cmd/internal/ld: output dwarf in external link mode on darwin

Fixes #8973

Change-Id: Idd53fc6d9e6971ae31ed72a3df3cfdce0bfbc1fd
Reviewed-on: https://go-review.googlesource.com/8661Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

src/cmd/5l/asm.go

@@ -533,14 +533,12 @@ func asmb() {
 			fmt.Fprintf(&ld.Bso, "%5.2f dwarf\n", obj.Cputime())
 		}
 
-		if ld.Debug['w'] == 0 { // TODO(minux): enable DWARF Support
 		dwarfoff := uint32(ld.Rnd(int64(uint64(ld.HEADR)+ld.Segtext.Length), int64(ld.INITRND)) + ld.Rnd(int64(ld.Segdata.Filelen), int64(ld.INITRND)))
 		ld.Cseek(int64(dwarfoff))
 
 		ld.Segdwarf.Fileoff = uint64(ld.Cpos())
 		ld.Dwarfemitdebugsections()
 		ld.Segdwarf.Filelen = uint64(ld.Cpos()) - ld.Segdwarf.Fileoff
-		}
 
 		machlink = uint32(ld.Domacholink())
 	}
@@ -567,7 +565,7 @@ func asmb() {
 			symo = uint32(ld.Segdata.Fileoff + ld.Segdata.Filelen)
 
 		case obj.Hdarwin:
-			symo = uint32(ld.Rnd(int64(uint64(ld.HEADR)+ld.Segtext.Filelen), int64(ld.INITRND)) + ld.Rnd(int64(ld.Segdata.Filelen), int64(ld.INITRND)) + int64(machlink))
+			symo = uint32(ld.Segdwarf.Fileoff + uint64(ld.Rnd(int64(ld.Segdwarf.Filelen), int64(ld.INITRND))) + uint64(machlink))
 		}
 
 		ld.Cseek(int64(symo))

src/cmd/6l/asm.go

@@ -710,7 +710,7 @@ func asmb() {
 			symo = int64(ld.Segdata.Fileoff + ld.Segdata.Filelen)
 
 		case obj.Hdarwin:
-			symo = int64(ld.Segdata.Fileoff + uint64(ld.Rnd(int64(ld.Segdata.Filelen), int64(ld.INITRND))) + uint64(machlink))
+			symo = int64(ld.Segdwarf.Fileoff + uint64(ld.Rnd(int64(ld.Segdwarf.Filelen), int64(ld.INITRND))) + uint64(machlink))
 
 		case obj.Hlinux,
 			obj.Hfreebsd,

src/cmd/7l/asm.go

@@ -317,14 +317,12 @@ func asmb() {
 			fmt.Fprintf(&ld.Bso, "%5.2f dwarf\n", obj.Cputime())
 		}
 
-		if ld.Debug['w'] == 0 { // TODO(minux): enable DWARF Support
 		dwarfoff := uint32(ld.Rnd(int64(uint64(ld.HEADR)+ld.Segtext.Length), int64(ld.INITRND)) + ld.Rnd(int64(ld.Segdata.Filelen), int64(ld.INITRND)))
 		ld.Cseek(int64(dwarfoff))
 
 		ld.Segdwarf.Fileoff = uint64(ld.Cpos())
 		ld.Dwarfemitdebugsections()
 		ld.Segdwarf.Filelen = uint64(ld.Cpos()) - ld.Segdwarf.Fileoff
-		}
 
 		machlink = uint32(ld.Domacholink())
 	}
@@ -351,7 +349,7 @@ func asmb() {
 			symo = uint32(ld.Segdata.Fileoff + ld.Segdata.Filelen)
 
 		case obj.Hdarwin:
-			symo = uint32(ld.Rnd(int64(uint64(ld.HEADR)+ld.Segtext.Filelen), int64(ld.INITRND)) + ld.Rnd(int64(ld.Segdata.Filelen), int64(ld.INITRND)) + int64(machlink))
+			symo = uint32(ld.Segdwarf.Fileoff + uint64(ld.Rnd(int64(ld.Segdwarf.Filelen), int64(ld.INITRND))) + uint64(machlink))
 		}
 
 		ld.Cseek(int64(symo))

src/cmd/8l/asm.go

@@ -551,7 +551,7 @@ func asmb() {
 			symo = uint32(ld.Segdata.Fileoff + ld.Segdata.Filelen)
 
 		case obj.Hdarwin:
-			symo = uint32(ld.Segdata.Fileoff + uint64(ld.Rnd(int64(ld.Segdata.Filelen), int64(ld.INITRND))) + uint64(machlink))
+			symo = uint32(ld.Segdwarf.Fileoff + uint64(ld.Rnd(int64(ld.Segdwarf.Filelen), int64(ld.INITRND))) + uint64(machlink))
 
 		case obj.Hwindows:
 			symo = uint32(ld.Segdata.Fileoff + ld.Segdata.Filelen)

src/cmd/internal/ld/dwarf.go

@@ -17,6 +17,7 @@ package ld
 import (
 	"cmd/internal/obj"
 	"fmt"
+	"os"
 	"strings"
 )
 
@@ -240,6 +241,7 @@ var abbrevs = [DW_NABRV]DWAbbrev{
 			{DW_AT_low_pc, DW_FORM_addr},
 			{DW_AT_high_pc, DW_FORM_addr},
 			{DW_AT_stmt_list, DW_FORM_data4},
+			{DW_AT_comp_dir, DW_FORM_string},
 		},
 	},
 
@@ -694,6 +696,9 @@ func adddwarfrel(sec *LSym, sym *LSym, offsetbase int64, siz int, addend int64)
 	if Iself && Thearch.Thechar == '6' {
 		addend = 0
 	}
+	if HEADTYPE == obj.Hdarwin {
+		addend += sym.Value
+	}
 	switch siz {
 	case 4:
 		Thearch.Lput(uint32(addend))
@@ -1547,6 +1552,13 @@ func flushunit(dwinfo *DWDie, pc int64, pcsym *LSym, unitstart int64, header_len
 	}
 }
 
+func getCompilationDir() string {
+	if dir, err := os.Getwd(); err == nil {
+		return dir
+	}
+	return "/"
+}
+
 func writelines() {
 	if linesec == nil {
 		linesec = Linklookup(Ctxt, ".dwarfline", 0)
@@ -1571,6 +1583,9 @@ func writelines() {
 	newattr(dwinfo, DW_AT_language, DW_CLS_CONSTANT, int64(lang), 0)
 	newattr(dwinfo, DW_AT_stmt_list, DW_CLS_PTR, unitstart-lineo, 0)
 	newattr(dwinfo, DW_AT_low_pc, DW_CLS_ADDRESS, s.Value, s)
+	// OS X linker requires compilation dir or absolute path in comp unit name to output debug info.
+	compDir := getCompilationDir()
+	newattr(dwinfo, DW_AT_comp_dir, DW_CLS_STRING, int64(len(compDir)), compDir)
 
 	// Write .debug_line Line Number Program Header (sec 6.2.4)
 	// Fields marked with (*) must be changed for 64-bit dwarf
@@ -2083,6 +2098,14 @@ func writedwarfreloc(s *LSym) int64 {
 	return start
 }
 
+func addmachodwarfsect(prev *Section, name string) *Section {
+	sect := addsection(&Segdwarf, name, 04)
+	sect.Extnum = prev.Extnum + 1
+	sym := Linklookup(Ctxt, name, 0)
+	sym.Sect = sect
+	return sect
+}
+
 /*
  * This is the main entry point for generating dwarf.  After emitting
  * the mandatory debug_abbrev section, it calls writelines() to set up
@@ -2097,9 +2120,33 @@ func Dwarfemitdebugsections() {
 		return
 	}
 
-	if Linkmode == LinkExternal && !Iself {
+	if Linkmode == LinkExternal {
+		if !Iself && HEADTYPE != obj.Hdarwin {
 			return
 		}
+		if HEADTYPE == obj.Hdarwin {
+			sect := Segdata.Sect
+			// find the last section.
+			for sect.Next != nil {
+				sect = sect.Next
+			}
+			sect = addmachodwarfsect(sect, ".debug_abbrev")
+			sect = addmachodwarfsect(sect, ".debug_line")
+			sect = addmachodwarfsect(sect, ".debug_frame")
+			sect = addmachodwarfsect(sect, ".debug_info")
+		}
+		infosym = Linklookup(Ctxt, ".debug_info", 0)
+		infosym.Hide = 1
+
+		abbrevsym = Linklookup(Ctxt, ".debug_abbrev", 0)
+		abbrevsym.Hide = 1
+
+		linesym = Linklookup(Ctxt, ".debug_line", 0)
+		linesym.Hide = 1
+
+		framesym = Linklookup(Ctxt, ".debug_frame", 0)
+		framesym.Hide = 1
+	}
 
 	// For diagnostic messages.
 	newattr(&dwtypes, DW_AT_name, DW_CLS_STRING, int64(len("dwtypes")), "dwtypes")
@@ -2191,6 +2238,15 @@ func Dwarfemitdebugsections() {
 	for Cpos()&7 != 0 {
 		Cput(0)
 	}
+	if HEADTYPE != obj.Hdarwin {
+		dwarfemitreloc()
+	}
+}
+
+func dwarfemitreloc() {
+	if Debug['w'] != 0 { // disable dwarf
+		return
+	}
 	inforeloco = writedwarfreloc(infosec)
 	inforelocsize = Cpos() - inforeloco
 	align(inforelocsize)
@@ -2263,18 +2319,6 @@ func dwarfaddshstrings(shstrtab *LSym) {
 			elfstrdbg[ElfStrRelDebugLine] = Addstring(shstrtab, ".rel.debug_line")
 			elfstrdbg[ElfStrRelDebugFrame] = Addstring(shstrtab, ".rel.debug_frame")
 		}
-
-		infosym = Linklookup(Ctxt, ".debug_info", 0)
-		infosym.Hide = 1
-
-		abbrevsym = Linklookup(Ctxt, ".debug_abbrev", 0)
-		abbrevsym.Hide = 1
-
-		linesym = Linklookup(Ctxt, ".debug_line", 0)
-		linesym.Hide = 1
-
-		framesym = Linklookup(Ctxt, ".debug_frame", 0)
-		framesym.Hide = 1
 	}
 }
 
@@ -2420,14 +2464,15 @@ func dwarfaddelfheaders() {
 /*
  * Macho
  */
-func dwarfaddmachoheaders() {
+func dwarfaddmachoheaders(ms *MachoSeg) {
 	if Debug['w'] != 0 { // disable dwarf
 		return
 	}
 
 	// Zero vsize segments won't be loaded in memory, even so they
 	// have to be page aligned in the file.
-	fakestart := abbrevo &^ 0xfff
+	fakestart := Rnd(int64(Segdwarf.Fileoff), 0x1000)
+	addr := Segdata.Vaddr + Segdata.Length
 
 	nsect := 4
 	if pubnamessize > 0 {
@@ -2443,57 +2488,94 @@ func dwarfaddmachoheaders() {
 		nsect++
 	}
 
-	ms := newMachoSeg("__DWARF", nsect)
+	if Linkmode != LinkExternal {
+		ms = newMachoSeg("__DWARF", nsect)
 		ms.fileoffset = uint64(fakestart)
-	ms.filesize = uint64(abbrevo) - uint64(fakestart)
-	ms.vaddr = ms.fileoffset + Segdata.Vaddr - Segdata.Fileoff
+		ms.filesize = Segdwarf.Filelen
+		ms.vaddr = addr
+	}
 
 	msect := newMachoSect(ms, "__debug_abbrev", "__DWARF")
 	msect.off = uint32(abbrevo)
 	msect.size = uint64(abbrevsize)
-	msect.addr = uint64(msect.off) + Segdata.Vaddr - Segdata.Fileoff
-	ms.filesize += msect.size
+	msect.addr = addr
+	addr += msect.size
+	msect.flag = 0x02000000
+	if abbrevsym != nil {
+		abbrevsym.Value = int64(msect.addr)
+	}
 
 	msect = newMachoSect(ms, "__debug_line", "__DWARF")
 	msect.off = uint32(lineo)
 	msect.size = uint64(linesize)
-	msect.addr = uint64(msect.off) + Segdata.Vaddr - Segdata.Fileoff
-	ms.filesize += msect.size
+	msect.addr = addr
+	addr += msect.size
+	msect.flag = 0x02000000
+	if linesym != nil {
+		linesym.Value = int64(msect.addr)
+	}
+	if linerelocsize > 0 {
+		msect.nreloc = uint32(len(linesec.R))
+		msect.reloc = uint32(linereloco)
+	}
 
 	msect = newMachoSect(ms, "__debug_frame", "__DWARF")
 	msect.off = uint32(frameo)
 	msect.size = uint64(framesize)
-	msect.addr = uint64(msect.off) + Segdata.Vaddr - Segdata.Fileoff
-	ms.filesize += msect.size
+	msect.addr = addr
+	addr += msect.size
+	msect.flag = 0x02000000
+	if framesym != nil {
+		framesym.Value = int64(msect.addr)
+	}
+	if framerelocsize > 0 {
+		msect.nreloc = uint32(len(framesec.R))
+		msect.reloc = uint32(framereloco)
+	}
 
 	msect = newMachoSect(ms, "__debug_info", "__DWARF")
 	msect.off = uint32(infoo)
 	msect.size = uint64(infosize)
-	msect.addr = uint64(msect.off) + Segdata.Vaddr - Segdata.Fileoff
-	ms.filesize += msect.size
+	msect.addr = addr
+	addr += msect.size
+	msect.flag = 0x02000000
+	if infosym != nil {
+		infosym.Value = int64(msect.addr)
+	}
+	if inforelocsize > 0 {
+		msect.nreloc = uint32(len(infosec.R))
+		msect.reloc = uint32(inforeloco)
+	}
 
 	if pubnamessize > 0 {
 		msect := newMachoSect(ms, "__debug_pubnames", "__DWARF")
 		msect.off = uint32(pubnameso)
 		msect.size = uint64(pubnamessize)
-		msect.addr = uint64(msect.off) + Segdata.Vaddr - Segdata.Fileoff
-		ms.filesize += msect.size
+		msect.addr = addr
+		addr += msect.size
+		msect.flag = 0x02000000
 	}
 
 	if pubtypessize > 0 {
 		msect := newMachoSect(ms, "__debug_pubtypes", "__DWARF")
 		msect.off = uint32(pubtypeso)
 		msect.size = uint64(pubtypessize)
-		msect.addr = uint64(msect.off) + Segdata.Vaddr - Segdata.Fileoff
-		ms.filesize += msect.size
+		msect.addr = addr
+		addr += msect.size
+		msect.flag = 0x02000000
 	}
 
 	if arangessize > 0 {
 		msect := newMachoSect(ms, "__debug_aranges", "__DWARF")
 		msect.off = uint32(arangeso)
 		msect.size = uint64(arangessize)
-		msect.addr = uint64(msect.off) + Segdata.Vaddr - Segdata.Fileoff
-		ms.filesize += msect.size
+		msect.addr = addr
+		addr += msect.size
+		msect.flag = 0x02000000
+		if arangesrelocsize > 0 {
+			msect.nreloc = uint32(len(arangessec.R))
+			msect.reloc = uint32(arangesreloco)
+		}
 	}
 
 	// TODO(lvd) fix gdb/python to load MachO (16 char section name limit)
@@ -2501,8 +2583,9 @@ func dwarfaddmachoheaders() {
 		msect := newMachoSect(ms, "__debug_gdb_scripts", "__DWARF")
 		msect.off = uint32(gdbscripto)
 		msect.size = uint64(gdbscriptsize)
-		msect.addr = uint64(msect.off) + Segdata.Vaddr - Segdata.Fileoff
-		ms.filesize += msect.size
+		msect.addr = addr
+		addr += msect.size
+		msect.flag = 0x02000000
 	}
 }
 

src/cmd/internal/ld/lib.go

@@ -922,7 +922,7 @@ func hostlink() {
 	}
 
 	if HEADTYPE == obj.Hdarwin {
-		argv = append(argv, "-Wl,-no_pie,-pagezero_size,4000000")
+		argv = append(argv, "-Wl,-no_pie,-pagezero_size,4000000,-headerpad,1144")
 	}
 	if HEADTYPE == obj.Hopenbsd {
 		argv = append(argv, "-Wl,-nopie")
@@ -1029,6 +1029,25 @@ func hostlink() {
 	if out, err := exec.Command(argv[0], argv[1:]...).CombinedOutput(); err != nil {
 		Exitf("running %s failed: %v\n%s", argv[0], err, out)
 	}
+
+	if Debug['s'] == 0 && debug_s == 0 && HEADTYPE == obj.Hdarwin {
+		dsym := fmt.Sprintf("%s/go.dwarf", tmpdir)
+		if out, err := exec.Command("dsymutil", "-f", outfile, "-o", dsym).CombinedOutput(); err != nil {
+			Ctxt.Cursym = nil
+			Exitf("%s: running dsymutil failed: %v\n%s", os.Args[0], err, out)
+		}
+		combinedOutput := fmt.Sprintf("%s/go.combined", tmpdir)
+		if err := machoCombineDwarf(outfile, dsym, combinedOutput); err != nil {
+			Ctxt.Cursym = nil
+			Exitf("%s: combining dwarf failed: %v", os.Args[0], err)
+		}
+		origOutput := fmt.Sprintf("%s/go.orig", tmpdir)
+		os.Rename(outfile, origOutput)
+		if err := os.Rename(combinedOutput, outfile); err != nil {
+			Ctxt.Cursym = nil
+			Exitf("%s: rename(%s, %s) failed: %v", os.Args[0], combinedOutput, outfile, err)
+		}
+	}
 }
 
 func ldobj(f *obj.Biobuf, pkg string, length int64, pn string, file string, whence int) {

src/cmd/internal/ld/macho.go

@@ -443,7 +443,8 @@ func Asmbmacho() {
 		ms = newMachoSeg("", 40)
 
 		ms.fileoffset = Segtext.Fileoff
-		ms.filesize = Segdata.Fileoff + Segdata.Filelen - Segtext.Fileoff
+		ms.filesize = Segdwarf.Fileoff + Segdwarf.Filelen - Segtext.Fileoff
+		ms.vsize = ms.filesize
 	}
 
 	/* segment for zero page */
@@ -561,8 +562,8 @@ func Asmbmacho() {
 	}
 
 	// TODO: dwarf headers go in ms too
-	if Debug['s'] == 0 && Linkmode != LinkExternal {
-		dwarfaddmachoheaders()
+	if Debug['s'] == 0 {
+		dwarfaddmachoheaders(ms)
 	}
 
 	a := machowrite()
@@ -850,4 +851,5 @@ func Machoemitreloc() {
 	for sect := Segdata.Sect; sect != nil; sect = sect.Next {
 		machorelocsect(sect, datap)
 	}
+	dwarfemitreloc()
 }

src/cmd/internal/ld/macho_combine_dwarf.go

+// Copyright 2015 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 ld
+
+import (
+	"bytes"
+	"debug/macho"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"os"
+	"reflect"
+	"unsafe"
+)
+
+var fakedwarf, realdwarf, linkseg *macho.Segment
+var dwarfstart, linkstart int64
+var linkoffset uint32
+var machHeader *macho.FileHeader
+var mappedHeader []byte
+
+const (
+	LC_LOAD_DYLINKER        = 0xe
+	LC_PREBOUND_DYLIB       = 0x10
+	LC_LOAD_WEAK_DYLIB      = 0x18
+	LC_UUID                 = 0x1b
+	LC_RPATH                = 0x8000001c
+	LC_CODE_SIGNATURE       = 0x1d
+	LC_SEGMENT_SPLIT_INFO   = 0x1e
+	LC_REEXPORT_DYLIB       = 0x8000001f
+	LC_ENCRYPTION_INFO      = 0x21
+	LC_DYLD_INFO            = 0x22
+	LC_DYLD_INFO_ONLY       = 0x80000022
+	LC_VERSION_MIN_MACOSX   = 0x24
+	LC_VERSION_MIN_IPHONEOS = 0x25
+	LC_FUNCTION_STARTS      = 0x26
+	LC_MAIN                 = 0x80000028
+	LC_DATA_IN_CODE         = 0x29
+	LC_SOURCE_VERSION       = 0x2A
+	LC_DYLIB_CODE_SIGN_DRS  = 0x2B
+	LC_ENCRYPTION_INFO_64   = 0x2C
+
+	dwarfMinAlign = 6  // 64 = 1 << 6
+	pageAlign     = 12 // 4096 = 1 << 12
+)
+
+type loadCmd struct {
+	Cmd macho.LoadCmd
+	Len uint32
+}
+
+type dyldInfoCmd struct {
+	Cmd                      macho.LoadCmd
+	Len                      uint32
+	RebaseOff, RebaseLen     uint32
+	BindOff, BindLen         uint32
+	WeakBindOff, WeakBindLen uint32
+	LazyBindOff, LazyBindLen uint32
+	ExportOff, ExportLen     uint32
+}
+
+type linkEditDataCmd struct {
+	Cmd              macho.LoadCmd
+	Len              uint32
+	DataOff, DataLen uint32
+}
+
+type encryptionInfoCmd struct {
+	Cmd                macho.LoadCmd
+	Len                uint32
+	CryptOff, CryptLen uint32
+	CryptId            uint32
+}
+
+type loadCmdReader struct {
+	offset, next int64
+	f            *os.File
+	order        binary.ByteOrder
+}
+
+func (r *loadCmdReader) Next() (cmd loadCmd, err error) {
+	r.offset = r.next
+	if _, err = r.f.Seek(r.offset, 0); err != nil {
+		return
+	}
+	if err = binary.Read(r.f, r.order, &cmd); err != nil {
+		return
+	}
+	r.next = r.offset + int64(cmd.Len)
+	return
+}
+
+func (r loadCmdReader) ReadAt(offset int64, data interface{}) error {
+	if _, err := r.f.Seek(r.offset+offset, 0); err != nil {
+		return err
+	}
+	return binary.Read(r.f, r.order, data)
+}
+
+func (r loadCmdReader) WriteAt(offset int64, data interface{}) error {
+	if _, err := r.f.Seek(r.offset+offset, 0); err != nil {
+		return err
+	}
+	return binary.Write(r.f, r.order, data)
+}
+
+// machoCombineDwarf merges dwarf info generated by dsymutil into a macho executable.
+// With internal linking, DWARF is embedded into the executable, this lets us do the
+// same for external linking.
+// inexe is the path to the executable with no DWARF. It must have enough room in the macho
+// header to add the DWARF sections. (Use ld's -headerpad option)
+// dsym is the path to the macho file containing DWARF from dsymutil.
+// outexe is the path where the combined executable should be saved.
+func machoCombineDwarf(inexe, dsym, outexe string) error {
+	exef, err := os.Open(inexe)
+	if err != nil {
+		return err
+	}
+	dwarff, err := os.Open(dsym)
+	if err != nil {
+		return err
+	}
+	outf, err := os.Create(outexe)
+	if err != nil {
+		return err
+	}
+	outf.Chmod(0755)
+
+	exem, err := macho.NewFile(exef)
+	if err != nil {
+		return err
+	}
+	dwarfm, err := macho.NewFile(dwarff)
+	if err != nil {
+		return err
+	}
+
+	// The string table needs to be the last thing in the file
+	// for code signing to work. So we'll need to move the
+	// linkedit section, but all the others can be copied directly.
+	linkseg = exem.Segment("__LINKEDIT")
+	if linkseg == nil {
+		return fmt.Errorf("missing __LINKEDIT segment")
+	}
+
+	if _, err = exef.Seek(0, 0); err != nil {
+		return err
+	}
+	if _, err := io.CopyN(outf, exef, int64(linkseg.Offset)); err != nil {
+		return err
+	}
+
+	realdwarf = dwarfm.Segment("__DWARF")
+	if realdwarf == nil {
+		return fmt.Errorf("missing __DWARF segment")
+	}
+
+	// Now copy the dwarf data into the output.
+	maxalign := uint32(dwarfMinAlign) //
+	for _, sect := range dwarfm.Sections {
+		if sect.Align > maxalign {
+			maxalign = sect.Align
+		}
+	}
+	dwarfstart = machoCalcStart(realdwarf.Offset, linkseg.Offset, maxalign)
+	if _, err = outf.Seek(dwarfstart, 0); err != nil {
+		return err
+	}
+
+	if _, err = dwarff.Seek(int64(realdwarf.Offset), 0); err != nil {
+		return err
+	}
+	if _, err := io.CopyN(outf, dwarff, int64(realdwarf.Filesz)); err != nil {
+		return err
+	}
+
+	// And finally the linkedit section.
+	if _, err = exef.Seek(int64(linkseg.Offset), 0); err != nil {
+		return err
+	}
+	linkstart = machoCalcStart(linkseg.Offset, uint64(dwarfstart)+realdwarf.Filesz, pageAlign)
+	linkoffset = uint32(linkstart - int64(linkseg.Offset))
+	if _, err = outf.Seek(linkstart, 0); err != nil {
+		return err
+	}
+	if _, err := io.Copy(outf, exef); err != nil {
+		return err
+	}
+
+	// Now we need to update the headers.
+	cmdOffset := unsafe.Sizeof(exem.FileHeader)
+	is64bit := exem.Magic == macho.Magic64
+	if is64bit {
+		// mach_header_64 has one extra uint32.
+		cmdOffset += unsafe.Sizeof(exem.Magic)
+	}
+
+	textsect := exem.Section("__text")
+	if linkseg == nil {
+		return fmt.Errorf("missing __text section")
+	}
+
+	dwarfCmdOffset := int64(cmdOffset) + int64(exem.FileHeader.Cmdsz)
+	availablePadding := int64(textsect.Offset) - dwarfCmdOffset
+	if availablePadding < int64(realdwarf.Len) {
+		return fmt.Errorf("No room to add dwarf info. Need at least %d padding bytes, found %d", realdwarf.Len, availablePadding)
+	}
+	// First, copy the dwarf load command into the header
+	if _, err = outf.Seek(dwarfCmdOffset, 0); err != nil {
+		return err
+	}
+	if _, err := io.CopyN(outf, bytes.NewReader(realdwarf.Raw()), int64(realdwarf.Len)); err != nil {
+		return err
+	}
+
+	if _, err = outf.Seek(int64(unsafe.Offsetof(exem.FileHeader.Ncmd)), 0); err != nil {
+		return err
+	}
+	if err = binary.Write(outf, exem.ByteOrder, exem.Ncmd+1); err != nil {
+		return err
+	}
+	if err = binary.Write(outf, exem.ByteOrder, exem.Cmdsz+realdwarf.Len); err != nil {
+		return err
+	}
+
+	reader := loadCmdReader{next: int64(cmdOffset), f: outf, order: exem.ByteOrder}
+	for i := uint32(0); i < exem.Ncmd; i++ {
+		cmd, err := reader.Next()
+		if err != nil {
+			return err
+		}
+		switch cmd.Cmd {
+		case macho.LoadCmdSegment64:
+			err = machoUpdateSegment(reader, &macho.Segment64{}, &macho.Section64{})
+		case macho.LoadCmdSegment:
+			err = machoUpdateSegment(reader, &macho.Segment32{}, &macho.Section32{})
+		case LC_DYLD_INFO, LC_DYLD_INFO_ONLY:
+			err = machoUpdateLoadCommand(reader, &dyldInfoCmd{}, "RebaseOff", "BindOff", "WeakBindOff", "LazyBindOff", "ExportOff")
+		case macho.LoadCmdSymtab:
+			err = machoUpdateLoadCommand(reader, &macho.SymtabCmd{}, "Symoff", "Stroff")
+		case macho.LoadCmdDysymtab:
+			err = machoUpdateLoadCommand(reader, &macho.DysymtabCmd{}, "Tocoffset", "Modtaboff", "Extrefsymoff", "Indirectsymoff", "Extreloff", "Locreloff")
+		case LC_CODE_SIGNATURE, LC_SEGMENT_SPLIT_INFO, LC_FUNCTION_STARTS, LC_DATA_IN_CODE, LC_DYLIB_CODE_SIGN_DRS:
+			err = machoUpdateLoadCommand(reader, &linkEditDataCmd{}, "DataOff")
+		case LC_ENCRYPTION_INFO, LC_ENCRYPTION_INFO_64:
+			err = machoUpdateLoadCommand(reader, &encryptionInfoCmd{}, "CryptOff")
+		case macho.LoadCmdDylib, macho.LoadCmdThread, macho.LoadCmdUnixThread, LC_PREBOUND_DYLIB, LC_UUID, LC_VERSION_MIN_MACOSX, LC_VERSION_MIN_IPHONEOS, LC_SOURCE_VERSION, LC_MAIN, LC_LOAD_DYLINKER, LC_LOAD_WEAK_DYLIB, LC_REEXPORT_DYLIB, LC_RPATH:
+			// Nothing to update
+		default:
+			err = fmt.Errorf("Unknown load command 0x%x (%s)\n", int(cmd.Cmd), cmd.Cmd)
+		}
+		if err != nil {
+			return err
+		}
+	}
+	return machoUpdateDwarfHeader(&reader)
+}
+
+// machoUpdateSegment updates the load command for a moved segment.
+// Only the linkedit segment should move, and it should have 0 sections.
+// seg should be a macho.Segment32 or macho.Segment64 as appropriate.
+// sect should be a macho.Section32 or macho.Section64 as appropriate.
+func machoUpdateSegment(r loadCmdReader, seg, sect interface{}) error {
+	if err := r.ReadAt(0, seg); err != nil {
+		return err
+	}
+	segValue := reflect.ValueOf(seg)
+	offset := reflect.Indirect(segValue).FieldByName("Offset")
+
+	// Only the linkedit segment moved, any thing before that is fine.
+	if offset.Uint() < linkseg.Offset {
+		return nil
+	}
+	offset.SetUint(offset.Uint() + uint64(linkoffset))
+	if err := r.WriteAt(0, seg); err != nil {
+		return err
+	}
+	// There shouldn't be any sections, but just to make sure...
+	return machoUpdateSections(r, segValue, reflect.ValueOf(sect), uint64(linkoffset))
+}
+
+func machoUpdateSections(r loadCmdReader, seg, sect reflect.Value, delta uint64) error {
+	iseg := reflect.Indirect(seg)
+	nsect := iseg.FieldByName("Nsect").Uint()
+	if nsect == 0 {
+		return nil
+	}
+	sectOffset := int64(iseg.Type().Size())
+
+	isect := reflect.Indirect(sect)
+	offsetField := isect.FieldByName("Offset")
+	reloffField := isect.FieldByName("Reloff")
+	sectSize := int64(isect.Type().Size())
+	for i := uint64(0); i < nsect; i++ {
+		if err := r.ReadAt(sectOffset, sect.Interface()); err != nil {
+			return err
+		}
+		if offsetField.Uint() != 0 {
+			offsetField.SetUint(offsetField.Uint() + delta)
+		}
+		if reloffField.Uint() != 0 {
+			reloffField.SetUint(reloffField.Uint() + delta)
+		}
+		if err := r.WriteAt(sectOffset, sect.Interface()); err != nil {
+			return err
+		}
+		sectOffset += sectSize
+	}
+	return nil
+}
+
+// machoUpdateDwarfHeader updates the DWARF segment load command.
+func machoUpdateDwarfHeader(r *loadCmdReader) error {
+	var seg, sect interface{}
+	cmd, err := r.Next()
+	if err != nil {
+		return err
+	}
+	if cmd.Cmd == macho.LoadCmdSegment64 {
+		seg = new(macho.Segment64)
+		sect = new(macho.Section64)
+	} else {
+		seg = new(macho.Segment32)
+		sect = new(macho.Section32)
+	}
+	if err := r.ReadAt(0, seg); err != nil {
+		return err
+	}
+	segValue := reflect.ValueOf(seg)
+	offset := reflect.Indirect(segValue).FieldByName("Offset")
+
+	delta := uint64(dwarfstart) - realdwarf.Offset
+	offset.SetUint(offset.Uint() + delta)
+	if err := r.WriteAt(0, seg); err != nil {
+		return err
+	}
+	return machoUpdateSections(*r, segValue, reflect.ValueOf(sect), delta)
+}
+
+func machoUpdateLoadCommand(r loadCmdReader, cmd interface{}, fields ...string) error {
+	if err := r.ReadAt(0, cmd); err != nil {
+		return err
+	}
+	value := reflect.Indirect(reflect.ValueOf(cmd))
+
+	for _, name := range fields {
+		field := value.FieldByName(name)
+		fieldval := field.Uint()
+		if fieldval >= linkseg.Offset {
+			field.SetUint(fieldval + uint64(linkoffset))
+		}
+	}
+	if err := r.WriteAt(0, cmd); err != nil {
+		return err
+	}
+	return nil
+}
+
+func machoCalcStart(origAddr, newAddr uint64, alignExp uint32) int64 {
+	align := uint64(1 << alignExp)
+	if (origAddr % align) == (newAddr % align) {
+		return int64(newAddr)
+	}
+	padding := (align - (newAddr % align))
+	padding += origAddr % align
+	return int64(padding + newAddr)
+}