Subject: Re: [PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets in BPF

From: Florian Westphal <[email-masked]>
Date: Tue, 7 Mar 2023 21:11:56 +0100

Daniel Xu <[email-masked]> wrote:
> From my reading (I'll run some tests later) it looks like netfilter
> will defrag all ipv4/ipv6 packets in any netns with conntrack enabled.
> It appears to do so in NF_INET_PRE_ROUTING.

Yes, and output.

> One thing we would need though are (probably kfunc) wrappers around
> nf_defrag_ipv4_enable() and nf_defrag_ipv6_enable() to ensure BPF progs
> are not transitively depending on defrag support from other netfilter
> modules.
>
> The exact mechanism would probably need some thinking, as the above
> functions kinda rely on module_init() and module_exit() semantics. We
> cannot make the prog bump the refcnt every time it runs -- it would
> overflow.  And it would be nice to automatically free the refcnt when
> prog is unloaded.

Probably add a flag attribute that is evaluated at BPF_LINK time, so
progs can say they need defrag enabled.  Same could be used to request
conntrack enablement.

Will need some glue on netfilter side to handle DEFRAG=m, but we already
have plenty of those.