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Documentation / connector / connector.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:36 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_multi(struct cn_msg *msg, u16 len, u32 portid, u32 __group, int gfp_mask);
28	void cn_netlink_send(struct cn_msg *msg, u32 portid, u32 __group, int gfp_mask);
29	
30	struct cb_id
31	{
32		__u32			idx;
33		__u32			val;
34	};
35	
36	idx and val are unique identifiers which must be registered in the
37	connector.h header for in-kernel usage.  void (*callback) (void *) is a
38	callback function which will be called when a message with above idx.val
39	is received by the connector core.  The argument for that function must
40	be dereferenced to struct cn_msg *.
41	
42	struct cn_msg
43	{
44		struct cb_id		id;
45	
46		__u32			seq;
47		__u32			ack;
48	
49		__u32			len;		/* Length of the following data */
50		__u8			data[0];
51	};
52	
53	/*****************************************/
54	Connector interfaces.
55	/*****************************************/
56	
57	int cn_add_callback(struct cb_id *id, char *name, void (*callback) (struct cn_msg *, struct netlink_skb_parms *));
58	
59	 Registers new callback with connector core.
60	
61	 struct cb_id *id		- unique connector's user identifier.
62					  It must be registered in connector.h for legal in-kernel users.
63	 char *name			- connector's callback symbolic name.
64	 void (*callback) (struct cn..)	- connector's callback.
65					  cn_msg and the sender's credentials
66	
67	
68	void cn_del_callback(struct cb_id *id);
69	
70	 Unregisters new callback with connector core.
71	
72	 struct cb_id *id		- unique connector's user identifier.
73	
74	
75	int cn_netlink_send_multi(struct cn_msg *msg, u16 len, u32 portid, u32 __groups, int gfp_mask);
76	int cn_netlink_send(struct cn_msg *msg, u32 portid, u32 __groups, int gfp_mask);
77	
78	 Sends message to the specified groups.  It can be safely called from
79	 softirq context, but may silently fail under strong memory pressure.
80	 If there are no listeners for given group -ESRCH can be returned.
81	
82	 struct cn_msg *		- message header(with attached data).
83	 u16 len			- for *_multi multiple cn_msg messages can be sent
84	 u32 port			- destination port.
85	 				  If non-zero the message will be sent to the
86					  given port, which should be set to the
87					  original sender.
88	 u32 __group			- destination group.
89					  If port and __group is zero, then appropriate group will
90					  be searched through all registered connector users,
91					  and message will be delivered to the group which was
92					  created for user with the same ID as in msg.
93					  If __group is not zero, then message will be delivered
94					  to the specified group.
95	 int gfp_mask			- GFP mask.
96	
97	 Note: When registering new callback user, connector core assigns
98	 netlink group to the user which is equal to its id.idx.
99	
100	/*****************************************/
101	Protocol description.
102	/*****************************************/
103	
104	The current framework offers a transport layer with fixed headers.  The
105	recommended protocol which uses such a header is as following:
106	
107	msg->seq and msg->ack are used to determine message genealogy.  When
108	someone sends a message, they use a locally unique sequence and random
109	acknowledge number.  The sequence number may be copied into
110	nlmsghdr->nlmsg_seq too.
111	
112	The sequence number is incremented with each message sent.
113	
114	If you expect a reply to the message, then the sequence number in the
115	received message MUST be the same as in the original message, and the
116	acknowledge number MUST be the same + 1.
117	
118	If we receive a message and its sequence number is not equal to one we
119	are expecting, then it is a new message.  If we receive a message and
120	its sequence number is the same as one we are expecting, but its
121	acknowledge is not equal to the sequence number in the original
122	message + 1, then it is a new message.
123	
124	Obviously, the protocol header contains the above id.
125	
126	The connector allows event notification in the following form: kernel
127	driver or userspace process can ask connector to notify it when
128	selected ids will be turned on or off (registered or unregistered its
129	callback).  It is done by sending a special command to the connector
130	driver (it also registers itself with id={-1, -1}).
131	
132	As example of this usage can be found in the cn_test.c module which
133	uses the connector to request notification and to send messages.
134	
135	/*****************************************/
136	Reliability.
137	/*****************************************/
138	
139	Netlink itself is not a reliable protocol.  That means that messages can
140	be lost due to memory pressure or process' receiving queue overflowed,
141	so caller is warned that it must be prepared.  That is why the struct
142	cn_msg [main connector's message header] contains u32 seq and u32 ack
143	fields.
144	
145	/*****************************************/
146	Userspace usage.
147	/*****************************************/
148	
149	2.6.14 has a new netlink socket implementation, which by default does not
150	allow people to send data to netlink groups other than 1.
151	So, if you wish to use a netlink socket (for example using connector)
152	with a different group number, the userspace application must subscribe to
153	that group first.  It can be achieved by the following pseudocode:
154	
155	s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_CONNECTOR);
156	
157	l_local.nl_family = AF_NETLINK;
158	l_local.nl_groups = 12345;
159	l_local.nl_pid = 0;
160	
161	if (bind(s, (struct sockaddr *)&l_local, sizeof(struct sockaddr_nl)) == -1) {
162		perror("bind");
163		close(s);
164		return -1;
165	}
166	
167	{
168		int on = l_local.nl_groups;
169		setsockopt(s, 270, 1, &on, sizeof(on));
170	}
171	
172	Where 270 above is SOL_NETLINK, and 1 is a NETLINK_ADD_MEMBERSHIP socket
173	option.  To drop a multicast subscription, one should call the above socket
174	option with the NETLINK_DROP_MEMBERSHIP parameter which is defined as 0.
175	
176	2.6.14 netlink code only allows to select a group which is less or equal to
177	the maximum group number, which is used at netlink_kernel_create() time.
178	In case of connector it is CN_NETLINK_USERS + 0xf, so if you want to use
179	group number 12345, you must increment CN_NETLINK_USERS to that number.
180	Additional 0xf numbers are allocated to be used by non-in-kernel users.
181	
182	Due to this limitation, group 0xffffffff does not work now, so one can
183	not use add/remove connector's group notifications, but as far as I know, 
184	only cn_test.c test module used it.
185	
186	Some work in netlink area is still being done, so things can be changed in
187	2.6.15 timeframe, if it will happen, documentation will be updated for that
188	kernel.
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