skb->sb on driver xmit

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I see (kernel 2.4.26) that, sometimes, driver's xmit function gets packet with 
skb->cb containing something from higher level protocols, and non-NULL 
skb->destructor.

 Is it documented? If no, it is worth to do so. Like "In case driver want to 
do some non-trivial manipulations with skb, that involves skb->cb and 
destructor, one should call skb_orphan()."

Also,
"When passing  skb from driver to stack, driver should clear skb->cb. 
Otherwise, stack will get confused."

Vladimir.
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