can-raw: add msg_flags to distinguish local traffic

CAN has no addressing scheme. It is currently impossible for userspace
to tell is a received CAN frame comes from another process on the local
host, or from a remote CAN device.

This patch add support for userspace applications to distinguish between
'own', 'local' and 'remote' CAN traffic. The distinction is made by returning
flags in msg->msg_flags in the call to recvmsg().

The added documentation explains the introduced flags.

Signed-off-by: Kurt Van Dijck <kurt.van.dijck@eia.be>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/can.txt

@@ -22,6 +22,7 @@ This file contains
       4.1.2 RAW socket option CAN_RAW_ERR_FILTER
       4.1.3 RAW socket option CAN_RAW_LOOPBACK
       4.1.4 RAW socket option CAN_RAW_RECV_OWN_MSGS
+      4.1.5 RAW socket returned message flags
     4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)
     4.3 connected transport protocols (SOCK_SEQPACKET)
     4.4 unconnected transport protocols (SOCK_DGRAM)
@@ -471,6 +472,17 @@ solution for a couple of reasons:
     setsockopt(s, SOL_CAN_RAW, CAN_RAW_RECV_OWN_MSGS,
                &recv_own_msgs, sizeof(recv_own_msgs));
 
+  4.1.5 RAW socket returned message flags
+
+  When using recvmsg() call, the msg->msg_flags may contain following flags:
+
+    MSG_DONTROUTE: set when the received frame was created on the local host.
+
+    MSG_CONFIRM: set when the frame was sent via the socket it is received on.
+      This flag can be interpreted as a 'transmission confirmation' when the
+      CAN driver supports the echo of frames on driver level, see 3.2 and 6.2.
+      In order to receive such messages, CAN_RAW_RECV_OWN_MSGS must be set.
+
   4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)
   4.3 connected transport protocols (SOCK_SEQPACKET)
   4.4 unconnected transport protocols (SOCK_DGRAM)