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Date: Mon, 2 Oct 2023 10:20:35 -0600
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To: Eric Dumazet <edumazet@google.com>, Xin Long <lucien.xin@gmail.com>
Cc: "netdev@vger.kernel.org" <netdev@vger.kernel.org>
From: David Ahern <dsahern@kernel.org>
Subject: tcpdump and Big TCP
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Eric:

Looking at the tcpdump source code, it has a GUESS_TSO define that can
be enabled to dump IPv4 packets with tot_len = 0:

        if (len < hlen) {
#ifdef GUESS_TSO
            if (len) {
                ND_PRINT("bad-len %u", len);
                return;
            }
            else {
                /* we guess that it is a TSO send */
                len = length;
            }
#else
            ND_PRINT("bad-len %u", len);
            return;
#endif /* GUESS_TSO */
        }


The IPv6 version has a similar check but no compile change needed:
        /*
         * RFC 1883 says:
         *
         * The Payload Length field in the IPv6 header must be set to zero
         * in every packet that carries the Jumbo Payload option.  If a
         * packet is received with a valid Jumbo Payload option present and
         * a non-zero IPv6 Payload Length field, an ICMP Parameter Problem
         * message, Code 0, should be sent to the packet's source, pointing
         * to the Option Type field of the Jumbo Payload option.
         *
         * Later versions of the IPv6 spec don't discuss the Jumbo Payload
         * option.
         *
         * If the payload length is 0, we temporarily just set the total
         * length to the remaining data in the packet (which, for Ethernet,
         * could include frame padding, but if it's a Jumbo Payload frame,
         * it shouldn't even be sendable over Ethernet, so we don't worry
         * about that), so we can process the extension headers in order
         * to *find* a Jumbo Payload hop-by-hop option and, when we've
         * processed all the extension headers, check whether we found
         * a Jumbo Payload option, and fail if we haven't.
         */
        if (payload_len != 0) {
                len = payload_len + sizeof(struct ip6_hdr);
                if (length < len)
                        ND_PRINT("truncated-ip6 - %u bytes missing!",
                                len - length);
        } else
                len = length + sizeof(struct ip6_hdr);


Maybe I am missing something, but it appears that no code change to
tcpdump is needed for Linux Big TCP packets other than enabling that
macro when building. I did that in a local build and the large packets
were dumped just fine.