[PATCH 0/4] Devices accessibility control group (v2)

[sukadev at us.ibm.com]

| > I started playing with this and noticed that even if I try to
| > enable read access to device [c, 1:3] it also grants access
| > to device [c, 1:5]. 
| 
| Hm... I can't reproduce this:
| 
| # /bin/echo 'c 1:3 r-' > /cnt/dev/0/devices.permissions
| # /bin/echo -n $$ > /cnt/dev/0/tasks
| # cat /cnt/dev/0/devices.permissions 
| c 1:3 r-
| # hexdump /dev/null 
| # hexdump /dev/zero 
| hexdump: /dev/zero: No such device or address
| hexdump: /dev/zero: Bad file descriptor
| 
| Maybe you have played with devs cgroups before getting this?
| Can you show what's the contents of the devices.permissions file
| in your case?

Here is the repro again. I even tried after a reboot. Basically,
granting access to /dev/null is also granting access to /dev/zero.

	# cat devices.permissions
	# hexdump /dev/zero
	hexdump: /dev/zero: No such device or address
	hexdump: /dev/zero: Bad file descriptor
	# hexdump /dev/null
	hexdump: /dev/null: No such device or address
	hexdump: /dev/null: Bad file descriptor
	# echo 'c 1:3 r-' > devices.permissions
	# hexdump /dev/null
	# hexdump /dev/zero
	0000000 0000 0000 0000 0000 0000 0000 0000 0000
	*
	^C
	# cat tasks
	3279
	22266
	# ps
	  PID TTY          TIME CMD
	 3279 pts/0    00:00:00 bash
	22267 pts/0    00:00:00 ps