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	Version: linux-6.0.1 spark-3.0.0 hadoop-3.2.0-src hadoop-3.3.0-src spark-2.4.7 linux-4.15.13 hadoop-2.7.4-src Revert   Change

 .. SPDX-License-Identifier: GPL-2.0
 
 ================
 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(const struct cb_id *id, char *name, void (*callback) (struct cn_msg *, struct netlink_skb_parms *));
   void cn_netlink_send_mult(struct cn_msg *msg, u16 len, u32 portid, u32 __group, int gfp_mask);
   void cn_netlink_send(struct cn_msg *msg, u32 portid, 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;
 
         __u16                   len;    /* Length of the following data */
         __u16                   flags;
         __u8                    data[0];
   };
 
 Connector interfaces
 ====================
 
  .. kernel-doc:: include/linux/connector.h
 
  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 sequence 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.
 
 Code samples
 ============
 
 Sample code for a connector test module and user space can be found
 in samples/connector/. To build this code, enable CONFIG_CONNECTOR
 and CONFIG_SAMPLES.

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