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

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Date:   Tue, 10 Mar 2020 08:59:01 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     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 08:49:23AM -0400, Willem de Bruijn wrote:
> 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.

Oh right. That's good then, sorry about the noise.

> 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.

Oh I mean if we do want to add new types and want to pass them to
users, then virtio_net_hdr_from_skb will need to flag so it
knows whether that will or won't confuse userspace.

-- 
MST