mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-25 10:19:47 +00:00
![Evgeniy Polyakov](/assets/img/avatar_default.png)
Return -ESRCH from cn_netlink_send() when there are not listeners, just as it could be done by netlink_broadcast(). Propagate netlink_broadcast() error back to the caller. Signed-off-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru> Signed-off-by: David S. Miller <davem@davemloft.net>
178 lines
6.4 KiB
Text
178 lines
6.4 KiB
Text
/*****************************************/
|
|
Kernel Connector.
|
|
/*****************************************/
|
|
|
|
Kernel connector - new netlink based userspace <-> kernel space easy
|
|
to use communication module.
|
|
|
|
Connector driver adds possibility to connect various agents using
|
|
netlink based network. One must register callback and
|
|
identifier. When driver receives special netlink message with
|
|
appropriate identifier, appropriate callback will be called.
|
|
|
|
From the userspace point of view it's quite straightforward:
|
|
|
|
socket();
|
|
bind();
|
|
send();
|
|
recv();
|
|
|
|
But if kernelspace want to use full power of such connections, driver
|
|
writer must create special sockets, must know about struct sk_buff
|
|
handling... Connector 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) (void *));
|
|
void cn_netlink_send(struct cn_msg *msg, u32 __group, int gfp_mask);
|
|
|
|
struct cb_id
|
|
{
|
|
__u32 idx;
|
|
__u32 val;
|
|
};
|
|
|
|
idx and val are unique identifiers which must be registered in
|
|
connector.h for in-kernel usage. void (*callback) (void *) - is a
|
|
callback function which will be called when message with above idx.val
|
|
will be received by connector core. 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.
|
|
/*****************************************/
|
|
|
|
int cn_add_callback(struct cb_id *id, char *name, void (*callback) (void *));
|
|
|
|
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) (void *) - connector's callback.
|
|
Argument must be dereferenced to struct cn_msg *.
|
|
|
|
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(struct cn_msg *msg, 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).
|
|
u32 __group - destination group.
|
|
If __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 it's id.idx.
|
|
|
|
/*****************************************/
|
|
Protocol description.
|
|
/*****************************************/
|
|
|
|
Current offers transport layer with fixed header. Recommended
|
|
protocol which uses such header is following:
|
|
|
|
msg->seq and msg->ack are used to determine message genealogy. When
|
|
someone sends message it puts there locally unique sequence and random
|
|
acknowledge numbers. Sequence number may be copied into
|
|
nlmsghdr->nlmsg_seq too.
|
|
|
|
Sequence number is incremented with each message to be sent.
|
|
|
|
If we expect reply to our message, then sequence number in received
|
|
message MUST be the same as in original message, and acknowledge
|
|
number MUST be the same + 1.
|
|
|
|
If we receive message and it's sequence number is not equal to one we
|
|
are expecting, then it is new message. If we receive message and it's
|
|
sequence number is the same as one we are expecting, but it's
|
|
acknowledge is not equal acknowledge number in original message + 1,
|
|
then it is new message.
|
|
|
|
Obviously, protocol header contains above id.
|
|
|
|
connector allows event notification in the following form: kernel
|
|
driver or userspace process can ask connector to notify it when
|
|
selected id's will be turned on or off(registered or unregistered it's
|
|
callback). It is done by sending special command to connector
|
|
driver(it also registers itself with id={-1, -1}).
|
|
|
|
As example of usage Documentation/connector now contains cn_test.c -
|
|
testing module which uses connector to request notification and to
|
|
send messages.
|
|
|
|
/*****************************************/
|
|
Reliability.
|
|
/*****************************************/
|
|
|
|
Netlink itself is not reliable protocol, that means that messages can
|
|
be lost due to memory pressure or process' receiving queue overflowed,
|
|
so caller is warned must be prepared. That is why 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 to send data to netlink groups other than 1.
|
|
So, if to use netlink socket (for example using connector)
|
|
with different group number userspace application must subscribe to
|
|
that group. It can be achieved by 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 multicast subscription one should call above socket option
|
|
with 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.
|