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Documentation / connector

Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 EST.

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