cgroup: denying device doesn't work with 'rw' mode string

[Amos Kong]

# mount -t cgroup -o devices none /cgroup
# mkdir /cgroups/devices
# ls -l /dev/vg/lv
lrwxrwxrwx. 1 root root 7 Oct 14 19:03 /dev/vg/lv -> ../dm-3
# ls -l /dev/dm-3
brw-rw----. 1 root disk 253, 3 Oct 14 19:03 /dev/dm-3


------- test1
# echo a > devices/devices.allow
# echo 'b 253:3 rwm' > devices/devices.deny
                ^^^
# echo $$ > task
# dd if=/dev/zero of=/dev/dm-3 bs=1M count=1
couldn't write to /dev/dm-3 successfully

------- test2
deny read-write permission of dm-3, but it doesn't effect.

# echo a > devices/devices.allow
# echo 'b 253:2 rw' > devices.deny
                ^^
# echo $$ > task
# dd if=/dev/zero of=/dev/dm-3 bs=1M count=1
can write to /dev/dm-3 successfully ???

-----------

related upstream commit:
commit 08ce5f16ee466ffc5bf243800deeecd77d9eaf50
Author: Serge E. Hallyn <serue@xxxxxxxxxx>
Date:   Tue Apr 29 01:00:10 2008 -0700

    cgroups: implement device whitelist


cgroup tracks and enforces open and mknod restrictions on device files,
so 'm' are always needed in the mode string? 'rw' is uneffective?
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