ldmacho.go

// Copyright 2017 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 loadmacho implements a Mach-O file reader.
package loadmacho

import (
	"bytes"
	"github.com/bir3/gocompiler/src/cmd/internal/bio"
	"github.com/bir3/gocompiler/src/cmd/internal/objabi"
	"github.com/bir3/gocompiler/src/cmd/internal/sys"
	"github.com/bir3/gocompiler/src/cmd/link/internal/loader"
	"github.com/bir3/gocompiler/src/cmd/link/internal/sym"
	"encoding/binary"
	"fmt"
)

/*
Derived from Plan 9 from User Space's src/libmach/elf.h, elf.c
https://github.com/9fans/plan9port/tree/master/src/libmach/

	Copyright © 2004 Russ Cox.
	Portions Copyright © 2008-2010 Google Inc.
	Portions Copyright © 2010 The Go Authors.

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
*/

// TODO(crawshaw): de-duplicate these symbols with cmd/link/internal/ld
const (
	MACHO_X86_64_RELOC_UNSIGNED	= 0
	MACHO_X86_64_RELOC_SIGNED	= 1
	MACHO_ARM64_RELOC_ADDEND	= 10
)

type ldMachoObj struct {
	f		*bio.Reader
	base		int64	// off in f where Mach-O begins
	length		int64	// length of Mach-O
	is64		bool
	name		string
	e		binary.ByteOrder
	cputype		uint
	subcputype	uint
	filetype	uint32
	flags		uint32
	cmd		[]ldMachoCmd
	ncmd		uint
}

type ldMachoCmd struct {
	type_	int
	off	uint32
	size	uint32
	seg	ldMachoSeg
	sym	ldMachoSymtab
	dsym	ldMachoDysymtab
}

type ldMachoSeg struct {
	name		string
	vmaddr		uint64
	vmsize		uint64
	fileoff		uint32
	filesz		uint32
	maxprot		uint32
	initprot	uint32
	nsect		uint32
	flags		uint32
	sect		[]ldMachoSect
}

type ldMachoSect struct {
	name	string
	segname	string
	addr	uint64
	size	uint64
	off	uint32
	align	uint32
	reloff	uint32
	nreloc	uint32
	flags	uint32
	res1	uint32
	res2	uint32
	sym	loader.Sym
	rel	[]ldMachoRel
}

type ldMachoRel struct {
	addr		uint32
	symnum		uint32
	pcrel		uint8
	length		uint8
	extrn		uint8
	type_		uint8
	scattered	uint8
	value		uint32
}

type ldMachoSymtab struct {
	symoff	uint32
	nsym	uint32
	stroff	uint32
	strsize	uint32
	str	[]byte
	sym	[]ldMachoSym
}

type ldMachoSym struct {
	name	string
	type_	uint8
	sectnum	uint8
	desc	uint16
	kind	int8
	value	uint64
	sym	loader.Sym
}

type ldMachoDysymtab struct {
	ilocalsym	uint32
	nlocalsym	uint32
	iextdefsym	uint32
	nextdefsym	uint32
	iundefsym	uint32
	nundefsym	uint32
	tocoff		uint32
	ntoc		uint32
	modtaboff	uint32
	nmodtab		uint32
	extrefsymoff	uint32
	nextrefsyms	uint32
	indirectsymoff	uint32
	nindirectsyms	uint32
	extreloff	uint32
	nextrel		uint32
	locreloff	uint32
	nlocrel		uint32
	indir		[]uint32
}

// ldMachoSym.type_
const (
	N_EXT	= 0x01
	N_TYPE	= 0x1e
	N_STAB	= 0xe0
)

// ldMachoSym.desc
const (
	N_WEAK_REF	= 0x40
	N_WEAK_DEF	= 0x80
)

const (
	LdMachoCpuVax		= 1
	LdMachoCpu68000		= 6
	LdMachoCpu386		= 7
	LdMachoCpuAmd64		= 1<<24 | 7
	LdMachoCpuMips		= 8
	LdMachoCpu98000		= 10
	LdMachoCpuHppa		= 11
	LdMachoCpuArm		= 12
	LdMachoCpuArm64		= 1<<24 | 12
	LdMachoCpu88000		= 13
	LdMachoCpuSparc		= 14
	LdMachoCpu860		= 15
	LdMachoCpuAlpha		= 16
	LdMachoCpuPower		= 18
	LdMachoCmdSegment	= 1
	LdMachoCmdSymtab	= 2
	LdMachoCmdSymseg	= 3
	LdMachoCmdThread	= 4
	LdMachoCmdDysymtab	= 11
	LdMachoCmdSegment64	= 25
	LdMachoFileObject	= 1
	LdMachoFileExecutable	= 2
	LdMachoFileFvmlib	= 3
	LdMachoFileCore		= 4
	LdMachoFilePreload	= 5
)

func unpackcmd(p []byte, m *ldMachoObj, c *ldMachoCmd, type_ uint, sz uint) int {
	e4 := m.e.Uint32
	e8 := m.e.Uint64

	c.type_ = int(type_)
	c.size = uint32(sz)
	switch type_ {
	default:
		return -1

	case LdMachoCmdSegment:
		if sz < 56 {
			return -1
		}
		c.seg.name = cstring(p[8:24])
		c.seg.vmaddr = uint64(e4(p[24:]))
		c.seg.vmsize = uint64(e4(p[28:]))
		c.seg.fileoff = e4(p[32:])
		c.seg.filesz = e4(p[36:])
		c.seg.maxprot = e4(p[40:])
		c.seg.initprot = e4(p[44:])
		c.seg.nsect = e4(p[48:])
		c.seg.flags = e4(p[52:])
		c.seg.sect = make([]ldMachoSect, c.seg.nsect)
		if uint32(sz) < 56+c.seg.nsect*68 {
			return -1
		}
		p = p[56:]
		var s *ldMachoSect
		for i := 0; uint32(i) < c.seg.nsect; i++ {
			s = &c.seg.sect[i]
			s.name = cstring(p[0:16])
			s.segname = cstring(p[16:32])
			s.addr = uint64(e4(p[32:]))
			s.size = uint64(e4(p[36:]))
			s.off = e4(p[40:])
			s.align = e4(p[44:])
			s.reloff = e4(p[48:])
			s.nreloc = e4(p[52:])
			s.flags = e4(p[56:])
			s.res1 = e4(p[60:])
			s.res2 = e4(p[64:])
			p = p[68:]
		}

	case LdMachoCmdSegment64:
		if sz < 72 {
			return -1
		}
		c.seg.name = cstring(p[8:24])
		c.seg.vmaddr = e8(p[24:])
		c.seg.vmsize = e8(p[32:])
		c.seg.fileoff = uint32(e8(p[40:]))
		c.seg.filesz = uint32(e8(p[48:]))
		c.seg.maxprot = e4(p[56:])
		c.seg.initprot = e4(p[60:])
		c.seg.nsect = e4(p[64:])
		c.seg.flags = e4(p[68:])
		c.seg.sect = make([]ldMachoSect, c.seg.nsect)
		if uint32(sz) < 72+c.seg.nsect*80 {
			return -1
		}
		p = p[72:]
		var s *ldMachoSect
		for i := 0; uint32(i) < c.seg.nsect; i++ {
			s = &c.seg.sect[i]
			s.name = cstring(p[0:16])
			s.segname = cstring(p[16:32])
			s.addr = e8(p[32:])
			s.size = e8(p[40:])
			s.off = e4(p[48:])
			s.align = e4(p[52:])
			s.reloff = e4(p[56:])
			s.nreloc = e4(p[60:])
			s.flags = e4(p[64:])
			s.res1 = e4(p[68:])
			s.res2 = e4(p[72:])

			// p+76 is reserved
			p = p[80:]
		}

	case LdMachoCmdSymtab:
		if sz < 24 {
			return -1
		}
		c.sym.symoff = e4(p[8:])
		c.sym.nsym = e4(p[12:])
		c.sym.stroff = e4(p[16:])
		c.sym.strsize = e4(p[20:])

	case LdMachoCmdDysymtab:
		if sz < 80 {
			return -1
		}
		c.dsym.ilocalsym = e4(p[8:])
		c.dsym.nlocalsym = e4(p[12:])
		c.dsym.iextdefsym = e4(p[16:])
		c.dsym.nextdefsym = e4(p[20:])
		c.dsym.iundefsym = e4(p[24:])
		c.dsym.nundefsym = e4(p[28:])
		c.dsym.tocoff = e4(p[32:])
		c.dsym.ntoc = e4(p[36:])
		c.dsym.modtaboff = e4(p[40:])
		c.dsym.nmodtab = e4(p[44:])
		c.dsym.extrefsymoff = e4(p[48:])
		c.dsym.nextrefsyms = e4(p[52:])
		c.dsym.indirectsymoff = e4(p[56:])
		c.dsym.nindirectsyms = e4(p[60:])
		c.dsym.extreloff = e4(p[64:])
		c.dsym.nextrel = e4(p[68:])
		c.dsym.locreloff = e4(p[72:])
		c.dsym.nlocrel = e4(p[76:])
	}

	return 0
}

func macholoadrel(m *ldMachoObj, sect *ldMachoSect) int {
	if sect.rel != nil || sect.nreloc == 0 {
		return 0
	}
	rel := make([]ldMachoRel, sect.nreloc)
	m.f.MustSeek(m.base+int64(sect.reloff), 0)
	buf, _, err := m.f.Slice(uint64(sect.nreloc * 8))
	if err != nil {
		return -1
	}
	for i := uint32(0); i < sect.nreloc; i++ {
		r := &rel[i]
		p := buf[i*8:]
		r.addr = m.e.Uint32(p)

		// TODO(rsc): Wrong interpretation for big-endian bitfields?
		if r.addr&0x80000000 != 0 {
			// scatterbrained relocation
			r.scattered = 1

			v := r.addr >> 24
			r.addr &= 0xFFFFFF
			r.type_ = uint8(v & 0xF)
			v >>= 4
			r.length = 1 << (v & 3)
			v >>= 2
			r.pcrel = uint8(v & 1)
			r.value = m.e.Uint32(p[4:])
		} else {
			v := m.e.Uint32(p[4:])
			r.symnum = v & 0xFFFFFF
			v >>= 24
			r.pcrel = uint8(v & 1)
			v >>= 1
			r.length = 1 << (v & 3)
			v >>= 2
			r.extrn = uint8(v & 1)
			v >>= 1
			r.type_ = uint8(v)
		}
	}

	sect.rel = rel
	return 0
}

func macholoaddsym(m *ldMachoObj, d *ldMachoDysymtab) int {
	n := int(d.nindirectsyms)
	m.f.MustSeek(m.base+int64(d.indirectsymoff), 0)
	p, _, err := m.f.Slice(uint64(n * 4))
	if err != nil {
		return -1
	}

	d.indir = make([]uint32, n)
	for i := 0; i < n; i++ {
		d.indir[i] = m.e.Uint32(p[4*i:])
	}
	return 0
}

func macholoadsym(m *ldMachoObj, symtab *ldMachoSymtab) int {
	if symtab.sym != nil {
		return 0
	}

	m.f.MustSeek(m.base+int64(symtab.stroff), 0)
	strbuf, _, err := m.f.Slice(uint64(symtab.strsize))
	if err != nil {
		return -1
	}

	symsize := 12
	if m.is64 {
		symsize = 16
	}
	n := int(symtab.nsym * uint32(symsize))
	m.f.MustSeek(m.base+int64(symtab.symoff), 0)
	symbuf, _, err := m.f.Slice(uint64(n))
	if err != nil {
		return -1
	}
	sym := make([]ldMachoSym, symtab.nsym)
	p := symbuf
	for i := uint32(0); i < symtab.nsym; i++ {
		s := &sym[i]
		v := m.e.Uint32(p)
		if v >= symtab.strsize {
			return -1
		}
		s.name = cstring(strbuf[v:])
		s.type_ = p[4]
		s.sectnum = p[5]
		s.desc = m.e.Uint16(p[6:])
		if m.is64 {
			s.value = m.e.Uint64(p[8:])
		} else {
			s.value = uint64(m.e.Uint32(p[8:]))
		}
		p = p[symsize:]
	}

	symtab.str = strbuf
	symtab.sym = sym
	return 0
}

// Load the Mach-O file pn from f.
// Symbols are written into syms, and a slice of the text symbols is returned.
func Load(l *loader.Loader, arch *sys.Arch, localSymVersion int, f *bio.Reader, pkg string, length int64, pn string) (textp []loader.Sym, err error) {
	errorf := func(str string, args ...interface{}) ([]loader.Sym, error) {
		return nil, fmt.Errorf("loadmacho: %v: %v", pn, fmt.Sprintf(str, args...))
	}

	base := f.Offset()

	hdr, _, err := f.Slice(7 * 4)
	if err != nil {
		return errorf("reading hdr: %v", err)
	}

	var e binary.ByteOrder
	if binary.BigEndian.Uint32(hdr[:])&^1 == 0xFEEDFACE {
		e = binary.BigEndian
	} else if binary.LittleEndian.Uint32(hdr[:])&^1 == 0xFEEDFACE {
		e = binary.LittleEndian
	} else {
		return errorf("bad magic - not mach-o file")
	}

	is64 := e.Uint32(hdr[:]) == 0xFEEDFACF
	ncmd := e.Uint32(hdr[4*4:])
	cmdsz := e.Uint32(hdr[5*4:])
	if ncmd > 0x10000 || cmdsz >= 0x01000000 {
		return errorf("implausible mach-o header ncmd=%d cmdsz=%d", ncmd, cmdsz)
	}

	if is64 {
		f.MustSeek(4, 1)	// skip reserved word in header
	}

	m := &ldMachoObj{
		f:		f,
		e:		e,
		cputype:	uint(e.Uint32(hdr[1*4:])),
		subcputype:	uint(e.Uint32(hdr[2*4:])),
		filetype:	e.Uint32(hdr[3*4:]),
		ncmd:		uint(ncmd),
		flags:		e.Uint32(hdr[6*4:]),
		is64:		is64,
		base:		base,
		length:		length,
		name:		pn,
	}

	switch arch.Family {
	default:
		return errorf("mach-o %s unimplemented", arch.Name)
	case sys.AMD64:
		if e != binary.LittleEndian || m.cputype != LdMachoCpuAmd64 {
			return errorf("mach-o object but not amd64")
		}
	case sys.ARM64:
		if e != binary.LittleEndian || m.cputype != LdMachoCpuArm64 {
			return errorf("mach-o object but not arm64")
		}
	}

	m.cmd = make([]ldMachoCmd, ncmd)
	cmdp, _, err := f.Slice(uint64(cmdsz))
	if err != nil {
		return errorf("reading cmds: %v", err)
	}

	// read and parse load commands
	var c *ldMachoCmd

	var symtab *ldMachoSymtab
	var dsymtab *ldMachoDysymtab

	off := uint32(len(hdr))
	for i := uint32(0); i < ncmd; i++ {
		ty := e.Uint32(cmdp)
		sz := e.Uint32(cmdp[4:])
		m.cmd[i].off = off
		unpackcmd(cmdp, m, &m.cmd[i], uint(ty), uint(sz))
		cmdp = cmdp[sz:]
		off += sz
		if ty == LdMachoCmdSymtab {
			if symtab != nil {
				return errorf("multiple symbol tables")
			}

			symtab = &m.cmd[i].sym
			macholoadsym(m, symtab)
		}

		if ty == LdMachoCmdDysymtab {
			dsymtab = &m.cmd[i].dsym
			macholoaddsym(m, dsymtab)
		}

		if (is64 && ty == LdMachoCmdSegment64) || (!is64 && ty == LdMachoCmdSegment) {
			if c != nil {
				return errorf("multiple load commands")
			}

			c = &m.cmd[i]
		}
	}

	// load text and data segments into memory.
	// they are not as small as the load commands, but we'll need
	// the memory anyway for the symbol images, so we might
	// as well use one large chunk.
	if c == nil {
		return errorf("no load command")
	}

	if symtab == nil {
		// our work is done here - no symbols means nothing can refer to this file
		return
	}

	if int64(c.seg.fileoff+c.seg.filesz) >= length {
		return errorf("load segment out of range")
	}

	f.MustSeek(m.base+int64(c.seg.fileoff), 0)
	dat, readOnly, err := f.Slice(uint64(c.seg.filesz))
	if err != nil {
		return errorf("cannot load object data: %v", err)
	}

	for i := uint32(0); i < c.seg.nsect; i++ {
		sect := &c.seg.sect[i]
		if sect.segname != "__TEXT" && sect.segname != "__DATA" {
			continue
		}
		if sect.name == "__eh_frame" {
			continue
		}
		name := fmt.Sprintf("%s(%s/%s)", pkg, sect.segname, sect.name)
		s := l.LookupOrCreateSym(name, localSymVersion)
		bld := l.MakeSymbolUpdater(s)
		if bld.Type() != 0 {
			return errorf("duplicate %s/%s", sect.segname, sect.name)
		}

		if sect.flags&0xff == 1 {	// S_ZEROFILL
			bld.SetData(make([]byte, sect.size))
		} else {
			bld.SetReadOnly(readOnly)
			bld.SetData(dat[sect.addr-c.seg.vmaddr:][:sect.size])
		}
		bld.SetSize(int64(len(bld.Data())))

		if sect.segname == "__TEXT" {
			if sect.name == "__text" {
				bld.SetType(sym.STEXT)
			} else {
				bld.SetType(sym.SRODATA)
			}
		} else {
			if sect.name == "__bss" {
				bld.SetType(sym.SNOPTRBSS)
				bld.SetData(nil)
			} else {
				bld.SetType(sym.SNOPTRDATA)
			}
		}

		sect.sym = s
	}

	// enter sub-symbols into symbol table.
	// have to guess sizes from next symbol.
	for i := uint32(0); i < symtab.nsym; i++ {
		machsym := &symtab.sym[i]
		if machsym.type_&N_STAB != 0 {
			continue
		}

		// TODO: check sym->type against outer->type.
		name := machsym.name

		if name[0] == '_' && name[1] != '\x00' {
			name = name[1:]
		}
		v := 0
		if machsym.type_&N_EXT == 0 {
			v = localSymVersion
		}
		s := l.LookupOrCreateCgoExport(name, v)
		if machsym.type_&N_EXT == 0 {
			l.SetAttrDuplicateOK(s, true)
		}
		if machsym.desc&(N_WEAK_REF|N_WEAK_DEF) != 0 {
			l.SetAttrDuplicateOK(s, true)
		}
		machsym.sym = s
		if machsym.sectnum == 0 {	// undefined
			continue
		}
		if uint32(machsym.sectnum) > c.seg.nsect {
			return errorf("reference to invalid section %d", machsym.sectnum)
		}

		sect := &c.seg.sect[machsym.sectnum-1]
		bld := l.MakeSymbolUpdater(s)
		outer := sect.sym
		if outer == 0 {
			continue	// ignore reference to invalid section
		}

		if osym := l.OuterSym(s); osym != 0 {
			if l.AttrDuplicateOK(s) {
				continue
			}
			return errorf("duplicate symbol reference: %s in both %s and %s", l.SymName(s), l.SymName(osym), l.SymName(sect.sym))
		}

		bld.SetType(l.SymType(outer))
		if l.SymSize(outer) != 0 {	// skip empty section (0-sized symbol)
			l.AddInteriorSym(outer, s)
		}

		bld.SetValue(int64(machsym.value - sect.addr))
		if !l.AttrCgoExportDynamic(s) {
			bld.SetDynimplib("")	// satisfy dynimport
		}
		if l.SymType(outer) == sym.STEXT {
			if bld.External() && !bld.DuplicateOK() {
				return errorf("%v: duplicate symbol definition", s)
			}
			bld.SetExternal(true)
		}
	}

	// Sort outer lists by address, adding to textp.
	// This keeps textp in increasing address order.
	for i := 0; uint32(i) < c.seg.nsect; i++ {
		sect := &c.seg.sect[i]
		s := sect.sym
		if s == 0 {
			continue
		}
		bld := l.MakeSymbolUpdater(s)
		if bld.SubSym() != 0 {

			bld.SortSub()

			// assign sizes, now that we know symbols in sorted order.
			for s1 := bld.Sub(); s1 != 0; s1 = l.SubSym(s1) {
				s1Bld := l.MakeSymbolUpdater(s1)
				if sub := l.SubSym(s1); sub != 0 {
					s1Bld.SetSize(l.SymValue(sub) - l.SymValue(s1))
				} else {
					dlen := int64(len(l.Data(s)))
					s1Bld.SetSize(l.SymValue(s) + dlen - l.SymValue(s1))
				}
			}
		}

		if bld.Type() == sym.STEXT {
			if bld.OnList() {
				return errorf("symbol %s listed multiple times", bld.Name())
			}
			bld.SetOnList(true)
			textp = append(textp, s)
			for s1 := bld.Sub(); s1 != 0; s1 = l.SubSym(s1) {
				if l.AttrOnList(s1) {
					return errorf("symbol %s listed multiple times", l.SymName(s1))
				}
				l.SetAttrOnList(s1, true)
				textp = append(textp, s1)
			}
		}
	}

	// load relocations
	for i := 0; uint32(i) < c.seg.nsect; i++ {
		sect := &c.seg.sect[i]
		s := sect.sym
		if s == 0 {
			continue
		}
		macholoadrel(m, sect)
		if sect.rel == nil {
			continue
		}

		sb := l.MakeSymbolUpdater(sect.sym)
		var rAdd int64
		for j := uint32(0); j < sect.nreloc; j++ {
			var (
				rOff	int32
				rSize	uint8
				rType	objabi.RelocType
				rSym	loader.Sym
			)
			rel := &sect.rel[j]
			if rel.scattered != 0 {
				// mach-o only uses scattered relocation on 32-bit platforms,
				// which are no longer supported.
				return errorf("%v: unexpected scattered relocation", s)
			}

			if arch.Family == sys.ARM64 && rel.type_ == MACHO_ARM64_RELOC_ADDEND {
				// Two relocations. This addend will be applied to the next one.
				rAdd = int64(rel.symnum) << 40 >> 40	// convert unsigned 24-bit to signed 24-bit
				continue
			}

			rSize = rel.length
			rType = objabi.MachoRelocOffset + (objabi.RelocType(rel.type_) << 1) + objabi.RelocType(rel.pcrel)
			rOff = int32(rel.addr)

			// Handle X86_64_RELOC_SIGNED referencing a section (rel.extrn == 0).
			p := l.Data(s)
			if arch.Family == sys.AMD64 {
				if rel.extrn == 0 && rel.type_ == MACHO_X86_64_RELOC_SIGNED {
					// Calculate the addend as the offset into the section.
					//
					// The rip-relative offset stored in the object file is encoded
					// as follows:
					//
					//    movsd	0x00000360(%rip),%xmm0
					//
					// To get the absolute address of the value this rip-relative address is pointing
					// to, we must add the address of the next instruction to it. This is done by
					// taking the address of the relocation and adding 4 to it (since the rip-relative
					// offset can at most be 32 bits long).  To calculate the offset into the section the
					// relocation is referencing, we subtract the vaddr of the start of the referenced
					// section found in the original object file.
					//
					// [For future reference, see Darwin's /usr/include/mach-o/x86_64/reloc.h]
					secaddr := c.seg.sect[rel.symnum-1].addr
					rAdd = int64(uint64(int64(int32(e.Uint32(p[rOff:])))+int64(rOff)+4) - secaddr)
				} else {
					rAdd = int64(int32(e.Uint32(p[rOff:])))
				}
			}

			// An unsigned internal relocation has a value offset
			// by the section address.
			if arch.Family == sys.AMD64 && rel.extrn == 0 && rel.type_ == MACHO_X86_64_RELOC_UNSIGNED {
				secaddr := c.seg.sect[rel.symnum-1].addr
				rAdd -= int64(secaddr)
			}

			if rel.extrn == 0 {
				if rel.symnum < 1 || rel.symnum > c.seg.nsect {
					return errorf("invalid relocation: section reference out of range %d vs %d", rel.symnum, c.seg.nsect)
				}

				rSym = c.seg.sect[rel.symnum-1].sym
				if rSym == 0 {
					return errorf("invalid relocation: %s", c.seg.sect[rel.symnum-1].name)
				}
			} else {
				if rel.symnum >= symtab.nsym {
					return errorf("invalid relocation: symbol reference out of range")
				}

				rSym = symtab.sym[rel.symnum].sym
			}

			r, _ := sb.AddRel(rType)
			r.SetOff(rOff)
			r.SetSiz(rSize)
			r.SetSym(rSym)
			r.SetAdd(rAdd)

			rAdd = 0	// clear rAdd for next iteration
		}

		sb.SortRelocs()
	}

	return textp, nil
}

func cstring(x []byte) string {
	i := bytes.IndexByte(x, '\x00')
	if i >= 0 {
		x = x[:i]
	}
	return string(x)
}