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(struct cb_id *id, char *name, void (*callback) (struct cn_msg *, struct netlink_skb_parms *));
29  void cn_netlink_send_multi(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	__u32			len;	/* Length of the following data */
52	__u8			data[0];
53  };
54
55Connector interfaces
56====================
57
58 .. kernel-doc:: include/linux/connector.h
59
60 Note:
61   When registering new callback user, connector core assigns
62   netlink group to the user which is equal to its id.idx.
63
64Protocol description
65====================
66
67The current framework offers a transport layer with fixed headers.  The
68recommended protocol which uses such a header is as following:
69
70msg->seq and msg->ack are used to determine message genealogy.  When
71someone sends a message, they use a locally unique sequence and random
72acknowledge number.  The sequence number may be copied into
73nlmsghdr->nlmsg_seq too.
74
75The sequence number is incremented with each message sent.
76
77If you expect a reply to the message, then the sequence number in the
78received message MUST be the same as in the original message, and the
79acknowledge number MUST be the same + 1.
80
81If we receive a message and its sequence number is not equal to one we
82are expecting, then it is a new message.  If we receive a message and
83its sequence number is the same as one we are expecting, but its
84acknowledge is not equal to the sequence number in the original
85message + 1, then it is a new message.
86
87Obviously, the protocol header contains the above id.
88
89The connector allows event notification in the following form: kernel
90driver or userspace process can ask connector to notify it when
91selected ids will be turned on or off (registered or unregistered its
92callback).  It is done by sending a special command to the connector
93driver (it also registers itself with id={-1, -1}).
94
95As example of this usage can be found in the cn_test.c module which
96uses the connector to request notification and to send messages.
97
98Reliability
99===========
100
101Netlink itself is not a reliable protocol.  That means that messages can
102be lost due to memory pressure or process' receiving queue overflowed,
103so caller is warned that it must be prepared.  That is why the struct
104cn_msg [main connector's message header] contains u32 seq and u32 ack
105fields.
106
107Userspace usage
108===============
109
1102.6.14 has a new netlink socket implementation, which by default does not
111allow people to send data to netlink groups other than 1.
112So, if you wish to use a netlink socket (for example using connector)
113with a different group number, the userspace application must subscribe to
114that group first.  It can be achieved by the following pseudocode::
115
116  s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_CONNECTOR);
117
118  l_local.nl_family = AF_NETLINK;
119  l_local.nl_groups = 12345;
120  l_local.nl_pid = 0;
121
122  if (bind(s, (struct sockaddr *)&l_local, sizeof(struct sockaddr_nl)) == -1) {
123	perror("bind");
124	close(s);
125	return -1;
126  }
127
128  {
129	int on = l_local.nl_groups;
130	setsockopt(s, 270, 1, &on, sizeof(on));
131  }
132
133Where 270 above is SOL_NETLINK, and 1 is a NETLINK_ADD_MEMBERSHIP socket
134option.  To drop a multicast subscription, one should call the above socket
135option with the NETLINK_DROP_MEMBERSHIP parameter which is defined as 0.
136
1372.6.14 netlink code only allows to select a group which is less or equal to
138the maximum group number, which is used at netlink_kernel_create() time.
139In case of connector it is CN_NETLINK_USERS + 0xf, so if you want to use
140group number 12345, you must increment CN_NETLINK_USERS to that number.
141Additional 0xf numbers are allocated to be used by non-in-kernel users.
142
143Due to this limitation, group 0xffffffff does not work now, so one can
144not use add/remove connector's group notifications, but as far as I know,
145only cn_test.c test module used it.
146
147Some work in netlink area is still being done, so things can be changed in
1482.6.15 timeframe, if it will happen, documentation will be updated for that
149kernel.
150
151Code samples
152============
153
154Sample code for a connector test module and user space can be found
155in samples/connector/. To build this code, enable CONFIG_CONNECTOR
156and CONFIG_SAMPLES.
157