[bpf-next,V13,0/7] bpf: New approach for BPF MTU handling

Message

Jesper Dangaard Brouer Jan. 25, 2021, 5:09 p.m. UTC

This patchset drops all the MTU checks in TC BPF-helpers that limits
growing the packet size. This is done because these BPF-helpers doesn't
take redirect into account, which can result in their MTU check being done
against the wrong netdev.

The new approach is to give BPF-programs knowledge about the MTU on a
netdev (via ifindex) and fib route lookup level. Meaning some BPF-helpers
are added and extended to make it possible to do MTU checks in the
BPF-code.

If BPF-prog doesn't comply with the MTU then the packet will eventually
get dropped as some other layer. In some cases the existing kernel MTU
checks will drop the packet, but there are also cases where BPF can bypass
these checks. Specifically doing TC-redirect from ingress step
(sch_handle_ingress) into egress code path (basically calling
dev_queue_xmit()). It is left up to driver code to handle these kind of
MTU violations.

One advantage of this approach is that it ingress-to-egress BPF-prog can
send information via packet data. With the MTU checks removed in the
helpers, and also not done in skb_do_redirect() call, this allows for an
ingress BPF-prog to communicate with an egress BPF-prog via packet data,
as long as egress BPF-prog remove this prior to transmitting packet.

This patchset is primarily focused on TC-BPF, but I've made sure that the
MTU BPF-helpers also works for XDP BPF-programs.

V2: Change BPF-helper API from lookup to check.
V3: Drop enforcement of MTU in net-core, leave it to drivers.
V4: Keep sanity limit + netdev "up" checks + rename BPF-helper.
V5: Fix uninit variable + name struct output member mtu_result.
V6: Use bpf_check_mtu() in selftest
V7: Fix logic using tot_len and add another selftest
V8: Add better selftests for BPF-helper bpf_check_mtu
V9: Remove patch that use skb_set_redirected
V10: Fix selftests and 'tot_len' MTU check like XDP
V11: Fix nitpicks in selftests
V12: Adjustments requested by Daniel
V13: More adjustments requested by Daniel

---

Jesper Dangaard Brouer (7):
      bpf: Remove MTU check in __bpf_skb_max_len
      bpf: fix bpf_fib_lookup helper MTU check for SKB ctx
      bpf: bpf_fib_lookup return MTU value as output when looked up
      bpf: add BPF-helper for MTU checking
      bpf: drop MTU check when doing TC-BPF redirect to ingress
      selftests/bpf: use bpf_check_mtu in selftest test_cls_redirect
      selftests/bpf: tests using bpf_check_mtu BPF-helper


 include/linux/netdevice.h                          |   32 +++
 include/uapi/linux/bpf.h                           |   78 +++++++
 net/core/dev.c                                     |   32 +--
 net/core/filter.c                                  |  204 +++++++++++++++----
 tools/include/uapi/linux/bpf.h                     |   78 +++++++
 tools/testing/selftests/bpf/prog_tests/check_mtu.c |  216 ++++++++++++++++++++
 tools/testing/selftests/bpf/progs/test_check_mtu.c |  198 ++++++++++++++++++
 .../selftests/bpf/progs/test_cls_redirect.c        |    7 +
 8 files changed, 781 insertions(+), 64 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/check_mtu.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_check_mtu.c

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