netdev - Re: BUG_ON in skb_segment, after bpf_skb_change

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Date:   Tue, 3 Sep 2019 20:03:12 +0300
From:   Shmulik Ladkani <shmulik@...anetworks.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <eric.dumazet@...il.com>,
        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>,
        eyal@...anetworks.com
Subject: Re: BUG_ON in skb_segment, after bpf_skb_change_proto was applied

On Tue, 3 Sep 2019 12:23:54 -0400
Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:

> This is a lot more code change. Especially for stable fixes that need
> to be backported, a smaller patch is preferable.

Indeed. Thanks for the feedback.

> My suggestion only tested the first frag_skb length. If a list can be
> created where the first frag_skb is head_frag but a later one is not,
> it will fail short. I kind of doubt that.
> 
> By default skb_gro_receive builds GSO skbs that can be segmented
> along the original gso_size boundaries. We have so far only observed
> this issue when messing with gso_size.

The rationale was based on inputs specified in 43170c4e0ba7, where a GRO
skb has a fraglist with different amounts of payloads.

> We can easily refine the test to fall back on to copying only if
> skb_headlen(list_skb) != mss.

I'm concerned this is too generic; innocent skbs may fall victim to our
skb copy fallback. Probably those mentioned in 43170c4e0ba7.

> Alternatively, only on SKB_GSO_DODGY is fine, too.
> 
> I suggest we stick with the two-liner.

OK.
So lets refine your original codition, testing only the first
frag_skb, but also ensuring SKB_GSO_DODGY *and* 'skb_headlen(list_skb) != mss'
(we know existing code DOES work OK for unchanged gso_size, even if frags
have linear, non head_frag, data).

This hits the known, reproducable case of the mentioned BUG_ON, and is
tightly scoped to that case.

If that's agreed, I'll submit a proper patch.

Best,
Shmulik