netdev - XDP question - how much can BPF change in xdp

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Message-ID: <BY2PR07MB23090DE460B0AE6AA23CD7368DAE0@BY2PR07MB2309.namprd07.prod.outlook.com>
Date:   Mon, 31 Oct 2016 18:31:30 +0000
From:   "Mintz, Yuval" <Yuval.Mintz@...ium.com>
To:     netdev <netdev@...r.kernel.org>
Subject: XDP question - how much can BPF change in xdp_buff?

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]