From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: XDP question - how much can BPF change in xdp_buff? Date: Mon, 31 Oct 2016 14:57:34 -0400 (EDT) Message-ID: <20161031.145734.1243544464767231445.davem@davemloft.net> References: Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org To: Yuval.Mintz@cavium.com Return-path: Received: from shards.monkeyblade.net ([184.105.139.130]:55940 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S945655AbcJaS5g (ORCPT ); Mon, 31 Oct 2016 14:57:36 -0400 In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: From: "Mintz, Yuval" Date: Mon, 31 Oct 2016 18:31:30 +0000 > So I've [finally] started looking into implementing XDP > for qede, and there's one thing I feel like I'm missing in > regard to XDP_TX - what's the guarantee/requirement > that the bpf program isn't going to transmute some fields > of the rx packet in a way that would prevent the forwarding? > > E.g., can a BPF change the TCP payload of an incoming packet > without correcting its TCP checksum, and then expect the > driver to transmit it [via XDP_TX]? If not, how is this enforced [if at all]? > > [Looked at samples/bpf/xdp2_kern.c which manipulates the > UDP header; so I'm not certain what prevents it from doing > the same when checksum modifications would be required] My understanding is that the eBPF program would be responsible for updating the checksum if it mangles the packet in such a way that such a fixup would be required.