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for the loader. bpftool can be used to load BPF flow dissector program as well... SPDX-License-Identifier: GPL-2.0==================BPF Flow Dissector==================Overview========Flow dissector is a routine that parses metadata out of the packets. It'sused in the various places in the networking subsystem (RFS, flow hash, etc).BPF flow dissector is an attempt to reimplement C-based flow dissector logicin BPF to gain all the benefits of BPF verifier (namely, limits on thenumber of instructions and tail calls).API===BPF flow dissector programs operate on an ``__sk_buff``. However, only thelimited set of fields is allowed: ``data``, ``data_end`` and ``flow_keys``.``flow_keys`` is ``struct bpf_flow_keys`` and contains flow dissector inputand output arguments.The inputs are: * ``nhoff`` - initial offset of the networking header * ``thoff`` - initial offset of the transport header, initialized to nhoff * ``n_proto`` - L3 protocol type, parsed out of L2 headerFlow dissector BPF program should fill out the rest of the ``structbpf_flow_keys`` fields. Input arguments ``nhoff/thoff/n_proto`` should bealso adjusted accordingly.The return code of the BPF program is either BPF_OK to indicate successfuldissection, or BPF_DROP to indicate parsing error.__sk_buff->data===============In the VLAN-less case, this is what the initial state of the BPF flowdissector looks like:: +------+------+------------+-----------+ | DMAC | SMAC | ETHER_TYPE | L3_HEADER | +------+------+------------+-----------+ ^ | +-- flow dissector starts here.. code:: c skb->data + flow_keys->nhoff point to the first byte of L3_HEADER flow_keys->thoff = nhoff flow_keys->n_proto = ETHER_TYPEIn case of VLAN, flow dissector can be called with the two different states.Pre-VLAN parsing:: +------+------+------+-----+-----------+-----------+ | DMAC | SMAC | TPID | TCI |ETHER_TYPE | L3_HEADER | +------+------+------+-----+-----------+-----------+ ^ | +-- flow dissector starts here.. code:: c skb->data + flow_keys->nhoff point the to first byte of TCI flow_keys->thoff = nhoff flow_keys->n_proto = TPIDPlease note that TPID can be 802.1AD and, hence, BPF program wouldhave to parse VLAN information twice for double tagged packets.Post-VLAN parsing:: +------+------+------+-----+-----------+-----------+ | DMAC | SMAC | TPID | TCI |ETHER_TYPE | L3_HEADER | +------+------+------+-----+-----------+-----------+ ^ | +-- flow dissector starts here.. code:: c skb->data + flow_keys->nhoff point the to first byte of L3_HEADER flow_keys->thoff = nhoff flow_keys->n_proto = ETHER_TYPE