Merge branch 'bpf-ipv6-seg6-bpf-action'

Mathieu Xhonneux says:

====================
As of Linux 4.14, it is possible to define advanced local processing for
IPv6 packets with a Segment Routing Header through the seg6local LWT
infrastructure. This LWT implements the network programming principles
defined in the IETF "SRv6 Network Programming" draft.

The implemented operations are generic, and it would be very interesting to
be able to implement user-specific seg6local actions, without having to
modify the kernel directly. To do so, this patchset adds an End.BPF action
to seg6local, powered by some specific Segment Routing-related helpers,
which provide SR functionalities that can be applied on the packet. This
BPF hook would then allow to implement specific actions at native kernel
speed such as OAM features, advanced SR SDN policies, SRv6 actions like
Segment Routing Header (SRH) encapsulation depending on the content of
the packet, etc.

This patchset is divided in 6 patches, whose main features are :

- A new seg6local action End.BPF with the corresponding new BPF program
  type BPF_PROG_TYPE_LWT_SEG6LOCAL. Such attached BPF program can be
  passed to the LWT seg6local through netlink, the same way as the LWT
  BPF hook operates.
- 3 new BPF helpers for the seg6local BPF hook, allowing to edit/grow/
  shrink a SRH and apply on a packet some of the generic SRv6 actions.
- 1 new BPF helper for the LWT BPF IN hook, allowing to add a SRH through
  encapsulation (via IPv6 encapsulation or inlining if the packet contains
  already an IPv6 header).

As this patchset adds a new LWT BPF hook, I took into account the result
of the discussions when the LWT BPF infrastructure got merged. Hence, the
seg6local BPF hook doesn't allow write access to skb->data directly, only
the SRH can be modified through specific helpers, which ensures that the
integrity of the packet is maintained. More details are available in the
related patches messages.

The performances of this BPF hook have been assessed with the BPF JIT
enabled on an Intel Xeon X3440 processors with 4 cores and 8 threads
clocked at 2.53 GHz. No throughput losses are noted with the seg6local
BPF hook when the BPF program does nothing (440kpps). Adding a 8-bytes
TLV (1 call each to bpf_lwt_seg6_adjust_srh and bpf_lwt_seg6_store_bytes)
drops the throughput to 410kpps, and inlining a SRH via bpf_lwt_seg6_action
drops the throughput to 420kpps. All throughputs are stable.

Changelog:

v2: move the SRH integrity state from skb->cb to a per-cpu buffer
v3: - document helpers in man-page style
    - fix kbuild bugs
    - un-break BPF LWT out hook
    - bpf_push_seg6_encap is now static
    - preempt_enable is now called when the packet is dropped in
      input_action_end_bpf
v4: fix kbuild bugs when CONFIG_IPV6=m
v5: fix kbuild sparse warnings when CONFIG_IPV6=m
v6: fix skb pointers-related bugs in helpers
v7: - fix memory leak in error path of End.BPF setup
    - add freeing of BPF data in seg6_local_destroy_state
    - new enums SEG6_LOCAL_BPF_* instead of re-using ones of lwt bpf for
      netlink nested bpf attributes
    - SEG6_LOCAL_BPF_PROG attr now contains prog->aux->id when dumping
      state
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>