Hello, This is v3 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 kfunc design are that a cgroup_skb program can call this function N times, thus send N ICMP unreach control messages and that the program can return from the BPF filter with SK_PASS leading to a potential confusing situation where the TCP connection was established while the client received ICMP_DEST_UNREACH messages. Another more sophisticated design idea would be for the kfunc to set the kernel to send an ICMP_HOST_UNREACH control message with the appropriate code when the cgroup_skb program terminates with SK_DROP. Creating a new 'SK_REJECT' return code for cgroup_skb program was generally rejected and would be too limited for other program types support. We should bear in mind that we want to add a TCP reset kfunc next and also could extend this kfunc to other program types if wanted. v2 updates: - fix a build error from a missing function call rename; - avoid changing return line in bpf_kfunc_init; - return SK_DROP from the kfunc (similarly to bpf_redirect); - check the return value in the selftest. v3 update: - fix an undefined reference build error. [^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 | 61 ++++++++++++ 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 | 99 +++++++++++++++++++ .../selftests/bpf/progs/icmp_send_unreach.c | 36 +++++++ 9 files changed, 235 insertions(+), 33 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
