Add relocation CRUD and dyld bind kinds

add remove APIs for ELF/Mach-O/PE relocations and relocation sections
allow ELF alloc relocation sections to grow via new PT_LOAD; stabilize section write order
add Mach-O scattered relocs plus weak/lazy bind stream support
add PE COFF relocation helpers (symbol lookup)
update README and tests

README.md

@@ -2,9 +2,9 @@
 We have forked the debug/ folder from the standard library, to take direct control of the debug/elf, debug/macho, and debug/pe binary format parsers. To these parsers, we have added the ability to also generate executable files from the parsed intermediate data structures. This lets us load a file with debug parsers, make changes by interacting with the parser structures, and then write those changes back out to a new file.
 
 ## Relocation editing
-- `debug/elf`: `AddRelocation`, `AddRelocations`, `ReplaceRelocations`, `AddRelocationForSymbol`, `AddRelocationForAddr`, `AddRelocationsToRelocSection`
-- `debug/macho`: `AddRelocation`, `AddRelocations`, `ReplaceRelocations`, `AddRelocationForSymbol`, `AddRelocationForSymbolWithDylibOrdinal`, `SetDylibOrdinalForSymbol`
-- `debug/pe`: `AddBaseRelocation`, `ReplaceBaseRelocations`, `AddBaseReloc`, `AddSectionRelocation`, `ReplaceSectionRelocations`
+- `debug/elf`: `AddRelocation`, `AddRelocations`, `ReplaceRelocations`, `RemoveRelocations`, `AddRelocationForSymbol`, `AddRelocationForAddr`, `AddRelocationsToRelocSection`, `RemoveRelocationsFromRelocSection` (alloc relocs can now grow via a new PT_LOAD when needed)
+- `debug/macho`: `AddRelocation`, `AddRelocations`, `ReplaceRelocations`, `RemoveRelocations`, `AddScatteredRelocation`, `AddRelocationForSymbol`, `AddRelocationForSymbolWithDylibOrdinal`, `SetDylibOrdinalForSymbol`, `SetBindKindForSymbol`
+- `debug/pe`: `AddBaseRelocation`, `ReplaceBaseRelocations`, `RemoveBaseRelocations`, `AddBaseReloc`, `AddSectionRelocation`, `AddSectionRelocationForSymbol`, `ReplaceSectionRelocations`, `RemoveSectionRelocations`
 
 
 ## Read more about the project here:

elf/reloc_edit.go

@@ -23,6 +23,30 @@ func (f *File) ReplaceRelocations(target *Section, rels interface{}) error {
 	return f.addRelocations(target, rels, true, nil)
 }
 
+// RemoveRelocations clears relocation entries for the target section.
+func (f *File) RemoveRelocations(target *Section) error {
+	if target == nil {
+		return errors.New("target section is nil")
+	}
+	targetIndex, ok := f.sectionIndex(target)
+	if !ok {
+		return errors.New("target section not found in file")
+	}
+	relocSecRel, _ := f.relocationSection(targetIndex, SHT_REL)
+	relocSecRela, _ := f.relocationSection(targetIndex, SHT_RELA)
+	if relocSecRel == nil && relocSecRela == nil {
+		return nil
+	}
+	if relocSecRel != nil {
+		relocSecRel.Replace(bytes.NewReader(nil), 0)
+	}
+	if relocSecRela != nil {
+		relocSecRela.Replace(bytes.NewReader(nil), 0)
+	}
+	f.updateDynamicRelocTags()
+	return nil
+}
+
 // AddRelocationsToRelocSection appends relocation entries to a specific relocation section
 // (e.g. ".rela.dyn" or ".rel.plt").
 func (f *File) AddRelocationsToRelocSection(sectionName string, rels interface{}) error {
@@ -67,6 +91,20 @@ func (f *File) AddRelocationsToRelocSection(sectionName string, rels interface{}
 	return nil
 }
 
+// RemoveRelocationsFromRelocSection clears relocation entries for the named relocation section.
+func (f *File) RemoveRelocationsFromRelocSection(sectionName string) error {
+	relocSec := f.Section(sectionName)
+	if relocSec == nil {
+		return fmt.Errorf("relocation section %q not found", sectionName)
+	}
+	if relocSec.Type != SHT_REL && relocSec.Type != SHT_RELA {
+		return fmt.Errorf("section %q is not a relocation section", sectionName)
+	}
+	relocSec.Replace(bytes.NewReader(nil), 0)
+	f.updateDynamicRelocTags()
+	return nil
+}
+
 // AddRelocationForSymbol builds and adds a relocation for the named symbol.
 // If addend is nil, a REL entry is created. Otherwise a RELA entry is used.
 func (f *File) AddRelocationForSymbol(sectionName, symbolName string, offset uint64, rType uint32, addend *int64) error {
@@ -476,7 +514,7 @@ func (f *File) relayoutAllocRelocationSection(section *Section) error {
 	newOff := alignUp(prog.Off+prog.Filesz, align)
 	newEnd := newOff + section.FileSize
 	if nextLoadOff != 0 && newEnd > nextLoadOff {
-		return fmt.Errorf("no room to grow %q within PT_LOAD", section.Name)
+		return f.relayoutAllocRelocationSectionNewLoad(section)
 	}
 	newAddr := prog.Vaddr + (newOff - prog.Off)
 	section.Offset = newOff
@@ -496,6 +534,67 @@ func (f *File) relayoutAllocRelocationSection(section *Section) error {
 	return nil
 }
 
+func (f *File) relayoutAllocRelocationSectionNewLoad(section *Section) error {
+	align := section.Addralign
+	if align == 0 {
+		align = relocationAlign(f.Class)
+	}
+	var lastLoad *Prog
+	for _, p := range f.Progs {
+		if p.Type != PT_LOAD {
+			continue
+		}
+		if lastLoad == nil || p.Off > lastLoad.Off {
+			lastLoad = p
+		}
+	}
+	if lastLoad == nil {
+		return fmt.Errorf("no PT_LOAD segments available")
+	}
+	baseOff := lastLoad.Off + lastLoad.Filesz
+	fileEnd := f.maxFileEnd()
+	if fileEnd > baseOff {
+		baseOff = fileEnd
+	}
+	newOff := alignUp(baseOff, align)
+	newAddr := lastLoad.Vaddr + (newOff - lastLoad.Off)
+	section.Offset = newOff
+	section.Addr = newAddr
+	end := newOff + section.FileSize
+	if end > lastLoad.Off+lastLoad.Filesz {
+		delta := end - (lastLoad.Off + lastLoad.Filesz)
+		lastLoad.Filesz += delta
+		lastLoad.Memsz += delta
+	}
+
+	if end := section.Offset + section.FileSize; int64(end) > f.SHTOffset {
+		shtAlign := uint64(4)
+		if f.Class == ELFCLASS64 {
+			shtAlign = 8
+		}
+		f.SHTOffset = int64(alignUp(end, shtAlign))
+	}
+	return nil
+}
+
+func (f *File) maxFileEnd() uint64 {
+	var maxEnd uint64
+	for _, s := range f.Sections {
+		if s.Type == SHT_NOBITS || s.FileSize == 0 {
+			continue
+		}
+		end := s.Offset + s.FileSize
+		if end > maxEnd {
+			maxEnd = end
+		}
+	}
+	if f.SHTOffset > int64(maxEnd) {
+		maxEnd = uint64(f.SHTOffset)
+	}
+	return maxEnd
+}
+
+
 func (f *File) updateDynamicRelocTags() {
 	if len(f.DynTags) == 0 {
 		return

elf/reloc_edit_test.go

@@ -3,7 +3,6 @@ package elf
 import (
 	"bytes"
 	"path"
-	"strings"
 	"testing"
 )
 
@@ -63,15 +62,13 @@ func TestAddDynamicRelocationUpdatesTags(t *testing.T) {
 	if relDyn == nil {
 		t.Fatalf("rela.dyn missing in test binary")
 	}
+	origSize := relDyn.Size
 	rel := Rela64{
 		Off:    0,
 		Info:   R_INFO(0, uint32(R_X86_64_RELATIVE)),
 		Addend: 0,
 	}
 	if err := f.AddRelocationsToRelocSection(".rela.dyn", []Rela64{rel}); err != nil {
-		if strings.Contains(err.Error(), "no room to grow") {
-			t.Skip("test binary has no space to grow .rela.dyn")
-		}
 		t.Fatalf("add to rela.dyn: %v", err)
 	}
 
@@ -85,7 +82,7 @@ func TestAddDynamicRelocationUpdatesTags(t *testing.T) {
 	}
 
 	relDyn2 := f2.Section(".rela.dyn")
-	if relDyn2 == nil || relDyn2.Size <= relDyn.Size {
+	if relDyn2 == nil || relDyn2.Size <= origSize {
 		t.Fatalf("rela.dyn not grown")
 	}
 
@@ -130,3 +127,95 @@ func TestAddRelocationCreatesRelText(t *testing.T) {
 		t.Fatalf("rel.text size invalid: size=%d entsize=%d", rel.Size, rel.Entsize)
 	}
 }
+
+func TestPltRelocationUpdatesTags(t *testing.T) {
+	f, err := Open(path.Join("testdata", "gcc-amd64-linux-exec"))
+	if err != nil {
+		t.Fatalf("open elf: %v", err)
+	}
+	defer f.Close()
+
+	relPlt := f.Section(".rela.plt")
+	if relPlt == nil {
+		t.Skip("no .rela.plt section in test binary")
+	}
+	origSize := relPlt.Size
+	rel := Rela64{
+		Off:    0,
+		Info:   R_INFO(0, uint32(R_X86_64_JMP_SLOT)),
+		Addend: 0,
+	}
+	if err := f.AddRelocationsToRelocSection(".rela.plt", []Rela64{rel}); err != nil {
+		t.Fatalf("add to rela.plt: %v", err)
+	}
+
+	out, err := f.Bytes()
+	if err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	f2, err := NewFile(bytes.NewReader(out))
+	if err != nil {
+		t.Fatalf("reopen: %v", err)
+	}
+	relPlt2 := f2.Section(".rela.plt")
+	if relPlt2 == nil || relPlt2.Size <= origSize {
+		t.Fatalf("rela.plt not grown")
+	}
+	dynTags := map[DynTag]uint64{}
+	for _, tag := range f2.DynTags {
+		dynTags[tag.Tag] = tag.Value
+	}
+	if dynTags[DT_JMPREL] != relPlt2.Addr {
+		t.Fatalf("DT_JMPREL not updated")
+	}
+	if dynTags[DT_PLTRELSZ] != relPlt2.Size {
+		t.Fatalf("DT_PLTRELSZ not updated")
+	}
+	if dynTags[DT_PLTREL] != uint64(DT_RELA) {
+		t.Fatalf("DT_PLTREL not updated")
+	}
+}
+
+func TestRemoveRelocationsFromSection(t *testing.T) {
+	f, err := Open(path.Join("testdata", "gcc-amd64-linux-exec"))
+	if err != nil {
+		t.Fatalf("open elf: %v", err)
+	}
+	defer f.Close()
+
+	addend := int64(0)
+	syms, err := f.Symbols()
+	if err != nil {
+		t.Fatalf("symbols: %v", err)
+	}
+	var symName string
+	for _, sym := range syms {
+		if sym.Name != "" && sym.Section != SHN_UNDEF {
+			symName = sym.Name
+			break
+		}
+	}
+	if symName == "" {
+		t.Skip("no suitable symbol found")
+	}
+	if err := f.AddRelocationForSymbol(".text", symName, 0, uint32(R_X86_64_64), &addend); err != nil {
+		t.Fatalf("add relocation: %v", err)
+	}
+	if err := f.RemoveRelocations(f.Section(".text")); err != nil {
+		t.Fatalf("remove relocations: %v", err)
+	}
+	out, err := f.Bytes()
+	if err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	f2, err := NewFile(bytes.NewReader(out))
+	if err != nil {
+		t.Fatalf("reopen: %v", err)
+	}
+	if sec := f2.Section(".rela.text"); sec != nil && sec.Size != 0 {
+		t.Fatalf("expected .rela.text cleared")
+	}
+	if sec := f2.Section(".rel.text"); sec != nil && sec.Size != 0 {
+		t.Fatalf("expected .rel.text cleared")
+	}
+}

elf/write.go

@@ -7,6 +7,7 @@ import (
 	"io/ioutil"
 	"log"
 	"os"
+	"sort"
 )
 
 // Bytes - returns the bytes of an Elf file
@@ -152,8 +153,10 @@ func (elfFile *File) Bytes() ([]byte, error) {
 		}
 	}
 
-	sortedSections := elfFile.Sections[:]
-	//sort.Slice(sortedSections, func(a, b int) bool { return elfFile.Sections[a].Link < elfFile.Sections[b].Link })
+	sortedSections := append([]*Section(nil), elfFile.Sections...)
+	sort.SliceStable(sortedSections, func(i, j int) bool {
+		return sortedSections[i].Offset < sortedSections[j].Offset
+	})
 	for _, s := range sortedSections {
 
 		//log.Printf("Writing section: %s type: %+v\n", s.Name, s.Type)

macho/file.go

@@ -36,6 +36,7 @@ type File struct {
 	Insertion  []byte
 
 	dylibOrdinalBySymbol map[uint32]uint8
+	bindKindBySymbol     map[uint32]BindKind
 	closer io.Closer
 }
 

macho/reloc_edit.go

@@ -5,6 +5,15 @@ import (
 	"fmt"
 )
 
+// BindKind controls which dyld binding stream is used for an extern relocation.
+type BindKind uint8
+
+const (
+	BindNormal BindKind = iota
+	BindWeak
+	BindLazy
+)
+
 // AddRelocation appends a relocation to the named section.
 func (f *File) AddRelocation(sectionName string, rel Reloc) error {
 	section := f.Section(sectionName)
@@ -15,6 +24,25 @@ func (f *File) AddRelocation(sectionName string, rel Reloc) error {
 	return nil
 }
 
+// AddScatteredRelocation appends a scattered relocation to the named section.
+func (f *File) AddScatteredRelocation(sectionName string, addr uint32, value uint32, relType uint8, length uint8, pcrel bool) error {
+	section := f.Section(sectionName)
+	if section == nil {
+		return fmt.Errorf("section %q not found", sectionName)
+	}
+	rel := Reloc{
+		Addr:      addr,
+		Value:     value,
+		Type:      relType,
+		Len:       length,
+		Pcrel:     pcrel,
+		Extern:    false,
+		Scattered: true,
+	}
+	section.Relocs = append(section.Relocs, rel)
+	return nil
+}
+
 // AddRelocations appends relocations to the named section.
 func (f *File) AddRelocations(sectionName string, rels []Reloc) error {
 	section := f.Section(sectionName)
@@ -35,6 +63,16 @@ func (f *File) ReplaceRelocations(sectionName string, rels []Reloc) error {
 	return nil
 }
 
+// RemoveRelocations clears relocations for the named section.
+func (f *File) RemoveRelocations(sectionName string) error {
+	section := f.Section(sectionName)
+	if section == nil {
+		return fmt.Errorf("section %q not found", sectionName)
+	}
+	section.Relocs = section.Relocs[:0]
+	return nil
+}
+
 // AddRelocationForSymbol creates a non-scattered relocation that references a symbol.
 func (f *File) AddRelocationForSymbol(sectionName, symbolName string, addr uint32, relType uint8, length uint8, pcrel bool) error {
 	if f.Symtab == nil {
@@ -108,3 +146,25 @@ func (f *File) SetDylibOrdinalForSymbolIndex(index uint32, ordinal uint8) error
 	f.dylibOrdinalBySymbol[index] = ordinal
 	return nil
 }
+
+// SetBindKindForSymbol records a bind kind for the named symbol.
+func (f *File) SetBindKindForSymbol(symbolName string, kind BindKind) error {
+	if f.Symtab == nil {
+		return errors.New("symbol table not available")
+	}
+	for i, sym := range f.Symtab.Syms {
+		if sym.Name == symbolName {
+			return f.SetBindKindForSymbolIndex(uint32(i), kind)
+		}
+	}
+	return fmt.Errorf("symbol %q not found", symbolName)
+}
+
+// SetBindKindForSymbolIndex records a bind kind for the symbol index.
+func (f *File) SetBindKindForSymbolIndex(index uint32, kind BindKind) error {
+	if f.bindKindBySymbol == nil {
+		f.bindKindBySymbol = map[uint32]BindKind{}
+	}
+	f.bindKindBySymbol[index] = kind
+	return nil
+}

macho/reloc_edit_test.go

@@ -91,3 +91,136 @@ func TestDyldBindOrdinal(t *testing.T) {
 		t.Fatalf("bind info missing ordinal opcode")
 	}
 }
+
+func TestScatteredRelocationRoundTrip(t *testing.T) {
+	f, err := Open(path.Join("testdata", "gcc-amd64-darwin-exec"))
+	if err != nil {
+		t.Fatalf("open macho: %v", err)
+	}
+	defer f.Close()
+
+	sec := f.Section("__text")
+	if sec == nil {
+		t.Fatalf("missing __text section")
+	}
+	origCount := len(sec.Relocs)
+	if err := f.AddScatteredRelocation("__text", 0, 0, 0, 2, false); err != nil {
+		t.Fatalf("add scattered relocation: %v", err)
+	}
+
+	out, err := f.Bytes()
+	if err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	f2, err := NewFile(bytes.NewReader(out))
+	if err != nil {
+		t.Fatalf("reopen: %v", err)
+	}
+	sec2 := f2.Section("__text")
+	if sec2 == nil || len(sec2.Relocs) != origCount+1 {
+		t.Fatalf("relocations not persisted")
+	}
+	found := false
+	for _, rel := range sec2.Relocs {
+		if rel.Scattered {
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Fatalf("scattered relocation not preserved")
+	}
+}
+
+func TestRemoveRelocations(t *testing.T) {
+	f, err := Open(path.Join("testdata", "gcc-amd64-darwin-exec"))
+	if err != nil {
+		t.Fatalf("open macho: %v", err)
+	}
+	defer f.Close()
+
+	sec := f.Section("__text")
+	if sec == nil {
+		t.Fatalf("missing __text section")
+	}
+	if err := f.AddRelocation("__text", Reloc{Addr: 0, Type: 0, Len: 3, Pcrel: false, Extern: false}); err != nil {
+		t.Fatalf("add relocation: %v", err)
+	}
+	if err := f.RemoveRelocations("__text"); err != nil {
+		t.Fatalf("remove relocation: %v", err)
+	}
+
+	out, err := f.Bytes()
+	if err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	f2, err := NewFile(bytes.NewReader(out))
+	if err != nil {
+		t.Fatalf("reopen: %v", err)
+	}
+	sec2 := f2.Section("__text")
+	if sec2 == nil {
+		t.Fatalf("missing __text section")
+	}
+	if sec2.Nreloc != 0 || len(sec2.Relocs) != 0 {
+		t.Fatalf("relocations not removed")
+	}
+}
+
+func TestWeakAndLazyBindKinds(t *testing.T) {
+	f, err := Open(path.Join("testdata", "gcc-amd64-darwin-exec"))
+	if err != nil {
+		t.Fatalf("open macho: %v", err)
+	}
+	defer f.Close()
+
+	if f.DylinkInfo == nil || f.Symtab == nil {
+		t.Skip("missing dyld info or symbol table")
+	}
+	sec := f.Section("__text")
+	if sec == nil {
+		t.Fatalf("missing __text section")
+	}
+
+	var symName string
+	for _, sym := range f.Symtab.Syms {
+		if sym.Name != "" {
+			symName = sym.Name
+			break
+		}
+	}
+	if symName == "" {
+		t.Skip("no suitable symbol found")
+	}
+
+	if err := f.AddRelocationForSymbol("__text", symName, 0, 0, 3, false); err != nil {
+		t.Fatalf("add relocation: %v", err)
+	}
+	if err := f.SetBindKindForSymbol(symName, BindWeak); err != nil {
+		t.Fatalf("set weak bind: %v", err)
+	}
+	if err := f.AddRelocationForSymbol("__text", symName, 4, 0, 3, false); err != nil {
+		t.Fatalf("add relocation: %v", err)
+	}
+	if err := f.SetBindKindForSymbol(symName, BindLazy); err != nil {
+		t.Fatalf("set lazy bind: %v", err)
+	}
+
+	out, err := f.Bytes()
+	if err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	f2, err := NewFile(bytes.NewReader(out))
+	if err != nil {
+		t.Fatalf("reopen: %v", err)
+	}
+	if f2.DylinkInfo == nil {
+		t.Fatalf("missing dyld info")
+	}
+	if len(f2.DylinkInfo.WeakBindingDat) == 0 {
+		t.Fatalf("weak binding info missing")
+	}
+	if len(f2.DylinkInfo.LazyBindingDat) == 0 {
+		t.Fatalf("lazy binding info missing")
+	}
+}

macho/reloc_write.go

@@ -73,17 +73,17 @@ func (f *File) prepareDyldInfoFromRelocs() error {
 	if f.DylinkInfo == nil {
 		return nil
 	}
-	rebaseDat, bindDat, err := f.encodeDyldInfoFromRelocs()
+	rebaseDat, bindDat, weakBindDat, lazyBindDat, err := f.encodeDyldInfoFromRelocs()
 	if err != nil {
 		return err
 	}
-	if len(rebaseDat) == 0 && len(bindDat) == 0 {
+	if len(rebaseDat) == 0 && len(bindDat) == 0 && len(weakBindDat) == 0 && len(lazyBindDat) == 0 {
 		return nil
 	}
 
 	start := alignUp64(f.endOfSections(), 4)
 	limit := f.dyldInfoEndLimit()
-	total := uint64(len(rebaseDat) + len(bindDat))
+	total := uint64(len(rebaseDat) + len(bindDat) + len(weakBindDat) + len(lazyBindDat))
 	if limit != 0 && start+total > limit {
 		return fmt.Errorf("not enough room for dyld info")
 	}
@@ -97,6 +97,16 @@ func (f *File) prepareDyldInfoFromRelocs() error {
 	f.DylinkInfo.BindingInfoDat = bindDat
 	f.DylinkInfo.BindingInfoLen = uint32(len(bindDat))
 	f.DylinkInfo.BindingInfoOffset = offset
+	offset += uint64(len(bindDat))
+
+	f.DylinkInfo.WeakBindingDat = weakBindDat
+	f.DylinkInfo.WeakBindingLen = uint32(len(weakBindDat))
+	f.DylinkInfo.WeakBindingOffset = offset
+	offset += uint64(len(weakBindDat))
+
+	f.DylinkInfo.LazyBindingDat = lazyBindDat
+	f.DylinkInfo.LazyBindingLen = uint32(len(lazyBindDat))
+	f.DylinkInfo.LazyBindingOffset = offset
 
 	return f.refreshDylinkInfoLoadBytes()
 }
@@ -145,19 +155,28 @@ func (f *File) dyldInfoEndLimit() uint64 {
 	return limit
 }
 
-func (f *File) encodeDyldInfoFromRelocs() ([]byte, []byte, error) {
+func (f *File) encodeDyldInfoFromRelocs() ([]byte, []byte, []byte, []byte, error) {
 	segments := f.segmentOrdinals()
 	if len(segments) == 0 {
-		return nil, nil, nil
+		return nil, nil, nil, nil, nil
 	}
 
 	var rebase bytes.Buffer
 	var bind bytes.Buffer
+	var weak bytes.Buffer
+	var lazy bytes.Buffer
 
 	rebase.WriteByte(rebaseOpcodeSetTypeImm | rebaseTypePointer)
 	bind.WriteByte(bindOpcodeSetTypeImm | bindTypePointer)
+	weak.WriteByte(bindOpcodeSetTypeImm | bindTypePointer)
+	lazy.WriteByte(bindOpcodeSetTypeImm | bindTypePointer)
+
 	currentOrdinal := uint8(0)
+	weakOrdinal := uint8(0)
+	lazyOrdinal := uint8(0)
 	bind.WriteByte(bindOpcodeSetDylibOrdinalImm | currentOrdinal)
+	weak.WriteByte(bindOpcodeSetDylibOrdinalImm | weakOrdinal)
+	lazy.WriteByte(bindOpcodeSetDylibOrdinalImm | lazyOrdinal)
 
 	for _, s := range f.Sections {
 		if len(s.Relocs) == 0 {
@@ -165,35 +184,45 @@ func (f *File) encodeDyldInfoFromRelocs() ([]byte, []byte, error) {
 		}
 		ordinal, ok := segments[s.Seg]
 		if !ok {
-			return nil, nil, fmt.Errorf("unknown segment for section %q", s.Name)
+			return nil, nil, nil, nil, fmt.Errorf("unknown segment for section %q", s.Name)
 		}
 		seg := byte(ordinal & 0x0f)
 		for _, rel := range s.Relocs {
 			offset := uint64(s.Addr) + uint64(rel.Addr)
 			segBase := f.segmentAddr(s.Seg)
 			if offset < segBase {
-				return nil, nil, fmt.Errorf("relocation offset underflows segment %q", s.Seg)
+				return nil, nil, nil, nil, fmt.Errorf("relocation offset underflows segment %q", s.Seg)
 			}
 			segOffset := offset - segBase
 			if rel.Extern {
 				ordinal, err := f.dylibOrdinalForSymbol(rel.Value)
 				if err != nil {
-					return nil, nil, err
+					return nil, nil, nil, nil, err
 				}
-				if ordinal != currentOrdinal {
-					bind.WriteByte(bindOpcodeSetDylibOrdinalImm | ordinal)
-					currentOrdinal = ordinal
+				stream := &bind
+				streamOrdinal := &currentOrdinal
+				switch f.bindKindForSymbol(rel.Value) {
+				case BindWeak:
+					stream = &weak
+					streamOrdinal = &weakOrdinal
+				case BindLazy:
+					stream = &lazy
+					streamOrdinal = &lazyOrdinal
+				}
+				if ordinal != *streamOrdinal {
+					stream.WriteByte(bindOpcodeSetDylibOrdinalImm | ordinal)
+					*streamOrdinal = ordinal
 				}
 				name, err := f.symbolName(rel.Value)
 				if err != nil {
-					return nil, nil, err
+					return nil, nil, nil, nil, err
 				}
-				bind.WriteByte(bindOpcodeSetSegmentAndOffsetULEB | seg)
-				bind.Write(encodeULEB128(segOffset))
-				bind.WriteByte(bindOpcodeSetSymbolTrailingFlags | 0)
-				bind.WriteString(name)
-				bind.WriteByte(0)
-				bind.WriteByte(bindOpcodeDoBind)
+				stream.WriteByte(bindOpcodeSetSegmentAndOffsetULEB | seg)
+				stream.Write(encodeULEB128(segOffset))
+				stream.WriteByte(bindOpcodeSetSymbolTrailingFlags | 0)
+				stream.WriteString(name)
+				stream.WriteByte(0)
+				stream.WriteByte(bindOpcodeDoBind)
 			} else {
 				rebase.WriteByte(rebaseOpcodeSetSegmentAndOffsetULEB | seg)
 				rebase.Write(encodeULEB128(segOffset))
@@ -203,9 +232,17 @@ func (f *File) encodeDyldInfoFromRelocs() ([]byte, []byte, error) {
 	}
 
 	rebase.WriteByte(rebaseOpcodeDone)
-	bind.WriteByte(bindOpcodeDone)
+	if bind.Len() > 1 {
+		bind.WriteByte(bindOpcodeDone)
+	}
+	if weak.Len() > 1 {
+		weak.WriteByte(bindOpcodeDone)
+	}
+	if lazy.Len() > 1 {
+		lazy.WriteByte(bindOpcodeDone)
+	}
 
-	return rebase.Bytes(), bind.Bytes(), nil
+	return rebase.Bytes(), bind.Bytes(), weak.Bytes(), lazy.Bytes(), nil
 }
 
 func (f *File) segmentOrdinals() map[string]int {
@@ -250,6 +287,13 @@ func (f *File) dylibOrdinalForSymbol(index uint32) (uint8, error) {
 	return ordinal, nil
 }
 
+func (f *File) bindKindForSymbol(index uint32) BindKind {
+	if f.bindKindBySymbol == nil {
+		return BindNormal
+	}
+	return f.bindKindBySymbol[index]
+}
+
 func encodeULEB128(value uint64) []byte {
 	var out []byte
 	for {

pe/reloc_edit.go

@@ -1,6 +1,8 @@
 package pe
 
-import "fmt"
+import (
+	"fmt"
+)
 
 // AddBaseRelocation appends a relocation block to the base relocation table.
 func (f *File) AddBaseRelocation(block RelocationTableEntry) {
@@ -20,6 +22,15 @@ func (f *File) ReplaceBaseRelocations(blocks []RelocationTableEntry) {
 	*f.BaseRelocationTable = append((*f.BaseRelocationTable)[:0], blocks...)
 }
 
+// RemoveBaseRelocations clears all base relocation entries.
+func (f *File) RemoveBaseRelocations() {
+	if f.BaseRelocationTable == nil {
+		entries := []RelocationTableEntry{}
+		f.BaseRelocationTable = &entries
+	}
+	*f.BaseRelocationTable = (*f.BaseRelocationTable)[:0]
+}
+
 // AddBaseReloc adds a single base relocation to the block for the containing page.
 func (f *File) AddBaseReloc(rva uint32, typ byte) {
 	page := rva &^ 0x0fff
@@ -52,6 +63,33 @@ func (f *File) AddSectionRelocation(sectionName string, rel Reloc) error {
 	return nil
 }
 
+// AddSectionRelocationForSymbol appends a COFF relocation using a symbol name lookup.
+func (f *File) AddSectionRelocationForSymbol(sectionName string, symbolName string, rel Reloc) error {
+	index, err := f.SymbolIndexByName(symbolName)
+	if err != nil {
+		return err
+	}
+	rel.SymbolTableIndex = index
+	return f.AddSectionRelocation(sectionName, rel)
+}
+
+// SymbolIndexByName returns the COFF symbol table index for the given name.
+func (f *File) SymbolIndexByName(symbolName string) (uint32, error) {
+	if len(f.COFFSymbols) == 0 {
+		return 0, fmt.Errorf("no COFF symbols available")
+	}
+	for i := range f.COFFSymbols {
+		name, err := f.COFFSymbols[i].FullName(f.StringTable)
+		if err != nil {
+			return 0, err
+		}
+		if name == symbolName {
+			return uint32(i), nil
+		}
+	}
+	return 0, fmt.Errorf("symbol %q not found", symbolName)
+}
+
 // ReplaceSectionRelocations replaces COFF relocations for a section.
 func (f *File) ReplaceSectionRelocations(sectionName string, rels []Reloc) error {
 	section := f.Section(sectionName)
@@ -61,3 +99,13 @@ func (f *File) ReplaceSectionRelocations(sectionName string, rels []Reloc) error
 	section.Relocs = append(section.Relocs[:0], rels...)
 	return nil
 }
+
+// RemoveSectionRelocations clears COFF relocations for a section.
+func (f *File) RemoveSectionRelocations(sectionName string) error {
+	section := f.Section(sectionName)
+	if section == nil {
+		return fmt.Errorf("section %q not found", sectionName)
+	}
+	section.Relocs = section.Relocs[:0]
+	return nil
+}

pe/reloc_edit_test.go

@@ -58,3 +58,101 @@ func TestAddBaseAndSectionRelocations(t *testing.T) {
 		t.Fatalf("relocs stripped flag still set")
 	}
 }
+
+func TestAddSectionRelocationForSymbol(t *testing.T) {
+	f, err := Open(path.Join("testdata", "gcc-386-mingw-exec"))
+	if err != nil {
+		t.Fatalf("open pe: %v", err)
+	}
+	defer f.Close()
+
+	if len(f.COFFSymbols) == 0 {
+		t.Skip("no COFF symbols available")
+	}
+	var symName string
+	for i := range f.COFFSymbols {
+		name, err := f.COFFSymbols[i].FullName(f.StringTable)
+		if err != nil {
+			t.Fatalf("symbol name: %v", err)
+		}
+		if name != "" {
+			symName = name
+			break
+		}
+	}
+	if symName == "" {
+		t.Skip("no suitable symbol found")
+	}
+
+	sec := f.Sections[0]
+	rel := Reloc{VirtualAddress: 0, Type: 0}
+	if err := f.AddSectionRelocationForSymbol(sec.Name, symName, rel); err != nil {
+		t.Fatalf("add relocation: %v", err)
+	}
+	out, err := f.Bytes()
+	if err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	f2, err := NewFile(bytes.NewReader(out))
+	if err != nil {
+		t.Fatalf("reopen: %v", err)
+	}
+	sec2 := f2.Section(sec.Name)
+	if sec2 == nil || len(sec2.Relocs) == 0 {
+		t.Fatalf("relocations not written")
+	}
+	idx, err := f.SymbolIndexByName(symName)
+	if err != nil {
+		t.Fatalf("symbol index: %v", err)
+	}
+	found := false
+	for _, r := range sec2.Relocs {
+		if r.SymbolTableIndex == idx {
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Fatalf("relocation with symbol index not found")
+	}
+}
+
+func TestRemoveRelocations(t *testing.T) {
+	f, err := Open(path.Join("testdata", "gcc-386-mingw-exec"))
+	if err != nil {
+		t.Fatalf("open pe: %v", err)
+	}
+	defer f.Close()
+
+	if len(f.Sections) == 0 {
+		t.Fatalf("no sections")
+	}
+	sec := f.Sections[0]
+	f.AddBaseReloc(sec.VirtualAddress, IMAGE_REL_BASED_HIGHLOW)
+	if err := f.AddSectionRelocation(sec.Name, Reloc{VirtualAddress: 0, SymbolTableIndex: 0, Type: 0}); err != nil {
+		t.Fatalf("add relocation: %v", err)
+	}
+	f.RemoveBaseRelocations()
+	if err := f.RemoveSectionRelocations(sec.Name); err != nil {
+		t.Fatalf("remove relocation: %v", err)
+	}
+
+	out, err := f.Bytes()
+	if err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	f2, err := NewFile(bytes.NewReader(out))
+	if err != nil {
+		t.Fatalf("reopen: %v", err)
+	}
+	if f2.BaseRelocationTable != nil && len(*f2.BaseRelocationTable) != 0 {
+		t.Fatalf("base relocations not removed")
+	}
+	sec2 := f2.Section(sec.Name)
+	if sec2 == nil {
+		t.Fatalf("missing section")
+	}
+	if len(sec2.Relocs) != 0 || sec2.NumberOfRelocations != 0 {
+		t.Fatalf("section relocations not removed")
+	}
+}