Hello, I'm working on some application-specific NIC driver. On the TX path, it must remove a custom tag that sits between the Ethernet type field and the actual Ethernet payload; then it must add a different tag in front of the Ethernet header (the MAC DA field) before it hands over the frame to the hardware for delivery.
I'm wondering if skb_cow_head() is enough to safely modify the skb as described above, or I should use skb_cow() instead. I've been going through the skbuff code and I still can't figure out the difference. Comment blocks around the definitions of skb_cow(), skb_cow_head() and SKB_DATAREF_SHIFT mention a "header" part and a "payload" part of the skb, but it's still unclear to me which is what. As far as I understand, there is only "data" and it sits between the headroom and tailroom. If the skb is fragmented, additional data is stored in separate pages (in frags[]) or skbs (in frag_list). Last but not least, is there any *recent* documentation that explains the subtleties of skbs? Most of the documentation I could find is very old and/or covers only the basic concepts, such as head/data/tail/end. Thank you in advance, Radu Rendec
