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:

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On Thu, Feb 13, 2020 at 07:44:06AM -0800, Eric Dumazet wrote:

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On 2/13/20 2:00 AM, Michael S. Tsirkin wrote:

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On Wed, Feb 12, 2020 at 05:38:09PM +0000, Anton Ivanov wrote:

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On 11/02/2020 10:37, Michael S. Tsirkin wrote:

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On Tue, Feb 11, 2020 at 07:42:37AM +0000, Anton Ivanov wrote:

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On 11/02/2020 02:51, Jason Wang wrote:

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On 2020/2/11 上午12:55, Anton Ivanov wrote:

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On 09/12/2019 10:48, anton.ivanov@cambridgegreys.com wrote:

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From: Anton Ivanov <anton.ivanov@cambridgegreys.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@cambridgegreys.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.

Want to submit a patch to address this for now?

quoted

?


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Anton R. Ivanov
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