linux/Documentation/connector/connector.txt
<<
>>
Prefs
   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(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 the
  36connector.h header for in-kernel usage.  void (*callback) (void *) is a
  37callback function which will be called when a message with above idx.val
  38is received by the connector core.  The 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) (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
  67void 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
  74int 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 its id.idx.
  92
  93/*****************************************/
  94Protocol description.
  95/*****************************************/
  96
  97The current framework offers a transport layer with fixed headers.  The
  98recommended protocol which uses such a header is as following:
  99
 100msg->seq and msg->ack are used to determine message genealogy.  When
 101someone sends a message, they use a locally unique sequence and random
 102acknowledge number.  The sequence number may be copied into
 103nlmsghdr->nlmsg_seq too.
 104
 105The sequence number is incremented with each message sent.
 106
 107If you expect a reply to the message, then the sequence number in the
 108received message MUST be the same as in the original message, and the
 109acknowledge number MUST be the same + 1.
 110
 111If we receive a message and its sequence number is not equal to one we
 112are expecting, then it is a new message.  If we receive a message and
 113its sequence number is the same as one we are expecting, but its
 114acknowledge is not equal to the acknowledge number in the original
 115message + 1, then it is a new message.
 116
 117Obviously, the protocol header contains the above id.
 118
 119The connector allows event notification in the following form: kernel
 120driver or userspace process can ask connector to notify it when
 121selected ids will be turned on or off (registered or unregistered its
 122callback).  It is done by sending a special command to the connector
 123driver (it also registers itself with id={-1, -1}).
 124
 125As example of this usage can be found in the cn_test.c module which
 126uses the connector to request notification and to send messages.
 127
 128/*****************************************/
 129Reliability.
 130/*****************************************/
 131
 132Netlink itself is not a reliable protocol.  That means that messages can
 133be lost due to memory pressure or process' receiving queue overflowed,
 134so caller is warned that it must be prepared.  That is why the struct
 135cn_msg [main connector's message header] contains u32 seq and u32 ack
 136fields.
 137
 138/*****************************************/
 139Userspace usage.
 140/*****************************************/
 141
 1422.6.14 has a new netlink socket implementation, which by default does not
 143allow people to send data to netlink groups other than 1.
 144So, if you wish to use a netlink socket (for example using connector)
 145with a different group number, the userspace application must subscribe to
 146that group first.  It can be achieved by the following pseudocode:
 147
 148s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_CONNECTOR);
 149
 150l_local.nl_family = AF_NETLINK;
 151l_local.nl_groups = 12345;
 152l_local.nl_pid = 0;
 153
 154if (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
 165Where 270 above is SOL_NETLINK, and 1 is a NETLINK_ADD_MEMBERSHIP socket
 166option.  To drop a multicast subscription, one should call the above socket
 167option with the NETLINK_DROP_MEMBERSHIP parameter which is defined as 0.
 168
 1692.6.14 netlink code only allows to select a group which is less or equal to
 170the maximum group number, which is used at netlink_kernel_create() time.
 171In case of connector it is CN_NETLINK_USERS + 0xf, so if you want to use
 172group number 12345, you must increment CN_NETLINK_USERS to that number.
 173Additional 0xf numbers are allocated to be used by non-in-kernel users.
 174
 175Due to this limitation, group 0xffffffff does not work now, so one can
 176not use add/remove connector's group notifications, but as far as I know, 
 177only cn_test.c test module used it.
 178
 179Some work in netlink area is still being done, so things can be changed in
 1802.6.15 timeframe, if it will happen, documentation will be updated for that
 181kernel.
 182
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.