99 строки
3.8 KiB
ReStructuredText
99 строки
3.8 KiB
ReStructuredText
.. SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
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=====================
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BPF sk_lookup program
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=====================
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BPF sk_lookup program type (``BPF_PROG_TYPE_SK_LOOKUP``) introduces programmability
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into the socket lookup performed by the transport layer when a packet is to be
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delivered locally.
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When invoked BPF sk_lookup program can select a socket that will receive the
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incoming packet by calling the ``bpf_sk_assign()`` BPF helper function.
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Hooks for a common attach point (``BPF_SK_LOOKUP``) exist for both TCP and UDP.
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Motivation
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==========
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BPF sk_lookup program type was introduced to address setup scenarios where
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binding sockets to an address with ``bind()`` socket call is impractical, such
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as:
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1. receiving connections on a range of IP addresses, e.g. 192.0.2.0/24, when
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binding to a wildcard address ``INADRR_ANY`` is not possible due to a port
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conflict,
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2. receiving connections on all or a wide range of ports, i.e. an L7 proxy use
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case.
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Such setups would require creating and ``bind()``'ing one socket to each of the
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IP address/port in the range, leading to resource consumption and potential
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latency spikes during socket lookup.
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Attachment
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==========
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BPF sk_lookup program can be attached to a network namespace with
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``bpf(BPF_LINK_CREATE, ...)`` syscall using the ``BPF_SK_LOOKUP`` attach type and a
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netns FD as attachment ``target_fd``.
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Multiple programs can be attached to one network namespace. Programs will be
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invoked in the same order as they were attached.
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Hooks
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=====
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The attached BPF sk_lookup programs run whenever the transport layer needs to
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find a listening (TCP) or an unconnected (UDP) socket for an incoming packet.
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Incoming traffic to established (TCP) and connected (UDP) sockets is delivered
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as usual without triggering the BPF sk_lookup hook.
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The attached BPF programs must return with either ``SK_PASS`` or ``SK_DROP``
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verdict code. As for other BPF program types that are network filters,
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``SK_PASS`` signifies that the socket lookup should continue on to regular
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hashtable-based lookup, while ``SK_DROP`` causes the transport layer to drop the
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packet.
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A BPF sk_lookup program can also select a socket to receive the packet by
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calling ``bpf_sk_assign()`` BPF helper. Typically, the program looks up a socket
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in a map holding sockets, such as ``SOCKMAP`` or ``SOCKHASH``, and passes a
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``struct bpf_sock *`` to ``bpf_sk_assign()`` helper to record the
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selection. Selecting a socket only takes effect if the program has terminated
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with ``SK_PASS`` code.
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When multiple programs are attached, the end result is determined from return
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codes of all the programs according to the following rules:
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1. If any program returned ``SK_PASS`` and selected a valid socket, the socket
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is used as the result of the socket lookup.
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2. If more than one program returned ``SK_PASS`` and selected a socket, the last
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selection takes effect.
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3. If any program returned ``SK_DROP``, and no program returned ``SK_PASS`` and
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selected a socket, socket lookup fails.
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4. If all programs returned ``SK_PASS`` and none of them selected a socket,
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socket lookup continues on.
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API
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===
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In its context, an instance of ``struct bpf_sk_lookup``, BPF sk_lookup program
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receives information about the packet that triggered the socket lookup. Namely:
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* IP version (``AF_INET`` or ``AF_INET6``),
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* L4 protocol identifier (``IPPROTO_TCP`` or ``IPPROTO_UDP``),
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* source and destination IP address,
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* source and destination L4 port,
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* the socket that has been selected with ``bpf_sk_assign()``.
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Refer to ``struct bpf_sk_lookup`` declaration in ``linux/bpf.h`` user API
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header, and `bpf-helpers(7)
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<https://man7.org/linux/man-pages/man7/bpf-helpers.7.html>`_ man-page section
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for ``bpf_sk_assign()`` for details.
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Example
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=======
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See ``tools/testing/selftests/bpf/prog_tests/sk_lookup.c`` for the reference
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implementation.
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