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([2a0f:6480:3:1:d65d:64ff:fed0:4a9d]) by smtp.gmail.com with ESMTPSA id m27sm8322088eji.64.2020.11.09.01.05.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 09 Nov 2020 01:05:16 -0800 (PST) Subject: Re: [PATCH] IPv6: Set SIT tunnel hard_header_len to zero To: Willem de Bruijn Cc: Network Development , David Miller , Alexey Kuznetsov , Hideaki YOSHIFUJI , Jakub Kicinski References: <20201103104133.GA1573211@tws> From: Oliver Herms Message-ID: <66145819-f0aa-794f-4045-1c203b260f47@gmail.com> Date: Mon, 9 Nov 2020 10:05:15 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 04.11.20 20:52, Willem de Bruijn wrote: >>>> Fixes: c54419321455 ("GRE: Refactor GRE tunneling code.") >>> >>> How did you arrive at this SHA1? >> I think the legacy usage of hard_header_len in ipv6/sit.c was overseen in c54419321455. >> Please correct me if I'm wrong. > > I don't see anything in that patch assign or modify hard_header_len. > It's not assigning or modifying it but changing expectations about how dev->hard_header_len is to be used. The patch changed the MTU calculation from: mtu = dst_mtu(&rt->dst) - dev->hard_header_len - tunnel->hlen; to this: mtu = dst_mtu(&rt->dst) - dev->hard_header_len - sizeof(struct iphdr); Later is became this (in patch 23a3647. This is the current implementation.): mtu = dst_mtu(&rt->dst) - dev->hard_header_len - sizeof(struct iphdr) - tunnel_hlen; Apparently the initial usage of dev->hard_header_len was that it contains the length of all headers before the tunnel payload. c54419321455 changed it to assuming dev->hard_header_len does not contain the tunnels outter IP header. Thus I think the bug was introduced by c54419321455. Kind Regards Oliver