Is it all right for Netfilter modules to use control buffers (skb->cb)?

[Alberto Leiva]

Hi

I was assuming yes, since I've seen at least one module doing this. An
example is nf_defrag_ipv6 (ipv6_defrag() calls nf_ct_frag6_gather()
which calls NFCT_FRAG6_CB()).

What confuses me is that the IPv6 subsystem seems to have its own
control buffer:

- ipv6_rcv() calls IP6CB():
http://lxr.free-electrons.com/source/net/ipv6/ip6_input.c?v=3.17#L102
- then it calls the netfilter chain, which might include
nf_defrag_ipv6:
http://lxr.free-electrons.com/source/net/ipv6/ip6_input.c?v=3.17#L186
- then ip6_rcv_finish() comes into play, which calls dst_input(),
which might well call ip6_input(), which calls ip6_input_finish().
- ip6_input_finish() once again dereferences the IPv6 version of the
control buffer:
http://lxr.free-electrons.com/source/net/ipv6/ip6_input.c?v=3.17#L219

If Netfilter modules are allowed to wash out control buffers, how does
the IPv6 private data seems to be surviving the Netfilter chain?

Thanks