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

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Date:   Tue, 7 Mar 2023 21:11:56 +0100
From:   Florian Westphal <fw@...len.de>
To:     Daniel Xu <dxu@...uu.xyz>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        bpf <bpf@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, pablo@...filter.org,
        kadlec@...filter.org, fw@...len.de, daniel@...earbox.net
Subject: Re: [PATCH bpf-next v2 0/8] Support defragmenting IPv(4|6) packets
 in BPF

Daniel Xu <dxu@...uu.xyz> 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.