1.. SPDX-License-Identifier: GPL-2.0
  2
  3================
  4Kernel Connector
  5================
  6
  7Kernel connector - new netlink based userspace <-> kernel space easy
  8to use communication module.
  9
 10The Connector driver makes it easy to connect various agents using a
 11netlink based network.  One must register a callback and an identifier.
 12When the driver receives a special netlink message with the appropriate
 13identifier, the appropriate callback will be called.
 14
 15From the userspace point of view it's quite straightforward:
 16
 17	- socket();
 18	- bind();
 19	- send();
 20	- recv();
 21
 22But if kernelspace wants to use the full power of such connections, the
 23driver writer must create special sockets, must know about struct sk_buff
 24handling, etc...  The Connector driver allows any kernelspace agents to use
 25netlink based networking for inter-process communication in a significantly
 26easier way::
 27
 28  int cn_add_callback(const struct cb_id *id, char *name, void (*callback) (struct cn_msg *, struct netlink_skb_parms *));
 29  void cn_netlink_send_mult(struct cn_msg *msg, u16 len, u32 portid, u32 __group, int gfp_mask);
 30  void cn_netlink_send(struct cn_msg *msg, u32 portid, u32 __group, int gfp_mask);
 31
 32  struct cb_id
 33  {
 34	__u32			idx;
 35	__u32			val;
 36  };
 37
 38idx and val are unique identifiers which must be registered in the
 39connector.h header for in-kernel usage.  `void (*callback) (void *)` is a
 40callback function which will be called when a message with above idx.val
 41is received by the connector core.  The argument for that function must
 42be dereferenced to `struct cn_msg *`::
 43
 44  struct cn_msg
 45  {
 46	struct cb_id		id;
 47
 48	__u32			seq;
 49	__u32			ack;
 50
 51	__u16			len;	/* Length of the following data */
 52	__u16			flags;
 53	__u8			data[0];
 54  };
 55
 56Connector interfaces
 57====================
 58
 59 .. kernel-doc:: include/linux/connector.h
 60
 61 Note:
 62   When registering new callback user, connector core assigns
 63   netlink group to the user which is equal to its id.idx.
 64
 65Protocol description
 66====================
 67
 68The current framework offers a transport layer with fixed headers.  The
 69recommended protocol which uses such a header is as following:
 70
 71msg->seq and msg->ack are used to determine message genealogy.  When
 72someone sends a message, they use a locally unique sequence and random
 73acknowledge number.  The sequence number may be copied into
 74nlmsghdr->nlmsg_seq too.
 75
 76The sequence number is incremented with each message sent.
 77
 78If you expect a reply to the message, then the sequence number in the
 79received message MUST be the same as in the original message, and the
 80acknowledge number MUST be the same + 1.
 81
 82If we receive a message and its sequence number is not equal to one we
 83are expecting, then it is a new message.  If we receive a message and
 84its sequence number is the same as one we are expecting, but its
 85acknowledge is not equal to the sequence number in the original
 86message + 1, then it is a new message.
 87
 88Obviously, the protocol header contains the above id.
 89
 90The connector allows event notification in the following form: kernel
 91driver or userspace process can ask connector to notify it when
 92selected ids will be turned on or off (registered or unregistered its
 93callback).  It is done by sending a special command to the connector
 94driver (it also registers itself with id={-1, -1}).
 95
 96As example of this usage can be found in the cn_test.c module which
 97uses the connector to request notification and to send messages.
 98
 99Reliability
100===========
101
102Netlink itself is not a reliable protocol.  That means that messages can
103be lost due to memory pressure or process' receiving queue overflowed,
104so caller is warned that it must be prepared.  That is why the struct
105cn_msg [main connector's message header] contains u32 seq and u32 ack
106fields.
107
108Userspace usage
109===============
110
1112.6.14 has a new netlink socket implementation, which by default does not
112allow people to send data to netlink groups other than 1.
113So, if you wish to use a netlink socket (for example using connector)
114with a different group number, the userspace application must subscribe to
115that group first.  It can be achieved by the following pseudocode::
116
117  s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_CONNECTOR);
118
119  l_local.nl_family = AF_NETLINK;
120  l_local.nl_groups = 12345;
121  l_local.nl_pid = 0;
122
123  if (bind(s, (struct sockaddr *)&l_local, sizeof(struct sockaddr_nl)) == -1) {
124	perror("bind");
125	close(s);
126	return -1;
127  }
128
129  {
130	int on = l_local.nl_groups;
131	setsockopt(s, 270, 1, &on, sizeof(on));
132  }
133
134Where 270 above is SOL_NETLINK, and 1 is a NETLINK_ADD_MEMBERSHIP socket
135option.  To drop a multicast subscription, one should call the above socket
136option with the NETLINK_DROP_MEMBERSHIP parameter which is defined as 0.
137
1382.6.14 netlink code only allows to select a group which is less or equal to
139the maximum group number, which is used at netlink_kernel_create() time.
140In case of connector it is CN_NETLINK_USERS + 0xf, so if you want to use
141group number 12345, you must increment CN_NETLINK_USERS to that number.
142Additional 0xf numbers are allocated to be used by non-in-kernel users.
143
144Due to this limitation, group 0xffffffff does not work now, so one can
145not use add/remove connector's group notifications, but as far as I know,
146only cn_test.c test module used it.
147
148Some work in netlink area is still being done, so things can be changed in
1492.6.15 timeframe, if it will happen, documentation will be updated for that
150kernel.
151
152Code samples
153============
154
155Sample code for a connector test module and user space can be found
156in samples/connector/. To build this code, enable CONFIG_CONNECTOR
157and CONFIG_SAMPLES.