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

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Date:   Mon, 31 Oct 2016 14:57:34 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     Yuval.Mintz@...ium.com
Cc:     netdev@...r.kernel.org
Subject: Re: XDP question - how much can BPF change in xdp_buff?

From: "Mintz, Yuval" <Yuval.Mintz@...ium.com>
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.