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+/*****************************************/
+Kernel Connector.
+/*****************************************/
+
+Kernel connector - new netlink based userspace <-> kernel space easy
+to use communication module.
+
+The Connector driver makes it easy to connect various agents using a
+netlink based network. One must register a callback and an identifier.
+When the driver receives a special netlink message with the appropriate
+identifier, the appropriate callback will be called.
+
+From the userspace point of view it's quite straightforward:
+
+ socket();
+ bind();
+ send();
+ recv();
+
+But if kernelspace wants to use the full power of such connections, the
+driver writer must create special sockets, must know about struct sk_buff
+handling, etc... The Connector driver allows any kernelspace agents to use
+netlink based networking for inter-process communication in a significantly
+easier way:
+
+int cn_add_callback(struct cb_id *id, char *name, void (*callback) (struct cn_msg *, struct netlink_skb_parms *));
+void cn_netlink_send(struct cn_msg *msg, u32 __group, int gfp_mask);
+
+struct cb_id
+{
+ __u32 idx;
+ __u32 val;
+};
+
+idx and val are unique identifiers which must be registered in the
+connector.h header for in-kernel usage. void (*callback) (void *) is a
+callback function which will be called when a message with above idx.val
+is received by the connector core. The argument for that function must
+be dereferenced to struct cn_msg *.
+
+struct cn_msg
+{
+ struct cb_id id;
+
+ __u32 seq;
+ __u32 ack;
+
+ __u32 len; /* Length of the following data */
+ __u8 data[0];
+};
+
+/*****************************************/
+Connector interfaces.
+/*****************************************/
+
+int cn_add_callback(struct cb_id *id, char *name, void (*callback) (struct cn_msg *, struct netlink_skb_parms *));
+
+ Registers new callback with connector core.
+
+ struct cb_id *id - unique connector's user identifier.
+ It must be registered in connector.h for legal in-kernel users.
+ char *name - connector's callback symbolic name.
+ void (*callback) (struct cn..) - connector's callback.
+ cn_msg and the sender's credentials
+
+
+void cn_del_callback(struct cb_id *id);
+
+ Unregisters new callback with connector core.
+
+ struct cb_id *id - unique connector's user identifier.
+
+
+int cn_netlink_send(struct cn_msg *msg, u32 __groups, int gfp_mask);
+
+ Sends message to the specified groups. It can be safely called from
+ softirq context, but may silently fail under strong memory pressure.
+ If there are no listeners for given group -ESRCH can be returned.
+
+ struct cn_msg * - message header(with attached data).
+ u32 __group - destination group.
+ If __group is zero, then appropriate group will
+ be searched through all registered connector users,
+ and message will be delivered to the group which was
+ created for user with the same ID as in msg.
+ If __group is not zero, then message will be delivered
+ to the specified group.
+ int gfp_mask - GFP mask.
+
+ Note: When registering new callback user, connector core assigns
+ netlink group to the user which is equal to its id.idx.
+
+/*****************************************/
+Protocol description.
+/*****************************************/
+
+The current framework offers a transport layer with fixed headers. The
+recommended protocol which uses such a header is as following:
+
+msg->seq and msg->ack are used to determine message genealogy. When
+someone sends a message, they use a locally unique sequence and random
+acknowledge number. The sequence number may be copied into
+nlmsghdr->nlmsg_seq too.
+
+The sequence number is incremented with each message sent.
+
+If you expect a reply to the message, then the sequence number in the
+received message MUST be the same as in the original message, and the
+acknowledge number MUST be the same + 1.
+
+If we receive a message and its sequence number is not equal to one we
+are expecting, then it is a new message. If we receive a message and
+its sequence number is the same as one we are expecting, but its
+acknowledge is not equal to the acknowledge number in the original
+message + 1, then it is a new message.
+
+Obviously, the protocol header contains the above id.
+
+The connector allows event notification in the following form: kernel
+driver or userspace process can ask connector to notify it when
+selected ids will be turned on or off (registered or unregistered its
+callback). It is done by sending a special command to the connector
+driver (it also registers itself with id={-1, -1}).
+
+As example of this usage can be found in the cn_test.c module which
+uses the connector to request notification and to send messages.
+
+/*****************************************/
+Reliability.
+/*****************************************/
+
+Netlink itself is not a reliable protocol. That means that messages can
+be lost due to memory pressure or process' receiving queue overflowed,
+so caller is warned that it must be prepared. That is why the struct
+cn_msg [main connector's message header] contains u32 seq and u32 ack
+fields.
+
+/*****************************************/
+Userspace usage.
+/*****************************************/
+
+2.6.14 has a new netlink socket implementation, which by default does not
+allow people to send data to netlink groups other than 1.
+So, if you wish to use a netlink socket (for example using connector)
+with a different group number, the userspace application must subscribe to
+that group first. It can be achieved by the following pseudocode:
+
+s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_CONNECTOR);
+
+l_local.nl_family = AF_NETLINK;
+l_local.nl_groups = 12345;
+l_local.nl_pid = 0;
+
+if (bind(s, (struct sockaddr *)&l_local, sizeof(struct sockaddr_nl)) == -1) {
+ perror("bind");
+ close(s);
+ return -1;
+}
+
+{
+ int on = l_local.nl_groups;
+ setsockopt(s, 270, 1, &on, sizeof(on));
+}
+
+Where 270 above is SOL_NETLINK, and 1 is a NETLINK_ADD_MEMBERSHIP socket
+option. To drop a multicast subscription, one should call the above socket
+option with the NETLINK_DROP_MEMBERSHIP parameter which is defined as 0.
+
+2.6.14 netlink code only allows to select a group which is less or equal to
+the maximum group number, which is used at netlink_kernel_create() time.
+In case of connector it is CN_NETLINK_USERS + 0xf, so if you want to use
+group number 12345, you must increment CN_NETLINK_USERS to that number.
+Additional 0xf numbers are allocated to be used by non-in-kernel users.
+
+Due to this limitation, group 0xffffffff does not work now, so one can
+not use add/remove connector's group notifications, but as far as I know,
+only cn_test.c test module used it.
+
+Some work in netlink area is still being done, so things can be changed in
+2.6.15 timeframe, if it will happen, documentation will be updated for that
+kernel.