Root/
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 it's 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. |
182 |
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