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Date: Mon, 8 Dec 2025 15:18:25 +0100
From: Florian Westphal <fw@strlen.de>
To: Nick Wood <nwood@cloudflare.com>
Cc: Jesper Dangaard Brouer <hawk@kernel.org>,
	netfilter-devel@vger.kernel.org,
	Pablo Neira Ayuso <pablo@netfilter.org>, netdev@vger.kernel.org,
	phil@nwl.cc, Eric Dumazet <eric.dumazet@gmail.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	kernel-team@cloudflare.com, mfleming@cloudflare.com,
	matt@readmodwrite.com, aforster@cloudflare.com
Subject: Re: [PATCH nf-next RFC 1/3] xt_statistic: taking GRO/GSO into
 account for nth-match
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 <176424683595.194326.16910514346485415528.stgit@firesoul>
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 <c38966ab-4a3c-4a72-a3c1-5c0301408609@kernel.org>
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Nick Wood <nwood@cloudflare.com> wrote:
> > It was easier to change the nth code (and easier for me to reason about)
> > than dealing with converting the the formula to use an integer
> > approximation (given we don't have floating point calc in kernel).
> 
> with s = integer sample rate (i.e s=100 if we're sampling 1/100)
> and n = nr_packets:
> 
> sample_threshold = [ 2**32 //s //s ] * [n*(s - (n-1))] ;
> 
> if get_random_u32 < sample_threshold {
>     sample_single_subpacket
> }*

Thanks for clarifying.  But we can't do that within the limitations
imposed by the netfilter framework.

We can only flag the entire skb/aggregate as either matching or not
matching.  We can't signal a 'match this subpacket'.

While we could split the aggregate within the conditional, we can't
(re)inject splitted packets back into the processing pipeline.

If there is a way, I can't think of anything.  At least not without
major rework of the entire netfilter stack :-|