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Summary:DAHLIN-00381: Bypassed HDLC handler for outgoing packets
Reporter:Alexei A Smekalkine (ikle)Labels:patch
Date Opened:2020-08-30 11:29:31Date Closed:
Priority:MajorRegression?
Status:Triage/NewComponents:dahdi (the module)
Versions:2.11.1 3.0.0 3.1.0 Frequency of
Occurrence
Related
Issues:
Environment:Attachments:( 0) 0001-dahdi-base-netdev-send-frame-via-protocol-handlers-f.patch
Description:The HDLC encapsulation module can add its own headers and trailers to the packet. For correct operation in this case, it is necessary to send a packet from DAHDI to the appropriate HDLC handler. The HDLC handler, in turn, should call the device driver procedure to transfer the packet.

An example of such a module is our [Cisco HDLC Ethernet encapsulation|https://github.com/ikle/ds/blob/master/linux/hdlc_cisco_eth.c#L80].

Thus, for correct processing of outgoing packets, we should use function hdlc_start_xmit from Linux HDLC stack as a ndo_start_xmit procedure.

(Note that if the protocol handler does not provide a xmit procedure, then function hdlc_start_xmit will call the hardware driver directly.)
   
In the case of DAHDI, the packet transfer procedure (device driver xmit procedure) is set in function dahdi_ioctl_chanconfig to dahdi_xmit:
{code}
dev_to_hdlc(chan->hdlcnetdev->netdev)->xmit = dahdi_xmit;
{code}

P.S. [The attached patch|^0001-dahdi-base-netdev-send-frame-via-protocol-handlers-f.patch] has beed tested on real hardware and used in production for more then two years.
Comments:By: Alexei A Smekalkine (ikle) 2020-08-30 12:13:36.017-0500

drivers/net/wan/hdlc.c:
{code}
netdev_tx_t hdlc_start_xmit(struct sk_buff *skb, struct net_device *dev)
{
       hdlc_device *hdlc = dev_to_hdlc(dev);

       if (hdlc->proto->xmit)
               return hdlc->proto->xmit(skb, dev);

       return hdlc->xmit(skb, dev); /* call hardware driver directly */
}
{code}

# Use case #1: HDLC protocol handler does not provide xmit procedure. In such case hdlc_start_xmit will call hdlc->xmit = dahdi_xmit — nothing changed.
# Use case #2: Cisco HDLC Ethernet encapsulation [provide xmit procedure|https://github.com/ikle/ds/blob/master/linux/hdlc_cisco_eth.c#L80]. In such case HDLC handler pad Ethernet packet to 60 bytes if required, add Cisco HDLC header, and call hdlc->xmit = dahdi_xmit with modified packet.
By: Harald Welte (laforge) 2020-09-06 07:06:55.296-0500

I completely agree with the Alexei on the bug and the correctness of this patch