Re: kernel BUG at net/core/skbuff.c:4219

quoted

Hi,

One of our Flatcar users has been hitting the kernel BUG in the subject line
for the past year (https://github.com/flatcar/Flatcar/issues/378). This was
first reported when on 5.10.25, but has been happening across kernel updates,
most recently with 5.15.63. The nodes where this happens are AWS EC2 instances,
using ENA and calico networking in eBPF mode with VXLAN encapsulation. When
GRO/GSO is enabled, the host hits this bug and prints the following stacktrace:

I suspect eBPF code lowers gso_size ?

gso stack is not able to arbitrarily segment a GRO packet after gso_size
being changed.

Based on the stack trace, it happens for a tcp packet. Since it seems
like it is on egress from a host, I suspect that it is encapsulating a
tcp packet into a vxlan tunnel and useds bpf_skb_adjust_room() here
https://github.com/projectcalico/calico/blob/master/felix/bpf-gpl/nat.h#L77-L80
Not sure if it should use BPF_F_ADJ_ROOM_FIXED_GSO in that case.

Calico uses the flag when decapsulating packets here
https://github.com/projectcalico/calico/blob/master/felix/bpf-gpl/nat.h#L143
and it uses the flag as the packet is UDP.

Any of that could cause the BUG?


Tomas