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
   8The Connector driver makes it easy to connect various agents using a
   9netlink based network.  One must register a callback and an identifier.
  10When the driver receives a special netlink message with the appropriate
  11identifier, the 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 wants to use the full power of such connections, the
  21driver writer must create special sockets, must know about struct sk_buff
  22handling, etc...  The Connector driver allows any kernelspace agents to use
  23netlink based networking for inter-process communication in a significantly
  24easier way:
  25
  26int cn_add_callback(struct cb_id *id, char *name, void (*callback) (struct cn_msg *, struct netlink_skb_parms *));
  27void cn_netlink_send_multi(struct cn_msg *msg, u16 len, u32 portid, u32 __group, int gfp_mask);
  28void cn_netlink_send(struct cn_msg *msg, u32 portid, u32 __group, int gfp_mask);
  29
  30struct cb_id
  31{
  32        __u32                   idx;
  33        __u32                   val;
  34};
  35
  36idx and val are unique identifiers which must be registered in the
  37connector.h header for in-kernel usage.  void (*callback) (void *) is a
  38callback function which will be called when a message with above idx.val
  39is received by the connector core.  The argument for that function must
  40be dereferenced to struct cn_msg *.
  41
  42struct 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/*****************************************/
  54Connector interfaces.
  55/*****************************************/
  56
  57int 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
  68void 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
  75int cn_netlink_send_multi(struct cn_msg *msg, u16 len, u32 portid, u32 __groups, int gfp_mask);
  76int 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/*****************************************/
 101Protocol description.
 102/*****************************************/
 103
 104The current framework offers a transport layer with fixed headers.  The
 105recommended protocol which uses such a header is as following:
 106
 107msg->seq and msg->ack are used to determine message genealogy.  When
 108someone sends a message, they use a locally unique sequence and random
 109acknowledge number.  The sequence number may be copied into
 110nlmsghdr->nlmsg_seq too.
 111
 112The sequence number is incremented with each message sent.
 113
 114If you expect a reply to the message, then the sequence number in the
 115received message MUST be the same as in the original message, and the
 116acknowledge number MUST be the same + 1.
 117
 118If we receive a message and its sequence number is not equal to one we
 119are expecting, then it is a new message.  If we receive a message and
 120its sequence number is the same as one we are expecting, but its
 121acknowledge is not equal to the sequence number in the original
 122message + 1, then it is a new message.
 123
 124Obviously, the protocol header contains the above id.
 125
 126The connector allows event notification in the following form: kernel
 127driver or userspace process can ask connector to notify it when
 128selected ids will be turned on or off (registered or unregistered its
 129callback).  It is done by sending a special command to the connector
 130driver (it also registers itself with id={-1, -1}).
 131
 132As example of this usage can be found in the cn_test.c module which
 133uses the connector to request notification and to send messages.
 134
 135/*****************************************/
 136Reliability.
 137/*****************************************/
 138
 139Netlink itself is not a reliable protocol.  That means that messages can
 140be lost due to memory pressure or process' receiving queue overflowed,
 141so caller is warned that it must be prepared.  That is why the struct
 142cn_msg [main connector's message header] contains u32 seq and u32 ack
 143fields.
 144
 145/*****************************************/
 146Userspace usage.
 147/*****************************************/
 148
 1492.6.14 has a new netlink socket implementation, which by default does not
 150allow people to send data to netlink groups other than 1.
 151So, if you wish to use a netlink socket (for example using connector)
 152with a different group number, the userspace application must subscribe to
 153that group first.  It can be achieved by the following pseudocode:
 154
 155s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_CONNECTOR);
 156
 157l_local.nl_family = AF_NETLINK;
 158l_local.nl_groups = 12345;
 159l_local.nl_pid = 0;
 160
 161if (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
 172Where 270 above is SOL_NETLINK, and 1 is a NETLINK_ADD_MEMBERSHIP socket
 173option.  To drop a multicast subscription, one should call the above socket
 174option with the NETLINK_DROP_MEMBERSHIP parameter which is defined as 0.
 175
 1762.6.14 netlink code only allows to select a group which is less or equal to
 177the maximum group number, which is used at netlink_kernel_create() time.
 178In case of connector it is CN_NETLINK_USERS + 0xf, so if you want to use
 179group number 12345, you must increment CN_NETLINK_USERS to that number.
 180Additional 0xf numbers are allocated to be used by non-in-kernel users.
 181
 182Due to this limitation, group 0xffffffff does not work now, so one can
 183not use add/remove connector's group notifications, but as far as I know, 
 184only cn_test.c test module used it.
 185
 186Some work in netlink area is still being done, so things can be changed in
 1872.6.15 timeframe, if it will happen, documentation will be updated for that
 188kernel.
 189