[PATCH bpf-next v1 0/4] bpf: add icmp_send_unreach kfunc

Mahe Tardy
2025-07-25T18:55:50Z

Hello,

This is v2 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.

[^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 | 59 +++++++++++
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, 233 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

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2.34.1