linux/Documentation/networking/tc-actions-env-rules.txt
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   2The "environmental" rules for authors of any new tc actions are:
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   41) If you stealeth or borroweth any packet thou shalt be branching
   5from the righteous path and thou shalt cloneth.
   6
   7For example if your action queues a packet to be processed later,
   8or intentionally branches by redirecting a packet, then you need to
   9clone the packet.
  10
  11There are certain fields in the skb tc_verd that need to be reset so we
  12avoid loops, etc.  A few are generic enough that skb_act_clone()
  13resets them for you, so invoke skb_act_clone() rather than skb_clone().
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  152) If you munge any packet thou shalt call pskb_expand_head in the case
  16someone else is referencing the skb. After that you "own" the skb.
  17You must also tell us if it is ok to munge the packet (TC_OK2MUNGE),
  18this way any action downstream can stomp on the packet.
  19
  203) Dropping packets you don't own is a no-no. You simply return
  21TC_ACT_SHOT to the caller and they will drop it.
  22
  23The "environmental" rules for callers of actions (qdiscs etc) are:
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  25*) Thou art responsible for freeing anything returned as being
  26TC_ACT_SHOT/STOLEN/QUEUED. If none of TC_ACT_SHOT/STOLEN/QUEUED is
  27returned, then all is great and you don't need to do anything.
  28
  29Post on netdev if something is unclear.
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