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

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Message-ID: <CA+FuTSfqctjnMw2pg5s10MW-7fQJx_rkhrtZ2YgBr1ywq1CtLw@mail.gmail.com>
Date:   Tue, 10 Mar 2020 12:14:34 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.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 11:38 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Tue, Mar 10, 2020 at 10:44 AM Michael S. Tsirkin <mst@...hat.com> wrote:
> >
> > On Tue, Mar 10, 2020 at 10:16:56AM -0400, Willem de Bruijn wrote:
> > > On Tue, Mar 10, 2020 at 8:59 AM Michael S. Tsirkin <mst@...hat.com> wrote:
> > > >
> > > > 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.
> > >
> > > Not at all. Thanks for taking a look!
> > >
> > > > > 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.
> > >
> > > I'm not sure how that would work. Ignoring other tun/tap/virtio for
> > > now, just looking at tpacket, a new variant of socket option for
> > > PACKET_VNET_HDR, for every new GSO type?
> >
> > Maybe a single one with a bitmap of legal types?
> >
> > > In practice the userspace I'm aware of, and any sane implementation,
> > > will be future proof to drop and account packets whose type it cannot
> > > process. So I think we can just add new types.
> >
> > Well if packets are just dropped then userspace breaks right?
>
> It is an improvement over the current silent discard in the kernel.
>
> If it can count these packets, userspace becomes notified that it
> should perhaps upgrade or use ethtool to stop the kernel from
> generating certain packets.
>
> Specifically for packet sockets, it wants to receive packets as they
> appear "on the wire". It does not have to drop these today even, but
> can easily parse the headers.
>
> For packet sockets at least, I don't think that we want transparent
> segmentation.

Or more succinct:

For packet sockets I think we should pass everything up to userspace
as it is. Then virtio_net_hdr_from_skb never has to fail. And this
patch is unnecessary.

But we would need at least one new VIRTIO_NET_HDR_GSO_UNKNOWN type.
And only use that on packet sockets, keeping existing EINVAL in the
(indeed unexpected) case such packets make it to virtio_net or tun/tap
ndo_start_xmit.