[Bug Report] WAN IPv6 UDP dropped due to SKB_DROP_REASON_PKT_TOO_SMALL

[jschwinger233]

Checks

• I have searched the existing issues
• I have read the documentation
• Is it your first time sumbitting an issue

Current Behavior

dig @2001:4860:4860::8844 one.one.one.one couldn't get response.

dae trace log says it never receive any IPv6 UDP request.

pwru tracing says packets are dropped by kernel due to SKB_DROP_REASON_PKT_TOO_SMALL:

0xffff8a63853a3600      3     [dig(52934)]                udpv6_rcv netns=4026534242 mark=0x0 iface=16(eth0) proto=0x86dd mtu=1500 len=64 [2001:4860:4860::8844]:46396->[2001:db8::3]:12345(udp)
0xffff8a63853a3600      3     [dig(52934)]           __udp6_lib_rcv netns=4026534242 mark=0x0 iface=16(eth0) proto=0x86dd mtu=1500 len=64 [2001:4860:4860::8844]:46396->[2001:db8::3]:12345(udp)
0xffff8a63853a3600      3     [dig(52934)] kfree_skb_reason(SKB_DROP_REASON_PKT_TOO_SMALL) netns=4026534242 mark=0x0 iface=16(eth0) proto=0x86dd mtu=1500 len=64 [2001:4860:4860::8844]:46396->[2001:db8::3]:12345(udp)

Kernel source says skb->len doesn't match udp->len:
https://github.com/torvalds/linux/blob/v6.5/net/ipv6/udp.c#L969-L970

I suspect it's caused by adjust_ipv4_len during tproxy_wan_egress. It looks even IPv6 UDP packets are handled by adjust_ipv4_len, not sure what consequence it is.

Expected Behavior

No response

Steps to Reproduce

No response

Environment

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Anything else?

No response

[dae-prow]

Thanks for opening this issue!

[mzz2017]

Thank you so much for investigating it. It is related to #102.

[mzz2017]

@jschwinger233 If we use bpf_assign, this problem should be bypass.