netdev - Re: [PATCH v3] virtio: Work around frames incorrectly marked as gso

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Message-ID: <CA+FuTSdOCJZCVS4xohx+BQmkmq8JALnK=gGc0=qy1TbjY707ag@mail.gmail.com>
Date:   Mon, 24 Feb 2020 15:20:53 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Anton Ivanov <anton.ivanov@...bridgegreys.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        linux-um@...ts.infradead.org,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v3] virtio: Work around frames incorrectly marked as gso

On Mon, Feb 24, 2020 at 2:55 PM Anton Ivanov
<anton.ivanov@...bridgegreys.com> wrote:
>
> On 24/02/2020 19:27, Willem de Bruijn wrote:
> > On Mon, Feb 24, 2020 at 8:26 AM <anton.ivanov@...bridgegreys.com> wrote:
> >>
> >> From: Anton Ivanov <anton.ivanov@...bridgegreys.com>
> >>
> >> Some of the locally generated frames marked as GSO which
> >> arrive at virtio_net_hdr_from_skb() have no GSO_TYPE, no
> >> fragments (data_len = 0) and length significantly shorter
> >> than the MTU (752 in my experiments).
> >
> > Do we understand how these packets are generated?
>
> No, we have not been able to trace them.
>
> The only thing we know is that this is specific to locally generated
> packets. Something arriving from the network does not show this.
>
> > Else it seems this
> > might be papering over a deeper problem.
> >
> > The stack should not create GSO packets less than or equal to
> > skb_shinfo(skb)->gso_size. See for instance the check in
> > tcp_gso_segment after pulling the tcp header:
> >
> >          mss = skb_shinfo(skb)->gso_size;
> >          if (unlikely(skb->len <= mss))
> >                  goto out;
> >
> > What is the gso_type, and does it include SKB_GSO_DODGY?
> >
>
>
> 0 - not set.

Thanks for the follow-up details. Is this something that you can trigger easily?

An skb_dump() + dump_stack() when the packet socket gets such a
packet may point us to the root cause and fix that.