netdev - Re: [PATCH net] net: gso: Fix skb_segment splat when splitting gso_size mangled skb having linear-headed frag

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Date:   Fri, 6 Sep 2019 09:20:42 +0300
From:   Shmulik Ladkani <shmulik@...anetworks.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        eyal@...anetworks.com, netdev <netdev@...r.kernel.org>,
        Shmulik Ladkani <shmulik.ladkani@...il.com>
Subject: Re: [PATCH net] net: gso: Fix skb_segment splat when splitting
 gso_size mangled skb having linear-headed frag_list

On Thu, 5 Sep 2019 14:49:44 -0700
Alexander Duyck <alexander.duyck@...il.com> wrote:

> I would change the order of the tests you use here so that we can
> eliminate the possibility of needing to perform many tests for the
> more common cases. You could probably swap "list_skb" and "mss !=
> GSO_BY_FRAGS" since list_skb is more likely to be false for many of
> the common cases such as a standard TSO send from a socket. You might
> even consider moving the GSO_BY_FRAGS check toward the end of your
> checks since SCTP is the only protocol that I believe uses it and the
> likelihood of encountering it is much lower compared to other
> protocols.
> 
> You could probably test for !list_skb->head_frag before seeing if
> there is a headlen since many NICs would be generating frames using
> head_frag, so in the GRO case you mentioned above it could probably
> save you some effort on a number of NICs.
> 
> You might also consider moving this code up before we push the mac
> header back on and instead of setting sg to false you could just clear
> the NETIF_F_SG flag from features. It would save you from having to
> then remove doffset in your last check.

Thanks Alexander for the input. Will encorporate as much as possible
into a v2 patch.

BTW, I've strugged with myself regarding order of tests, and came
up with this suggestion, as my motivation was to have the tests order
tell a coherent logical story when read top-to-bottom left-to-right, FWIW.
For example, although 'mss != skb_headlen(head_skb)' could be tested
earlier, the condition by itself isn't meaningful unless we have an
existing frag_list and with a !head_frag.

Best
Shmulik