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In commit da70314917 ("bpf: Document BPF_PROG_TYPE_CGROUP_SYSCTL")
Andrey proposes to put per-prog type docs under Documentation/bpf/
Let's move flow dissector documentation there as well.
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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4.3 KiB
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127 lines
4.3 KiB
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.. SPDX-License-Identifier: GPL-2.0
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============================
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BPF_PROG_TYPE_FLOW_DISSECTOR
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============================
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Overview
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========
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Flow dissector is a routine that parses metadata out of the packets. It's
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used in the various places in the networking subsystem (RFS, flow hash, etc).
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BPF flow dissector is an attempt to reimplement C-based flow dissector logic
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in BPF to gain all the benefits of BPF verifier (namely, limits on the
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number of instructions and tail calls).
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API
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===
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BPF flow dissector programs operate on an ``__sk_buff``. However, only the
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limited set of fields is allowed: ``data``, ``data_end`` and ``flow_keys``.
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``flow_keys`` is ``struct bpf_flow_keys`` and contains flow dissector input
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and output arguments.
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The inputs are:
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* ``nhoff`` - initial offset of the networking header
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* ``thoff`` - initial offset of the transport header, initialized to nhoff
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* ``n_proto`` - L3 protocol type, parsed out of L2 header
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Flow dissector BPF program should fill out the rest of the ``struct
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bpf_flow_keys`` fields. Input arguments ``nhoff/thoff/n_proto`` should be
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also adjusted accordingly.
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The return code of the BPF program is either BPF_OK to indicate successful
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dissection, or BPF_DROP to indicate parsing error.
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__sk_buff->data
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===============
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In the VLAN-less case, this is what the initial state of the BPF flow
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dissector looks like::
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+------+------+------------+-----------+
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| DMAC | SMAC | ETHER_TYPE | L3_HEADER |
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+------+------+------------+-----------+
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^
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+-- flow dissector starts here
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.. code:: c
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skb->data + flow_keys->nhoff point to the first byte of L3_HEADER
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flow_keys->thoff = nhoff
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flow_keys->n_proto = ETHER_TYPE
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In case of VLAN, flow dissector can be called with the two different states.
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Pre-VLAN parsing::
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+------+------+------+-----+-----------+-----------+
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| DMAC | SMAC | TPID | TCI |ETHER_TYPE | L3_HEADER |
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+------+------+------+-----+-----------+-----------+
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^
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+-- flow dissector starts here
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.. code:: c
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skb->data + flow_keys->nhoff point the to first byte of TCI
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flow_keys->thoff = nhoff
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flow_keys->n_proto = TPID
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Please note that TPID can be 802.1AD and, hence, BPF program would
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have to parse VLAN information twice for double tagged packets.
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Post-VLAN parsing::
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+------+------+------+-----+-----------+-----------+
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| DMAC | SMAC | TPID | TCI |ETHER_TYPE | L3_HEADER |
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+------+------+------+-----+-----------+-----------+
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^
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+-- flow dissector starts here
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.. code:: c
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skb->data + flow_keys->nhoff point the to first byte of L3_HEADER
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flow_keys->thoff = nhoff
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flow_keys->n_proto = ETHER_TYPE
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In this case VLAN information has been processed before the flow dissector
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and BPF flow dissector is not required to handle it.
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The takeaway here is as follows: BPF flow dissector program can be called with
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the optional VLAN header and should gracefully handle both cases: when single
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or double VLAN is present and when it is not present. The same program
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can be called for both cases and would have to be written carefully to
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handle both cases.
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Reference Implementation
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========================
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See ``tools/testing/selftests/bpf/progs/bpf_flow.c`` for the reference
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implementation and ``tools/testing/selftests/bpf/flow_dissector_load.[hc]``
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for the loader. bpftool can be used to load BPF flow dissector program as well.
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The reference implementation is organized as follows:
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* ``jmp_table`` map that contains sub-programs for each supported L3 protocol
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* ``_dissect`` routine - entry point; it does input ``n_proto`` parsing and
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does ``bpf_tail_call`` to the appropriate L3 handler
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Since BPF at this point doesn't support looping (or any jumping back),
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jmp_table is used instead to handle multiple levels of encapsulation (and
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IPv6 options).
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Current Limitations
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===================
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BPF flow dissector doesn't support exporting all the metadata that in-kernel
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C-based implementation can export. Notable example is single VLAN (802.1Q)
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and double VLAN (802.1AD) tags. Please refer to the ``struct bpf_flow_keys``
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for a set of information that's currently can be exported from the BPF context.
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