Re: XDP question - how much can BPF change in xdp_buff?

On 16-10-31 11:57 AM, David Miller wrote:

From: "Mintz, Yuval" <redacted>
Date: Mon, 31 Oct 2016 18:31:30 +0000

quoted

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.

For XDP we will probably need to add support for at minimum the
following helpers,

	bpf_l3_csum_replace
	bpf_l4_csum_replace

Thanks,
John