netdev - Re: [PATCH net] net/packet: tpacket_rcv: do not increment ring index on drop

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Date:   Tue, 10 Mar 2020 08:49:23 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net] net/packet: tpacket_rcv: do not increment ring index
 on drop

On Tue, Mar 10, 2020 at 2:43 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Mon, Mar 09, 2020 at 11:34:35AM -0400, Willem de Bruijn wrote:
> > From: Willem de Bruijn <willemb@...gle.com>
> >
> > In one error case, tpacket_rcv drops packets after incrementing the
> > ring producer index.
> >
> > If this happens, it does not update tp_status to TP_STATUS_USER and
> > thus the reader is stalled for an iteration of the ring, causing out
> > of order arrival.
> >
> > The only such error path is when virtio_net_hdr_from_skb fails due
> > to encountering an unknown GSO type.
> >
> > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> >
> > ---
> >
> > I wonder whether it should drop packets with unknown GSO types at all.
> > This consistently blinds the reader to certain packets, including
> > recent UDP and SCTP GSO types.
>
> Ugh it looks like you have found a bug.  Consider a legacy userspace -
> it was actually broken by adding USD and SCTP GSO.  I suspect the right
> thing to do here is actually to split these packets up, not drop them.

In the main virtio users, virtio_net/tun/tap, the packets will always
arrive segmented, due to these devices not advertising hardware
segmentation for these protocols.

So the issue is limited to users of tpacket_rcv, which is relatively
new. There too it is limited on egress to devices that do advertise
h/w offload. And on r/x to GRO.

The UDP GSO issue precedes the fraglist GRO patch, by the way, and
goes back to my (argh!) introduction of the feature on the egress
path.

>
> > The peer function virtio_net_hdr_to_skb already drops any packets with
> > unknown types, so it should be fine to add an SKB_GSO_UNKNOWN type and
> > let the peer at least be aware of failure.
> >
> > And possibly add SKB_GSO_UDP_L4 and SKB_GSO_SCTP types to virtio too.
>
> This last one is possible for sure, but for virtio_net_hdr_from_skb
> we'll need more flags to know whether it's safe to pass
> these types to userspace.

Can you elaborate? Since virtio_net_hdr_to_skb users already returns
-EINVAL on unknown GSO types and its callers just drop these packets,
it looks to me that the infra is future proof wrt adding new GSO
types.