can: j1939: j1939_sk_send_loop(): fix unable to send messages with data length zero

The J1939 standard requires the transmission of messages of length 0.

For example proprietary messages are specified with a data length of 0
to 1785. The transmission of such messages is not possible. Sending
results in no error being returned but no corresponding can frame
being generated.

Enable the transmission of zero length J1939 messages. In order to
facilitate this two changes are necessary:

1) If the transmission of a new message is requested from user space
the message is segmented in j1939_sk_send_loop(). Let the segmentation
take into account zero length messages, do not terminate immediately,
queue the corresponding skb.

2) j1939_session_skb_get_by_offset() selects the next skb to transmit
for a session. Take into account that there might be zero length skbs
in the queue.

Signed-off-by: Alexander Hölzl <[email protected]>
Acked-by: Oleksij Rempel <[email protected]>
Link: https://patch.msgid.link/[email protected]
Fixes: 9d71dd0 ("can: add support of SAE J1939 protocol")
Cc: [email protected]
[mkl: commit message rephrased]
Signed-off-by: Marc Kleine-Budde <[email protected]>

Author: xile273

net/can/j1939/socket.c

@@ -1132,7 +1132,7 @@ static int j1939_sk_send_loop(struct j1939_priv *priv,  struct sock *sk,
 
 	todo_size = size;
 
-	while (todo_size) {
+	do {
 		struct j1939_sk_buff_cb *skcb;
 
 		segment_size = min_t(size_t, J1939_MAX_TP_PACKET_SIZE,
@@ -1177,7 +1177,7 @@ static int j1939_sk_send_loop(struct j1939_priv *priv,  struct sock *sk,
 
 		todo_size -= segment_size;
 		session->total_queued_size += segment_size;
-	}
+	} while (todo_size);
 
 	switch (ret) {
 	case 0: /* OK */

net/can/j1939/transport.c

@@ -382,8 +382,9 @@ sk_buff *j1939_session_skb_get_by_offset(struct j1939_session *session,
 	skb_queue_walk(&session->skb_queue, do_skb) {
 		do_skcb = j1939_skb_to_cb(do_skb);
 
-		if (offset_start >= do_skcb->offset &&
-		    offset_start < (do_skcb->offset + do_skb->len)) {
+		if ((offset_start >= do_skcb->offset &&
+		     offset_start < (do_skcb->offset + do_skb->len)) ||
+		     (offset_start == 0 && do_skcb->offset == 0 && do_skb->len == 0)) {
 			skb = do_skb;
 		}
 	}