158 lines
5.4 KiB
ReStructuredText
158 lines
5.4 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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================
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Kernel Connector
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================
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Kernel connector - new netlink based userspace <-> kernel space easy
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to use communication module.
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The Connector driver makes it easy to connect various agents using a
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netlink based network. One must register a callback and an identifier.
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When the driver receives a special netlink message with the appropriate
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identifier, the appropriate callback will be called.
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From the userspace point of view it's quite straightforward:
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- socket();
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- bind();
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- send();
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- recv();
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But if kernelspace wants to use the full power of such connections, the
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driver writer must create special sockets, must know about struct sk_buff
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handling, etc... The Connector driver allows any kernelspace agents to use
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netlink based networking for inter-process communication in a significantly
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easier way::
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int cn_add_callback(const struct cb_id *id, char *name, void (*callback) (struct cn_msg *, struct netlink_skb_parms *));
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void cn_netlink_send_mult(struct cn_msg *msg, u16 len, u32 portid, u32 __group, int gfp_mask);
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void cn_netlink_send(struct cn_msg *msg, u32 portid, u32 __group, int gfp_mask);
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struct cb_id
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{
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__u32 idx;
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__u32 val;
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};
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idx and val are unique identifiers which must be registered in the
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connector.h header for in-kernel usage. `void (*callback) (void *)` is a
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callback function which will be called when a message with above idx.val
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is received by the connector core. The argument for that function must
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be dereferenced to `struct cn_msg *`::
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struct cn_msg
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{
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struct cb_id id;
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__u32 seq;
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__u32 ack;
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__u16 len; /* Length of the following data */
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__u16 flags;
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__u8 data[0];
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};
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Connector interfaces
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====================
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.. kernel-doc:: include/linux/connector.h
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Note:
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When registering new callback user, connector core assigns
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netlink group to the user which is equal to its id.idx.
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Protocol description
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====================
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The current framework offers a transport layer with fixed headers. The
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recommended protocol which uses such a header is as following:
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msg->seq and msg->ack are used to determine message genealogy. When
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someone sends a message, they use a locally unique sequence and random
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acknowledge number. The sequence number may be copied into
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nlmsghdr->nlmsg_seq too.
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The sequence number is incremented with each message sent.
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If you expect a reply to the message, then the sequence number in the
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received message MUST be the same as in the original message, and the
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acknowledge number MUST be the same + 1.
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If we receive a message and its sequence number is not equal to one we
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are expecting, then it is a new message. If we receive a message and
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its sequence number is the same as one we are expecting, but its
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acknowledge is not equal to the sequence number in the original
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message + 1, then it is a new message.
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Obviously, the protocol header contains the above id.
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The connector allows event notification in the following form: kernel
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driver or userspace process can ask connector to notify it when
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selected ids will be turned on or off (registered or unregistered its
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callback). It is done by sending a special command to the connector
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driver (it also registers itself with id={-1, -1}).
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As example of this usage can be found in the cn_test.c module which
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uses the connector to request notification and to send messages.
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Reliability
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===========
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Netlink itself is not a reliable protocol. That means that messages can
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be lost due to memory pressure or process' receiving queue overflowed,
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so caller is warned that it must be prepared. That is why the struct
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cn_msg [main connector's message header] contains u32 seq and u32 ack
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fields.
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Userspace usage
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===============
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2.6.14 has a new netlink socket implementation, which by default does not
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allow people to send data to netlink groups other than 1.
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So, if you wish to use a netlink socket (for example using connector)
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with a different group number, the userspace application must subscribe to
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that group first. It can be achieved by the following pseudocode::
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s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_CONNECTOR);
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l_local.nl_family = AF_NETLINK;
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l_local.nl_groups = 12345;
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l_local.nl_pid = 0;
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if (bind(s, (struct sockaddr *)&l_local, sizeof(struct sockaddr_nl)) == -1) {
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perror("bind");
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close(s);
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return -1;
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}
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{
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int on = l_local.nl_groups;
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setsockopt(s, 270, 1, &on, sizeof(on));
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}
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Where 270 above is SOL_NETLINK, and 1 is a NETLINK_ADD_MEMBERSHIP socket
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option. To drop a multicast subscription, one should call the above socket
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option with the NETLINK_DROP_MEMBERSHIP parameter which is defined as 0.
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2.6.14 netlink code only allows to select a group which is less or equal to
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the maximum group number, which is used at netlink_kernel_create() time.
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In case of connector it is CN_NETLINK_USERS + 0xf, so if you want to use
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group number 12345, you must increment CN_NETLINK_USERS to that number.
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Additional 0xf numbers are allocated to be used by non-in-kernel users.
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Due to this limitation, group 0xffffffff does not work now, so one can
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not use add/remove connector's group notifications, but as far as I know,
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only cn_test.c test module used it.
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Some work in netlink area is still being done, so things can be changed in
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2.6.15 timeframe, if it will happen, documentation will be updated for that
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kernel.
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Code samples
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============
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Sample code for a connector test module and user space can be found
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in samples/connector/. To build this code, enable CONFIG_CONNECTOR
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and CONFIG_SAMPLES.
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