linux-kernel - Re: [PATCH bpf-next 0/7] Support defragmenting IPv(4|6) packets in BPF

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Message-ID: <hwsdt5pjygunx7lxrvnwjerugchzpu5mpb442hlwf2h5szqq2h@cuqbteb6bqyh>
Date:   Tue, 27 Jun 2023 08:51:47 -0600
From:   Daniel Xu <dxu@...uu.xyz>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        coreteam@...filter.org, netfilter-devel@...r.kernel.org,
        fw@...len.de, daniel@...earbox.net, dsahern@...nel.org
Subject: Re: [PATCH bpf-next 0/7] Support defragmenting IPv(4|6) packets in
 BPF

Hi Toke,

Thanks for taking a look at the patchset.

On Tue, Jun 27, 2023 at 04:25:13PM +0200, Toke Høiland-Jørgensen wrote:
> > The basic idea is we bump a refcnt on the netfilter defrag module and
> > then run the bpf prog after the defrag module runs. This allows bpf
> > progs to transparently see full, reassembled packets. The nice thing
> > about this is that progs don't have to carry around logic to detect
> > fragments.
> 
> One high-level comment after glancing through the series: Instead of
> allocating a flag specifically for the defrag module, why not support
> loading (and holding) arbitrary netfilter modules in the UAPI? If we
> need to allocate a new flag every time someone wants to use a netfilter
> module along with BPF we'll run out of flags pretty quickly :)

I don't have enough context on netfilter in general to say if it'd be
generically useful -- perhaps Florian can comment on that.

However, I'm not sure such a mechanism removes the need for a flag. The
netfilter defrag modules still need to be called into to bump the refcnt.

The module could export some kfuncs to inc/dec the refcnt, but it'd be
rather odd for prog code to think about the lifetime of the attachment
(as inc/dec for _each_ prog execution seems wasteful and slow).  AFAIK
all the other resource acquire/release APIs are for a single prog
execution.

So a flag for link attach feels the most natural to me. We could always
add a flag2 field or something right?

[...]

Thanks,
Daniel