mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-12-28 16:56:26 +00:00
07ff4f0126
Describe the purpose of BPF sk_lookup program, how it can be attached, when it gets invoked, and what information gets passed to it. Point the reader to examples and further documentation. Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Link: https://lore.kernel.org/bpf/20200821100226.403844-1-jakub@cloudflare.com
99 lines
3.8 KiB
ReStructuredText
99 lines
3.8 KiB
ReStructuredText
.. 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.
|