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Miller" , Jakub Kicinski , Paolo Abeni , Simon Horman , netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH net] amt: do not offer software GSO on the amt device Date: Thu, 1 Oct 2026 15:51:36 +0300 Message-ID: <20261001125139.17059-1-omar@blockcast.net> X-Mailer: git-send-email 2.50.1 In-Reply-To: References: <20260928181554.85766-1-omar@blockcast.net> <179061959167.31693.11895969550331516671@kernel.org> <20260930222700.45470-1-omar@blockcast.net> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit (Resending with the list and Cc's restored; my first copy went to Eric only by mistake.) On Thu, Oct 1, 2026, Eric Dumazet wrote: > 1) Note that tx checksum offload is off by default on amt, so the bug > would not trigger by default. You're right, and I should have checked that before sending. With tx-checksum-ip-generic off, netdev_fix_features() drops GSO_UDP_L4 from amt's features, so validate_xmit_skb() segments any UDP GSO skb before amt_dev_xmit() sees it. It takes "ethtool -K tx on" first. The commit message, and my reply to the bot, also overstate the symptom. Even with tx on, the per-tunnel copy keeps csum_start on the inner UDP header, so where it is segmented in software, __udp_gso_segment() rejects it and the copy is dropped, rather than being cut into malformed AMT messages. That is from reading the code, not from a reproducer. > 2) Please take a look at Documentation/process/coding-assistants.rst Thanks, I have. Anything further I send that an LLM helped with will carry an Assisted-by: tag and say what was and was not reproduced. > 3) A better fix would be (for net-next) to call udp_tunnel_handle_offloads() > in amt_send_multicast_data() like other UDP tunnels do Agreed, please drop this patch. I'll do it that way for net-next, with an amt.sh case that sends a UDP_SEGMENT burst through the relay with tx on. pw-bot: cr