Hello, This is v1 of adding the icmp_send_unreach kfunc, as suggested during LSF/MM/BPF 2025[^1]. The goal is to allow cgroup_skb programs to actively reject east-west traffic, similarly to what is possible to do with netfilter reject target. The first step to implement this is using ICMP control messages, with the ICMP_DEST_UNREACH type with various code ICMP_NET_UNREACH, ICMP_HOST_UNREACH, ICMP_PROT_UNREACH, etc. This is easier to implement than a TCP RST reply and will already hint the client TCP stack to abort the connection and not retry extensively. Note that this is different than the sock_destroy kfunc, that along calls tcp_abort and thus sends a reset, destroying the underlying socket. Caveats of this design are that a cgroup_skb program can call this function N times, and thus send N ICMP unreach control messages, and that the program can also finish the BPF filter with SK_PASS leading to a potential confusing situation where the TCP connection was established while the client receive ICMP_DEST_UNREACH messages. Other design ideas (to prevent above issues) could be: * Extend the return codes for the cgroup_skb program to trigger the reject after completion (SK_REJECT). * Adding a kfunc to set the kernel to send an ICMP_HOST_UNREACH control message with appropriate code when the cgroup_skb program eventually terminates with SK_DROP. We should bear in mind that we want to extend this with TCP reset next. Please tell me what's your opinion on above ideas: if adding new return codes could be considered and/or the other alternatives would be better than this patch series and thus proposed instead. v1 updates (from Daniel's offline review): - rename netfilter moved functions to ip(6)_route_reply_fetch_dst; - explain why nf_ip(6)_route are replaced with core route functions; - remove useless IP frag checks; - add KF_TRUSTED_ARGS to the kfunc; - simplify declarations of structs, initialize fd to -1 in tests; - insist on why SK_PASS is easier for testing in BPF prog. [^1]: https://lwn.net/Articles/1022034/ Mahe Tardy (4): net: move netfilter nf_reject_fill_skb_dst to core ipv4 net: move netfilter nf_reject6_fill_skb_dst to core ipv6 bpf: add bpf_icmp_send_unreach cgroup_skb kfunc selftests/bpf: add icmp_send_unreach kfunc tests include/net/ip6_route.h | 2 + include/net/route.h | 1 + net/core/filter.c | 63 +++++++++++- net/ipv4/netfilter/nf_reject_ipv4.c | 19 +--- net/ipv4/route.c | 15 +++ net/ipv6/netfilter/nf_reject_ipv6.c | 17 +--- net/ipv6/route.c | 18 ++++ .../bpf/prog_tests/icmp_send_unreach_kfunc.c | 96 +++++++++++++++++++ .../selftests/bpf/progs/icmp_send_unreach.c | 35 +++++++ 9 files changed, 232 insertions(+), 34 deletions(-) create mode 100644 tools/testing/selftests/bpf/prog_tests/icmp_send_unreach_kfunc.c create mode 100644 tools/testing/selftests/bpf/progs/icmp_send_unreach.c -- 2.34.1
