TCP SYN/ACK packets of connections from processes/sockets outside a cgroup on the same host are not received by the cgroup's installed cgroup_skb filters.
There were two BPF cgroup_skb programs attached to a cgroup named "my_cgroup".
SEC("cgroup_skb/ingress") int ingress(struct __sk_buff *skb) { /* .... process skb ... */ return 1; }
SEC("cgroup_skb/egress") int egress(struct __sk_buff *skb) { /* .... process skb ... */ return 1;
}
We discovered that when running the command "nc -6 -l 8000" in "my_group" and connecting to it from outside of "my_cgroup" with the command "nc -6 localhost 8000", the egress filter did not detect the SYN/ACK packet. However, we did observe the SYN/ACK packet at the ingress when connecting from a socket in "my_cgroup" to a socket outside of it.
We came across BPF_CGROUP_RUN_PROG_INET_EGRESS(). This macro is responsible for calling BPF programs that are attached to the egress hook of a cgroup and it skips programs if the sending socket is not the owner of the skb. Specifically, in our situation, the SYN/ACK skb is owned by a struct request_sock instance, but the sending socket is the listener socket we use to receive incoming connections. The request_sock is created to manage an incoming connection.
It has been determined that checking the owner of a skb against the sending socket is not required. Removing this check will allow the filters to receive SYN/ACK packets.
To ensure that cgroup_skb filters can receive all signaling packets, including SYN, SYN/ACK, ACK, FIN, and FIN/ACK. A new self-test has been added as well.
Changes from v2:
- Remove redundant blank lines.
Changes from v1:
- Check the number of observed packets instead of just sleeping.
- Use ASSERT_XXX() instead of CHECK()/
[v1] https://lore.kernel.org/all/20230612191641.441774-1-kuifeng@meta.com/ [v2] https://lore.kernel.org/all/20230617052756.640916-2-kuifeng@meta.com/
Kui-Feng Lee (2): net: bpf: Always call BPF cgroup filters for egress. selftests/bpf: Verify that the cgroup_skb filters receive expected packets.
include/linux/bpf-cgroup.h | 2 +- tools/testing/selftests/bpf/cgroup_helpers.c | 12 + tools/testing/selftests/bpf/cgroup_helpers.h | 1 + tools/testing/selftests/bpf/cgroup_tcp_skb.h | 35 ++ .../selftests/bpf/prog_tests/cgroup_tcp_skb.c | 399 ++++++++++++++++++ .../selftests/bpf/progs/cgroup_tcp_skb.c | 382 +++++++++++++++++ 6 files changed, 830 insertions(+), 1 deletion(-) create mode 100644 tools/testing/selftests/bpf/cgroup_tcp_skb.h create mode 100644 tools/testing/selftests/bpf/prog_tests/cgroup_tcp_skb.c create mode 100644 tools/testing/selftests/bpf/progs/cgroup_tcp_skb.c