Re: BPF redirect API design issue for BPF-prog MTU feedback?

On Sat, 19 Sep 2020 at 00:06, Maciej Żenczykowski [off-list ref] wrote:

quoted

This is a good point.  As bpf_skb_adjust_room() can just be run after
bpf_redirect() call, then a MTU check in bpf_redirect() actually
doesn't make much sense.  As clever/bad BPF program can then avoid the
MTU check anyhow.  This basically means that we have to do the MTU
check (again) on kernel side anyhow to catch such clever/bad BPF
programs.  (And I don't like wasting cycles on doing the same check two
times).

If you get rid of the check in bpf_redirect() you might as well get
rid of *all* the checks for excessive mtu in all the helpers that
adjust packet size one way or another way.  They *all* then become
useless overhead.

I don't like that.  There may be something the bpf program could do to
react to the error condition (for example in my case, not modify
things and just let the core stack deal with things - which will
probably just generate packet too big icmp error).

btw. right now our forwarding programs first adjust the packet size
then call bpf_redirect() and almost immediately return what it
returned.

but this could I think easily be changed to reverse the ordering, so
we wouldn't increase packet size before the core stack was informed we
would be forwarding via a different interface.

We do the same, except that we also use XDP_TX when appropriate. This
complicates the matter, because there is no helper call we could
return an error from.

My preference would be to have three helpers: get MTU for a device,
redirect ctx to a device (with MTU check), resize ctx (without MTU
check) but that doesn't work with XDP_TX. Your idea of doing checks in
redirect and adjust_room is pragmatic and seems easier to implement.

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