linux/linux-5.18.11/Documentation/bpf/prog_sk_lookup.rst

.. SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)

=====================
BPF sk_lookup program
=====================

BPF sk_lookup program type (``BPF_PROG_TYPE_SK_LOOKUP``) introduces programmability
into the socket lookup performed by the transport layer when a packet is to be
delivered locally.

When invoked BPF sk_lookup program can select a socket that will receive the
incoming packet by calling the ``bpf_sk_assign()`` BPF helper function.

Hooks for a common attach point (``BPF_SK_LOOKUP``) exist for both TCP and UDP.

Motivation
==========

BPF sk_lookup program type was introduced to address setup scenarios where
binding sockets to an address with ``bind()`` socket call is impractical, such
as:

1. receiving connections on a range of IP addresses, e.g. 192.0.2.0/24, when
   binding to a wildcard address ``INADRR_ANY`` is not possible due to a port
   conflict,
2. receiving connections on all or a wide range of ports, i.e. an L7 proxy use
   case.

Such setups would require creating and ``bind()``'ing one socket to each of the
IP address/port in the range, leading to resource consumption and potential
latency spikes during socket lookup.

Attachment
==========

BPF sk_lookup program can be attached to a network namespace with
``bpf(BPF_LINK_CREATE, ...)`` syscall using the ``BPF_SK_LOOKUP`` attach type and a
netns FD as attachment ``target_fd``.

Multiple programs can be attached to one network namespace. Programs will be
invoked in the same order as they were attached.

Hooks
=====

The attached BPF sk_lookup programs run whenever the transport layer needs to
find a listening (TCP) or an unconnected (UDP) socket for an incoming packet.

Incoming traffic to established (TCP) and connected (UDP) sockets is delivered
as usual without triggering the BPF sk_lookup hook.

The attached BPF programs must return with either ``SK_PASS`` or ``SK_DROP``
verdict code. As for other BPF program types that are network filters,
``SK_PASS`` signifies that the socket lookup should continue on to regular
hashtable-based lookup, while ``SK_DROP`` causes the transport layer to drop the
packet.

A BPF sk_lookup program can also select a socket to receive the packet by
calling ``bpf_sk_assign()`` BPF helper. Typically, the program looks up a socket
in a map holding sockets, such as ``SOCKMAP`` or ``SOCKHASH``, and passes a
``struct bpf_sock *`` to ``bpf_sk_assign()`` helper to record the
selection. Selecting a socket only takes effect if the program has terminated
with ``SK_PASS`` code.

When multiple programs are attached, the end result is determined from return
codes of all the programs according to the following rules:

1. If any program returned ``SK_PASS`` and selected a valid socket, the socket
   is used as the result of the socket lookup.
2. If more than one program returned ``SK_PASS`` and selected a socket, the last
   selection takes effect.
3. If any program returned ``SK_DROP``, and no program returned ``SK_PASS`` and
   selected a socket, socket lookup fails.
4. If all programs returned ``SK_PASS`` and none of them selected a socket,
   socket lookup continues on.

API
===

In its context, an instance of ``struct bpf_sk_lookup``, BPF sk_lookup program
receives information about the packet that triggered the socket lookup. Namely:

* IP version (``AF_INET`` or ``AF_INET6``),
* L4 protocol identifier (``IPPROTO_TCP`` or ``IPPROTO_UDP``),
* source and destination IP address,
* source and destination L4 port,
* the socket that has been selected with ``bpf_sk_assign()``.

Refer to ``struct bpf_sk_lookup`` declaration in ``linux/bpf.h`` user API
header, and `bpf-helpers(7)
<https://man7.org/linux/man-pages/man7/bpf-helpers.7.html>`_ man-page section
for ``bpf_sk_assign()`` for details.

Example
=======

See ``tools/testing/selftests/bpf/prog_tests/sk_lookup.c`` for the reference
implementation.