track skb pointer

[borkmann]

Would be great to track the pointer itself after the initial pkt classification match, and then keep following based on pointer value. Example was filtering on specific port:

[...]
0xffff8882262bf600    [ksoftirqd/1]             tcf_classify    4840296765444
0xffff8882262bf600    [ksoftirqd/1]      skb_ensure_writable    4840296788712
0xffff8882262bf600    [ksoftirqd/1] inet_proto_csum_replace4    4840296792231
0xffff8882262bf600    [ksoftirqd/1]      skb_ensure_writable    4840296794062         <--- last occurrence before out of sight
0xffff8882262bf900        [<empty>]             ip_local_out    4856424668947
0xffff8882262bf900        [<empty>]           __ip_local_out    4856424679151
0xffff8882262bf900        [<empty>]             nf_hook_slow    4856424682172
[...]

What can be seen, we lost track of pointer 0xffff8882262bf600 after skb_ensure_writable because in the tc BPF program we did NAT'ing of the packet.