linux/Documentation/connector/connector.txt
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   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