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| -rw-r--r-- | Documentation/networking/bpf_flow_dissector.rst | 126 | ||||
| -rw-r--r-- | Documentation/networking/bpf_flow_dissector.txt | 115 | ||||
| -rw-r--r-- | Documentation/networking/index.rst | 1 |
3 files changed, 127 insertions, 115 deletions
diff --git a/Documentation/networking/bpf_flow_dissector.rst b/Documentation/networking/bpf_flow_dissector.rst new file mode 100644 index 000000000000..b375ae2ec2c4 --- /dev/null +++ b/Documentation/networking/bpf_flow_dissector.rst @@ -0,0 +1,126 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================== +BPF Flow Dissector +================== + +Overview +======== + +Flow dissector is a routine that parses metadata out of the packets. It's +used in the various places in the networking subsystem (RFS, flow hash, etc). + +BPF flow dissector is an attempt to reimplement C-based flow dissector logic +in BPF to gain all the benefits of BPF verifier (namely, limits on the +number of instructions and tail calls). + +API +=== + +BPF flow dissector programs operate on an ``__sk_buff``. However, only the +limited set of fields is allowed: ``data``, ``data_end`` and ``flow_keys``. +``flow_keys`` is ``struct bpf_flow_keys`` and contains flow dissector input +and 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 header + +Flow dissector BPF program should fill out the rest of the ``struct +bpf_flow_keys`` fields. Input arguments ``nhoff/thoff/n_proto`` should be +also adjusted accordingly. + +The return code of the BPF program is either BPF_OK to indicate successful +dissection, or BPF_DROP to indicate parsing error. + +__sk_buff->data +=============== + +In the VLAN-less case, this is what the initial state of the BPF flow +dissector 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_TYPE + +In 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 = TPID + +Please note that TPID can be 802.1AD and, hence, BPF program would +have 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 + +In this case VLAN information has been processed before the flow dissector +and BPF flow dissector is not required to handle it. + + +The takeaway here is as follows: BPF flow dissector program can be called with +the optional VLAN header and should gracefully handle both cases: when single +or double VLAN is present and when it is not present. The same program +can be called for both cases and would have to be written carefully to +handle both cases. + + +Reference Implementation +======================== + +See ``tools/testing/selftests/bpf/progs/bpf_flow.c`` for the reference +implementation and ``tools/testing/selftests/bpf/flow_dissector_load.[hc]`` +for the loader. bpftool can be used to load BPF flow dissector program as well. + +The reference implementation is organized as follows: + * ``jmp_table`` map that contains sub-programs for each supported L3 protocol + * ``_dissect`` routine - entry point; it does input ``n_proto`` parsing and + does ``bpf_tail_call`` to the appropriate L3 handler + +Since BPF at this point doesn't support looping (or any jumping back), +jmp_table is used instead to handle multiple levels of encapsulation (and +IPv6 options). + + +Current Limitations +=================== +BPF flow dissector doesn't support exporting all the metadata that in-kernel +C-based implementation can export. Notable example is single VLAN (802.1Q) +and double VLAN (802.1AD) tags. Please refer to the ``struct bpf_flow_keys`` +for a set of information that's currently can be exported from the BPF context. diff --git a/Documentation/networking/bpf_flow_dissector.txt b/Documentation/networking/bpf_flow_dissector.txt deleted file mode 100644 index 70f66a2d57e7..000000000000 --- a/Documentation/networking/bpf_flow_dissector.txt +++ /dev/null @@ -1,115 +0,0 @@ -================== -BPF Flow Dissector -================== - -Overview -======== - -Flow dissector is a routine that parses metadata out of the packets. It's -used in the various places in the networking subsystem (RFS, flow hash, etc). - -BPF flow dissector is an attempt to reimplement C-based flow dissector logic -in BPF to gain all the benefits of BPF verifier (namely, limits on the -number of instructions and tail calls). - -API -=== - -BPF flow dissector programs operate on an __sk_buff. However, only the -limited set of fields is allowed: data, data_end and flow_keys. flow_keys -is 'struct bpf_flow_keys' and contains flow dissector input and -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 header - -Flow dissector BPF program should fill out the rest of the 'struct -bpf_flow_keys' fields. Input arguments nhoff/thoff/n_proto should be also -adjusted accordingly. - -The return code of the BPF program is either BPF_OK to indicate successful -dissection, or BPF_DROP to indicate parsing error. - -__sk_buff->data -=============== - -In the VLAN-less case, this is what the initial state of the BPF flow -dissector looks like: -+------+------+------------+-----------+ -| DMAC | SMAC | ETHER_TYPE | L3_HEADER | -+------+------+------------+-----------+ - ^ - | - +-- flow dissector starts here - -skb->data + flow_keys->nhoff point to the first byte of L3_HEADER. -flow_keys->thoff = nhoff -flow_keys->n_proto = ETHER_TYPE - - -In 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 - -skb->data + flow_keys->nhoff point the to first byte of TCI. -flow_keys->thoff = nhoff -flow_keys->n_proto = TPID - -Please note that TPID can be 802.1AD and, hence, BPF program would -have to parse VLAN information twice for double tagged packets. - - -Post-VLAN parsing: -+------+------+------+-----+-----------+-----------+ -| DMAC | SMAC | TPID | TCI |ETHER_TYPE | L3_HEADER | -+------+------+------+-----+-----------+-----------+ - ^ - | - +-- flow dissector starts here - -skb->data + flow_keys->nhoff point the to first byte of L3_HEADER. -flow_keys->thoff = nhoff -flow_keys->n_proto = ETHER_TYPE - -In this case VLAN information has been processed before the flow dissector -and BPF flow dissector is not required to handle it. - - -The takeaway here is as follows: BPF flow dissector program can be called with -the optional VLAN header and should gracefully handle both cases: when single -or double VLAN is present and when it is not present. The same program -can be called for both cases and would have to be written carefully to -handle both cases. - - -Reference Implementation -======================== - -See tools/testing/selftests/bpf/progs/bpf_flow.c for the reference -implementation and tools/testing/selftests/bpf/flow_dissector_load.[hc] for -the loader. bpftool can be used to load BPF flow dissector program as well. - -The reference implementation is organized as follows: -* jmp_table map that contains sub-programs for each supported L3 protocol -* _dissect routine - entry point; it does input n_proto parsing and does - bpf_tail_call to the appropriate L3 handler - -Since BPF at this point doesn't support looping (or any jumping back), -jmp_table is used instead to handle multiple levels of encapsulation (and -IPv6 options). - - -Current Limitations -=================== -BPF flow dissector doesn't support exporting all the metadata that in-kernel -C-based implementation can export. Notable example is single VLAN (802.1Q) -and double VLAN (802.1AD) tags. Please refer to the 'struct bpf_flow_keys' -for a set of information that's currently can be exported from the BPF context. diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst index 5449149be496..984e68f9e026 100644 --- a/Documentation/networking/index.rst +++ b/Documentation/networking/index.rst @@ -9,6 +9,7 @@ Contents: netdev-FAQ af_xdp batman-adv + bpf_flow_dissector can can_ucan_protocol device_drivers/freescale/dpaa2/index |