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

[sukadev at us.ibm.com]

Pavel Emelianov [xemul at openvz.org] wrote:
| The first version was posted long ago 
| (http://openvz.org/pipermail/devel/2007-September/007647.html)
| and since then there are many (good I hope) changes:
| 
| * Added the block devices support :) It turned out to
|   be a bit simpler than the char one (or I missed
|   something significant);
| * Now we can enable/disable not just individual devices,
|   but the whole major with all its minors (see the TODO
|   list beyond as well);
| * Added the ability to restrict the read/write permissions
|   to devices, not just visible/invisible state.
| 
| That is - the main features I wished to implement right
| after the v1 was sent. Some minor changes are:
| 
| * I merged the devices.char and devices.block files into
|   one - devices.permissions;
| * As the result of the change above - the strings passed
|   to this file has changed. Now they are
|          [bc] <major>:{<minor>|*} [r-][w-]
|   E.g. b 5:2 r- will grant the read permissions to the
|   block 5:2 device and c 3:* -w will grant the write-only
|   access to all the character devices with the major 5.
| 
| However, there are some things to be done:
| 
| * Make the /proc/devices show relevant info depending on
|   who is reading it. This seems to be easy to do, since
|   I already have the support to dump similar info into the
|   devices.permissions file, but I haven't tried to use
|   this in /proc/devices yet;
| * Add the support for devices ranges. I.e. someone might
|   wish to tell smth like b 5:[0-10] r- to this subsystem.
|   Currently this is not supported and I'm afraid that if we
|   start support minor ranges we'll have smth similar to
|   VMA-s or FLOCK-s ranges management in one more place in the
|   kernel.
| * One more question is - are there any other permissions to
|   work with? E.g. in OpenVZ we have a separate bit for 
|   quota management, maybe we can invent some more...
| 
| Currently I didn't pay much attention to split this set well,
| so this will most likely won't work with git-bisect, but I 
| think this is OK for now. I will sure split it better when I
| send the v3 and further.
| 
| The set is prepared against the 2.6.24-rc5-mm1.
| 
| All this is minimally tested and seems to work. Hope to hear
| you comments, wishes and patches soon :)
| 
| To play with it - run a standard procedure:
| 
|  # mount -t container none /cont/devs -o devices

This should be '-t cgroup'

|  # mkdir /cont/devs/0
|  # echo -n $$ > /cont/devs/0/tasks
| 
| and tune device permissions.

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]. 

i.e the access restrictions seem to apply to all devices with
a given major number. Is that really the intent ?

Both devices accessible here:
	# hexdump /dev/null
	# hexdump /dev/zero
	0000000 0000 0000 0000 0000 0000 0000 0000 0000
	*
	^C

Neither device accessible:

	# echo $$ > /container/devs/0/tasks
	# 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

Grant read access to /dev/null, but /dev/zero is also readable

	# echo c 1:3 r- > /container/devs/0/devices.permissions
	# hexdump /dev/null
	# hexdump /dev/zero
	0000000 0000 0000 0000 0000 0000 0000 0000 0000
	*
	^C

Remove read access to /dev/null, but /dev/zero is also not
readable.

	# echo c 1:3 -- > /container/devs/0/devices.permissions
	# hexdump /dev/zero
	hexdump: /dev/zero: No such device or address
	hexdump: /dev/zero: Bad file descriptor

BTW, a question about cgroups: If we 'echo $$ > /container/devs/0/tasks'
is there a way to remove/undo it later  (so that the process has access
as before) ?

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