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// Copyright 2018 Google Inc.
//
// 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 (
"fmt"
"math"
"gvisor.googlesource.com/gvisor/pkg/abi/linux"
)
const (
labelTarget = math.MaxUint8
labelDirectTarget = math.MaxUint32
)
// ProgramBuilder assists with building a BPF program with jump
// labels that are resolved to their proper offsets.
type ProgramBuilder struct {
// Maps label names to label objects.
labels map[string]*label
// Array of BPF instructions that makes up the program.
instructions []linux.BPFInstruction
}
// NewProgramBuilder creates a new ProgramBuilder instance.
func NewProgramBuilder() *ProgramBuilder {
return &ProgramBuilder{labels: map[string]*label{}}
}
// label contains information to resolve a label to an offset.
type label struct {
// List of locations that reference the label in the program.
sources []source
// Program line when the label is located.
target int
}
type jmpType int
const (
jDirect jmpType = iota
jTrue
jFalse
)
// source contains information about a single reference to a label.
type source struct {
// Program line where the label reference is present.
line int
// True if label reference is in the 'jump if true' part of the jump.
// False if label reference is in the 'jump if false' part of the jump.
jt jmpType
}
// AddStmt adds a new statement to the program.
func (b *ProgramBuilder) AddStmt(code uint16, k uint32) {
b.instructions = append(b.instructions, Stmt(code, k))
}
// AddJump adds a new jump to the program.
func (b *ProgramBuilder) AddJump(code uint16, k uint32, jt, jf uint8) {
b.instructions = append(b.instructions, Jump(code, k, jt, jf))
}
// AddDirectJumpLabel adds a new jump to the program where is labelled.
func (b *ProgramBuilder) AddDirectJumpLabel(labelName string) {
b.addLabelSource(labelName, jDirect)
b.AddJump(Jmp|Ja, labelDirectTarget, 0, 0)
}
// AddJumpTrueLabel adds a new jump to the program where 'jump if true' is a label.
func (b *ProgramBuilder) AddJumpTrueLabel(code uint16, k uint32, jtLabel string, jf uint8) {
b.addLabelSource(jtLabel, jTrue)
b.AddJump(code, k, labelTarget, jf)
}
// AddJumpFalseLabel adds a new jump to the program where 'jump if false' is a label.
func (b *ProgramBuilder) AddJumpFalseLabel(code uint16, k uint32, jt uint8, jfLabel string) {
b.addLabelSource(jfLabel, jFalse)
b.AddJump(code, k, jt, labelTarget)
}
// AddJumpLabels adds a new jump to the program where both jump targets are labels.
func (b *ProgramBuilder) AddJumpLabels(code uint16, k uint32, jtLabel, jfLabel string) {
b.addLabelSource(jtLabel, jTrue)
b.addLabelSource(jfLabel, jFalse)
b.AddJump(code, k, labelTarget, labelTarget)
}
// AddLabel sets the given label name at the current location. The next instruction is executed
// when the any code jumps to this label. More than one label can be added to the same location.
func (b *ProgramBuilder) AddLabel(name string) error {
l, ok := b.labels[name]
if !ok {
// This is done to catch jump backwards cases, but it's not strictly wrong
// to have unused labels.
return fmt.Errorf("Adding a label that hasn't been used is not allowed: %v", name)
}
if l.target != -1 {
return fmt.Errorf("label %q target already set: %v", name, l.target)
}
l.target = len(b.instructions)
return nil
}
// Instructions returns an array of BPF instructions representing the program with all labels
// resolved. Return error in case label resolution failed due to an invalid program.
//
// N.B. Partial results will be returned in the error case, which is useful for debugging.
func (b *ProgramBuilder) Instructions() ([]linux.BPFInstruction, error) {
if err := b.resolveLabels(); err != nil {
return b.instructions, err
}
return b.instructions, nil
}
func (b *ProgramBuilder) addLabelSource(labelName string, t jmpType) {
l, ok := b.labels[labelName]
if !ok {
l = &label{sources: make([]source, 0), target: -1}
b.labels[labelName] = l
}
l.sources = append(l.sources, source{line: len(b.instructions), jt: t})
}
func (b *ProgramBuilder) resolveLabels() error {
for key, v := range b.labels {
if v.target == -1 {
return fmt.Errorf("label target not set: %v", key)
}
if v.target >= len(b.instructions) {
return fmt.Errorf("target is beyond end of ProgramBuilder")
}
for _, s := range v.sources {
// Finds jump instruction that references the label.
inst := b.instructions[s.line]
if s.line >= v.target {
return fmt.Errorf("cannot jump backwards")
}
// Calculates the jump offset from current line.
offset := v.target - s.line - 1
// Sets offset into jump instruction.
switch s.jt {
case jDirect:
if offset > labelDirectTarget {
return fmt.Errorf("jump offset to label '%v' is too large: %v, inst: %v, lineno: %v", key, offset, inst, s.line)
}
if inst.K != labelDirectTarget {
return fmt.Errorf("jump target is not a label")
}
inst.K = uint32(offset)
case jTrue:
if offset > labelTarget {
return fmt.Errorf("jump offset to label '%v' is too large: %v, inst: %v, lineno: %v", key, offset, inst, s.line)
}
if inst.JumpIfTrue != labelTarget {
return fmt.Errorf("jump target is not a label")
}
inst.JumpIfTrue = uint8(offset)
case jFalse:
if offset > labelTarget {
return fmt.Errorf("jump offset to label '%v' is too large: %v, inst: %v, lineno: %v", key, offset, inst, s.line)
}
if inst.JumpIfFalse != labelTarget {
return fmt.Errorf("jump target is not a label")
}
inst.JumpIfFalse = uint8(offset)
}
b.instructions[s.line] = inst
}
}
b.labels = map[string]*label{}
return nil
}
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