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// 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
import (
"errors"
"fmt"
)
// A VM is an emulated BPF virtual machine.
type VM struct {
filter []Instruction
}
// NewVM returns a new VM using the input BPF program.
func NewVM(filter []Instruction) (*VM, error) {
if len(filter) == 0 {
return nil, errors.New("one or more Instructions must be specified")
}
for i, ins := range filter {
check := len(filter) - (i + 1)
switch ins := ins.(type) {
// Check for out-of-bounds jumps in instructions
case Jump:
if check <= int(ins.Skip) {
return nil, fmt.Errorf("cannot jump %d instructions; jumping past program bounds", ins.Skip)
}
case JumpIf:
if check <= int(ins.SkipTrue) {
return nil, fmt.Errorf("cannot jump %d instructions in true case; jumping past program bounds", ins.SkipTrue)
}
if check <= int(ins.SkipFalse) {
return nil, fmt.Errorf("cannot jump %d instructions in false case; jumping past program bounds", ins.SkipFalse)
}
// Check for division or modulus by zero
case ALUOpConstant:
if ins.Val != 0 {
break
}
switch ins.Op {
case ALUOpDiv, ALUOpMod:
return nil, errors.New("cannot divide by zero using ALUOpConstant")
}
// Check for unknown extensions
case LoadExtension:
switch ins.Num {
case ExtLen:
default:
return nil, fmt.Errorf("extension %d not implemented", ins.Num)
}
}
}
// Make sure last instruction is a return instruction
switch filter[len(filter)-1].(type) {
case RetA, RetConstant:
default:
return nil, errors.New("BPF program must end with RetA or RetConstant")
}
// Though our VM works using disassembled instructions, we
// attempt to assemble the input filter anyway to ensure it is compatible
// with an operating system VM.
_, err := Assemble(filter)
return &VM{
filter: filter,
}, err
}
// Run runs the VM's BPF program against the input bytes.
// Run returns the number of bytes accepted by the BPF program, and any errors
// which occurred while processing the program.
func (v *VM) Run(in []byte) (int, error) {
var (
// Registers of the virtual machine
regA uint32
regX uint32
regScratch [16]uint32
// OK is true if the program should continue processing the next
// instruction, or false if not, causing the loop to break
ok = true
)
// TODO(mdlayher): implement:
// - NegateA:
// - would require a change from uint32 registers to int32
// registers
// TODO(mdlayher): add interop tests that check signedness of ALU
// operations against kernel implementation, and make sure Go
// implementation matches behavior
for i := 0; i < len(v.filter) && ok; i++ {
ins := v.filter[i]
switch ins := ins.(type) {
case ALUOpConstant:
regA = aluOpConstant(ins, regA)
case ALUOpX:
regA, ok = aluOpX(ins, regA, regX)
case Jump:
i += int(ins.Skip)
case JumpIf:
jump := jumpIf(ins, regA)
i += jump
case LoadAbsolute:
regA, ok = loadAbsolute(ins, in)
case LoadConstant:
regA, regX = loadConstant(ins, regA, regX)
case LoadExtension:
regA = loadExtension(ins, in)
case LoadIndirect:
regA, ok = loadIndirect(ins, in, regX)
case LoadMemShift:
regX, ok = loadMemShift(ins, in)
case LoadScratch:
regA, regX = loadScratch(ins, regScratch, regA, regX)
case RetA:
return int(regA), nil
case RetConstant:
return int(ins.Val), nil
case StoreScratch:
regScratch = storeScratch(ins, regScratch, regA, regX)
case TAX:
regX = regA
case TXA:
regA = regX
default:
return 0, fmt.Errorf("unknown Instruction at index %d: %T", i, ins)
}
}
return 0, nil
}
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