virtio BPF offers incorrect packet length

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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
#!/usr/bin/python
# Copyright (c) PLUMgrid, Inc.
# Licensed under the Apache License, Version 2.0 (the "License")
import sys
from bcc import BPF
from pyroute2 import IPRoute

ipr = IPRoute()

text = """
    int pkt_len_test(struct __sk_buff *skb) {
    void *data_end = (void *) (long)skb->data_end;
    void *data = (void *) (long)skb->data;
    bpf_trace_printk("data: %u end: %u len: %d \\n", data, data_end, skb->len);
    return 1;
}
"""

try:
    b = BPF(text=text, debug=0)
    fn = b.load_func("pkt_len_test", BPF.SCHED_CLS)
    idx = ipr.link_lookup(ifname=str(sys.argv[1]))[0]

    ipr.tc("add", "ingress", idx, "ffff:")
    ipr.tc("add-filter", "bpf", idx, ":1", fd=fn.fd,
           name=fn.name, parent="ffff:", action="ok", classid=1)
    ipr.tc("add", "sfq", idx, "1:")
    ipr.tc("add-filter", "bpf", idx, ":1", fd=fn.fd,
           name=fn.name, parent="1:", action="ok", classid=1)
    foo = input("Press any key to remove the filters and stop the program")
    ipr.tc("del", "ingress", idx, "ffff:")
    ipr.tc("del", "sfq", idx, "1:")
    print("Filters deleted")
except Exception as e:
    print("Error: %s", e)

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