netdev - Re: BUG_ON in skb_segment, after bpf_skb_change

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Date:   Tue, 27 Aug 2019 14:10:35 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Shmulik Ladkani <shmulik.ladkani@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     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 8/27/19 1:42 PM, Shmulik Ladkani wrote:
[...]
> - 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.

Given first point above wrt hitting rarely, it would be good to first get a
better understanding for writing a reproducer. Back then Yonghong added one
to the BPF kernel test suite [0], so it would be desirable to extend it for
the case you're hitting. Given NAT64 use-case is needed and used by multiple
parties, we should try to (fully) fix it generically.

Thanks,
Daniel

   [0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=76db8087c4c991dcd17f5ea8ac0eafd0696ab450

> Shmulik
>