netdev - Re: BUG_ON in skb_segment, after bpf_skb_change

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Date:   Tue, 27 Aug 2019 14:42:18 +0300
From:   Shmulik Ladkani <shmulik.ladkani@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        netdev <netdev@...r.kernel.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Yonghong Song <yhs@...com>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        shmulik@...anetworks.com, eyal@...anetworks.com
Subject: Re: BUG_ON in skb_segment, after bpf_skb_change_proto was applied

On Mon, 26 Aug 2019 19:47:40 +0200
Eric Dumazet <eric.dumazet@...il.com> wrote:

> On 8/26/19 4:07 PM, Shmulik Ladkani wrote:
> >   - ipv4 forwarding to dummy1, where eBPF nat4-to-6 program is attached
> >     at TC Egress (calls 'bpf_skb_change_proto()'), then redirect to ingress
> >     on same device.
> >     NOTE: 'bpf_skb_proto_4_to_6()' mangles 'shinfo->gso_size'  
> 
> Doing this on an skb with a frag_list is doomed, in current gso_segment() state.
> 
> A rewrite  would be needed (I believe I did so at some point, but Herbert Xu fought hard against it)

Thanks Eric,

- If a rewrite is still considered out of the question, how can one use
  eBPF's bpf_skb_change_proto() safely without disabling GRO completely?
  - e.g. is there a way to force the GROed skbs to fit into a layout that is
    tolerated by skb_segment?
  - alternatively can eBPF layer somehow enforce segmentation of the
    original GROed skb before mangling the gso_size?

- Another thing that puzzles me is that we hit the BUG_ON rather rarely
  and cannot yet reproduce synthetically. If skb_segment's handling of
  skbs with a frag_list (that have gso_size mangled) is broken, I'd expect
  to hit this more often... Any ideas?

- Suppose going for a rewrite, care to elaborate what's exactly missing
  in skb_segment's logic?
  I must admit I do not fully understand all the different code flows in
  this function, it seems to support many different input skbs - any
  assistance is highly appreciated.

Shmulik