lwt: fix return values of BPF ops

lwt xmit hook does not expect positive return values in function ip_finish_output2 and ip6_finish_output. However, BPF programs can directly return positive statuses such like NET_XMIT_DROP, NET_RX_DROP, and etc to the caller. Such return values would make the kernel continue processing already freed skbs and eventually panic.

This set fixes the return values from BPF ops to unexpected continue processing, checks strictly on the correct continue condition for future proof. In addition, add missing selftests for BPF redirect and reroute cases for BPF-CI.

v5: https://lore.kernel.org/bpf/[email protected]/
v4: https://lore.kernel.org/bpf/[email protected]/T/
v3: https://lore.kernel.org/bpf/[email protected]/
v2: https://lore.kernel.org/netdev/[email protected]/
v1: https://lore.kernel.org/bpf/[email protected]/

changes since v5:
* fix BPF-CI failures due to missing config and busybox ping issue

changes since v4:
* fixed same error on BPF_REROUTE path
* re-implemented selftests under BPF-CI requirement

changes since v3:
* minor change in commit message and changelogs
* tested by Jakub Sitnicki

changes since v2:
* subject name changed
* also covered redirect to ingress case
* added selftests

changes since v1:
* minor code style changes

Yan Zhai (4):
lwt: fix return values of BPF xmit ops
lwt: check LWTUNNEL_XMIT_CONTINUE strictly selftests/bpf: add lwt_xmit tests for BPF_REDIRECT selftests/bpf: add lwt_xmit tests for BPF_REROUTE

 include/net/lwtunnel.h                        |   5 +-
 net/core/lwt_bpf.c                            |   7 +-
 net/ipv4/ip_output.c                          |   2 +-
 net/ipv6/ip6_output.c                         |   2 +-
 tools/testing/selftests/bpf/config            |   2 +
 .../selftests/bpf/prog_tests/lwt_helpers.h    | 139 ++++++++
 .../selftests/bpf/prog_tests/lwt_redirect.c   | 330 ++++++++++++++++++
 .../selftests/bpf/prog_tests/lwt_reroute.c    | 262 ++++++++++++++
 .../selftests/bpf/progs/test_lwt_redirect.c   |  90 +++++
 .../selftests/bpf/progs/test_lwt_reroute.c    |  36 ++
 10 files changed, 868 insertions(+), 7 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/lwt_helpers.h
 create mode 100644 tools/testing/selftests/bpf/prog_tests/lwt_redirect.c
 create mode 100644 tools/testing/selftests/bpf/prog_tests/lwt_reroute.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_lwt_redirect.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_lwt_reroute.c

2.30.2