Unexpected change of file owner:group

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I'm seeing some file ownership behavior that concerns me. Near as I can tell, a non-root user who's a member of a group can change ownership of a file that's owned by another member of the same group, even if the group perms for the file are read-only. I need to know if this is expected behavior. I also saw the behavior today in SLES9, although I need to verify the details more carefully tomorrow.

On my Fedora machine I added my non-root self to group "users", then, as root, created a directory with root:users ownership. I then added a file inside that directory called "junk" with 644 perms and owned by
root:users.  Next, as myself (non-root) I opened the file with vi and
was able to save changes to it. When I exit the file, it's no longer owned by root: it's owned by my non-root account. Behold:

[jcliburn@osprey ~]$ uname -a
Linux osprey 2.6.14-1.1656_FC4.netdev.8 #1 Wed Jan 11 23:24:33 EST 2006 x86_64 x86_64 x86_64 GNU/Linux
[jcliburn@osprey ~]$ id
uid=500(jcliburn) gid=500(jcliburn) groups=100(users),500(jcliburn)
[jcliburn@osprey ~]$ cd /opt
[jcliburn@osprey opt]$ ls -l | grep test
drwxrwx---   2 root     users     4096 Jan 19 19:19 test
[jcliburn@osprey opt]$ cd test
[jcliburn@osprey test]$ ls -l
total 4
-rw-r--r--  1 root users 56 Jan 19 19:19 junk
[jcliburn@osprey test]$ cat junk
Hello there non-root mortal.  Can you modify this file?
[jcliburn@osprey test]$ chown jcliburn:jcliburn junk
chown: changing ownership of `junk': Operation not permitted
[jcliburn@osprey test]$ vi junk

##  Save changes within vi, then quit.  ##

[jcliburn@osprey test]$ ls -l
total 4
-rw-r--r--  1 jcliburn jcliburn 80 Jan 19 19:25 junk
[jcliburn@osprey test]$ cat junk
Hello there non-root mortal.  Can you modify this file?
Why yes, I think I can.
[jcliburn@osprey test]$

This means that any group member can take ownership of another group member's file -- even, as was demonstrated here, a file owned by root -- simply by using vi to edit the file!

It ain't supposed to be that way, right?  What am I overlooking?


p.s.  SELinux is disabled.

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