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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