Tuesday, February 22, 2022

[SOLVED] Transmit second sk_buff using current virtual device

Issue

I modify the transmit function (ndo_start_xmit) of a virtual network device which gets an struct sk_buff skb_one as input.

At one point in the transmission flow I generate a second struct sk_buff skb_second. This one should go through the same flow skb_one was going, so it should be sent by the virtual device I am currently in. Furthermore it should be sent after skb_one.

I thought about two different methods. One is modyfing the queue skb_one is in:

    skb_second->next = skb_one->next;
    skb_second->prev = skb_one;
    skb_one->next = skb_frag;

Using this approach it is sent after skb_one but it is not going through the transmission flow of the virtual network device.

The second approach is just using

dev_queue_xmit(skb_second);

With this approach the skb_second goes through the transmission flow "again", but it is sent before skb_one.

Is there a possibility to reach both goals?


Solution

dev_queue_xmit would eventually try to get the device's xmit_lock, but it would fail because being in ndo_start_xmit you should already hold it.

So the frame would be queued to run in the NET_TX_SOFTIRQ after skb_one is sent out and ndo_start_xmit has returned.

Am I missing something?



Answered By - a3f
Answer Checked By - Gilberto Lyons (WPSolving Admin)
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