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

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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 <alexander.hoelzl@gmx.net>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://patch.msgid.link/20250205174651.103238-1-alexander.hoelzl@gmx.net
Fixes: 9d71dd0c70 ("can: add support of SAE J1939 protocol")
Cc: stable@vger.kernel.org
[mkl: commit message rephrased]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
This commit is contained in:
Alexander Hölzl 2025-02-05 18:46:51 +01:00 • committed by Marc Kleine-Budde
parent d0b197b650
commit 44de577e61
2 changed files with 5 additions and 4 deletions
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4
net/can/j1939/socket.c
View file

@ -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 */

5
net/can/j1939/transport.c
View file

@ -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;
}
}