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Date: Mon, 25 May 2026 23:28:25 +0200
From: Florian Westphal <fw@strlen.de>
To: Kacper Kokot <kacper.kokot.44@gmail.com>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>, Phil Sutter <phil@nwl.cc>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Simon Horman <horms@kernel.org>, netfilter-devel@vger.kernel.org,
	coreteam@netfilter.org, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH] netfilter: TCPMSS: fix dropped packets when MSS option
 is unaligned
Message-ID: <ahS--cPlhv6NHAcO@strlen.de>
References: <20260525201116.407338-2-kacper.kokot.44@gmail.com>
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In-Reply-To: <20260525201116.407338-2-kacper.kokot.44@gmail.com>

Kacper Kokot <kacper.kokot.44@gmail.com> wrote:
> Padding TCP options with NOPs is optional, so it is legal to send an
> MSS option that is not aligned to a word boundary and therefore not
> aligned for checksum calculation. The current TCPMSS target is not
> robust to this: when the MSS option is unaligned it produces an
> invalid checksum, and the packet is dropped.

Is this an actual, real world bug?  This code is 20+ years old, all that
this hints at is that they are always aligned in reality?

(Not disputing theoretical problem).