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Based on kernel version 3.13. Page generated on 2014-01-20 22:00 EST.

1	/*****************************************/
2	Kernel Connector.
3	/*****************************************/
4	
5	Kernel connector - new netlink based userspace <-> kernel space easy
6	to use communication module.
7	
8	The Connector driver makes it easy to connect various agents using a
9	netlink based network.  One must register a callback and an identifier.
10	When the driver receives a special netlink message with the appropriate
11	identifier, the appropriate callback will be called.
12	
13	From the userspace point of view it's quite straightforward:
14	
15		socket();
16		bind();
17		send();
18		recv();
19	
20	But if kernelspace wants to use the full power of such connections, the
21	driver writer must create special sockets, must know about struct sk_buff
22	handling, etc...  The Connector driver allows any kernelspace agents to use
23	netlink based networking for inter-process communication in a significantly
24	easier way:
25	
26	int cn_add_callback(struct cb_id *id, char *name, void (*callback) (struct cn_msg *, struct netlink_skb_parms *));
27	void cn_netlink_send(struct cn_msg *msg, u32 __group, int gfp_mask);
28	
29	struct cb_id
30	{
31		__u32			idx;
32		__u32			val;
33	};
34	
35	idx and val are unique identifiers which must be registered in the
36	connector.h header for in-kernel usage.  void (*callback) (void *) is a
37	callback function which will be called when a message with above idx.val
38	is received by the connector core.  The argument for that function must
39	be dereferenced to struct cn_msg *.
40	
41	struct cn_msg
42	{
43		struct cb_id		id;
44	
45		__u32			seq;
46		__u32			ack;
47	
48		__u32			len;		/* Length of the following data */
49		__u8			data[0];
50	};
51	
52	/*****************************************/
53	Connector interfaces.
54	/*****************************************/
55	
56	int cn_add_callback(struct cb_id *id, char *name, void (*callback) (struct cn_msg *, struct netlink_skb_parms *));
57	
58	 Registers new callback with connector core.
59	
60	 struct cb_id *id		- unique connector's user identifier.
61					  It must be registered in connector.h for legal in-kernel users.
62	 char *name			- connector's callback symbolic name.
63	 void (*callback) (struct cn..)	- connector's callback.
64					  cn_msg and the sender's credentials
65	
66	
67	void cn_del_callback(struct cb_id *id);
68	
69	 Unregisters new callback with connector core.
70	
71	 struct cb_id *id		- unique connector's user identifier.
72	
73	
74	int cn_netlink_send(struct cn_msg *msg, u32 __groups, int gfp_mask);
75	
76	 Sends message to the specified groups.  It can be safely called from
77	 softirq context, but may silently fail under strong memory pressure.
78	 If there are no listeners for given group -ESRCH can be returned.
79	
80	 struct cn_msg *		- message header(with attached data).
81	 u32 __group			- destination group.
82					  If __group is zero, then appropriate group will
83					  be searched through all registered connector users,
84					  and message will be delivered to the group which was
85					  created for user with the same ID as in msg.
86					  If __group is not zero, then message will be delivered
87					  to the specified group.
88	 int gfp_mask			- GFP mask.
89	
90	 Note: When registering new callback user, connector core assigns
91	 netlink group to the user which is equal to its id.idx.
92	
93	/*****************************************/
94	Protocol description.
95	/*****************************************/
96	
97	The current framework offers a transport layer with fixed headers.  The
98	recommended protocol which uses such a header is as following:
99	
100	msg->seq and msg->ack are used to determine message genealogy.  When
101	someone sends a message, they use a locally unique sequence and random
102	acknowledge number.  The sequence number may be copied into
103	nlmsghdr->nlmsg_seq too.
104	
105	The sequence number is incremented with each message sent.
106	
107	If you expect a reply to the message, then the sequence number in the
108	received message MUST be the same as in the original message, and the
109	acknowledge number MUST be the same + 1.
110	
111	If we receive a message and its sequence number is not equal to one we
112	are expecting, then it is a new message.  If we receive a message and
113	its sequence number is the same as one we are expecting, but its
114	acknowledge is not equal to the acknowledge number in the original
115	message + 1, then it is a new message.
116	
117	Obviously, the protocol header contains the above id.
118	
119	The connector allows event notification in the following form: kernel
120	driver or userspace process can ask connector to notify it when
121	selected ids will be turned on or off (registered or unregistered its
122	callback).  It is done by sending a special command to the connector
123	driver (it also registers itself with id={-1, -1}).
124	
125	As example of this usage can be found in the cn_test.c module which
126	uses the connector to request notification and to send messages.
127	
128	/*****************************************/
129	Reliability.
130	/*****************************************/
131	
132	Netlink itself is not a reliable protocol.  That means that messages can
133	be lost due to memory pressure or process' receiving queue overflowed,
134	so caller is warned that it must be prepared.  That is why the struct
135	cn_msg [main connector's message header] contains u32 seq and u32 ack
136	fields.
137	
138	/*****************************************/
139	Userspace usage.
140	/*****************************************/
141	
142	2.6.14 has a new netlink socket implementation, which by default does not
143	allow people to send data to netlink groups other than 1.
144	So, if you wish to use a netlink socket (for example using connector)
145	with a different group number, the userspace application must subscribe to
146	that group first.  It can be achieved by the following pseudocode:
147	
148	s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_CONNECTOR);
149	
150	l_local.nl_family = AF_NETLINK;
151	l_local.nl_groups = 12345;
152	l_local.nl_pid = 0;
153	
154	if (bind(s, (struct sockaddr *)&l_local, sizeof(struct sockaddr_nl)) == -1) {
155		perror("bind");
156		close(s);
157		return -1;
158	}
159	
160	{
161		int on = l_local.nl_groups;
162		setsockopt(s, 270, 1, &on, sizeof(on));
163	}
164	
165	Where 270 above is SOL_NETLINK, and 1 is a NETLINK_ADD_MEMBERSHIP socket
166	option.  To drop a multicast subscription, one should call the above socket
167	option with the NETLINK_DROP_MEMBERSHIP parameter which is defined as 0.
168	
169	2.6.14 netlink code only allows to select a group which is less or equal to
170	the maximum group number, which is used at netlink_kernel_create() time.
171	In case of connector it is CN_NETLINK_USERS + 0xf, so if you want to use
172	group number 12345, you must increment CN_NETLINK_USERS to that number.
173	Additional 0xf numbers are allocated to be used by non-in-kernel users.
174	
175	Due to this limitation, group 0xffffffff does not work now, so one can
176	not use add/remove connector's group notifications, but as far as I know, 
177	only cn_test.c test module used it.
178	
179	Some work in netlink area is still being done, so things can be changed in
180	2.6.15 timeframe, if it will happen, documentation will be updated for that
181	kernel.
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