/
macho.go
380 lines (339 loc) · 8.77 KB
/
macho.go
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// Copyright 2014 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.
// Mach-O (Darwin) object file writing.
package main
import (
"debug/macho"
"encoding/binary"
"io"
"strings"
)
// machoFormat is the implementation of formatter.
type machoFormat struct{}
// machoHeader and friends are data structures
// corresponding to the Mach-O file header
// to be written to disk.
const (
macho64Bit = 1 << 24
machoSubCPU386 = 3
)
// machoArch describes a Mach-O target architecture.
type machoArch struct {
CPU uint32
SubCPU uint32
}
// machoHeader is the Mach-O file header.
type machoHeader struct {
machoArch
FileType uint32
Loads []*machoLoad
Segments []*machoSegment
p *Prog // for reporting errors
}
// machoLoad is a Mach-O load command.
type machoLoad struct {
Type uint32
Data []uint32
}
// machoSegment is a Mach-O segment.
type machoSegment struct {
Name string
VirtAddr Addr
VirtSize Addr
FileOffset Addr
FileSize Addr
Prot1 uint32
Prot2 uint32
Flags uint32
Sections []*machoSection
}
// machoSection is a Mach-O section, inside a segment.
type machoSection struct {
Name string
Segment string
Addr Addr
Size Addr
Offset uint32
Align uint32
Reloc uint32
Nreloc uint32
Flags uint32
Res1 uint32
Res2 uint32
}
// layout positions the segments and sections in p
// to make room for the Mach-O file header.
// That is, it edits their VirtAddr fields to adjust for the presence
// of the Mach-O header at the beginning of the address space.
func (machoFormat) headerSize(p *Prog) (virt, file Addr) {
var h machoHeader
h.init(p)
size := Addr(h.size())
size = round(size, 4096)
p.HeaderSize = size
return size, size
}
// write writes p to w as a Mach-O executable.
// layout(p) must have already been called,
// and the number, sizes, and addresses of the segments
// and sections must not have been modified since the call.
func (machoFormat) write(w io.Writer, p *Prog) {
var h machoHeader
h.init(p)
off := Addr(0)
enc := h.encode()
w.Write(enc)
off += Addr(len(enc))
for _, seg := range p.Segments {
if seg.FileOffset < off {
h.p.errorf("mach-o error: invalid file offset")
}
w.Write(make([]byte, int(seg.FileOffset-off)))
if seg.FileSize != Addr(len(seg.Data)) {
h.p.errorf("mach-o error: invalid file size")
}
w.Write(seg.Data)
off = seg.FileOffset + Addr(len(seg.Data))
}
}
// Conversion of Prog to macho data structures.
// machoArches maps from GOARCH to machoArch.
var machoArches = map[string]machoArch{
"amd64": {
CPU: uint32(macho.CpuAmd64),
SubCPU: uint32(machoSubCPU386),
},
}
// init initializes the header h to describe p.
func (h *machoHeader) init(p *Prog) {
h.p = p
h.Segments = nil
h.Loads = nil
var ok bool
h.machoArch, ok = machoArches[p.GOARCH]
if !ok {
p.errorf("mach-o: unknown target GOARCH %q", p.GOARCH)
return
}
h.FileType = uint32(macho.TypeExec)
mseg := h.addSegment(p, "__PAGEZERO", nil)
mseg.VirtSize = p.UnmappedSize
for _, seg := range p.Segments {
h.addSegment(p, "__"+strings.ToUpper(seg.Name), seg)
}
var data []uint32
switch h.CPU {
default:
p.errorf("mach-o: unknown cpu %#x for GOARCH %q", h.CPU, p.GOARCH)
case uint32(macho.CpuAmd64):
data = make([]uint32, 2+42)
data[0] = 4 // thread type
data[1] = 42 // word count
data[2+32] = uint32(p.Entry) // RIP register, in two parts
data[2+32+1] = uint32(p.Entry >> 32)
}
h.Loads = append(h.Loads, &machoLoad{
Type: uint32(macho.LoadCmdUnixThread),
Data: data,
})
}
// addSegment adds to h a Mach-O segment like seg with the given name.
func (h *machoHeader) addSegment(p *Prog, name string, seg *Segment) *machoSegment {
mseg := &machoSegment{
Name: name,
}
h.Segments = append(h.Segments, mseg)
if seg == nil {
return mseg
}
mseg.VirtAddr = seg.VirtAddr
mseg.VirtSize = seg.VirtSize
mseg.FileOffset = round(seg.FileOffset, 4096)
mseg.FileSize = seg.FileSize
if name == "__TEXT" {
// Initially RWX, then just RX
mseg.Prot1 = 7
mseg.Prot2 = 5
// Text segment maps Mach-O header, needed by dynamic linker.
mseg.VirtAddr -= p.HeaderSize
mseg.VirtSize += p.HeaderSize
mseg.FileOffset -= p.HeaderSize
mseg.FileSize += p.HeaderSize
} else {
// RW
mseg.Prot1 = 3
mseg.Prot2 = 3
}
for _, sect := range seg.Sections {
h.addSection(mseg, seg, sect)
}
return mseg
}
// addSection adds to mseg a Mach-O section like sect, inside seg, with the given name.
func (h *machoHeader) addSection(mseg *machoSegment, seg *Segment, sect *Section) {
msect := &machoSection{
Name: "__" + sect.Name,
Segment: mseg.Name,
// Reloc: sect.RelocOffset,
// NumReloc: sect.RelocLen / 8,
Addr: sect.VirtAddr,
Size: sect.Size,
}
mseg.Sections = append(mseg.Sections, msect)
for 1<<msect.Align < sect.Align {
msect.Align++
}
if off := sect.VirtAddr - seg.VirtAddr; off < seg.FileSize {
// Data in file.
if sect.Size > seg.FileSize-off {
h.p.errorf("mach-o error: section crosses file boundary")
}
msect.Offset = uint32(seg.FileOffset + off)
} else {
// Zero filled.
msect.Flags |= 1
}
if sect.Name == "text" {
msect.Flags |= 0x400 // contains executable instructions
}
}
// A machoWriter helps write Mach-O headers.
// It is basically a buffer with some helper routines for writing integers.
type machoWriter struct {
dst []byte
tmp [8]byte
order binary.ByteOrder
is64 bool
p *Prog
}
// if64 returns x if w is writing a 64-bit object file; otherwise it returns y.
func (w *machoWriter) if64(x, y interface{}) interface{} {
if w.is64 {
return x
}
return y
}
// encode encodes each of the given arguments into the writer.
// It encodes uint32, []uint32, uint64, and []uint64 by writing each value
// in turn in the correct byte order for the output file.
// It encodes an Addr as a uint64 if writing a 64-bit output file, or else as a uint32.
// It encodes []byte and string by writing the raw bytes (no length prefix).
// It skips nil values in the args list.
func (w *machoWriter) encode(args ...interface{}) {
for _, arg := range args {
switch arg := arg.(type) {
default:
w.p.errorf("mach-o error: cannot encode %T", arg)
case nil:
// skip
case []byte:
w.dst = append(w.dst, arg...)
case string:
w.dst = append(w.dst, arg...)
case uint32:
w.order.PutUint32(w.tmp[:], arg)
w.dst = append(w.dst, w.tmp[:4]...)
case []uint32:
for _, x := range arg {
w.order.PutUint32(w.tmp[:], x)
w.dst = append(w.dst, w.tmp[:4]...)
}
case uint64:
w.order.PutUint64(w.tmp[:], arg)
w.dst = append(w.dst, w.tmp[:8]...)
case Addr:
if w.is64 {
w.order.PutUint64(w.tmp[:], uint64(arg))
w.dst = append(w.dst, w.tmp[:8]...)
} else {
if Addr(uint32(arg)) != arg {
w.p.errorf("mach-o error: truncating address %#x to uint32", arg)
}
w.order.PutUint32(w.tmp[:], uint32(arg))
w.dst = append(w.dst, w.tmp[:4]...)
}
}
}
}
// segmentSize returns the size of the encoding of seg in bytes.
func (w *machoWriter) segmentSize(seg *machoSegment) int {
if w.is64 {
return 18*4 + 20*4*len(seg.Sections)
}
return 14*4 + 22*4*len(seg.Sections)
}
// zeroPad returns the string s truncated or padded with NULs to n bytes.
func zeroPad(s string, n int) string {
if len(s) >= n {
return s[:n]
}
return s + strings.Repeat("\x00", n-len(s))
}
// size returns the encoded size of the header.
func (h *machoHeader) size() int {
// Could write separate code, but encoding is cheap; encode and throw it away.
return len(h.encode())
}
// encode returns the Mach-O encoding of the header.
func (h *machoHeader) encode() []byte {
w := &machoWriter{p: h.p}
w.is64 = h.CPU&macho64Bit != 0
w.order = w.p.byteorder
loadSize := 0
for _, seg := range h.Segments {
loadSize += w.segmentSize(seg)
}
for _, l := range h.Loads {
loadSize += 4 * (2 + len(l.Data))
}
w.encode(
w.if64(macho.Magic64, macho.Magic32),
uint32(h.CPU),
uint32(h.SubCPU),
uint32(h.FileType),
uint32(len(h.Loads)+len(h.Segments)),
uint32(loadSize),
uint32(1),
w.if64(uint32(0), nil),
)
for _, seg := range h.Segments {
w.encode(
w.if64(uint32(macho.LoadCmdSegment64), uint32(macho.LoadCmdSegment)),
uint32(w.segmentSize(seg)),
zeroPad(seg.Name, 16),
seg.VirtAddr,
seg.VirtSize,
seg.FileOffset,
seg.FileSize,
seg.Prot1,
seg.Prot2,
uint32(len(seg.Sections)),
seg.Flags,
)
for _, sect := range seg.Sections {
w.encode(
zeroPad(sect.Name, 16),
zeroPad(seg.Name, 16),
sect.Addr,
sect.Size,
sect.Offset,
sect.Align,
sect.Reloc,
sect.Nreloc,
sect.Flags,
sect.Res1,
sect.Res2,
w.if64(uint32(0), nil),
)
}
}
for _, load := range h.Loads {
w.encode(
load.Type,
uint32(4*(2+len(load.Data))),
load.Data,
)
}
return w.dst
}