the user is free to unplug and discard this device, i
> hardly think it matters if they use it as intended.)

Tested: the XO currently sets the group for /dev/ttyUSB0 to 'uucp'.
So, my proposal is that (a) the 'olpc' user be added to the 'uucp'
group, and (b) that the permissions.info file may be used to request
that the activity's "isolated" UID also be a member of the uucp group.
 (See http://wiki.laptop.org/go/Activity_bundles#activity.2Fpermissions.info
for Michael's documentation of the permissions.info file).

Does this seem right to you, Michael?  I think rainbow already has a
mechanism to create local copies of /dev/video etc for access to the
camera, etc; perhaps he'd prefer to use that mechanism instead.  (I
personally like trying to use the traditional groups for their
intended purposes whenever possible, to avoid surprises.)
 --scott

-- 
 ( http://cscott.net/ )