github.com/Andyfoo/golang/x/net@v0.0.0-20190901054642-57c1bf301704/bpf/doc.go (about)

     1  // Copyright 2016 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  /*
     6  
     7  Package bpf implements marshaling and unmarshaling of programs for the
     8  Berkeley Packet Filter virtual machine, and provides a Go implementation
     9  of the virtual machine.
    10  
    11  BPF's main use is to specify a packet filter for network taps, so that
    12  the kernel doesn't have to expensively copy every packet it sees to
    13  userspace. However, it's been repurposed to other areas where running
    14  user code in-kernel is needed. For example, Linux's seccomp uses BPF
    15  to apply security policies to system calls. For simplicity, this
    16  documentation refers only to packets, but other uses of BPF have their
    17  own data payloads.
    18  
    19  BPF programs run in a restricted virtual machine. It has almost no
    20  access to kernel functions, and while conditional branches are
    21  allowed, they can only jump forwards, to guarantee that there are no
    22  infinite loops.
    23  
    24  The virtual machine
    25  
    26  The BPF VM is an accumulator machine. Its main register, called
    27  register A, is an implicit source and destination in all arithmetic
    28  and logic operations. The machine also has 16 scratch registers for
    29  temporary storage, and an indirection register (register X) for
    30  indirect memory access. All registers are 32 bits wide.
    31  
    32  Each run of a BPF program is given one packet, which is placed in the
    33  VM's read-only "main memory". LoadAbsolute and LoadIndirect
    34  instructions can fetch up to 32 bits at a time into register A for
    35  examination.
    36  
    37  The goal of a BPF program is to produce and return a verdict (uint32),
    38  which tells the kernel what to do with the packet. In the context of
    39  packet filtering, the returned value is the number of bytes of the
    40  packet to forward to userspace, or 0 to ignore the packet. Other
    41  contexts like seccomp define their own return values.
    42  
    43  In order to simplify programs, attempts to read past the end of the
    44  packet terminate the program execution with a verdict of 0 (ignore
    45  packet). This means that the vast majority of BPF programs don't need
    46  to do any explicit bounds checking.
    47  
    48  In addition to the bytes of the packet, some BPF programs have access
    49  to extensions, which are essentially calls to kernel utility
    50  functions. Currently, the only extensions supported by this package
    51  are the Linux packet filter extensions.
    52  
    53  Examples
    54  
    55  This packet filter selects all ARP packets.
    56  
    57  	bpf.Assemble([]bpf.Instruction{
    58  		// Load "EtherType" field from the ethernet header.
    59  		bpf.LoadAbsolute{Off: 12, Size: 2},
    60  		// Skip over the next instruction if EtherType is not ARP.
    61  		bpf.JumpIf{Cond: bpf.JumpNotEqual, Val: 0x0806, SkipTrue: 1},
    62  		// Verdict is "send up to 4k of the packet to userspace."
    63  		bpf.RetConstant{Val: 4096},
    64  		// Verdict is "ignore packet."
    65  		bpf.RetConstant{Val: 0},
    66  	})
    67  
    68  This packet filter captures a random 1% sample of traffic.
    69  
    70  	bpf.Assemble([]bpf.Instruction{
    71  		// Get a 32-bit random number from the Linux kernel.
    72  		bpf.LoadExtension{Num: bpf.ExtRand},
    73  		// 1% dice roll?
    74  		bpf.JumpIf{Cond: bpf.JumpLessThan, Val: 2^32/100, SkipFalse: 1},
    75  		// Capture.
    76  		bpf.RetConstant{Val: 4096},
    77  		// Ignore.
    78  		bpf.RetConstant{Val: 0},
    79  	})
    80  
    81  */
    82  package bpf // import "github.com/Andyfoo/golang/x/net/bpf"