1、netlink 连接器 通信机制

使用netlink之前，先参考一下资料：http://www.ibm.com/developerworks/cn/linux/l-connector/

netlink通信机制介绍：资料来源 linux-4.8.13/Documentation/connector/connector.txt 2016-12-10 21:31:04

/*****************************************/
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();

用户空间使用netlink相对简单一些。

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_multi(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;

    __u32            len;        /* Length of the following data */
    __u8            data[0];
};

/*****************************************/
Connector interfaces.
/*****************************************/
注：

cn应该是connector的缩写

cb：call back

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_multi(struct cn_msg *msg, u16 len, u32 portid, u32 __groups, int gfp_mask);
int cn_netlink_send(struct cn_msg *msg, u32 portid, 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).
 u16 len            - for *_multi multiple cn_msg messages can be sent
 u32 port            - destination port.
                   If non-zero the message will be sent to the
                  given port, which should be set to the
                  original sender.
 u32 __group            - destination group.
                  If port and __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 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}).

内核自身提供了示例程序cn_test.c
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.