Short doc on what BPF flow dissector should expect in the input __sk_buff and flow_keys. Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxx> --- .../networking/bpf_flow_dissector.txt | 115 ++++++++++++++++++ 1 file changed, 115 insertions(+) create mode 100644 Documentation/networking/bpf_flow_dissector.txt diff --git a/Documentation/networking/bpf_flow_dissector.txt b/Documentation/networking/bpf_flow_dissector.txt new file mode 100644 index 000000000000..513be8e20afb --- /dev/null +++ b/Documentation/networking/bpf_flow_dissector.txt @@ -0,0 +1,115 @@ +================== +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. -- 2.21.0.392.gf8f6787159e-goog
