Changes to group membership not respected by existing sessions

Question

I'm encountering an odd behavior with Linux permissions and group membership that's got me scratching my head. Here's the situation:

1. I have two users: alice and bob

   alice@KAL:~$ id alice
   uid=3000(alice) gid=3000(alice) groups=3000(alice)
   alice@KAL:~$ id bob
   uid=3001(bob) gid=3001(bob) groups=3001(bob)
   

2. In alice's home directory, there is a subdirectory that I want to give write permission to bob.

   (as alice)
   alice@KAL:~$ mkdir shared
   alice@KAL:~$ chmod g+w shared
   alice@KAL:~$ ls -l
   total 4
   drwxrwxr-x 2 alice alice 4096 2012-05-15 23:56 shared
   

3. I add group alice (gid=3000) as one of bob's secondary groups

   (as root)
   root@KAL:~# id bob
   uid=3001(bob) gid=3001(bob) groups=3001(bob)
   root@KAL:~# usermod -G 3000 bob
   root@KAL:~# id bob
   uid=3001(bob) gid=3001(bob) groups=3001(bob),3000(alice)
   

4. I open a new terminal, and su as bob, and test my permissions in alice's home directory.

   (initially as kp, su'ing as bob)
   kp@KAL:~$ sudo su bob
   bob@KAL:/home/kp$ cd /home/alice
   bob@KAL:/home/alice$ ls -l
   total 4
   drwxrwxr-x 2 alice alice 4096 2012-05-15 23:56 shared
   bob@KAL:/home/alice$ touch test
   touch: cannot touch `test': Permission denied    <-- fails as expected
   bob@KAL:/home/alice$ cd shared
   bob@KAL:/home/alice/shared$ touch test     <-- succeeds as expected
   bob@KAL:/home/alice/shared$ ls -l
   total 0
   -rw-r--r-- 1 bob bob 0 2012-05-16 00:02 test
   

5. In a separate terminal, and as root, I revoke bob's membership in group alice.

   (root)
   root@KAL:~# usermod -G 3001 bob
   root@KAL:~# id bob
   uid=3001(bob) gid=3001(bob) groups=3001(bob)
   

6. Now, going back to the terminal where I'm su'ed as bob, it's clear that the membership revocation is recognized but not respected.

   (as bob)
   bob@KAL:/home/alice/shared$ id bob
   uid=3001(bob) gid=3001(bob) groups=3001(bob)   <-- group 3000 no longer secondary group
   bob@KAL:/home/alice/shared$ touch test2        <-- should fail
   bob@KAL:/home/alice/shared$ ls -l
   total 0
   -rw-r--r-- 1 bob bob 0 2012-05-16 00:02 test
   -rw-r--r-- 1 bob bob 0 2012-05-16 00:20 test2
   bob@KAL:/home/alice/shared$ rm test            <-- this should also fail
   bob@KAL:/home/alice/shared$ ls -l
   total 0
   -rw-r--r-- 1 bob bob 0 2012-05-16 00:20 test2
   

7. If I now exit, and su as bob again, the change is group membership is now respected.

   (as bob)
   bob@KAL:/home/alice/shared$ exit
   exit
   kp@KAL:~$ sudo su bob
   bob@KAL:/home/kp$ cd /home/alice/shared
   bob@KAL:/home/alice/shared$ ls -l
   total 0
   -rw-r--r-- 1 bob bob 0 2012-05-16 00:20 test2
   bob@KAL:/home/alice/shared$ touch test3
   touch: cannot touch `test3': Permission denied   <-- now fails as expected
   bob@KAL:/home/alice/shared$ id bob
   uid=3001(bob) gid=3001(bob) groups=3001(bob)
   bob@KAL:/home/alice/shared$ 
   

Is this some artifact of using su? Are group memberships only determined at start of the shell?

(This is on a machine running Ubuntu Maverick 10.10 x86_64 2.6.35-32-generic and running bash shell.)

Answer 1

Group memberships persist during sessions as they are applied to a process, i.e., your current shell.