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// Copyright 2018 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package bpf
import (
"bytes"
"fmt"
"gvisor.googlesource.com/gvisor/pkg/abi/linux"
)
// DecodeProgram translates an array of BPF instructions into text format.
func DecodeProgram(program []linux.BPFInstruction) (string, error) {
var ret bytes.Buffer
for line, s := range program {
ret.WriteString(fmt.Sprintf("%v: ", line))
if err := decode(s, line, &ret); err != nil {
return "", err
}
ret.WriteString("\n")
}
return ret.String(), nil
}
// Decode translates BPF instruction into text format.
func Decode(inst linux.BPFInstruction) (string, error) {
var ret bytes.Buffer
err := decode(inst, -1, &ret)
return ret.String(), err
}
func decode(inst linux.BPFInstruction, line int, w *bytes.Buffer) error {
var err error
switch inst.OpCode & instructionClassMask {
case Ld:
err = decodeLd(inst, w)
case Ldx:
err = decodeLdx(inst, w)
case St:
w.WriteString(fmt.Sprintf("M[%v] <- A", inst.K))
case Stx:
w.WriteString(fmt.Sprintf("M[%v] <- X", inst.K))
case Alu:
err = decodeAlu(inst, w)
case Jmp:
err = decodeJmp(inst, line, w)
case Ret:
err = decodeRet(inst, w)
case Misc:
err = decodeMisc(inst, w)
default:
return fmt.Errorf("invalid BPF instruction: %v", inst)
}
return err
}
// A <- P[k:4]
func decodeLd(inst linux.BPFInstruction, w *bytes.Buffer) error {
w.WriteString("A <- ")
switch inst.OpCode & loadModeMask {
case Imm:
w.WriteString(fmt.Sprintf("%v", inst.K))
case Abs:
w.WriteString(fmt.Sprintf("P[%v:", inst.K))
if err := decodeLdSize(inst, w); err != nil {
return err
}
w.WriteString("]")
case Ind:
w.WriteString(fmt.Sprintf("P[X+%v:", inst.K))
if err := decodeLdSize(inst, w); err != nil {
return err
}
w.WriteString("]")
case Mem:
w.WriteString(fmt.Sprintf("M[%v]", inst.K))
case Len:
w.WriteString("len")
default:
return fmt.Errorf("invalid BPF LD instruction: %v", inst)
}
return nil
}
func decodeLdSize(inst linux.BPFInstruction, w *bytes.Buffer) error {
switch inst.OpCode & loadSizeMask {
case W:
w.WriteString("4")
case H:
w.WriteString("2")
case B:
w.WriteString("1")
default:
return fmt.Errorf("Invalid BPF LD size: %v", inst)
}
return nil
}
// X <- P[k:4]
func decodeLdx(inst linux.BPFInstruction, w *bytes.Buffer) error {
w.WriteString("X <- ")
switch inst.OpCode & loadModeMask {
case Imm:
w.WriteString(fmt.Sprintf("%v", inst.K))
case Mem:
w.WriteString(fmt.Sprintf("M[%v]", inst.K))
case Len:
w.WriteString("len")
case Msh:
w.WriteString(fmt.Sprintf("4*(P[%v:1]&0xf)", inst.K))
default:
return fmt.Errorf("invalid BPF LDX instruction: %v", inst)
}
return nil
}
// A <- A + k
func decodeAlu(inst linux.BPFInstruction, w *bytes.Buffer) error {
code := inst.OpCode & aluMask
if code == Neg {
w.WriteString("A <- -A")
return nil
}
w.WriteString("A <- A ")
switch code {
case Add:
w.WriteString("+ ")
case Sub:
w.WriteString("- ")
case Mul:
w.WriteString("* ")
case Div:
w.WriteString("/ ")
case Or:
w.WriteString("| ")
case And:
w.WriteString("& ")
case Lsh:
w.WriteString("<< ")
case Rsh:
w.WriteString(">> ")
case Mod:
w.WriteString("% ")
case Xor:
w.WriteString("^ ")
default:
return fmt.Errorf("invalid BPF ALU instruction: %v", inst)
}
return decodeSource(inst, w)
}
func decodeSource(inst linux.BPFInstruction, w *bytes.Buffer) error {
switch inst.OpCode & srcAluJmpMask {
case K:
w.WriteString(fmt.Sprintf("%v", inst.K))
case X:
w.WriteString("X")
default:
return fmt.Errorf("invalid BPF ALU/JMP source instruction: %v", inst)
}
return nil
}
// pc += (A > k) ? jt : jf
func decodeJmp(inst linux.BPFInstruction, line int, w *bytes.Buffer) error {
code := inst.OpCode & jmpMask
w.WriteString("pc += ")
if code == Ja {
w.WriteString(printJmpTarget(inst.K, line))
} else {
w.WriteString("(A ")
switch code {
case Jeq:
w.WriteString("== ")
case Jgt:
w.WriteString("> ")
case Jge:
w.WriteString(">= ")
case Jset:
w.WriteString("& ")
default:
return fmt.Errorf("invalid BPF ALU instruction: %v", inst)
}
if err := decodeSource(inst, w); err != nil {
return err
}
w.WriteString(
fmt.Sprintf(") ? %s : %s",
printJmpTarget(uint32(inst.JumpIfTrue), line),
printJmpTarget(uint32(inst.JumpIfFalse), line)))
}
return nil
}
func printJmpTarget(target uint32, line int) string {
if line == -1 {
return fmt.Sprintf("%v", target)
}
return fmt.Sprintf("%v [%v]", target, int(target)+line+1)
}
// ret k
func decodeRet(inst linux.BPFInstruction, w *bytes.Buffer) error {
w.WriteString("ret ")
code := inst.OpCode & srcRetMask
switch code {
case K:
w.WriteString(fmt.Sprintf("%v", inst.K))
case A:
w.WriteString("A")
default:
return fmt.Errorf("invalid BPF RET source instruction: %v", inst)
}
return nil
}
func decodeMisc(inst linux.BPFInstruction, w *bytes.Buffer) error {
code := inst.OpCode & miscMask
switch code {
case Tax:
w.WriteString("X <- A")
case Txa:
w.WriteString("A <- X")
default:
return fmt.Errorf("invalid BPF ALU/JMP source instruction: %v", inst)
}
return nil
}
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