github.com/miolini/go@v0.0.0-20160405192216-fca68c8cb408/src/cmd/link/internal/ld/lib.go

// Inferno utils/8l/asm.c
// http://code.google.com/p/inferno-os/source/browse/utils/8l/asm.c
//
//	Copyright © 1994-1999 Lucent Technologies Inc.  All rights reserved.
//	Portions Copyright © 1995-1997 C H Forsyth (forsyth@terzarima.net)
//	Portions Copyright © 1997-1999 Vita Nuova Limited
//	Portions Copyright © 2000-2007 Vita Nuova Holdings Limited (www.vitanuova.com)
//	Portions Copyright © 2004,2006 Bruce Ellis
//	Portions Copyright © 2005-2007 C H Forsyth (forsyth@terzarima.net)
//	Revisions Copyright © 2000-2007 Lucent Technologies Inc. and others
//	Portions Copyright © 2009 The Go Authors.  All rights reserved.
//
// 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.

package ld

import (
	"bufio"
	"bytes"
	"cmd/internal/obj"
	"crypto/sha1"
	"debug/elf"
	"encoding/binary"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"os"
	"os/exec"
	"path/filepath"
	"runtime"
	"strings"
	"sync"
)

// Data layout and relocation.

// Derived from Inferno utils/6l/l.h
// http://code.google.com/p/inferno-os/source/browse/utils/6l/l.h
//
//	Copyright © 1994-1999 Lucent Technologies Inc.  All rights reserved.
//	Portions Copyright © 1995-1997 C H Forsyth (forsyth@terzarima.net)
//	Portions Copyright © 1997-1999 Vita Nuova Limited
//	Portions Copyright © 2000-2007 Vita Nuova Holdings Limited (www.vitanuova.com)
//	Portions Copyright © 2004,2006 Bruce Ellis
//	Portions Copyright © 2005-2007 C H Forsyth (forsyth@terzarima.net)
//	Revisions Copyright © 2000-2007 Lucent Technologies Inc. and others
//	Portions Copyright © 2009 The Go Authors.  All rights reserved.
//
// 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.

type Arch struct {
	Thechar          int
	Ptrsize          int
	Intsize          int
	Regsize          int
	Funcalign        int
	Maxalign         int
	Minalign         int
	Minlc            int
	Dwarfregsp       int
	Dwarfreglr       int
	Linuxdynld       string
	Freebsddynld     string
	Netbsddynld      string
	Openbsddynld     string
	Dragonflydynld   string
	Solarisdynld     string
	Adddynrel        func(*LSym, *Reloc)
	Archinit         func()
	Archreloc        func(*Reloc, *LSym, *int64) int
	Archrelocvariant func(*Reloc, *LSym, int64) int64
	Asmb             func()
	Elfreloc1        func(*Reloc, int64) int
	Elfsetupplt      func()
	Gentext          func()
	Machoreloc1      func(*Reloc, int64) int
	PEreloc1         func(*Reloc, int64) bool
	Wput             func(uint16)
	Lput             func(uint32)
	Vput             func(uint64)
	Append16         func(b []byte, v uint16) []byte
	Append32         func(b []byte, v uint32) []byte
	Append64         func(b []byte, v uint64) []byte
}

type Rpath struct {
	set bool
	val string
}

func (r *Rpath) Set(val string) error {
	r.set = true
	r.val = val
	return nil
}

func (r *Rpath) String() string {
	return r.val
}

var (
	Thearch Arch
	datap   *LSym
	Debug   [128]int
	Lcsize  int32
	rpath   Rpath
	Spsize  int32
	Symsize int32
)

// Terrible but standard terminology.
// A segment describes a block of file to load into memory.
// A section further describes the pieces of that block for
// use in debuggers and such.

const (
	MINFUNC = 16 // minimum size for a function
)

type Segment struct {
	Rwx     uint8  // permission as usual unix bits (5 = r-x etc)
	Vaddr   uint64 // virtual address
	Length  uint64 // length in memory
	Fileoff uint64 // file offset
	Filelen uint64 // length on disk
	Sect    *Section
}

type Section struct {
	Rwx     uint8
	Extnum  int16
	Align   int32
	Name    string
	Vaddr   uint64
	Length  uint64
	Next    *Section
	Seg     *Segment
	Elfsect *ElfShdr
	Reloff  uint64
	Rellen  uint64
}

// DynlinkingGo returns whether we are producing Go code that can live
// in separate shared libraries linked together at runtime.
func DynlinkingGo() bool {
	return Buildmode == BuildmodeShared || Linkshared
}

// UseRelro returns whether to make use of "read only relocations" aka
// relro.
func UseRelro() bool {
	switch Buildmode {
	case BuildmodeCShared, BuildmodeShared, BuildmodePIE:
		return Iself
	default:
		return Linkshared
	}
}

var (
	Thestring          string
	Thelinkarch        *LinkArch
	outfile            string
	dynexp             []*LSym
	dynlib             []string
	ldflag             []string
	havedynamic        int
	Funcalign          int
	iscgo              bool
	elfglobalsymndx    int
	flag_installsuffix string
	flag_race          int
	flag_msan          int
	Buildmode          BuildMode
	Linkshared         bool
	tracksym           string
	interpreter        string
	tmpdir             string
	extld              string
	extldflags         string
	extar              string
	libgccfile         string
	debug_s            int // backup old value of debug['s']
	Ctxt               *Link
	HEADR              int32
	HEADTYPE           int32
	INITRND            int32
	INITTEXT           int64
	INITDAT            int64
	INITENTRY          string /* entry point */
	nerrors            int
	Linkmode           int
	liveness           int64
)

var (
	Segtext   Segment
	Segrodata Segment
	Segdata   Segment
	Segdwarf  Segment
)

/* set by call to mywhatsys() */

/* whence for ldpkg */
const (
	FileObj = 0 + iota
	ArchiveObj
	Pkgdef
)

var (
	headstring string
	// buffered output
	Bso obj.Biobuf
)

type outBuf struct {
	w   *bufio.Writer
	f   *os.File
	off int64
}

func (w *outBuf) Write(p []byte) (n int, err error) {
	n, err = w.w.Write(p)
	w.off += int64(n)
	return n, err
}

func (w *outBuf) WriteString(s string) (n int, err error) {
	n, err = coutbuf.w.WriteString(s)
	w.off += int64(n)
	return n, err
}

var coutbuf outBuf

const pkgname = "__.PKGDEF"

var (
	// Set if we see an object compiled by the host compiler that is not
	// from a package that is known to support internal linking mode.
	externalobj = false
	goroot      string
	goarch      string
	goos        string
	theline     string
)

func Lflag(arg string) {
	Ctxt.Libdir = append(Ctxt.Libdir, arg)
}

// A BuildMode indicates the sort of object we are building:
//   "exe": build a main package and everything it imports into an executable.
//   "c-shared": build a main package, plus all packages that it imports, into a
//     single C shared library. The only callable symbols will be those functions
//     marked as exported.
//   "shared": combine all packages passed on the command line, and their
//     dependencies, into a single shared library that will be used when
//     building with the -linkshared option.
type BuildMode uint8

const (
	BuildmodeUnset BuildMode = iota
	BuildmodeExe
	BuildmodePIE
	BuildmodeCArchive
	BuildmodeCShared
	BuildmodeShared
)

func (mode *BuildMode) Set(s string) error {
	goos := obj.Getgoos()
	goarch := obj.Getgoarch()
	badmode := func() error {
		return fmt.Errorf("buildmode %s not supported on %s/%s", s, goos, goarch)
	}
	switch s {
	default:
		return fmt.Errorf("invalid buildmode: %q", s)
	case "exe":
		*mode = BuildmodeExe
	case "pie":
		switch goos {
		case "android", "linux":
		default:
			return badmode()
		}
		*mode = BuildmodePIE
	case "c-archive":
		switch goos {
		case "darwin", "linux":
		case "windows":
			switch goarch {
			case "amd64", "386":
			default:
				return badmode()
			}
		default:
			return badmode()
		}
		*mode = BuildmodeCArchive
	case "c-shared":
		switch goarch {
		case "386", "amd64", "arm", "arm64":
		default:
			return badmode()
		}
		*mode = BuildmodeCShared
	case "shared":
		switch goos {
		case "linux":
			switch goarch {
			case "386", "amd64", "arm", "arm64", "ppc64le", "s390x":
			default:
				return badmode()
			}
		default:
			return badmode()
		}
		*mode = BuildmodeShared
	}
	return nil
}

func (mode *BuildMode) String() string {
	switch *mode {
	case BuildmodeUnset:
		return "" // avoid showing a default in usage message
	case BuildmodeExe:
		return "exe"
	case BuildmodePIE:
		return "pie"
	case BuildmodeCArchive:
		return "c-archive"
	case BuildmodeCShared:
		return "c-shared"
	case BuildmodeShared:
		return "shared"
	}
	return fmt.Sprintf("BuildMode(%d)", uint8(*mode))
}

/*
 * Unix doesn't like it when we write to a running (or, sometimes,
 * recently run) binary, so remove the output file before writing it.
 * On Windows 7, remove() can force a subsequent create() to fail.
 * S_ISREG() does not exist on Plan 9.
 */
func mayberemoveoutfile() {
	if fi, err := os.Lstat(outfile); err == nil && !fi.Mode().IsRegular() {
		return
	}
	os.Remove(outfile)
}

func libinit() {
	Funcalign = Thearch.Funcalign
	mywhatsys() // get goroot, goarch, goos

	// add goroot to the end of the libdir list.
	suffix := ""

	suffixsep := ""
	if flag_installsuffix != "" {
		suffixsep = "_"
		suffix = flag_installsuffix
	} else if flag_race != 0 {
		suffixsep = "_"
		suffix = "race"
	} else if flag_msan != 0 {
		suffixsep = "_"
		suffix = "msan"
	}

	Lflag(filepath.Join(goroot, "pkg", fmt.Sprintf("%s_%s%s%s", goos, goarch, suffixsep, suffix)))

	mayberemoveoutfile()
	f, err := os.OpenFile(outfile, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0775)
	if err != nil {
		Exitf("cannot create %s: %v", outfile, err)
	}

	coutbuf.w = bufio.NewWriter(f)
	coutbuf.f = f

	if INITENTRY == "" {
		switch Buildmode {
		case BuildmodeCShared, BuildmodeCArchive:
			INITENTRY = fmt.Sprintf("_rt0_%s_%s_lib", goarch, goos)
		case BuildmodeExe, BuildmodePIE:
			INITENTRY = fmt.Sprintf("_rt0_%s_%s", goarch, goos)
		case BuildmodeShared:
			// No INITENTRY for -buildmode=shared
		default:
			Diag("unknown INITENTRY for buildmode %v", Buildmode)
		}
	}

	if !DynlinkingGo() {
		Linklookup(Ctxt, INITENTRY, 0).Type = obj.SXREF
	}
}

func Exitf(format string, a ...interface{}) {
	fmt.Fprintf(os.Stderr, os.Args[0]+": "+format+"\n", a...)
	if coutbuf.f != nil {
		coutbuf.f.Close()
		mayberemoveoutfile()
	}
	Exit(2)
}

func errorexit() {
	if coutbuf.f != nil {
		if nerrors != 0 {
			Cflush()
		}
		// For rmtemp run at atexit time on Windows.
		if err := coutbuf.f.Close(); err != nil {
			Exitf("close: %v", err)
		}
	}

	if nerrors != 0 {
		if coutbuf.f != nil {
			mayberemoveoutfile()
		}
		Exit(2)
	}

	Exit(0)
}

func loadinternal(name string) {
	found := 0
	for i := 0; i < len(Ctxt.Libdir); i++ {
		if Linkshared {
			shlibname := filepath.Join(Ctxt.Libdir[i], name+".shlibname")
			if Debug['v'] != 0 {
				fmt.Fprintf(&Bso, "searching for %s.a in %s\n", name, shlibname)
			}
			if _, err := os.Stat(shlibname); err == nil {
				addlibpath(Ctxt, "internal", "internal", "", name, shlibname)
				found = 1
				break
			}
		}
		pname := filepath.Join(Ctxt.Libdir[i], name+".a")
		if Debug['v'] != 0 {
			fmt.Fprintf(&Bso, "searching for %s.a in %s\n", name, pname)
		}
		if _, err := os.Stat(pname); err == nil {
			addlibpath(Ctxt, "internal", "internal", pname, name, "")
			found = 1
			break
		}
	}

	if found == 0 {
		fmt.Fprintf(&Bso, "warning: unable to find %s.a\n", name)
	}
}

func loadlib() {
	switch Buildmode {
	case BuildmodeCShared:
		s := Linklookup(Ctxt, "runtime.islibrary", 0)
		s.Attr |= AttrDuplicateOK
		Adduint8(Ctxt, s, 1)
	case BuildmodeCArchive:
		s := Linklookup(Ctxt, "runtime.isarchive", 0)
		s.Attr |= AttrDuplicateOK
		Adduint8(Ctxt, s, 1)
	}

	loadinternal("runtime")
	if Thearch.Thechar == '5' {
		loadinternal("math")
	}
	if flag_race != 0 {
		loadinternal("runtime/race")
	}
	if flag_msan != 0 {
		loadinternal("runtime/msan")
	}

	var i int
	for i = 0; i < len(Ctxt.Library); i++ {
		iscgo = iscgo || Ctxt.Library[i].Pkg == "runtime/cgo"
		if Ctxt.Library[i].Shlib == "" {
			if Debug['v'] > 1 {
				fmt.Fprintf(&Bso, "%5.2f autolib: %s (from %s)\n", obj.Cputime(), Ctxt.Library[i].File, Ctxt.Library[i].Objref)
			}
			objfile(Ctxt.Library[i])
		}
	}

	for i = 0; i < len(Ctxt.Library); i++ {
		if Ctxt.Library[i].Shlib != "" {
			if Debug['v'] > 1 {
				fmt.Fprintf(&Bso, "%5.2f autolib: %s (from %s)\n", obj.Cputime(), Ctxt.Library[i].Shlib, Ctxt.Library[i].Objref)
			}
			ldshlibsyms(Ctxt.Library[i].Shlib)
		}
	}

	if Linkmode == LinkAuto {
		if iscgo && externalobj {
			Linkmode = LinkExternal
		} else {
			Linkmode = LinkInternal
		}

		// Force external linking for android.
		if goos == "android" {
			Linkmode = LinkExternal
		}

		// Force external linking for PIE executables, as
		// internal linking does not support TLS_IE.
		if Buildmode == BuildmodePIE {
			Linkmode = LinkExternal
		}

		// cgo on Darwin must use external linking
		// we can always use external linking, but then there will be circular
		// dependency problems when compiling natively (external linking requires
		// runtime/cgo, runtime/cgo requires cmd/cgo, but cmd/cgo needs to be
		// compiled using external linking.)
		if (Thearch.Thechar == '5' || Thearch.Thechar == '7') && HEADTYPE == obj.Hdarwin && iscgo {
			Linkmode = LinkExternal
		}

		// Force external linking for msan.
		if flag_msan != 0 {
			Linkmode = LinkExternal
		}
	}

	// cmd/7l doesn't support cgo internal linking
	// This is https://golang.org/issue/10373.
	if iscgo && goarch == "arm64" {
		Linkmode = LinkExternal
	}

	if Linkmode == LinkExternal && !iscgo {
		// This indicates a user requested -linkmode=external.
		// The startup code uses an import of runtime/cgo to decide
		// whether to initialize the TLS.  So give it one. This could
		// be handled differently but it's an unusual case.
		loadinternal("runtime/cgo")

		if i < len(Ctxt.Library) {
			if Ctxt.Library[i].Shlib != "" {
				ldshlibsyms(Ctxt.Library[i].Shlib)
			} else {
				if DynlinkingGo() {
					Exitf("cannot implicitly include runtime/cgo in a shared library")
				}
				objfile(Ctxt.Library[i])
			}
		}
	}

	if Linkmode == LinkInternal {
		// Drop all the cgo_import_static declarations.
		// Turns out we won't be needing them.
		for _, s := range Ctxt.Allsym {
			if s.Type == obj.SHOSTOBJ {
				// If a symbol was marked both
				// cgo_import_static and cgo_import_dynamic,
				// then we want to make it cgo_import_dynamic
				// now.
				if s.Extname != "" && s.Dynimplib != "" && !s.Attr.CgoExport() {
					s.Type = obj.SDYNIMPORT
				} else {
					s.Type = 0
				}
			}
		}
	}

	tlsg := Linklookup(Ctxt, "runtime.tlsg", 0)

	// runtime.tlsg is used for external linking on platforms that do not define
	// a variable to hold g in assembly (currently only intel).
	if tlsg.Type == 0 {
		tlsg.Type = obj.STLSBSS
		tlsg.Size = int64(Thearch.Ptrsize)
	} else if tlsg.Type != obj.SDYNIMPORT {
		Diag("internal error: runtime declared tlsg variable %d", tlsg.Type)
	}
	tlsg.Attr |= AttrReachable
	Ctxt.Tlsg = tlsg

	moduledata := Linklookup(Ctxt, "runtime.firstmoduledata", 0)
	if moduledata.Type != 0 && moduledata.Type != obj.SDYNIMPORT {
		// If the module (toolchain-speak for "executable or shared
		// library") we are linking contains the runtime package, it
		// will define the runtime.firstmoduledata symbol and we
		// truncate it back to 0 bytes so we can define its entire
		// contents in symtab.go:symtab().
		moduledata.Size = 0

		// In addition, on ARM, the runtime depends on the linker
		// recording the value of GOARM.
		if Thearch.Thechar == '5' {
			s := Linklookup(Ctxt, "runtime.goarm", 0)

			s.Type = obj.SRODATA
			s.Size = 0
			Adduint8(Ctxt, s, uint8(Ctxt.Goarm))
		}
	} else {
		// If OTOH the module does not contain the runtime package,
		// create a local symbol for the moduledata.
		moduledata = Linklookup(Ctxt, "local.moduledata", 0)
		moduledata.Attr |= AttrLocal
	}
	// In all cases way we mark the moduledata as noptrdata to hide it from
	// the GC.
	moduledata.Type = obj.SNOPTRDATA
	moduledata.Attr |= AttrReachable
	Ctxt.Moduledata = moduledata

	// Now that we know the link mode, trim the dynexp list.
	x := AttrCgoExportDynamic

	if Linkmode == LinkExternal {
		x = AttrCgoExportStatic
	}
	w := 0
	for i := 0; i < len(dynexp); i++ {
		if dynexp[i].Attr&x != 0 {
			dynexp[w] = dynexp[i]
			w++
		}
	}
	dynexp = dynexp[:w]

	// In internal link mode, read the host object files.
	if Linkmode == LinkInternal {
		hostobjs()

		// If we have any undefined symbols in external
		// objects, try to read them from the libgcc file.
		any := false
		for _, s := range Ctxt.Allsym {
			for _, r := range s.R {
				if r.Sym != nil && r.Sym.Type&obj.SMASK == obj.SXREF && r.Sym.Name != ".got" {
					any = true
					break
				}
			}
		}
		if any {
			if libgccfile == "" {
				if extld == "" {
					extld = "gcc"
				}
				args := hostlinkArchArgs()
				args = append(args, "--print-libgcc-file-name")
				if Debug['v'] != 0 {
					fmt.Fprintf(&Bso, "%s %v\n", extld, args)
				}
				out, err := exec.Command(extld, args...).Output()
				if err != nil {
					if Debug['v'] != 0 {
						fmt.Fprintln(&Bso, "not using a libgcc file because compiler failed")
						fmt.Fprintf(&Bso, "%v\n%s\n", err, out)
					}
					libgccfile = "none"
				} else {
					libgccfile = strings.TrimSpace(string(out))
				}
			}

			if libgccfile != "none" {
				hostArchive(libgccfile)
			}
		}
	} else {
		hostlinksetup()
	}

	// We've loaded all the code now.
	// If there are no dynamic libraries needed, gcc disables dynamic linking.
	// Because of this, glibc's dynamic ELF loader occasionally (like in version 2.13)
	// assumes that a dynamic binary always refers to at least one dynamic library.
	// Rather than be a source of test cases for glibc, disable dynamic linking
	// the same way that gcc would.
	//
	// Exception: on OS X, programs such as Shark only work with dynamic
	// binaries, so leave it enabled on OS X (Mach-O) binaries.
	// Also leave it enabled on Solaris which doesn't support
	// statically linked binaries.
	switch Buildmode {
	case BuildmodeExe, BuildmodePIE:
		if havedynamic == 0 && HEADTYPE != obj.Hdarwin && HEADTYPE != obj.Hsolaris {
			Debug['d'] = 1
		}
	}

	importcycles()
}

/*
 * look for the next file in an archive.
 * adapted from libmach.
 */
func nextar(bp *obj.Biobuf, off int64, a *ArHdr) int64 {
	if off&1 != 0 {
		off++
	}
	obj.Bseek(bp, off, 0)
	buf := make([]byte, SAR_HDR)
	if n := obj.Bread(bp, buf); n < len(buf) {
		if n >= 0 {
			return 0
		}
		return -1
	}

	a.name = artrim(buf[0:16])
	a.date = artrim(buf[16:28])
	a.uid = artrim(buf[28:34])
	a.gid = artrim(buf[34:40])
	a.mode = artrim(buf[40:48])
	a.size = artrim(buf[48:58])
	a.fmag = artrim(buf[58:60])

	arsize := atolwhex(a.size)
	if arsize&1 != 0 {
		arsize++
	}
	return int64(arsize) + SAR_HDR
}

func objfile(lib *Library) {
	pkg := pathtoprefix(lib.Pkg)

	if Debug['v'] > 1 {
		fmt.Fprintf(&Bso, "%5.2f ldobj: %s (%s)\n", obj.Cputime(), lib.File, pkg)
	}
	Bso.Flush()
	f, err := obj.Bopenr(lib.File)
	if err != nil {
		Exitf("cannot open file %s: %v", lib.File, err)
	}

	magbuf := make([]byte, len(ARMAG))
	if obj.Bread(f, magbuf) != len(magbuf) || !strings.HasPrefix(string(magbuf), ARMAG) {
		/* load it as a regular file */
		l := obj.Bseek(f, 0, 2)

		obj.Bseek(f, 0, 0)
		ldobj(f, pkg, l, lib.File, lib.File, FileObj)
		obj.Bterm(f)

		return
	}

	/* process __.PKGDEF */
	off := obj.Boffset(f)

	var arhdr ArHdr
	l := nextar(f, off, &arhdr)
	var pname string
	if l <= 0 {
		Diag("%s: short read on archive file symbol header", lib.File)
		goto out
	}

	if !strings.HasPrefix(arhdr.name, pkgname) {
		Diag("%s: cannot find package header", lib.File)
		goto out
	}

	if Buildmode == BuildmodeShared {
		before := obj.Boffset(f)
		pkgdefBytes := make([]byte, atolwhex(arhdr.size))
		obj.Bread(f, pkgdefBytes)
		hash := sha1.Sum(pkgdefBytes)
		lib.hash = hash[:]
		obj.Bseek(f, before, 0)
	}

	off += l

	ldpkg(f, pkg, atolwhex(arhdr.size), lib.File, Pkgdef)

	/*
	 * load all the object files from the archive now.
	 * this gives us sequential file access and keeps us
	 * from needing to come back later to pick up more
	 * objects.  it breaks the usual C archive model, but
	 * this is Go, not C.  the common case in Go is that
	 * we need to load all the objects, and then we throw away
	 * the individual symbols that are unused.
	 *
	 * loading every object will also make it possible to
	 * load foreign objects not referenced by __.PKGDEF.
	 */
	for {
		l = nextar(f, off, &arhdr)
		if l == 0 {
			break
		}
		if l < 0 {
			Exitf("%s: malformed archive", lib.File)
		}

		off += l

		pname = fmt.Sprintf("%s(%s)", lib.File, arhdr.name)
		l = atolwhex(arhdr.size)
		ldobj(f, pkg, l, pname, lib.File, ArchiveObj)
	}

out:
	obj.Bterm(f)
}

type Hostobj struct {
	ld     func(*obj.Biobuf, string, int64, string)
	pkg    string
	pn     string
	file   string
	off    int64
	length int64
}

var hostobj []Hostobj

// These packages can use internal linking mode.
// Others trigger external mode.
var internalpkg = []string{
	"crypto/x509",
	"net",
	"os/user",
	"runtime/cgo",
	"runtime/race",
	"runtime/msan",
}

func ldhostobj(ld func(*obj.Biobuf, string, int64, string), f *obj.Biobuf, pkg string, length int64, pn string, file string) *Hostobj {
	isinternal := false
	for i := 0; i < len(internalpkg); i++ {
		if pkg == internalpkg[i] {
			isinternal = true
			break
		}
	}

	// DragonFly declares errno with __thread, which results in a symbol
	// type of R_386_TLS_GD or R_X86_64_TLSGD. The Go linker does not
	// currently know how to handle TLS relocations, hence we have to
	// force external linking for any libraries that link in code that
	// uses errno. This can be removed if the Go linker ever supports
	// these relocation types.
	if HEADTYPE == obj.Hdragonfly {
		if pkg == "net" || pkg == "os/user" {
			isinternal = false
		}
	}

	if !isinternal {
		externalobj = true
	}

	hostobj = append(hostobj, Hostobj{})
	h := &hostobj[len(hostobj)-1]
	h.ld = ld
	h.pkg = pkg
	h.pn = pn
	h.file = file
	h.off = obj.Boffset(f)
	h.length = length
	return h
}

func hostobjs() {
	var f *obj.Biobuf
	var h *Hostobj

	for i := 0; i < len(hostobj); i++ {
		h = &hostobj[i]
		var err error
		f, err = obj.Bopenr(h.file)
		if f == nil {
			Exitf("cannot reopen %s: %v", h.pn, err)
		}

		obj.Bseek(f, h.off, 0)
		h.ld(f, h.pkg, h.length, h.pn)
		obj.Bterm(f)
	}
}

// provided by lib9

func rmtemp() {
	os.RemoveAll(tmpdir)
}

func hostlinksetup() {
	if Linkmode != LinkExternal {
		return
	}

	// For external link, record that we need to tell the external linker -s,
	// and turn off -s internally: the external linker needs the symbol
	// information for its final link.
	debug_s = Debug['s']
	Debug['s'] = 0

	// create temporary directory and arrange cleanup
	if tmpdir == "" {
		dir, err := ioutil.TempDir("", "go-link-")
		if err != nil {
			log.Fatal(err)
		}
		tmpdir = dir
		AtExit(rmtemp)
	}

	// change our output to temporary object file
	coutbuf.f.Close()
	mayberemoveoutfile()

	p := filepath.Join(tmpdir, "go.o")
	var err error
	f, err := os.OpenFile(p, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0775)
	if err != nil {
		Exitf("cannot create %s: %v", p, err)
	}

	coutbuf.w = bufio.NewWriter(f)
	coutbuf.f = f
}

// hostobjCopy creates a copy of the object files in hostobj in a
// temporary directory.
func hostobjCopy() (paths []string) {
	var wg sync.WaitGroup
	sema := make(chan struct{}, runtime.NumCPU()) // limit open file descriptors
	for i, h := range hostobj {
		h := h
		dst := filepath.Join(tmpdir, fmt.Sprintf("%06d.o", i))
		paths = append(paths, dst)

		wg.Add(1)
		go func() {
			sema <- struct{}{}
			defer func() {
				<-sema
				wg.Done()
			}()
			f, err := os.Open(h.file)
			if err != nil {
				Exitf("cannot reopen %s: %v", h.pn, err)
			}
			if _, err := f.Seek(h.off, 0); err != nil {
				Exitf("cannot seek %s: %v", h.pn, err)
			}

			w, err := os.Create(dst)
			if err != nil {
				Exitf("cannot create %s: %v", dst, err)
			}
			if _, err := io.CopyN(w, f, h.length); err != nil {
				Exitf("cannot write %s: %v", dst, err)
			}
			if err := w.Close(); err != nil {
				Exitf("cannot close %s: %v", dst, err)
			}
		}()
	}
	wg.Wait()
	return paths
}

// archive builds a .a archive from the hostobj object files.
func archive() {
	if Buildmode != BuildmodeCArchive {
		return
	}

	if extar == "" {
		extar = "ar"
	}

	mayberemoveoutfile()

	// Force the buffer to flush here so that external
	// tools will see a complete file.
	Cflush()
	if err := coutbuf.f.Close(); err != nil {
		Exitf("close: %v", err)
	}
	coutbuf.f = nil

	argv := []string{extar, "-q", "-c", "-s", outfile}
	argv = append(argv, filepath.Join(tmpdir, "go.o"))
	argv = append(argv, hostobjCopy()...)

	if Debug['v'] != 0 {
		fmt.Fprintf(&Bso, "archive: %s\n", strings.Join(argv, " "))
		Bso.Flush()
	}

	if out, err := exec.Command(argv[0], argv[1:]...).CombinedOutput(); err != nil {
		Exitf("running %s failed: %v\n%s", argv[0], err, out)
	}
}

func hostlink() {
	if Linkmode != LinkExternal || nerrors > 0 {
		return
	}
	if Buildmode == BuildmodeCArchive {
		return
	}

	if extld == "" {
		extld = "gcc"
	}

	var argv []string
	argv = append(argv, extld)
	argv = append(argv, hostlinkArchArgs()...)

	if Debug['s'] == 0 && debug_s == 0 {
		argv = append(argv, "-gdwarf-2")
	} else {
		argv = append(argv, "-s")
	}

	if HEADTYPE == obj.Hdarwin {
		argv = append(argv, "-Wl,-no_pie,-headerpad,1144")
	}
	if HEADTYPE == obj.Hopenbsd {
		argv = append(argv, "-Wl,-nopie")
	}
	if HEADTYPE == obj.Hwindows {
		if headstring == "windowsgui" {
			argv = append(argv, "-mwindows")
		} else {
			argv = append(argv, "-mconsole")
		}
	}

	switch Buildmode {
	case BuildmodeExe:
		if HEADTYPE == obj.Hdarwin {
			argv = append(argv, "-Wl,-pagezero_size,4000000")
		}
	case BuildmodePIE:
		argv = append(argv, "-pie")
	case BuildmodeCShared:
		if HEADTYPE == obj.Hdarwin {
			argv = append(argv, "-dynamiclib", "-Wl,-read_only_relocs,suppress")
		} else {
			// ELF.
			argv = append(argv, "-Wl,-Bsymbolic")
			if UseRelro() {
				argv = append(argv, "-Wl,-z,relro")
			}
			// Pass -z nodelete to mark the shared library as
			// non-closeable: a dlclose will do nothing.
			argv = append(argv, "-shared", "-Wl,-z,nodelete")
		}
	case BuildmodeShared:
		if UseRelro() {
			argv = append(argv, "-Wl,-z,relro")
		}
		argv = append(argv, "-shared")
	}

	if Iself && DynlinkingGo() {
		// We force all symbol resolution to be done at program startup
		// because lazy PLT resolution can use large amounts of stack at
		// times we cannot allow it to do so.
		argv = append(argv, "-Wl,-znow")
	}

	if Iself && len(buildinfo) > 0 {
		argv = append(argv, fmt.Sprintf("-Wl,--build-id=0x%x", buildinfo))
	}

	// On Windows, given -o foo, GCC will append ".exe" to produce
	// "foo.exe".  We have decided that we want to honor the -o
	// option. To make this work, we append a '.' so that GCC
	// will decide that the file already has an extension. We
	// only want to do this when producing a Windows output file
	// on a Windows host.
	outopt := outfile
	if goos == "windows" && runtime.GOOS == "windows" && filepath.Ext(outopt) == "" {
		outopt += "."
	}
	argv = append(argv, "-o")
	argv = append(argv, outopt)

	if rpath.val != "" {
		argv = append(argv, fmt.Sprintf("-Wl,-rpath,%s", rpath.val))
	}

	// Force global symbols to be exported for dlopen, etc.
	if Iself {
		argv = append(argv, "-rdynamic")
	}

	if strings.Contains(argv[0], "clang") {
		argv = append(argv, "-Qunused-arguments")
	}

	argv = append(argv, filepath.Join(tmpdir, "go.o"))
	argv = append(argv, hostobjCopy()...)

	if Linkshared {
		seenDirs := make(map[string]bool)
		seenLibs := make(map[string]bool)
		addshlib := func(path string) {
			dir, base := filepath.Split(path)
			if !seenDirs[dir] {
				argv = append(argv, "-L"+dir)
				if !rpath.set {
					argv = append(argv, "-Wl,-rpath="+dir)
				}
				seenDirs[dir] = true
			}
			base = strings.TrimSuffix(base, ".so")
			base = strings.TrimPrefix(base, "lib")
			if !seenLibs[base] {
				argv = append(argv, "-l"+base)
				seenLibs[base] = true
			}
		}
		for _, shlib := range Ctxt.Shlibs {
			addshlib(shlib.Path)
			for _, dep := range shlib.Deps {
				if dep == "" {
					continue
				}
				libpath := findshlib(dep)
				if libpath != "" {
					addshlib(libpath)
				}
			}
		}
	}

	argv = append(argv, ldflag...)

	for _, p := range strings.Fields(extldflags) {
		argv = append(argv, p)

		// clang, unlike GCC, passes -rdynamic to the linker
		// even when linking with -static, causing a linker
		// error when using GNU ld. So take out -rdynamic if
		// we added it. We do it in this order, rather than
		// only adding -rdynamic later, so that -extldflags
		// can override -rdynamic without using -static.
		if Iself && p == "-static" {
			for i := range argv {
				if argv[i] == "-rdynamic" {
					argv[i] = "-static"
				}
			}
		}
	}
	if HEADTYPE == obj.Hwindows {
		argv = append(argv, peimporteddlls()...)
	}

	if Debug['v'] != 0 {
		fmt.Fprintf(&Bso, "host link:")
		for _, v := range argv {
			fmt.Fprintf(&Bso, " %q", v)
		}
		fmt.Fprintf(&Bso, "\n")
		Bso.Flush()
	}

	if out, err := exec.Command(argv[0], argv[1:]...).CombinedOutput(); err != nil {
		Exitf("running %s failed: %v\n%s", argv[0], err, out)
	} else if Debug['v'] != 0 && len(out) > 0 {
		fmt.Fprintf(&Bso, "%s", out)
		Bso.Flush()
	}

	if Debug['s'] == 0 && debug_s == 0 && HEADTYPE == obj.Hdarwin {
		// Skip combining dwarf on arm.
		if Thearch.Thechar != '5' && Thearch.Thechar != '7' {
			dsym := filepath.Join(tmpdir, "go.dwarf")
			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)
			}
			// Skip combining if `dsymutil` didn't generate a file. See #11994.
			if _, err := os.Stat(dsym); os.IsNotExist(err) {
				return
			}
			// For os.Rename to work reliably, must be in same directory as outfile.
			combinedOutput := outfile + "~"
			if err := machoCombineDwarf(outfile, dsym, combinedOutput); err != nil {
				Ctxt.Cursym = nil
				Exitf("%s: combining dwarf failed: %v", os.Args[0], err)
			}
			os.Remove(outfile)
			if err := os.Rename(combinedOutput, outfile); err != nil {
				Ctxt.Cursym = nil
				Exitf("%s: %v", os.Args[0], err)
			}
		}
	}
}

// hostlinkArchArgs returns arguments to pass to the external linker
// based on the architecture.
func hostlinkArchArgs() []string {
	switch Thearch.Thechar {
	case '8':
		return []string{"-m32"}
	case '6', '9', 'z':
		return []string{"-m64"}
	case '5':
		return []string{"-marm"}
	case '7':
		// nothing needed
	}
	return nil
}

// ldobj loads an input object. If it is a host object (an object
// compiled by a non-Go compiler) it returns the Hostobj pointer. If
// it is a Go object, it returns nil.
func ldobj(f *obj.Biobuf, pkg string, length int64, pn string, file string, whence int) *Hostobj {
	eof := obj.Boffset(f) + length

	start := obj.Boffset(f)
	c1 := obj.Bgetc(f)
	c2 := obj.Bgetc(f)
	c3 := obj.Bgetc(f)
	c4 := obj.Bgetc(f)
	obj.Bseek(f, start, 0)

	magic := uint32(c1)<<24 | uint32(c2)<<16 | uint32(c3)<<8 | uint32(c4)
	if magic == 0x7f454c46 { // \x7F E L F
		return ldhostobj(ldelf, f, pkg, length, pn, file)
	}

	if magic&^1 == 0xfeedface || magic&^0x01000000 == 0xcefaedfe {
		return ldhostobj(ldmacho, f, pkg, length, pn, file)
	}

	if c1 == 0x4c && c2 == 0x01 || c1 == 0x64 && c2 == 0x86 {
		return ldhostobj(ldpe, f, pkg, length, pn, file)
	}

	/* check the header */
	line := obj.Brdline(f, '\n')
	if line == "" {
		if obj.Blinelen(f) > 0 {
			Diag("%s: not an object file", pn)
			return nil
		}
		Diag("truncated object file: %s", pn)
		return nil
	}

	if !strings.HasPrefix(line, "go object ") {
		if strings.HasSuffix(pn, ".go") {
			Exitf("%cl: input %s is not .%c file (use %cg to compile .go files)", Thearch.Thechar, pn, Thearch.Thechar, Thearch.Thechar)
		}

		if line == Thestring {
			// old header format: just $GOOS
			Diag("%s: stale object file", pn)
			return nil
		}

		Diag("%s: not an object file", pn)
		return nil
	}

	// First, check that the basic goos, goarch, and version match.
	t := fmt.Sprintf("%s %s %s ", goos, obj.Getgoarch(), obj.Getgoversion())

	line = strings.TrimRight(line, "\n")
	if !strings.HasPrefix(line[10:]+" ", t) && Debug['f'] == 0 {
		Diag("%s: object is [%s] expected [%s]", pn, line[10:], t)
		return nil
	}

	// Second, check that longer lines match each other exactly,
	// so that the Go compiler and write additional information
	// that must be the same from run to run.
	if len(line) >= len(t)+10 {
		if theline == "" {
			theline = line[10:]
		} else if theline != line[10:] {
			Diag("%s: object is [%s] expected [%s]", pn, line[10:], theline)
			return nil
		}
	}

	/* skip over exports and other info -- ends with \n!\n */
	import0 := obj.Boffset(f)

	c1 = '\n' // the last line ended in \n
	c2 = obj.Bgetc(f)
	c3 = obj.Bgetc(f)
	for c1 != '\n' || c2 != '!' || c3 != '\n' {
		c1 = c2
		c2 = c3
		c3 = obj.Bgetc(f)
		if c3 == obj.Beof {
			Diag("truncated object file: %s", pn)
			return nil
		}
	}

	import1 := obj.Boffset(f)

	obj.Bseek(f, import0, 0)
	ldpkg(f, pkg, import1-import0-2, pn, whence) // -2 for !\n
	obj.Bseek(f, import1, 0)

	LoadObjFile(Ctxt, f, pkg, eof-obj.Boffset(f), pn)
	return nil
}

func readelfsymboldata(f *elf.File, sym *elf.Symbol) []byte {
	data := make([]byte, sym.Size)
	sect := f.Sections[sym.Section]
	if sect.Type != elf.SHT_PROGBITS && sect.Type != elf.SHT_NOTE {
		Diag("reading %s from non-data section", sym.Name)
	}
	n, err := sect.ReadAt(data, int64(sym.Value-sect.Addr))
	if uint64(n) != sym.Size {
		Diag("reading contents of %s: %v", sym.Name, err)
	}
	return data
}

func readwithpad(r io.Reader, sz int32) ([]byte, error) {
	data := make([]byte, Rnd(int64(sz), 4))
	_, err := io.ReadFull(r, data)
	if err != nil {
		return nil, err
	}
	data = data[:sz]
	return data, nil
}

func readnote(f *elf.File, name []byte, typ int32) ([]byte, error) {
	for _, sect := range f.Sections {
		if sect.Type != elf.SHT_NOTE {
			continue
		}
		r := sect.Open()
		for {
			var namesize, descsize, noteType int32
			err := binary.Read(r, f.ByteOrder, &namesize)
			if err != nil {
				if err == io.EOF {
					break
				}
				return nil, fmt.Errorf("read namesize failed: %v", err)
			}
			err = binary.Read(r, f.ByteOrder, &descsize)
			if err != nil {
				return nil, fmt.Errorf("read descsize failed: %v", err)
			}
			err = binary.Read(r, f.ByteOrder, &noteType)
			if err != nil {
				return nil, fmt.Errorf("read type failed: %v", err)
			}
			noteName, err := readwithpad(r, namesize)
			if err != nil {
				return nil, fmt.Errorf("read name failed: %v", err)
			}
			desc, err := readwithpad(r, descsize)
			if err != nil {
				return nil, fmt.Errorf("read desc failed: %v", err)
			}
			if string(name) == string(noteName) && typ == noteType {
				return desc, nil
			}
		}
	}
	return nil, nil
}

func findshlib(shlib string) string {
	for _, libdir := range Ctxt.Libdir {
		libpath := filepath.Join(libdir, shlib)
		if _, err := os.Stat(libpath); err == nil {
			return libpath
		}
	}
	Diag("cannot find shared library: %s", shlib)
	return ""
}

func ldshlibsyms(shlib string) {
	libpath := findshlib(shlib)
	if libpath == "" {
		return
	}
	for _, processedlib := range Ctxt.Shlibs {
		if processedlib.Path == libpath {
			return
		}
	}
	if Ctxt.Debugvlog > 1 && Ctxt.Bso != nil {
		fmt.Fprintf(Ctxt.Bso, "%5.2f ldshlibsyms: found library with name %s at %s\n", obj.Cputime(), shlib, libpath)
		Ctxt.Bso.Flush()
	}

	f, err := elf.Open(libpath)
	if err != nil {
		Diag("cannot open shared library: %s", libpath)
		return
	}

	hash, err := readnote(f, ELF_NOTE_GO_NAME, ELF_NOTE_GOABIHASH_TAG)
	if err != nil {
		Diag("cannot read ABI hash from shared library %s: %v", libpath, err)
		return
	}

	depsbytes, err := readnote(f, ELF_NOTE_GO_NAME, ELF_NOTE_GODEPS_TAG)
	if err != nil {
		Diag("cannot read dep list from shared library %s: %v", libpath, err)
		return
	}
	deps := strings.Split(string(depsbytes), "\n")

	syms, err := f.DynamicSymbols()
	if err != nil {
		Diag("cannot read symbols from shared library: %s", libpath)
		return
	}
	gcdata_locations := make(map[uint64]*LSym)
	for _, elfsym := range syms {
		if elf.ST_TYPE(elfsym.Info) == elf.STT_NOTYPE || elf.ST_TYPE(elfsym.Info) == elf.STT_SECTION {
			continue
		}
		lsym := Linklookup(Ctxt, elfsym.Name, 0)
		// Because loadlib above loads all .a files before loading any shared
		// libraries, any symbols we find that duplicate symbols already
		// loaded should be ignored (the symbols from the .a files "win").
		if lsym.Type != 0 {
			continue
		}
		lsym.Type = obj.SDYNIMPORT
		lsym.ElfType = elf.ST_TYPE(elfsym.Info)
		lsym.Size = int64(elfsym.Size)
		if elfsym.Section != elf.SHN_UNDEF {
			// Set .File for the library that actually defines the symbol.
			lsym.File = libpath
			// The decodetype_* functions in decodetype.go need access to
			// the type data.
			if strings.HasPrefix(lsym.Name, "type.") && !strings.HasPrefix(lsym.Name, "type..") {
				lsym.P = readelfsymboldata(f, &elfsym)
				gcdata_locations[elfsym.Value+2*uint64(Thearch.Ptrsize)+8+1*uint64(Thearch.Ptrsize)] = lsym
			}
		}
	}
	gcdata_addresses := make(map[*LSym]uint64)
	if Thearch.Thechar == '7' {
		for _, sect := range f.Sections {
			if sect.Type == elf.SHT_RELA {
				var rela elf.Rela64
				rdr := sect.Open()
				for {
					err := binary.Read(rdr, f.ByteOrder, &rela)
					if err == io.EOF {
						break
					} else if err != nil {
						Diag("reading relocation failed %v", err)
						return
					}
					t := elf.R_AARCH64(rela.Info & 0xffff)
					if t != elf.R_AARCH64_RELATIVE {
						continue
					}
					if lsym, ok := gcdata_locations[rela.Off]; ok {
						gcdata_addresses[lsym] = uint64(rela.Addend)
					}
				}
			}
		}
	}

	// We might have overwritten some functions above (this tends to happen for the
	// autogenerated type equality/hashing functions) and we don't want to generated
	// pcln table entries for these any more so unstitch them from the Textp linked
	// list.
	var last *LSym

	for s := Ctxt.Textp; s != nil; s = s.Next {
		if s.Type == obj.SDYNIMPORT {
			continue
		}

		if last == nil {
			Ctxt.Textp = s
		} else {
			last.Next = s
		}
		last = s
	}

	if last == nil {
		Ctxt.Textp = nil
		Ctxt.Etextp = nil
	} else {
		last.Next = nil
		Ctxt.Etextp = last
	}

	Ctxt.Shlibs = append(Ctxt.Shlibs, Shlib{Path: libpath, Hash: hash, Deps: deps, File: f, gcdata_addresses: gcdata_addresses})
}

func mywhatsys() {
	goroot = obj.Getgoroot()
	goos = obj.Getgoos()
	goarch = obj.Getgoarch()

	if !strings.HasPrefix(goarch, Thestring) {
		log.Fatalf("cannot use %cc with GOARCH=%s", Thearch.Thechar, goarch)
	}
}

// Copied from ../gc/subr.c:/^pathtoprefix; must stay in sync.
/*
 * Convert raw string to the prefix that will be used in the symbol table.
 * Invalid bytes turn into %xx.	 Right now the only bytes that need
 * escaping are %, ., and ", but we escape all control characters too.
 *
 * If you edit this, edit ../gc/subr.c:/^pathtoprefix too.
 * If you edit this, edit ../../debug/goobj/read.go:/importPathToPrefix too.
 */
func pathtoprefix(s string) string {
	slash := strings.LastIndex(s, "/")
	for i := 0; i < len(s); i++ {
		c := s[i]
		if c <= ' ' || i >= slash && c == '.' || c == '%' || c == '"' || c >= 0x7F {
			var buf bytes.Buffer
			for i := 0; i < len(s); i++ {
				c := s[i]
				if c <= ' ' || i >= slash && c == '.' || c == '%' || c == '"' || c >= 0x7F {
					fmt.Fprintf(&buf, "%%%02x", c)
					continue
				}
				buf.WriteByte(c)
			}
			return buf.String()
		}
	}
	return s
}

func addsection(seg *Segment, name string, rwx int) *Section {
	var l **Section

	for l = &seg.Sect; *l != nil; l = &(*l).Next {
	}
	sect := new(Section)
	sect.Rwx = uint8(rwx)
	sect.Name = name
	sect.Seg = seg
	sect.Align = int32(Thearch.Ptrsize) // everything is at least pointer-aligned
	*l = sect
	return sect
}

func Le16(b []byte) uint16 {
	return uint16(b[0]) | uint16(b[1])<<8
}

func Le32(b []byte) uint32 {
	return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
}

func Le64(b []byte) uint64 {
	return uint64(Le32(b)) | uint64(Le32(b[4:]))<<32
}

func Be16(b []byte) uint16 {
	return uint16(b[0])<<8 | uint16(b[1])
}

func Be32(b []byte) uint32 {
	return uint32(b[0])<<24 | uint32(b[1])<<16 | uint32(b[2])<<8 | uint32(b[3])
}

type Chain struct {
	sym   *LSym
	up    *Chain
	limit int // limit on entry to sym
}

var morestack *LSym

// TODO: Record enough information in new object files to
// allow stack checks here.

func haslinkregister() bool {
	return Ctxt.FixedFrameSize() != 0
}

func callsize() int {
	if haslinkregister() {
		return 0
	}
	return Thearch.Regsize
}

func dostkcheck() {
	var ch Chain

	morestack = Linklookup(Ctxt, "runtime.morestack", 0)

	// Every splitting function ensures that there are at least StackLimit
	// bytes available below SP when the splitting prologue finishes.
	// If the splitting function calls F, then F begins execution with
	// at least StackLimit - callsize() bytes available.
	// Check that every function behaves correctly with this amount
	// of stack, following direct calls in order to piece together chains
	// of non-splitting functions.
	ch.up = nil

	ch.limit = obj.StackLimit - callsize()

	// Check every function, but do the nosplit functions in a first pass,
	// to make the printed failure chains as short as possible.
	for s := Ctxt.Textp; s != nil; s = s.Next {
		// runtime.racesymbolizethunk is called from gcc-compiled C
		// code running on the operating system thread stack.
		// It uses more than the usual amount of stack but that's okay.
		if s.Name == "runtime.racesymbolizethunk" {
			continue
		}

		if s.Attr.NoSplit() {
			Ctxt.Cursym = s
			ch.sym = s
			stkcheck(&ch, 0)
		}
	}

	for s := Ctxt.Textp; s != nil; s = s.Next {
		if !s.Attr.NoSplit() {
			Ctxt.Cursym = s
			ch.sym = s
			stkcheck(&ch, 0)
		}
	}
}

func stkcheck(up *Chain, depth int) int {
	limit := up.limit
	s := up.sym

	// Don't duplicate work: only need to consider each
	// function at top of safe zone once.
	top := limit == obj.StackLimit-callsize()
	if top {
		if s.Attr.StackCheck() {
			return 0
		}
		s.Attr |= AttrStackCheck
	}

	if depth > 100 {
		Diag("nosplit stack check too deep")
		stkbroke(up, 0)
		return -1
	}

	if s.Attr.External() || s.Pcln == nil {
		// external function.
		// should never be called directly.
		// only diagnose the direct caller.
		// TODO(mwhudson): actually think about this.
		if depth == 1 && s.Type != obj.SXREF && !DynlinkingGo() &&
			Buildmode != BuildmodePIE && Buildmode != BuildmodeCShared {
			Diag("call to external function %s", s.Name)
		}
		return -1
	}

	if limit < 0 {
		stkbroke(up, limit)
		return -1
	}

	// morestack looks like it calls functions,
	// but it switches the stack pointer first.
	if s == morestack {
		return 0
	}

	var ch Chain
	ch.up = up

	if !s.Attr.NoSplit() {
		// Ensure we have enough stack to call morestack.
		ch.limit = limit - callsize()
		ch.sym = morestack
		if stkcheck(&ch, depth+1) < 0 {
			return -1
		}
		if !top {
			return 0
		}
		// Raise limit to allow frame.
		limit = int(obj.StackLimit+s.Locals) + int(Ctxt.FixedFrameSize())
	}

	// Walk through sp adjustments in function, consuming relocs.
	ri := 0

	endr := len(s.R)
	var ch1 Chain
	var pcsp Pciter
	var r *Reloc
	for pciterinit(Ctxt, &pcsp, &s.Pcln.Pcsp); pcsp.done == 0; pciternext(&pcsp) {
		// pcsp.value is in effect for [pcsp.pc, pcsp.nextpc).

		// Check stack size in effect for this span.
		if int32(limit)-pcsp.value < 0 {
			stkbroke(up, int(int32(limit)-pcsp.value))
			return -1
		}

		// Process calls in this span.
		for ; ri < endr && uint32(s.R[ri].Off) < pcsp.nextpc; ri++ {
			r = &s.R[ri]
			switch r.Type {
			// Direct call.
			case obj.R_CALL, obj.R_CALLARM, obj.R_CALLARM64, obj.R_CALLPOWER, obj.R_CALLMIPS:
				ch.limit = int(int32(limit) - pcsp.value - int32(callsize()))
				ch.sym = r.Sym
				if stkcheck(&ch, depth+1) < 0 {
					return -1
				}

			// Indirect call. Assume it is a call to a splitting function,
			// so we have to make sure it can call morestack.
			// Arrange the data structures to report both calls, so that
			// if there is an error, stkprint shows all the steps involved.
			case obj.R_CALLIND:
				ch.limit = int(int32(limit) - pcsp.value - int32(callsize()))

				ch.sym = nil
				ch1.limit = ch.limit - callsize() // for morestack in called prologue
				ch1.up = &ch
				ch1.sym = morestack
				if stkcheck(&ch1, depth+2) < 0 {
					return -1
				}
			}
		}
	}

	return 0
}

func stkbroke(ch *Chain, limit int) {
	Diag("nosplit stack overflow")
	stkprint(ch, limit)
}

func stkprint(ch *Chain, limit int) {
	var name string

	if ch.sym != nil {
		name = ch.sym.Name
		if ch.sym.Attr.NoSplit() {
			name += " (nosplit)"
		}
	} else {
		name = "function pointer"
	}

	if ch.up == nil {
		// top of chain.  ch->sym != nil.
		if ch.sym.Attr.NoSplit() {
			fmt.Printf("\t%d\tassumed on entry to %s\n", ch.limit, name)
		} else {
			fmt.Printf("\t%d\tguaranteed after split check in %s\n", ch.limit, name)
		}
	} else {
		stkprint(ch.up, ch.limit+callsize())
		if !haslinkregister() {
			fmt.Printf("\t%d\ton entry to %s\n", ch.limit, name)
		}
	}

	if ch.limit != limit {
		fmt.Printf("\t%d\tafter %s uses %d\n", limit, name, ch.limit-limit)
	}
}

func Cflush() {
	if err := coutbuf.w.Flush(); err != nil {
		Exitf("flushing %s: %v", coutbuf.f.Name(), err)
	}
}

func Cpos() int64 {
	return coutbuf.off
}

func Cseek(p int64) {
	if p == coutbuf.off {
		return
	}
	Cflush()
	if _, err := coutbuf.f.Seek(p, 0); err != nil {
		Exitf("seeking in output [0, 1]: %v", err)
	}
	coutbuf.off = p
}

func Cwritestring(s string) {
	coutbuf.WriteString(s)
}

func Cwrite(p []byte) {
	coutbuf.Write(p)
}

func Cput(c uint8) {
	coutbuf.w.WriteByte(c)
	coutbuf.off++
}

func usage() {
	fmt.Fprintf(os.Stderr, "usage: link [options] main.o\n")
	obj.Flagprint(2)
	Exit(2)
}

func setheadtype(s string) {
	h := headtype(s)
	if h < 0 {
		Exitf("unknown header type -H %s", s)
	}

	headstring = s
	HEADTYPE = int32(headtype(s))
}

func setinterp(s string) {
	Debug['I'] = 1 // denote cmdline interpreter override
	interpreter = s
}

func doversion() {
	Exitf("version %s", obj.Getgoversion())
}

func genasmsym(put func(*LSym, string, int, int64, int64, int, *LSym)) {
	// These symbols won't show up in the first loop below because we
	// skip STEXT symbols. Normal STEXT symbols are emitted by walking textp.
	s := Linklookup(Ctxt, "runtime.text", 0)
	if s.Type == obj.STEXT {
		put(s, s.Name, 'T', s.Value, s.Size, int(s.Version), nil)
	}
	s = Linklookup(Ctxt, "runtime.etext", 0)
	if s.Type == obj.STEXT {
		put(s, s.Name, 'T', s.Value, s.Size, int(s.Version), nil)
	}

	for _, s := range Ctxt.Allsym {
		if s.Attr.Hidden() {
			continue
		}
		if (s.Name == "" || s.Name[0] == '.') && s.Version == 0 && s.Name != ".rathole" && s.Name != ".TOC." {
			continue
		}
		switch s.Type & obj.SMASK {
		case obj.SCONST,
			obj.SRODATA,
			obj.SSYMTAB,
			obj.SPCLNTAB,
			obj.SINITARR,
			obj.SDATA,
			obj.SNOPTRDATA,
			obj.SELFROSECT,
			obj.SMACHOGOT,
			obj.STYPE,
			obj.SSTRING,
			obj.SGOSTRING,
			obj.SGOSTRINGHDR,
			obj.SGOFUNC,
			obj.SGCBITS,
			obj.STYPERELRO,
			obj.SSTRINGRELRO,
			obj.SGOSTRINGRELRO,
			obj.SGOSTRINGHDRRELRO,
			obj.SGOFUNCRELRO,
			obj.SGCBITSRELRO,
			obj.SRODATARELRO,
			obj.STYPELINK,
			obj.SITABLINK,
			obj.SWINDOWS:
			if !s.Attr.Reachable() {
				continue
			}
			put(s, s.Name, 'D', Symaddr(s), s.Size, int(s.Version), s.Gotype)

		case obj.SBSS, obj.SNOPTRBSS:
			if !s.Attr.Reachable() {
				continue
			}
			if len(s.P) > 0 {
				Diag("%s should not be bss (size=%d type=%d special=%v)", s.Name, int(len(s.P)), s.Type, s.Attr.Special())
			}
			put(s, s.Name, 'B', Symaddr(s), s.Size, int(s.Version), s.Gotype)

		case obj.SFILE:
			put(nil, s.Name, 'f', s.Value, 0, int(s.Version), nil)

		case obj.SHOSTOBJ:
			if HEADTYPE == obj.Hwindows || Iself {
				put(s, s.Name, 'U', s.Value, 0, int(s.Version), nil)
			}

		case obj.SDYNIMPORT:
			if !s.Attr.Reachable() {
				continue
			}
			put(s, s.Extname, 'U', 0, 0, int(s.Version), nil)

		case obj.STLSBSS:
			if Linkmode == LinkExternal && HEADTYPE != obj.Hopenbsd {
				put(s, s.Name, 't', Symaddr(s), s.Size, int(s.Version), s.Gotype)
			}
		}
	}

	var off int32
	for s := Ctxt.Textp; s != nil; s = s.Next {
		put(s, s.Name, 'T', s.Value, s.Size, int(s.Version), s.Gotype)

		// NOTE(ality): acid can't produce a stack trace without .frame symbols
		put(nil, ".frame", 'm', int64(s.Locals)+int64(Thearch.Ptrsize), 0, 0, nil)

		for _, a := range s.Autom {
			// Emit a or p according to actual offset, even if label is wrong.
			// This avoids negative offsets, which cannot be encoded.
			if a.Name != obj.A_AUTO && a.Name != obj.A_PARAM {
				continue
			}

			// compute offset relative to FP
			if a.Name == obj.A_PARAM {
				off = a.Aoffset
			} else {
				off = a.Aoffset - int32(Thearch.Ptrsize)
			}

			// FP
			if off >= 0 {
				put(nil, a.Asym.Name, 'p', int64(off), 0, 0, a.Gotype)
				continue
			}

			// SP
			if off <= int32(-Thearch.Ptrsize) {
				put(nil, a.Asym.Name, 'a', -(int64(off) + int64(Thearch.Ptrsize)), 0, 0, a.Gotype)
				continue
			}
		}
	}

	// Otherwise, off is addressing the saved program counter.
	// Something underhanded is going on. Say nothing.
	if Debug['v'] != 0 || Debug['n'] != 0 {
		fmt.Fprintf(&Bso, "%5.2f symsize = %d\n", obj.Cputime(), uint32(Symsize))
	}
	Bso.Flush()
}

func Symaddr(s *LSym) int64 {
	if !s.Attr.Reachable() {
		Diag("unreachable symbol in symaddr - %s", s.Name)
	}
	return s.Value
}

func xdefine(p string, t int, v int64) {
	s := Linklookup(Ctxt, p, 0)
	s.Type = int16(t)
	s.Value = v
	s.Attr |= AttrReachable
	s.Attr |= AttrSpecial
	s.Attr |= AttrLocal
}

func datoff(addr int64) int64 {
	if uint64(addr) >= Segdata.Vaddr {
		return int64(uint64(addr) - Segdata.Vaddr + Segdata.Fileoff)
	}
	if uint64(addr) >= Segtext.Vaddr {
		return int64(uint64(addr) - Segtext.Vaddr + Segtext.Fileoff)
	}
	Diag("datoff %#x", addr)
	return 0
}

func Entryvalue() int64 {
	a := INITENTRY
	if a[0] >= '0' && a[0] <= '9' {
		return atolwhex(a)
	}
	s := Linklookup(Ctxt, a, 0)
	if s.Type == 0 {
		return INITTEXT
	}
	if s.Type != obj.STEXT {
		Diag("entry not text: %s", s.Name)
	}
	return s.Value
}

func undefsym(s *LSym) {
	var r *Reloc

	Ctxt.Cursym = s
	for i := 0; i < len(s.R); i++ {
		r = &s.R[i]
		if r.Sym == nil { // happens for some external ARM relocs
			continue
		}
		if r.Sym.Type == obj.Sxxx || r.Sym.Type == obj.SXREF {
			Diag("undefined: %s", r.Sym.Name)
		}
		if !r.Sym.Attr.Reachable() {
			Diag("use of unreachable symbol: %s", r.Sym.Name)
		}
	}
}

func undef() {
	for s := Ctxt.Textp; s != nil; s = s.Next {
		undefsym(s)
	}
	for s := datap; s != nil; s = s.Next {
		undefsym(s)
	}
	if nerrors > 0 {
		errorexit()
	}
}

func callgraph() {
	if Debug['c'] == 0 {
		return
	}

	var i int
	var r *Reloc
	for s := Ctxt.Textp; s != nil; s = s.Next {
		for i = 0; i < len(s.R); i++ {
			r = &s.R[i]
			if r.Sym == nil {
				continue
			}
			if (r.Type == obj.R_CALL || r.Type == obj.R_CALLARM || r.Type == obj.R_CALLPOWER || r.Type == obj.R_CALLMIPS) && r.Sym.Type == obj.STEXT {
				fmt.Fprintf(&Bso, "%s calls %s\n", s.Name, r.Sym.Name)
			}
		}
	}
}

func Diag(format string, args ...interface{}) {
	tn := ""
	sep := ""
	if Ctxt.Cursym != nil {
		tn = Ctxt.Cursym.Name
		sep = ": "
	}
	fmt.Printf("%s%s%s\n", tn, sep, fmt.Sprintf(format, args...))
	nerrors++
	if Debug['h'] != 0 {
		panic("error")
	}
	if nerrors > 20 {
		Exitf("too many errors")
	}
}

func Rnd(v int64, r int64) int64 {
	if r <= 0 {
		return v
	}
	v += r - 1
	c := v % r
	if c < 0 {
		c += r
	}
	v -= c
	return v
}