virtio BPF offers incorrect packet length

Arnau Verdaguer
Wed, 24 Jul 2019 16:44:43 -0000

Hi,

I suspect that the virtio network driver calls some skb BPF programs with
skb->data_end - skb->data != skb->len, but only for forwarded packets.

For instance, the attached sched_cls tc program prints skb->data,
skb->data_end and skb->len for each packet:

          <idle>-0     [000] ..s.   491.561727: 0: data: 3110080576
data_end: 3110080704 len: 262
          <idle>-0     [000] .Ns.   491.561752: 0: data: 3110080064
data_end: 3110080192 len: 250

As it can be seen, the frame length should be 262 and 250 bytes, but
data_end - data is always 128. For packets smaller than 128 it works fine.

I've tried the latest kernel (5.3-rc1) besides 4.14, 4.19, 5.2, etc., and
the error persists. Other drivers than virtio work as expected and I can
inspect every byte of the packet. Locally generated traffic also works as
expected.

To carry out this experiment I've used a Debian 10 virtual machine with
net.ipv4.ip_forward=1 and net.ipv4.conf.eth0.forwarding=1, forwarding
packets between its two virtio interfaces. The example program is run with
the following command, having BCC installed:

$> sudo python3 pkt_len_text.py eth0

Where "eth0" could be another ingress interface. The output is showed
at
/sys/kernel/debug/tracing/trace_pipe.

Can anyone confirm if this error is reproducible, and does it have any
solutions?

BR.

Arnau
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