Date:   Thu, 13 Feb 2020 11:37:16 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Anton Ivanov <anton.ivanov@...bridgegreys.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
        Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-um@...ts.infradead.org
Subject: Re: [PATCH] virtio: Work around frames incorrectly marked as gso

On Thu, Feb 13, 2020 at 04:23:24PM +0000, Anton Ivanov wrote:
> 
> On 13/02/2020 15:53, Michael S. Tsirkin wrote:
> > On Thu, Feb 13, 2020 at 07:44:06AM -0800, Eric Dumazet wrote:
> > > 
> > > On 2/13/20 2:00 AM, Michael S. Tsirkin wrote:
> > > > On Wed, Feb 12, 2020 at 05:38:09PM +0000, Anton Ivanov wrote:
> > > > > 
> > > > > On 11/02/2020 10:37, Michael S. Tsirkin wrote:
> > > > > > On Tue, Feb 11, 2020 at 07:42:37AM +0000, Anton Ivanov wrote:
> > > > > > > On 11/02/2020 02:51, Jason Wang wrote:
> > > > > > > > On 2020/2/11 上午12:55, Anton Ivanov wrote:
> > > > > > > > > 
> > > > > > > > > On 09/12/2019 10:48, anton.ivanov@...bridgegreys.com wrote:
> > > > > > > > > > From: Anton Ivanov <anton.ivanov@...bridgegreys.com>
> > > > > > > > > > 
> > > > > > > > > > Some of the 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).
> > > > > > > > > > 
> > > > > > > > > > This is observed on raw sockets reading off vEth interfaces
> > > > > > > > > > in all 4.x and 5.x kernels I tested.
> > > > > > > > > > 
> > > > > > > > > > These frames are reported as invalid while they are in fact
> > > > > > > > > > gso-less frames.
> > > > > > > > > > 
> > > > > > > > > > This patch marks the vnet header as no-GSO for them instead
> > > > > > > > > > of reporting it as invalid.
> > > > > > > > > > 
> > > > > > > > > > Signed-off-by: Anton Ivanov <anton.ivanov@...bridgegreys.com>
> > > > > > > > > > ---
> > > > > > > > > >     include/linux/virtio_net.h | 8 ++++++--
> > > > > > > > > >     1 file changed, 6 insertions(+), 2 deletions(-)
> > > > > > > > > > 
> > > > > > > > > > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> > > > > > > > > > index 0d1fe9297ac6..d90d5cff1b9a 100644
> > > > > > > > > > --- a/include/linux/virtio_net.h
> > > > > > > > > > +++ b/include/linux/virtio_net.h
> > > > > > > > > > @@ -112,8 +112,12 @@ static inline int
> > > > > > > > > > virtio_net_hdr_from_skb(const struct sk_buff *skb,
> > > > > > > > > >                 hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV4;
> > > > > > > > > >             else if (sinfo->gso_type & SKB_GSO_TCPV6)
> > > > > > > > > >                 hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV6;
> > > > > > > > > > -        else
> > > > > > > > > > -            return -EINVAL;
> > > > > > > > > > +        else {
> > > > > > > > > > +            if (skb->data_len == 0)
> > > > > > > > > > +                hdr->gso_type = VIRTIO_NET_HDR_GSO_NONE;
> > > > > > > > > > +            else
> > > > > > > > > > +                return -EINVAL;
> > > > > > > > > > +        }
> > > > > > > > > >             if (sinfo->gso_type & SKB_GSO_TCP_ECN)
> > > > > > > > > >                 hdr->gso_type |= VIRTIO_NET_HDR_GSO_ECN;
> > > > > > > > > >         } else
> > > > > > > > > > 
> > > > > > > > > ping.
> > > > > > > > > 
> > > > > > > > Do you mean gso_size is set but gso_type is not? Looks like a bug
> > > > > > > > elsewhere.
> > > > > > > > 
> > > > > > > > Thanks
> > > > > > > > 
> > > > > > > > 
> > > > > > > Yes.
> > > > > > > 
> > > > > > > I could not trace it where it is coming from.
> > > > > > > 
> > > > > > > I see it when doing recvmmsg on raw sockets in the UML vector network
> > > > > > > drivers.
> > > > > > > 
> > > > > > I think we need to find the culprit and fix it there, lots of other things
> > > > > > can break otherwise.
> > > > > > Just printing out skb->dev->name should do the trick, no?
> > > > > The printk in virtio_net_hdr_from_skb says NULL.
> > > > > 
> > > > > That is probably normal for a locally originated frame.
> > > > > 
> > > > > I cannot reproduce this with network traffic by the way - it happens only if the traffic is locally originated on the host.
> > > > > 
> > > > > A,
> > > > OK so is it code in __tcp_transmit_skb that sets gso_size to non-null
> > > > when gso_type is 0?
> > > > 
> > > Correct way to determine if a packet is a gso one is by looking at gso_size.
> > > Then only it is legal looking at gso_type
> > > 
> > > 
> > > static inline bool skb_is_gso(const struct sk_buff *skb)
> > > {
> > >      return skb_shinfo(skb)->gso_size;
> > > }
> > > 
> > > /* Note: Should be called only if skb_is_gso(skb) is true */
> > > static inline bool skb_is_gso_v6(const struct sk_buff *skb)
> > > ...
> > > 
> > > 
> > > There is absolutely no relation between GSO and skb->data_len, skb can be linearized
> > > for various orthogonal reasons.
> > The reported problem is that virtio gets a packet where gso_size
> > is !0 but gso_type is 0.
> > 
> > It currently drops these on the assumption that it's some type
> > of a gso packet it does not know how to handle.
> > 
> > 
> > So you are saying if skb_is_gso we can still have gso_type set to 0,
> > and that's an expected configuration?
> > 
> > So the patch should just be:
> > 
> > 
> > -        if (skb_is_gso(skb)) {
> > +        if (skb_is_gso(skb) && sinfo->gso_type) {
> > 
> Yes, provided that skb_is_gso(skb) and sinfo->gso_type == 0 is a valid state.
> 
> I agree with Jason, there may be something wrong going on here and we need to find the source which creates these packets.
> 
> A.

Well Eric will know for sure. Eric is skb_is_gso(skb) and
sinfo->gso_type == 0 a valid state?

> > 
> > ?
> > 
> > 
> > _______________________________________________
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> > linux-um@...ts.infradead.org
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> 
> -- 
> Anton R. Ivanov
> Cambridgegreys Limited. Registered in England. Company Number 10273661
> https://www.cambridgegreys.com/

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