[PATCH V2 net-next] can-raw: add msg_flags to distinguish local traffic

[Kurt Van Dijck <kurt.van.dijck-/BeEPy95v10@public.gmane.org>]

differences in V2:
1) use 'unsigned int' for flags, conform to the msg_flags
   in the kernel headers (glibc uses 'int')
2) fixed typo in comments
3) Add description in Documentation
4) correct email address in Signed-off-by

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.
Distinction is made by returning flags in msg->msg_flags
in the call to recvmsg.
The Documentation/...  explains the flags.

Signed-off-by: Kurt Van Dijck <kurt.van.dijck-/BeEPy95v10@public.gmane.org>
---
 Documentation/networking/can.txt |   11 +++++++++++
 net/can/raw.c                    |   33 ++++++++++++++++++++++++++++++---
 2 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/Documentation/networking/can.txt b/Documentation/networking/can.txt
index cd79735..95341aa 100644
--- a/Documentation/networking/can.txt
+++ b/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 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 +472,16 @@ 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 flags
+
+  When using recvmsg() call, the msg->msg_flags may contain following flags:
+
+    MSG_DONTROUTE: set when the frame was sent via the same physical device,
+	ie. a loopback frame.
+    MSG_CONFIRM: set when the frame was sent via the socket it is received on.
+	This flag acts as a 'transmission confirmation'.
+	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)
diff --git a/net/can/raw.c b/net/can/raw.c
index 7d77e67..9020c4f 100644
--- a/net/can/raw.c
+++ b/net/can/raw.c
@@ -90,23 +90,39 @@ struct raw_sock {
 	can_err_mask_t err_mask;
 };
 
+/*
+ * return some space to store extra msg flags in.
+ * We use 1 int beyond the 'struct sockaddr_can' in skb->cb
+ * to store those.
+ * These flags will be use in raw_recvmsg()
+ */
+static inline unsigned int *raw_flags(struct sk_buff *skb)
+{
+	BUILD_BUG_ON(sizeof(skb->cb) <= (sizeof(struct sockaddr_can)
+				+ sizeof(unsigned int)));
+	/* return pointer after struct sockaddr_can */
+	return (unsigned int *)(&((struct sockaddr_can *)skb->cb)[1]);
+}
+
 static inline struct raw_sock *raw_sk(const struct sock *sk)
 {
 	return (struct raw_sock *)sk;
 }
 
-static void raw_rcv(struct sk_buff *skb, void *data)
+static void raw_rcv(struct sk_buff *oskb, void *data)
 {
 	struct sock *sk = (struct sock *)data;
 	struct raw_sock *ro = raw_sk(sk);
 	struct sockaddr_can *addr;
+	struct sk_buff *skb;
+	unsigned int *pflags;
 
 	/* check the received tx sock reference */
-	if (!ro->recv_own_msgs && skb->sk == sk)
+	if (!ro->recv_own_msgs && oskb->sk == sk)
 		return;
 
 	/* clone the given skb to be able to enqueue it into the rcv queue */
-	skb = skb_clone(skb, GFP_ATOMIC);
+	skb = skb_clone(oskb, GFP_ATOMIC);
 	if (!skb)
 		return;
 
@@ -123,6 +139,14 @@ static void raw_rcv(struct sk_buff *skb, void *data)
 	addr->can_family  = AF_CAN;
 	addr->can_ifindex = skb->dev->ifindex;
 
+	/* prepare the flags for raw_recvmsg() */
+	pflags = raw_flags(skb);
+	*pflags = 0;
+	if (oskb->sk)
+		*pflags |= MSG_DONTROUTE;
+	if (oskb->sk == sk)
+		*pflags |= MSG_CONFIRM;
+
 	if (sock_queue_rcv_skb(sk, skb) < 0)
 		kfree_skb(skb);
 }
@@ -707,6 +731,9 @@ static int raw_recvmsg(struct kiocb *iocb, struct socket *sock,
 		memcpy(msg->msg_name, skb->cb, msg->msg_namelen);
 	}
 
+	/* assign the flags that have been recorded in raw_rcv() */
+	msg->msg_flags |= *(raw_flags(skb));
+
 	skb_free_datagram(sk, skb);
 
 	return size;