1/*****************************************/ 2Kernel Connector. 3/*****************************************/ 4 5Kernel connector - new netlink based userspace <-> kernel space easy 6to use communication module. 7 8Connector driver adds possibility to connect various agents using 9netlink based network. One must register callback and 10identifier. When driver receives special netlink message with 11appropriate identifier, appropriate callback will be called. 12 13From the userspace point of view it's quite straightforward: 14 15 socket(); 16 bind(); 17 send(); 18 recv(); 19 20But if kernelspace want to use full power of such connections, driver 21writer must create special sockets, must know about struct sk_buff 22handling... Connector allows any kernelspace agents to use netlink 23based networking for inter-process communication in a significantly 24easier way: 25 26int cn_add_callback(struct cb_id *id, char *name, void (*callback) (void *)); 27void cn_netlink_send(struct cn_msg *msg, u32 __group, int gfp_mask); 28 29struct cb_id 30{ 31 __u32 idx; 32 __u32 val; 33}; 34 35idx and val are unique identifiers which must be registered in 36connector.h for in-kernel usage. void (*callback) (void *) - is a 37callback function which will be called when message with above idx.val 38will be received by connector core. Argument for that function must 39be dereferenced to struct cn_msg *. 40 41struct 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/*****************************************/ 53Connector interfaces. 54/*****************************************/ 55 56int cn_add_callback(struct cb_id *id, char *name, void (*callback) (void *)); 57 58Registers new callback with connector core. 59 60struct cb_id *id - unique connector's user identifier. 61 It must be registered in connector.h for legal in-kernel users. 62char *name - connector's callback symbolic name. 63void (*callback) (void *) - connector's callback. 64 Argument must be dereferenced to struct cn_msg *. 65 66void cn_del_callback(struct cb_id *id); 67 68Unregisters new callback with connector core. 69 70struct cb_id *id - unique connector's user identifier. 71 72int cn_netlink_send(struct cn_msg *msg, u32 __groups, int gfp_mask); 73 74Sends message to the specified groups. It can be safely called from 75softirq context, but may silently fail under strong memory pressure. 76If there are no listeners for given group -ESRCH can be returned. 77 78struct cn_msg * - message header(with attached data). 79u32 __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. 86int gfp_mask - GFP mask. 87 88Note: When registering new callback user, connector core assigns 89netlink group to the user which is equal to it's id.idx. 90 91/*****************************************/ 92Protocol description. 93/*****************************************/ 94 95Current offers transport layer with fixed header. Recommended 96protocol which uses such header is following: 97 98msg->seq and msg->ack are used to determine message genealogy. When 99someone sends message it puts there locally unique sequence and random 100acknowledge numbers. Sequence number may be copied into 101nlmsghdr->nlmsg_seq too. 102 103Sequence number is incremented with each message to be sent. 104 105If we expect reply to our message, then sequence number in received 106message MUST be the same as in original message, and acknowledge 107number MUST be the same + 1. 108 109If we receive message and it's sequence number is not equal to one we 110are expecting, then it is new message. If we receive message and it's 111sequence number is the same as one we are expecting, but it's 112acknowledge is not equal acknowledge number in original message + 1, 113then it is new message. 114 115Obviously, protocol header contains above id. 116 117connector allows event notification in the following form: kernel 118driver or userspace process can ask connector to notify it when 119selected id's will be turned on or off(registered or unregistered it's 120callback). It is done by sending special command to connector 121driver(it also registers itself with id={-1, -1}). 122 123As example of usage Documentation/connector now contains cn_test.c - 124testing module which uses connector to request notification and to 125send messages. 126 127/*****************************************/ 128Reliability. 129/*****************************************/ 130 131Netlink itself is not reliable protocol, that means that messages can 132be lost due to memory pressure or process' receiving queue overflowed, 133so caller is warned must be prepared. That is why struct cn_msg [main 134connector's message header] contains u32 seq and u32 ack fields. 135 136/*****************************************/ 137Userspace usage. 138/*****************************************/ 1392.6.14 has a new netlink socket implementation, which by default does not 140allow to send data to netlink groups other than 1. 141So, if to use netlink socket (for example using connector) 142with different group number userspace application must subscribe to 143that group. It can be achieved by following pseudocode: 144 145s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_CONNECTOR); 146 147l_local.nl_family = AF_NETLINK; 148l_local.nl_groups = 12345; 149l_local.nl_pid = 0; 150 151if (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 162Where 270 above is SOL_NETLINK, and 1 is a NETLINK_ADD_MEMBERSHIP socket 163option. To drop multicast subscription one should call above socket option 164with NETLINK_DROP_MEMBERSHIP parameter which is defined as 0. 165 1662.6.14 netlink code only allows to select a group which is less or equal to 167the maximum group number, which is used at netlink_kernel_create() time. 168In case of connector it is CN_NETLINK_USERS + 0xf, so if you want to use 169group number 12345, you must increment CN_NETLINK_USERS to that number. 170Additional 0xf numbers are allocated to be used by non-in-kernel users. 171 172Due to this limitation, group 0xffffffff does not work now, so one can 173not use add/remove connector's group notifications, but as far as I know, 174only cn_test.c test module used it. 175 176Some work in netlink area is still being done, so things can be changed in 1772.6.15 timeframe, if it will happen, documentation will be updated for that 178kernel. 179