179 lines
6.4 KiB
Plaintext
179 lines
6.4 KiB
Plaintext
/*****************************************/
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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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Connector driver adds possibility to connect various agents using
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netlink based network. One must register callback and
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identifier. When driver receives special netlink message with
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appropriate identifier, 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 want to use full power of such connections, driver
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writer must create special sockets, must know about struct sk_buff
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handling... Connector allows any kernelspace agents to use netlink
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based networking for inter-process communication in a significantly
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easier way:
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int cn_add_callback(struct cb_id *id, char *name, void (*callback) (void *));
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void cn_netlink_send(struct cn_msg *msg, 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
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connector.h for in-kernel usage. void (*callback) (void *) - is a
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callback function which will be called when message with above idx.val
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will be received by connector core. 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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__u32 len; /* Length of the following data */
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__u8 data[0];
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};
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/*****************************************/
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Connector interfaces.
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/*****************************************/
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int cn_add_callback(struct cb_id *id, char *name, void (*callback) (void *));
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Registers new callback with connector core.
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struct cb_id *id - unique connector's user identifier.
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It must be registered in connector.h for legal in-kernel users.
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char *name - connector's callback symbolic name.
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void (*callback) (void *) - connector's callback.
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Argument must be dereferenced to struct cn_msg *.
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void cn_del_callback(struct cb_id *id);
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Unregisters new callback with connector core.
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struct cb_id *id - unique connector's user identifier.
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int cn_netlink_send(struct cn_msg *msg, u32 __groups, int gfp_mask);
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Sends message to the specified groups. It can be safely called from
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softirq context, but may silently fail under strong memory pressure.
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If there are no listeners for given group -ESRCH can be returned.
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struct cn_msg * - message header(with attached data).
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u32 __group - destination group.
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If __group is zero, then appropriate group will
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be searched through all registered connector users,
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and message will be delivered to the group which was
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created for user with the same ID as in msg.
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If __group is not zero, then message will be delivered
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to the specified group.
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int gfp_mask - GFP mask.
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Note: When registering new callback user, connector core assigns
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netlink group to the user which is equal to it's id.idx.
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/*****************************************/
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Protocol description.
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/*****************************************/
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Current offers transport layer with fixed header. Recommended
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protocol which uses such header is following:
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msg->seq and msg->ack are used to determine message genealogy. When
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someone sends message it puts there locally unique sequence and random
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acknowledge numbers. Sequence number may be copied into
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nlmsghdr->nlmsg_seq too.
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Sequence number is incremented with each message to be sent.
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If we expect reply to our message, then sequence number in received
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message MUST be the same as in original message, and acknowledge
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number MUST be the same + 1.
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If we receive message and it's sequence number is not equal to one we
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are expecting, then it is new message. If we receive message and it's
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sequence number is the same as one we are expecting, but it's
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acknowledge is not equal acknowledge number in original message + 1,
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then it is new message.
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Obviously, protocol header contains above id.
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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 id's will be turned on or off(registered or unregistered it's
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callback). It is done by sending special command to connector
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driver(it also registers itself with id={-1, -1}).
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As example of usage Documentation/connector now contains cn_test.c -
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testing module which uses connector to request notification and to
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send messages.
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/*****************************************/
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Reliability.
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/*****************************************/
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Netlink itself is not 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 must be prepared. That is why struct cn_msg [main
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connector's message header] contains u32 seq and u32 ack fields.
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/*****************************************/
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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 to send data to netlink groups other than 1.
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So, if to use netlink socket (for example using connector)
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with different group number userspace application must subscribe to
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that group. It can be achieved by 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 multicast subscription one should call above socket option
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with 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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