Add FOU support for externally controlled ipip devices

This patch set adds support for using FOU or GUE encapsulation with an ipip device operating in collect-metadata mode and a set of kfuncs for controlling encap parameters exposed to a BPF tc-hook.

BPF tc-hooks allow us to read tunnel metadata (like remote IP addresses) in the ingress path of an externally controlled tunnel interface via the bpf_skb_get_tunnel_{key,opt} bpf-helpers. Packets can then be redirected to the same or a different externally controlled tunnel interface by overwriting metadata via the bpf_skb_set_tunnel_{key,opt} helpers and a call to bpf_redirect. This enables us to redirect packets between tunnel interfaces - and potentially change the encapsulation type - using only a single BPF program.

Today this approach works fine for a couple of tunnel combinations. For example: redirecting packets between Geneve and GRE interfaces or GRE and plain ipip interfaces. However, redirecting using FOU or GUE is not supported today. The ip_tunnel module does not allow us to egress packets using additional UDP encapsulation from an ipip device in collect-metadata mode.

Patch 1 lifts this restriction by adding a struct ip_tunnel_encap to the tunnel metadata. It can be filled by a new BPF kfunc introduced in Patch 2 and evaluated by the ip_tunnel egress path. This will allow us to use FOU and GUE encap with externally controlled ipip devices.

Patch 2 introduces two new BPF kfuncs: bpf_skb_{set,get}_fou_encap. These helpers can be used to set and get UDP encap parameters from the BPF tc-hook doing the packet redirect.

Patch 3 adds BPF tunnel selftests using the two kfuncs.

---
v3:
- Integrate selftest into test_progs (Alexei)
v2:
- Fixes for checkpatch.pl
- Fixes for kernel test robot

Christian Ehrig (3):
ipip,ip_tunnel,sit: Add FOU support for externally controlled ipip
devices
bpf,fou: Add bpf_skb_{set,get}_fou_encap kfuncs selftests/bpf: Test FOU kfuncs for externally controlled ipip devices

 include/net/fou.h                             |   2 +
 include/net/ip_tunnels.h                      |  28 ++--
 net/ipv4/Makefile                             |   2 +-
 net/ipv4/fou_bpf.c                            | 119 ++++++++++++++
 net/ipv4/fou_core.c                           |   5 +
 net/ipv4/ip_tunnel.c                          |  22 ++-
 net/ipv4/ipip.c                               |   1 +
 net/ipv6/sit.c                                |   2 +-
 .../selftests/bpf/prog_tests/test_tunnel.c    | 153 +++++++++++++++++-
 .../selftests/bpf/progs/test_tunnel_kern.c    | 117 ++++++++++++++
 10 files changed, 432 insertions(+), 19 deletions(-)
 create mode 100644 net/ipv4/fou_bpf.c

2.39.2