/dev/ttyS0 permission error

[Mikkel L. Ellertson]

Jay Cliburn wrote:
> I've seen references to this scattered about on the web, but no solutions.
> 
> I installed FC6 on a Dell Inspiron 4000 laptop to use as a serial
> console for debugging kernel problems.  Whenever I start minicom on the
> laptop, however, I'm greeted with a permission denied error to
> /dev/ttyS0.  Sure enough, the perms on ttyS0 are 0660 and I'm not a
> member of the uucp group:
> 
> crw-rw---- 1 root uucp 4, 64 Dec  1 06:04 /dev/ttyS0
> crw-rw---- 1 root uucp 4, 65 Dec  1 06:03 /dev/ttyS1
> crw-rw---- 1 root uucp 4, 66 Dec  1 06:03 /dev/ttyS2
> crw-rw---- 1 root uucp 4, 67 Dec  1 06:03 /dev/ttyS3
> 
> No problem, I thought -- I'll just gin up a udev rule and set the perms
> to 0666.  Sorry; doesn't work.  As a matter of fact, it gets worse:
> perms on ttyS0 go to 0600 root:root as a result of my new local rule.
> 
> Okay, not an ideal solution, but let's modify 50-udev.rules and set the
> default rule for tty[A-Z]* to MODE="0666".  Nope.  Now they're *all*
> 0600 root:root.
> 
> Here's the rule:
> KERNEL=="ttyS0", NAME="%k", GROUP="uucp", MODE="0666"
> 
> I had no problems with this under Debian Etch and Knoppix Live running
> on the same laptop.  Debian was running a 2.6.17 kernel, and for the
> life of me I can't remember the Knoppix kernel version (the web site
> says 2.6.x -- big help), and I don't have the CD here at home.
> 
> The current workaround is to add non-root users to the uucp group.  (Or
> maybe this is the solution?)  Any ideas why I can't get a udev rule to
> change the perms to 0666 on /dev/ttyS0?
> 
> Thanks,
> Jay
> 
> 
The kernel version will not make a difference. The difference is
probably in the settings of console permissions. (man console.perms)
The permissions of some devices are changed when the first non-root
user does a local login to the machine. They are changed again when
they log out. That way, a user that is at the machine itself gets to
use things like the floppy drive, sound card, and what ever else you
configure. I was surprised to discover that FC5 didn't handle the
serial ports. As I don't have FC6 installed yet, I am not sure if it
does. (My laptop does not have serial ports, so it is not that
important to me.)

Now, if you want the user that is logged into the console to be able
to use the serial ports, you could probably do something like create
a file /etc/security/console.perms.d/10-serial.perms with something
like:

# Serial port rules.
<serial>=/dev/ttyS*

<console>  0660 <serial>     0660 root.uucp

If you just want to control /dev/ttyS0 then change ttyS* to ttyS0.

Mikkel
-- 

  Do not meddle in the affairs of dragons,
for thou art crunchy and taste good with Ketchup!