mirror of https://git.keinpfusch.net/loweel/zabov
83 lines
3.1 KiB
Go
83 lines
3.1 KiB
Go
|
// Copyright 2016 The Go Authors. All rights reserved.
|
||
|
|
// Use of this source code is governed by a BSD-style
|
||
|
|
// license that can be found in the LICENSE file.
|
||
|
|
|
||
|
|
/*
|
||
|
|
|
||
|
|
Package bpf implements marshaling and unmarshaling of programs for the
|
||
|
|
Berkeley Packet Filter virtual machine, and provides a Go implementation
|
||
|
|
of the virtual machine.
|
||
|
|
|
||
|
|
BPF's main use is to specify a packet filter for network taps, so that
|
||
|
|
the kernel doesn't have to expensively copy every packet it sees to
|
||
|
|
userspace. However, it's been repurposed to other areas where running
|
||
|
|
user code in-kernel is needed. For example, Linux's seccomp uses BPF
|
||
|
|
to apply security policies to system calls. For simplicity, this
|
||
|
|
documentation refers only to packets, but other uses of BPF have their
|
||
|
|
own data payloads.
|
||
|
|
|
||
|
|
BPF programs run in a restricted virtual machine. It has almost no
|
||
|
|
access to kernel functions, and while conditional branches are
|
||
|
|
allowed, they can only jump forwards, to guarantee that there are no
|
||
|
|
infinite loops.
|
||
|
|
|
||
|
|
The virtual machine
|
||
|
|
|
||
|
|
The BPF VM is an accumulator machine. Its main register, called
|
||
|
|
register A, is an implicit source and destination in all arithmetic
|
||
|
|
and logic operations. The machine also has 16 scratch registers for
|
||
|
|
temporary storage, and an indirection register (register X) for
|
||
|
|
indirect memory access. All registers are 32 bits wide.
|
||
|
|
|
||
|
|
Each run of a BPF program is given one packet, which is placed in the
|
||
|
|
VM's read-only "main memory". LoadAbsolute and LoadIndirect
|
||
|
|
instructions can fetch up to 32 bits at a time into register A for
|
||
|
|
examination.
|
||
|
|
|
||
|
|
The goal of a BPF program is to produce and return a verdict (uint32),
|
||
|
|
which tells the kernel what to do with the packet. In the context of
|
||
|
|
packet filtering, the returned value is the number of bytes of the
|
||
|
|
packet to forward to userspace, or 0 to ignore the packet. Other
|
||
|
|
contexts like seccomp define their own return values.
|
||
|
|
|
||
|
|
In order to simplify programs, attempts to read past the end of the
|
||
|
|
packet terminate the program execution with a verdict of 0 (ignore
|
||
|
|
packet). This means that the vast majority of BPF programs don't need
|
||
|
|
to do any explicit bounds checking.
|
||
|
|
|
||
|
|
In addition to the bytes of the packet, some BPF programs have access
|
||
|
|
to extensions, which are essentially calls to kernel utility
|
||
|
|
functions. Currently, the only extensions supported by this package
|
||
|
|
are the Linux packet filter extensions.
|
||
|
|
|
||
|
|
Examples
|
||
|
|
|
||
|
|
This packet filter selects all ARP packets.
|
||
|
|
|
||
|
|
bpf.Assemble([]bpf.Instruction{
|
||
|
|
// Load "EtherType" field from the ethernet header.
|
||
|
|
bpf.LoadAbsolute{Off: 12, Size: 2},
|
||
|
|
// Skip over the next instruction if EtherType is not ARP.
|
||
|
|
bpf.JumpIf{Cond: bpf.JumpNotEqual, Val: 0x0806, SkipTrue: 1},
|
||
|
|
// Verdict is "send up to 4k of the packet to userspace."
|
||
|
|
bpf.RetConstant{Val: 4096},
|
||
|
|
// Verdict is "ignore packet."
|
||
|
|
bpf.RetConstant{Val: 0},
|
||
|
|
})
|
||
|
|
|
||
|
|
This packet filter captures a random 1% sample of traffic.
|
||
|
|
|
||
|
|
bpf.Assemble([]bpf.Instruction{
|
||
|
|
// Get a 32-bit random number from the Linux kernel.
|
||
|
|
bpf.LoadExtension{Num: bpf.ExtRand},
|
||
|
|
// 1% dice roll?
|
||
|
|
bpf.JumpIf{Cond: bpf.JumpLessThan, Val: 2^32/100, SkipFalse: 1},
|
||
|
|
// Capture.
|
||
|
|
bpf.RetConstant{Val: 4096},
|
||
|
|
// Ignore.
|
||
|
|
bpf.RetConstant{Val: 0},
|
||
|
|
})
|
||
|
|
|
||
|
|
*/
|
||
|
|
package bpf // import "golang.org/x/net/bpf"
|