libpcap 的 bpf

Posted 2020-06-10

tcpdump -d //参看 bpf 编译后的代码 

# tcpdump -i eth0 -n ‘vlan && tcp‘ -d
(000) ldh      [12]
(001) jeq      #0x8100          jt 3    jf 2
(002) jeq      #0x9100          jt 3    jf 14
(003) ldh      [16]
(004) jeq      #0x86dd          jt 5    jf 10
(005) ldb      [24]
(006) jeq      #0x6             jt 13    jf 7
(007) jeq      #0x2c            jt 8    jf 14
(008) ldb      [58]
(009) jeq      #0x6             jt 13    jf 14
(010) jeq      #0x800           jt 11    jf 14
(011) ldb      [27]
(012) jeq      #0x6             jt 13    jf 14
(013) ret      #262144
(014) ret      #0

bpf 语法

The BPF architecture consists of the following basic elements:

  Element          Description

  A                32 bit wide accumulator
  X                32 bit wide X register
  M[]              16 x 32 bit wide misc registers aka "scratch memory
                   store", addressable from 0 to 15

 Instruction      Addressing mode      Description

  ld               1, 2, 3, 4, 10       Load word into A
  ldi              4                    Load word into A
  ldh              1, 2                 Load half-word into A
  ldb              1, 2                 Load byte into A
  ldx              3, 4, 5, 10          Load word into X
  ldxi             4                    Load word into X
  ldxb             5                    Load byte into X

  st               3                    Store A into M[]
  stx              3                    Store X into M[]

  jmp              6                    Jump to label
  ja               6                    Jump to label
  jeq              7, 8                 Jump on k == A
  jneq             8                    Jump on k != A
  jne              8                    Jump on k != A
  jlt              8                    Jump on k < A
  jle              8                    Jump on k <= A
  jgt              7, 8                 Jump on k > A
  jge              7, 8                 Jump on k >= A
  jset             7, 8                 Jump on k & A

  add              0, 4                 A + <x>
  sub              0, 4                 A - <x>
  mul              0, 4                 A * <x>
  div              0, 4                 A / <x>
  mod              0, 4                 A % <x>
  neg              0, 4                 !A
  and              0, 4                 A & <x>
  or               0, 4                 A | <x>
  xor              0, 4                 A ^ <x>
  lsh              0, 4                 A << <x>
  rsh              0, 4                 A >> <x>

  tax                                   Copy A into X
  txa                                   Copy X into A

  ret              4, 9                 Return

The next table shows addressing formats from the 2nd column:

  Addressing mode  Syntax               Description

   0               x/%x                 Register X
   1               [k]                  BHW at byte offset k in the packet
   2               [x + k]              BHW at the offset X + k in the packet
   3               M[k]                 Word at offset k in M[]
   4               #k                   Literal value stored in k
   5               4*([k]&0xf)          Lower nibble * 4 at byte offset k in the packet
   6               L                    Jump label L
   7               #k,Lt,Lf             Jump to Lt if true, otherwise jump to Lf
   8               #k,Lt                Jump to Lt if predicate is true
   9               a/%a                 Accumulator A
  10               extension            BPF extension

The Linux kernel also has a couple of BPF extensions that are used along
with the class of load instructions by "overloading" the k argument with
a negative offset + a particular extension offset. The result of such BPF
extensions are loaded into A.

Possible BPF extensions are shown in the following table:

  Extension                             Description

  len                                   skb->len
  proto                                 skb->protocol
  type                                  skb->pkt_type
  poff                                  Payload start offset
  ifidx                                 skb->dev->ifindex
  nla                                   Netlink attribute of type X with offset A
  nlan                                  Nested Netlink attribute of type X with offset A
  mark                                  skb->mark
  queue                                 skb->queue_mapping
  hatype                                skb->dev->type
  rxhash                                skb->hash
  cpu                                   raw_smp_processor_id()
  vlan_tci                              skb_vlan_tag_get(skb)
  vlan_avail                            skb_vlan_tag_present(skb)
  vlan_tpid                             skb->vlan_proto
  rand                                  prandom_u32()