doc.go (3217B)
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 Package bpf implements marshaling and unmarshaling of programs for the 7 Berkeley Packet Filter virtual machine, and provides a Go implementation 8 of the virtual machine. 9 10 BPF's main use is to specify a packet filter for network taps, so that 11 the kernel doesn't have to expensively copy every packet it sees to 12 userspace. However, it's been repurposed to other areas where running 13 user code in-kernel is needed. For example, Linux's seccomp uses BPF 14 to apply security policies to system calls. For simplicity, this 15 documentation refers only to packets, but other uses of BPF have their 16 own data payloads. 17 18 BPF programs run in a restricted virtual machine. It has almost no 19 access to kernel functions, and while conditional branches are 20 allowed, they can only jump forwards, to guarantee that there are no 21 infinite loops. 22 23 # The virtual machine 24 25 The BPF VM is an accumulator machine. Its main register, called 26 register A, is an implicit source and destination in all arithmetic 27 and logic operations. The machine also has 16 scratch registers for 28 temporary storage, and an indirection register (register X) for 29 indirect memory access. All registers are 32 bits wide. 30 31 Each run of a BPF program is given one packet, which is placed in the 32 VM's read-only "main memory". LoadAbsolute and LoadIndirect 33 instructions can fetch up to 32 bits at a time into register A for 34 examination. 35 36 The goal of a BPF program is to produce and return a verdict (uint32), 37 which tells the kernel what to do with the packet. In the context of 38 packet filtering, the returned value is the number of bytes of the 39 packet to forward to userspace, or 0 to ignore the packet. Other 40 contexts like seccomp define their own return values. 41 42 In order to simplify programs, attempts to read past the end of the 43 packet terminate the program execution with a verdict of 0 (ignore 44 packet). This means that the vast majority of BPF programs don't need 45 to do any explicit bounds checking. 46 47 In addition to the bytes of the packet, some BPF programs have access 48 to extensions, which are essentially calls to kernel utility 49 functions. Currently, the only extensions supported by this package 50 are the Linux packet filter extensions. 51 52 # Examples 53 54 This packet filter selects all ARP packets. 55 56 bpf.Assemble([]bpf.Instruction{ 57 // Load "EtherType" field from the ethernet header. 58 bpf.LoadAbsolute{Off: 12, Size: 2}, 59 // Skip over the next instruction if EtherType is not ARP. 60 bpf.JumpIf{Cond: bpf.JumpNotEqual, Val: 0x0806, SkipTrue: 1}, 61 // Verdict is "send up to 4k of the packet to userspace." 62 bpf.RetConstant{Val: 4096}, 63 // Verdict is "ignore packet." 64 bpf.RetConstant{Val: 0}, 65 }) 66 67 This packet filter captures a random 1% sample of traffic. 68 69 bpf.Assemble([]bpf.Instruction{ 70 // Get a 32-bit random number from the Linux kernel. 71 bpf.LoadExtension{Num: bpf.ExtRand}, 72 // 1% dice roll? 73 bpf.JumpIf{Cond: bpf.JumpLessThan, Val: 2^32/100, SkipFalse: 1}, 74 // Capture. 75 bpf.RetConstant{Val: 4096}, 76 // Ignore. 77 bpf.RetConstant{Val: 0}, 78 }) 79 */ 80 package bpf // import "golang.org/x/net/bpf"