Re: file permissions--something is changing it

From: Bryan Brock (bbrock_at_gmail.com)
Date: 12/21/04 

Date: 21 Dec 2004 11:03:03 -0800

What are the exact permissions on /dev/ttyS0 and the kppp executable?
What's the group membership of the user? Can you use minicom to access
/dev/ttyS0?

If kppp is changing /dev/ttyS0, you should be able to use strace to see
the system calls kppp made to change the permissions or ownership of
the file (maybe chown32 or chmod32).

Try "strace -o strace.out kppp". This should run kppp and save the
stack trace output in strace.out. In another window, monitor the
system calls with "tail -f strace.out". You could also do an "ls -l
/dev/ttyS0" to note when the permissions have changed.

If you're not sure whether or not kppp is the culprit, you can use the
"-r" switch with the lsof command to monitor which program is opening
/dev/ttyS0.

To check for any processes that have opened /dev/ttyS0 once every
second, you can use the "-r 1" like this:

bbrock@queue:~> lsof -r 1 /dev/ttyS0
=======
=======
=======
=======
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
minicom 13413 bbrock 3u CHR 4,64 34953 /dev/ttyS0
=======
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
minicom 13413 bbrock 3u CHR 4,64 34953 /dev/ttyS0
=======
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
minicom 13413 bbrock 3u CHR 4,64 34953 /dev/ttyS0
=======
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
minicom 13413 bbrock 3u CHR 4,64 34953 /dev/ttyS0
=======
=======
=======
=======

bbrock@queue:~>

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