src/cmd/link/internal/ld/lib.go

// Inferno utils/8l/asm.c
// https://bitbucket.org/inferno-os/inferno-os/src/master/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 (
	"bytes"
	"cmd/internal/bio"
	"cmd/internal/goobj2"
	"cmd/internal/obj"
	"cmd/internal/objabi"
	"cmd/internal/sys"
	"cmd/link/internal/loadelf"
	"cmd/link/internal/loader"
	"cmd/link/internal/loadmacho"
	"cmd/link/internal/loadpe"
	"cmd/link/internal/loadxcoff"
	"cmd/link/internal/sym"
	"crypto/sha1"
	"debug/elf"
	"debug/macho"
	"encoding/base64"
	"encoding/binary"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"os"
	"os/exec"
	"path/filepath"
	"runtime"
	"sort"
	"strings"
	"sync"
)

// Data layout and relocation.

// Derived from Inferno utils/6l/l.h
// https://bitbucket.org/inferno-os/inferno-os/src/master/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.

// ArchSyms holds a number of architecture specific symbols used during
// relocation.  Rather than allowing them universal access to all symbols,
// we keep a subset for relocation application.
type ArchSyms struct {
	Rel     loader.Sym
	Rela    loader.Sym
	RelPLT  loader.Sym
	RelaPLT loader.Sym

	LinkEditGOT loader.Sym
	LinkEditPLT loader.Sym

	TOC    loader.Sym
	DotTOC []loader.Sym // for each version

	GOT    loader.Sym
	PLT    loader.Sym
	GOTPLT loader.Sym

	Tlsg      loader.Sym
	Tlsoffset int

	Dynamic loader.Sym
	DynSym  loader.Sym
	DynStr  loader.Sym
}

// mkArchSym is a helper for setArchSyms, to set up a special symbol.
func (ctxt *Link) mkArchSym(name string, ver int, ls *loader.Sym) {
	*ls = ctxt.loader.LookupOrCreateSym(name, ver)
}

// mkArchVecSym is similar to  setArchSyms, but operates on elements within
// a slice, where each element corresponds to some symbol version.
func (ctxt *Link) mkArchSymVec(name string, ver int, ls []loader.Sym) {
	ls[ver] = ctxt.loader.LookupOrCreateSym(name, ver)
}

// setArchSyms sets up the ArchSyms structure, and must be called before
// relocations are applied.
func (ctxt *Link) setArchSyms() {
	ctxt.mkArchSym(".got", 0, &ctxt.GOT)
	ctxt.mkArchSym(".plt", 0, &ctxt.PLT)
	ctxt.mkArchSym(".got.plt", 0, &ctxt.GOTPLT)
	ctxt.mkArchSym(".dynamic", 0, &ctxt.Dynamic)
	ctxt.mkArchSym(".dynsym", 0, &ctxt.DynSym)
	ctxt.mkArchSym(".dynstr", 0, &ctxt.DynStr)

	if ctxt.IsPPC64() {
		ctxt.mkArchSym("TOC", 0, &ctxt.TOC)

		// NB: note the +2 below for DotTOC2 compared to the +1 for
		// DocTOC. This is because loadlibfull() creates an additional
		// syms version during conversion of loader.Sym symbols to
		// *sym.Symbol symbols. Symbols that are assigned this final
		// version are not going to have TOC references, so it should
		// be ok for them to inherit an invalid .TOC. symbol.
		// TODO: revisit the +2, now that loadlibfull is gone.
		ctxt.DotTOC = make([]loader.Sym, ctxt.MaxVersion()+2)
		for i := 0; i <= ctxt.MaxVersion(); i++ {
			if i >= 2 && i < sym.SymVerStatic { // these versions are not used currently
				continue
			}
			ctxt.mkArchSymVec(".TOC.", i, ctxt.DotTOC)
		}
	}
	if ctxt.IsElf() {
		ctxt.mkArchSym(".rel", 0, &ctxt.Rel)
		ctxt.mkArchSym(".rela", 0, &ctxt.Rela)
		ctxt.mkArchSym(".rel.plt", 0, &ctxt.RelPLT)
		ctxt.mkArchSym(".rela.plt", 0, &ctxt.RelaPLT)
	}
	if ctxt.IsDarwin() {
		ctxt.mkArchSym(".linkedit.got", 0, &ctxt.LinkEditGOT)
		ctxt.mkArchSym(".linkedit.plt", 0, &ctxt.LinkEditPLT)
	}
}

type Arch struct {
	Funcalign      int
	Maxalign       int
	Minalign       int
	Dwarfregsp     int
	Dwarfreglr     int
	Androiddynld   string
	Linuxdynld     string
	Freebsddynld   string
	Netbsddynld    string
	Openbsddynld   string
	Dragonflydynld string
	Solarisdynld   string

	// Empty spaces between codeblocks will be padded with this value.
	// For example an architecture might want to pad with a trap instruction to
	// catch wayward programs. Architectures that do not define a padding value
	// are padded with zeros.
	CodePad []byte

	// Set to true to write all text blocks in with CodeBlkWrite
	WriteTextBlocks bool

	// Plan 9 variables.
	Plan9Magic  uint32
	Plan9_64Bit bool

	Adddynrel func(*Target, *loader.Loader, *ArchSyms, loader.Sym, loader.Reloc2, int) bool
	Archinit  func(*Link)
	// Archreloc is an arch-specific hook that assists in relocation processing
	// (invoked by 'relocsym'); it handles target-specific relocation tasks.
	// Here "rel" is the current relocation being examined, "sym" is the symbol
	// containing the chunk of data to which the relocation applies, and "off"
	// is the contents of the to-be-relocated data item (from sym.P). Return
	// value is the appropriately relocated value (to be written back to the
	// same spot in sym.P), a boolean indicating if the external relocations'
	// been used, and a boolean indicating success/failure (a failing value
	// indicates a fatal error).
	Archreloc func(*Target, *loader.Loader, *ArchSyms, loader.Reloc2, *loader.ExtReloc,
		loader.Sym, int64) (relocatedOffset int64, needExtReloc bool, ok bool)
	// Archrelocvariant is a second arch-specific hook used for
	// relocation processing; it handles relocations where r.Type is
	// insufficient to describe the relocation (r.Variant !=
	// sym.RV_NONE). Here "rel" is the relocation being applied, "sym"
	// is the symbol containing the chunk of data to which the
	// relocation applies, and "off" is the contents of the
	// to-be-relocated data item (from sym.P). Return is an updated
	// offset value.
	Archrelocvariant func(target *Target, ldr *loader.Loader, rel loader.Reloc2,
		rv sym.RelocVariant, sym loader.Sym, offset int64) (relocatedOffset int64)

	// Generate a trampoline for a call from s to rs if necessary. ri is
	// index of the relocation.
	Trampoline func(ctxt *Link, ldr *loader.Loader, ri int, rs, s loader.Sym)

	// Assembling the binary breaks into two phases, writing the code/data/
	// dwarf information (which is rather generic), and some more architecture
	// specific work like setting up the elf headers/dynamic relocations, etc.
	// The phases are called "Asmb" and "Asmb2". Asmb2 needs to be defined for
	// every architecture, but only if architecture has an Asmb function will
	// it be used for assembly.  Otherwise a generic assembly Asmb function is
	// used.
	Asmb  func(*Link, *loader.Loader)
	Asmb2 func(*Link, *loader.Loader)

	Elfreloc1    func(*Link, *loader.Loader, loader.Sym, loader.ExtRelocView, int64) bool
	ElfrelocSize uint32 // size of an ELF relocation record, must match Elfreloc1. Currently this can be 0, meaning that the size is not fixed (a Go reloc may turn into multiple ELF reloc).
	Elfsetupplt  func(ctxt *Link, plt, gotplt *loader.SymbolBuilder, dynamic loader.Sym)
	Gentext      func(*Link, *loader.Loader)
	Machoreloc1  func(*sys.Arch, *OutBuf, *loader.Loader, loader.Sym, loader.ExtRelocView, int64) bool
	PEreloc1     func(*sys.Arch, *OutBuf, *loader.Loader, loader.Sym, loader.ExtRelocView, int64) bool
	Xcoffreloc1  func(*sys.Arch, *OutBuf, *loader.Loader, loader.Sym, loader.ExtRelocView, int64) bool

	// TLSIEtoLE converts a TLS Initial Executable relocation to
	// a TLS Local Executable relocation.
	//
	// This is possible when a TLS IE relocation refers to a local
	// symbol in an executable, which is typical when internally
	// linking PIE binaries.
	TLSIEtoLE func(P []byte, off, size int)

	// optional override for assignAddress
	AssignAddress func(ldr *loader.Loader, sect *sym.Section, n int, s loader.Sym, va uint64, isTramp bool) (*sym.Section, int, uint64)
}

var (
	thearch Arch
	lcSize  int32
	rpath   Rpath
	spSize  int32
	symSize int32
)

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

// DynlinkingGo reports whether we are producing Go code that can live
// in separate shared libraries linked together at runtime.
func (ctxt *Link) DynlinkingGo() bool {
	if !ctxt.Loaded {
		panic("DynlinkingGo called before all symbols loaded")
	}
	return ctxt.BuildMode == BuildModeShared || ctxt.linkShared || ctxt.BuildMode == BuildModePlugin || ctxt.canUsePlugins
}

// CanUsePlugins reports whether a plugins can be used
func (ctxt *Link) CanUsePlugins() bool {
	if !ctxt.Loaded {
		panic("CanUsePlugins called before all symbols loaded")
	}
	return ctxt.canUsePlugins
}

var (
	dynlib          []string
	ldflag          []string
	havedynamic     int
	Funcalign       int
	iscgo           bool
	elfglobalsymndx int
	interpreter     string

	debug_s bool // backup old value of debug['s']
	HEADR   int32

	nerrors  int
	liveness int64

	// See -strictdups command line flag.
	checkStrictDups   int // 0=off 1=warning 2=error
	strictDupMsgCount int
)

var (
	Segtext      sym.Segment
	Segrodata    sym.Segment
	Segrelrodata sym.Segment
	Segdata      sym.Segment
	Segdwarf     sym.Segment

	Segments = []*sym.Segment{&Segtext, &Segrodata, &Segrelrodata, &Segdata, &Segdwarf}
)

const pkgdef = "__.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
	theline     string
)

func Lflag(ctxt *Link, arg string) {
	ctxt.Libdir = append(ctxt.Libdir, arg)
}

/*
 * 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(*flagOutfile); err == nil && !fi.Mode().IsRegular() {
		return
	}
	os.Remove(*flagOutfile)
}

func libinit(ctxt *Link) {
	Funcalign = thearch.Funcalign

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

	suffixsep := ""
	if *flagInstallSuffix != "" {
		suffixsep = "_"
		suffix = *flagInstallSuffix
	} else if *flagRace {
		suffixsep = "_"
		suffix = "race"
	} else if *flagMsan {
		suffixsep = "_"
		suffix = "msan"
	}

	Lflag(ctxt, filepath.Join(objabi.GOROOT, "pkg", fmt.Sprintf("%s_%s%s%s", objabi.GOOS, objabi.GOARCH, suffixsep, suffix)))

	mayberemoveoutfile()

	if err := ctxt.Out.Open(*flagOutfile); err != nil {
		Exitf("cannot create %s: %v", *flagOutfile, err)
	}

	if *flagEntrySymbol == "" {
		switch ctxt.BuildMode {
		case BuildModeCShared, BuildModeCArchive:
			*flagEntrySymbol = fmt.Sprintf("_rt0_%s_%s_lib", objabi.GOARCH, objabi.GOOS)
		case BuildModeExe, BuildModePIE:
			*flagEntrySymbol = fmt.Sprintf("_rt0_%s_%s", objabi.GOARCH, objabi.GOOS)
		case BuildModeShared, BuildModePlugin:
			// No *flagEntrySymbol for -buildmode=shared and plugin
		default:
			Errorf(nil, "unknown *flagEntrySymbol for buildmode %v", ctxt.BuildMode)
		}
	}
}

func exitIfErrors() {
	if nerrors != 0 || checkStrictDups > 1 && strictDupMsgCount > 0 {
		mayberemoveoutfile()
		Exit(2)
	}

}

func errorexit() {
	exitIfErrors()
	Exit(0)
}

func loadinternal(ctxt *Link, name string) *sym.Library {
	zerofp := goobj2.FingerprintType{}
	if ctxt.linkShared && ctxt.PackageShlib != nil {
		if shlib := ctxt.PackageShlib[name]; shlib != "" {
			return addlibpath(ctxt, "internal", "internal", "", name, shlib, zerofp)
		}
	}
	if ctxt.PackageFile != nil {
		if pname := ctxt.PackageFile[name]; pname != "" {
			return addlibpath(ctxt, "internal", "internal", pname, name, "", zerofp)
		}
		ctxt.Logf("loadinternal: cannot find %s\n", name)
		return nil
	}

	for _, libdir := range ctxt.Libdir {
		if ctxt.linkShared {
			shlibname := filepath.Join(libdir, name+".shlibname")
			if ctxt.Debugvlog != 0 {
				ctxt.Logf("searching for %s.a in %s\n", name, shlibname)
			}
			if _, err := os.Stat(shlibname); err == nil {
				return addlibpath(ctxt, "internal", "internal", "", name, shlibname, zerofp)
			}
		}
		pname := filepath.Join(libdir, name+".a")
		if ctxt.Debugvlog != 0 {
			ctxt.Logf("searching for %s.a in %s\n", name, pname)
		}
		if _, err := os.Stat(pname); err == nil {
			return addlibpath(ctxt, "internal", "internal", pname, name, "", zerofp)
		}
	}

	ctxt.Logf("warning: unable to find %s.a\n", name)
	return nil
}

// extld returns the current external linker.
func (ctxt *Link) extld() string {
	if *flagExtld == "" {
		*flagExtld = "gcc"
	}
	return *flagExtld
}

// findLibPathCmd uses cmd command to find gcc library libname.
// It returns library full path if found, or "none" if not found.
func (ctxt *Link) findLibPathCmd(cmd, libname string) string {
	extld := ctxt.extld()
	args := hostlinkArchArgs(ctxt.Arch)
	args = append(args, cmd)
	if ctxt.Debugvlog != 0 {
		ctxt.Logf("%s %v\n", extld, args)
	}
	out, err := exec.Command(extld, args...).Output()
	if err != nil {
		if ctxt.Debugvlog != 0 {
			ctxt.Logf("not using a %s file because compiler failed\n%v\n%s\n", libname, err, out)
		}
		return "none"
	}
	return strings.TrimSpace(string(out))
}

// findLibPath searches for library libname.
// It returns library full path if found, or "none" if not found.
func (ctxt *Link) findLibPath(libname string) string {
	return ctxt.findLibPathCmd("--print-file-name="+libname, libname)
}

func (ctxt *Link) loadlib() {
	var flags uint32
	switch *FlagStrictDups {
	case 0:
		// nothing to do
	case 1, 2:
		flags = loader.FlagStrictDups
	default:
		log.Fatalf("invalid -strictdups flag value %d", *FlagStrictDups)
	}
	elfsetstring1 := func(str string, off int) { elfsetstring(ctxt, 0, str, off) }
	ctxt.loader = loader.NewLoader(flags, elfsetstring1, &ctxt.ErrorReporter.ErrorReporter)
	ctxt.ErrorReporter.SymName = func(s loader.Sym) string {
		return ctxt.loader.SymName(s)
	}

	ctxt.cgo_export_static = make(map[string]bool)
	ctxt.cgo_export_dynamic = make(map[string]bool)

	// ctxt.Library grows during the loop, so not a range loop.
	i := 0
	for ; i < len(ctxt.Library); i++ {
		lib := ctxt.Library[i]
		if lib.Shlib == "" {
			if ctxt.Debugvlog > 1 {
				ctxt.Logf("autolib: %s (from %s)\n", lib.File, lib.Objref)
			}
			loadobjfile(ctxt, lib)
		}
	}

	// load internal packages, if not already
	if *flagRace {
		loadinternal(ctxt, "runtime/race")
	}
	if *flagMsan {
		loadinternal(ctxt, "runtime/msan")
	}
	loadinternal(ctxt, "runtime")
	for ; i < len(ctxt.Library); i++ {
		lib := ctxt.Library[i]
		if lib.Shlib == "" {
			loadobjfile(ctxt, lib)
		}
	}
	// At this point, the Go objects are "preloaded". Not all the symbols are
	// added to the symbol table (only defined package symbols are). Looking
	// up symbol by name may not get expected result.

	iscgo = ctxt.LibraryByPkg["runtime/cgo"] != nil
	ctxt.canUsePlugins = ctxt.LibraryByPkg["plugin"] != nil

	// We now have enough information to determine the link mode.
	determineLinkMode(ctxt)

	if ctxt.LinkMode == LinkExternal && !iscgo && !(objabi.GOOS == "darwin" && ctxt.BuildMode != BuildModePlugin && ctxt.Arch.Family == sys.AMD64) {
		// 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.
		if lib := loadinternal(ctxt, "runtime/cgo"); lib != nil && lib.Shlib == "" {
			if ctxt.BuildMode == BuildModeShared || ctxt.linkShared {
				Exitf("cannot implicitly include runtime/cgo in a shared library")
			}
			loadobjfile(ctxt, lib)
		}
	}

	// Add non-package symbols and references of externally defined symbols.
	ctxt.loader.LoadNonpkgSyms(ctxt.Arch)

	// Load symbols from shared libraries, after all Go object symbols are loaded.
	for _, lib := range ctxt.Library {
		if lib.Shlib != "" {
			if ctxt.Debugvlog > 1 {
				ctxt.Logf("autolib: %s (from %s)\n", lib.Shlib, lib.Objref)
			}
			ldshlibsyms(ctxt, lib.Shlib)
		}
	}

	// Process cgo directives (has to be done before host object loading).
	ctxt.loadcgodirectives()

	// Conditionally load host objects, or setup for external linking.
	hostobjs(ctxt)
	hostlinksetup(ctxt)

	if ctxt.LinkMode == LinkInternal && len(hostobj) != 0 {
		// If we have any undefined symbols in external
		// objects, try to read them from the libgcc file.
		any := false
		undefs := ctxt.loader.UndefinedRelocTargets(1)
		if len(undefs) > 0 {
			any = true
		}
		if any {
			if *flagLibGCC == "" {
				*flagLibGCC = ctxt.findLibPathCmd("--print-libgcc-file-name", "libgcc")
			}
			if runtime.GOOS == "openbsd" && *flagLibGCC == "libgcc.a" {
				// On OpenBSD `clang --print-libgcc-file-name` returns "libgcc.a".
				// In this case we fail to load libgcc.a and can encounter link
				// errors - see if we can find libcompiler_rt.a instead.
				*flagLibGCC = ctxt.findLibPathCmd("--print-file-name=libcompiler_rt.a", "libcompiler_rt")
			}
			if *flagLibGCC != "none" {
				hostArchive(ctxt, *flagLibGCC)
			}
			if ctxt.HeadType == objabi.Hwindows {
				if p := ctxt.findLibPath("libmingwex.a"); p != "none" {
					hostArchive(ctxt, p)
				}
				if p := ctxt.findLibPath("libmingw32.a"); p != "none" {
					hostArchive(ctxt, p)
				}
				// Link libmsvcrt.a to resolve '__acrt_iob_func' symbol
				// (see https://golang.org/issue/23649 for details).
				if p := ctxt.findLibPath("libmsvcrt.a"); p != "none" {
					hostArchive(ctxt, p)
				}
				// TODO: maybe do something similar to peimporteddlls to collect all lib names
				// and try link them all to final exe just like libmingwex.a and libmingw32.a:
				/*
					for:
					#cgo windows LDFLAGS: -lmsvcrt -lm
					import:
					libmsvcrt.a libm.a
				*/
			}
		}
	}

	// We've loaded all the code now.
	ctxt.Loaded = true

	importcycles()

	strictDupMsgCount = ctxt.loader.NStrictDupMsgs()
}

// setupdynexp constructs ctxt.dynexp, a list of loader.Sym.
func setupdynexp(ctxt *Link) {
	dynexpMap := ctxt.cgo_export_dynamic
	if ctxt.LinkMode == LinkExternal {
		dynexpMap = ctxt.cgo_export_static
	}
	d := make([]loader.Sym, 0, len(dynexpMap))
	for exp := range dynexpMap {
		s := ctxt.loader.LookupOrCreateSym(exp, 0)
		d = append(d, s)
		// sanity check
		if !ctxt.loader.AttrReachable(s) {
			panic("dynexp entry not reachable")
		}
	}
	sort.Slice(d, func(i, j int) bool {
		return ctxt.loader.SymName(d[i]) < ctxt.loader.SymName(d[j])
	})

	// Resolve ABI aliases in the list of cgo-exported functions.
	// This is necessary because we load the ABI0 symbol for all
	// cgo exports.
	for i, s := range d {
		if ctxt.loader.SymType(s) != sym.SABIALIAS {
			continue
		}
		t := ctxt.loader.ResolveABIAlias(s)
		ctxt.loader.CopyAttributes(s, t)
		ctxt.loader.SetSymExtname(t, ctxt.loader.SymExtname(s))
		d[i] = t
	}
	ctxt.dynexp = d

	ctxt.cgo_export_static = nil
	ctxt.cgo_export_dynamic = nil
}

// loadcgodirectives reads the previously discovered cgo directives, creating
// symbols in preparation for host object loading or use later in the link.
func (ctxt *Link) loadcgodirectives() {
	l := ctxt.loader
	hostObjSyms := make(map[loader.Sym]struct{})
	for _, d := range ctxt.cgodata {
		setCgoAttr(ctxt, ctxt.loader.LookupOrCreateSym, d.file, d.pkg, d.directives, hostObjSyms)
	}
	ctxt.cgodata = nil

	if ctxt.LinkMode == LinkInternal {
		// Drop all the cgo_import_static declarations.
		// Turns out we won't be needing them.
		for symIdx := range hostObjSyms {
			if l.SymType(symIdx) == sym.SHOSTOBJ {
				// If a symbol was marked both
				// cgo_import_static and cgo_import_dynamic,
				// then we want to make it cgo_import_dynamic
				// now.
				su := l.MakeSymbolUpdater(symIdx)
				if l.SymExtname(symIdx) != "" && l.SymDynimplib(symIdx) != "" && !(l.AttrCgoExportStatic(symIdx) || l.AttrCgoExportDynamic(symIdx)) {
					su.SetType(sym.SDYNIMPORT)
				} else {
					su.SetType(0)
				}
			}
		}
	}
}

// Set up flags and special symbols depending on the platform build mode.
// This version works with loader.Loader.
func (ctxt *Link) linksetup() {
	switch ctxt.BuildMode {
	case BuildModeCShared, BuildModePlugin:
		symIdx := ctxt.loader.LookupOrCreateSym("runtime.islibrary", 0)
		sb := ctxt.loader.MakeSymbolUpdater(symIdx)
		sb.SetType(sym.SNOPTRDATA)
		sb.AddUint8(1)
	case BuildModeCArchive:
		symIdx := ctxt.loader.LookupOrCreateSym("runtime.isarchive", 0)
		sb := ctxt.loader.MakeSymbolUpdater(symIdx)
		sb.SetType(sym.SNOPTRDATA)
		sb.AddUint8(1)
	}

	// Recalculate pe parameters now that we have ctxt.LinkMode set.
	if ctxt.HeadType == objabi.Hwindows {
		Peinit(ctxt)
	}

	if ctxt.HeadType == objabi.Hdarwin && ctxt.LinkMode == LinkExternal {
		*FlagTextAddr = 0
	}

	// 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.
	if ctxt.BuildMode == BuildModeExe {
		if havedynamic == 0 && ctxt.HeadType != objabi.Hdarwin && ctxt.HeadType != objabi.Hsolaris {
			*FlagD = true
		}
	}

	if ctxt.LinkMode == LinkExternal && ctxt.Arch.Family == sys.PPC64 && objabi.GOOS != "aix" {
		toc := ctxt.loader.LookupOrCreateSym(".TOC.", 0)
		sb := ctxt.loader.MakeSymbolUpdater(toc)
		sb.SetType(sym.SDYNIMPORT)
	}

	// The Android Q linker started to complain about underalignment of the our TLS
	// section. We don't actually use the section on android, so don't
	// generate it.
	if objabi.GOOS != "android" {
		tlsg := ctxt.loader.LookupOrCreateSym("runtime.tlsg", 0)
		sb := ctxt.loader.MakeSymbolUpdater(tlsg)

		// runtime.tlsg is used for external linking on platforms that do not define
		// a variable to hold g in assembly (currently only intel).
		if sb.Type() == 0 {
			sb.SetType(sym.STLSBSS)
			sb.SetSize(int64(ctxt.Arch.PtrSize))
		} else if sb.Type() != sym.SDYNIMPORT {
			Errorf(nil, "runtime declared tlsg variable %v", sb.Type())
		}
		ctxt.loader.SetAttrReachable(tlsg, true)
		ctxt.Tlsg = tlsg
	}

	var moduledata loader.Sym
	var mdsb *loader.SymbolBuilder
	if ctxt.BuildMode == BuildModePlugin {
		moduledata = ctxt.loader.LookupOrCreateSym("local.pluginmoduledata", 0)
		mdsb = ctxt.loader.MakeSymbolUpdater(moduledata)
		ctxt.loader.SetAttrLocal(moduledata, true)
	} else {
		moduledata = ctxt.loader.LookupOrCreateSym("runtime.firstmoduledata", 0)
		mdsb = ctxt.loader.MakeSymbolUpdater(moduledata)
	}
	if mdsb.Type() != 0 && mdsb.Type() != sym.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().
		mdsb.SetSize(0)

		// In addition, on ARM, the runtime depends on the linker
		// recording the value of GOARM.
		if ctxt.Arch.Family == sys.ARM {
			goarm := ctxt.loader.LookupOrCreateSym("runtime.goarm", 0)
			sb := ctxt.loader.MakeSymbolUpdater(goarm)
			sb.SetType(sym.SDATA)
			sb.SetSize(0)
			sb.AddUint8(uint8(objabi.GOARM))
		}

		if objabi.Framepointer_enabled(objabi.GOOS, objabi.GOARCH) {
			fpe := ctxt.loader.LookupOrCreateSym("runtime.framepointer_enabled", 0)
			sb := ctxt.loader.MakeSymbolUpdater(fpe)
			sb.SetType(sym.SNOPTRDATA)
			sb.SetSize(0)
			sb.AddUint8(1)
		}
	} else {
		// If OTOH the module does not contain the runtime package,
		// create a local symbol for the moduledata.
		moduledata = ctxt.loader.LookupOrCreateSym("local.moduledata", 0)
		mdsb = ctxt.loader.MakeSymbolUpdater(moduledata)
		ctxt.loader.SetAttrLocal(moduledata, true)
	}
	// In all cases way we mark the moduledata as noptrdata to hide it from
	// the GC.
	mdsb.SetType(sym.SNOPTRDATA)
	ctxt.loader.SetAttrReachable(moduledata, true)
	ctxt.Moduledata = moduledata

	if ctxt.Arch == sys.Arch386 && ctxt.HeadType != objabi.Hwindows {
		if (ctxt.BuildMode == BuildModeCArchive && ctxt.IsELF) || ctxt.BuildMode == BuildModeCShared || ctxt.BuildMode == BuildModePIE || ctxt.DynlinkingGo() {
			got := ctxt.loader.LookupOrCreateSym("_GLOBAL_OFFSET_TABLE_", 0)
			sb := ctxt.loader.MakeSymbolUpdater(got)
			sb.SetType(sym.SDYNIMPORT)
			ctxt.loader.SetAttrReachable(got, true)
		}
	}

	// DWARF-gen and other phases require that the unit Textp slices
	// be populated, so that it can walk the functions in each unit.
	// Call into the loader to do this (requires that we collect the
	// set of internal libraries first). NB: might be simpler if we
	// moved isRuntimeDepPkg to cmd/internal and then did the test in
	// loader.AssignTextSymbolOrder.
	ctxt.Library = postorder(ctxt.Library)
	intlibs := []bool{}
	for _, lib := range ctxt.Library {
		intlibs = append(intlibs, isRuntimeDepPkg(lib.Pkg))
	}
	ctxt.Textp = ctxt.loader.AssignTextSymbolOrder(ctxt.Library, intlibs, ctxt.Textp)
}

// mangleTypeSym shortens the names of symbols that represent Go types
// if they are visible in the symbol table.
//
// As the names of these symbols are derived from the string of
// the type, they can run to many kilobytes long. So we shorten
// them using a SHA-1 when the name appears in the final binary.
// This also removes characters that upset external linkers.
//
// These are the symbols that begin with the prefix 'type.' and
// contain run-time type information used by the runtime and reflect
// packages. All Go binaries contain these symbols, but only
// those programs loaded dynamically in multiple parts need these
// symbols to have entries in the symbol table.
func (ctxt *Link) mangleTypeSym() {
	if ctxt.BuildMode != BuildModeShared && !ctxt.linkShared && ctxt.BuildMode != BuildModePlugin && !ctxt.CanUsePlugins() {
		return
	}

	ldr := ctxt.loader
	for s := loader.Sym(1); s < loader.Sym(ldr.NSym()); s++ {
		if !ldr.AttrReachable(s) {
			continue
		}
		name := ldr.SymName(s)
		newName := typeSymbolMangle(name)
		if newName != name {
			ldr.SetSymExtname(s, newName)

			// When linking against a shared library, the Go object file may
			// have reference to the original symbol name whereas the shared
			// library provides a symbol with the mangled name. We need to
			// copy the payload of mangled to original.
			// XXX maybe there is a better way to do this.
			dup := ldr.Lookup(newName, ldr.SymVersion(s))
			if dup != 0 {
				st := ldr.SymType(s)
				dt := ldr.SymType(dup)
				if st == sym.Sxxx && dt != sym.Sxxx {
					ldr.CopySym(dup, s)
				}
			}
		}
	}
}

// typeSymbolMangle mangles the given symbol name into something shorter.
//
// Keep the type.. prefix, which parts of the linker (like the
// DWARF generator) know means the symbol is not decodable.
// Leave type.runtime. symbols alone, because other parts of
// the linker manipulates them.
func typeSymbolMangle(name string) string {
	if !strings.HasPrefix(name, "type.") {
		return name
	}
	if strings.HasPrefix(name, "type.runtime.") {
		return name
	}
	if len(name) <= 14 && !strings.Contains(name, "@") { // Issue 19529
		return name
	}
	hash := sha1.Sum([]byte(name))
	prefix := "type."
	if name[5] == '.' {
		prefix = "type.."
	}
	return prefix + base64.StdEncoding.EncodeToString(hash[:6])
}

/*
 * look for the next file in an archive.
 * adapted from libmach.
 */
func nextar(bp *bio.Reader, off int64, a *ArHdr) int64 {
	if off&1 != 0 {
		off++
	}
	bp.MustSeek(off, 0)
	var buf [SAR_HDR]byte
	if n, err := io.ReadFull(bp, buf[:]); err != nil {
		if n == 0 && err != io.EOF {
			return -1
		}
		return 0
	}

	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 arsize + SAR_HDR
}

func loadobjfile(ctxt *Link, lib *sym.Library) {
	pkg := objabi.PathToPrefix(lib.Pkg)

	if ctxt.Debugvlog > 1 {
		ctxt.Logf("ldobj: %s (%s)\n", lib.File, pkg)
	}
	f, err := bio.Open(lib.File)
	if err != nil {
		Exitf("cannot open file %s: %v", lib.File, err)
	}
	defer f.Close()
	defer func() {
		if pkg == "main" && !lib.Main {
			Exitf("%s: not package main", lib.File)
		}
	}()

	for i := 0; i < len(ARMAG); i++ {
		if c, err := f.ReadByte(); err == nil && c == ARMAG[i] {
			continue
		}

		/* load it as a regular file */
		l := f.MustSeek(0, 2)
		f.MustSeek(0, 0)
		ldobj(ctxt, f, lib, l, lib.File, lib.File)
		return
	}

	/*
	 * 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.
	 */
	var arhdr ArHdr
	off := f.Offset()
	for {
		l := nextar(f, off, &arhdr)
		if l == 0 {
			break
		}
		if l < 0 {
			Exitf("%s: malformed archive", lib.File)
		}
		off += l

		// __.PKGDEF isn't a real Go object file, and it's
		// absent in -linkobj builds anyway. Skipping it
		// ensures consistency between -linkobj and normal
		// build modes.
		if arhdr.name == pkgdef {
			continue
		}

		// Skip other special (non-object-file) sections that
		// build tools may have added. Such sections must have
		// short names so that the suffix is not truncated.
		if len(arhdr.name) < 16 {
			if ext := filepath.Ext(arhdr.name); ext != ".o" && ext != ".syso" {
				continue
			}
		}

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

type Hostobj struct {
	ld     func(*Link, *bio.Reader, 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(*Link, *bio.Reader, string, int64, string), headType objabi.HeadType, f *bio.Reader, pkg string, length int64, pn string, file string) *Hostobj {
	isinternal := false
	for _, intpkg := range internalpkg {
		if pkg == intpkg {
			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 == objabi.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 = f.Offset()
	h.length = length
	return h
}

func hostobjs(ctxt *Link) {
	if ctxt.LinkMode != LinkInternal {
		return
	}
	var h *Hostobj

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

		f.MustSeek(h.off, 0)
		h.ld(ctxt, f, h.pkg, h.length, h.pn)
		f.Close()
	}
}

func hostlinksetup(ctxt *Link) {
	if ctxt.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 = *FlagS
	*FlagS = false

	// create temporary directory and arrange cleanup
	if *flagTmpdir == "" {
		dir, err := ioutil.TempDir("", "go-link-")
		if err != nil {
			log.Fatal(err)
		}
		*flagTmpdir = dir
		ownTmpDir = true
		AtExit(func() {
			ctxt.Out.Close()
			os.RemoveAll(*flagTmpdir)
		})
	}

	// change our output to temporary object file
	if err := ctxt.Out.Close(); err != nil {
		Exitf("error closing output file")
	}
	mayberemoveoutfile()

	p := filepath.Join(*flagTmpdir, "go.o")
	if err := ctxt.Out.Open(p); err != nil {
		Exitf("cannot create %s: %v", p, err)
	}
}

// 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(*flagTmpdir, 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)
			}
			defer f.Close()
			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
}

// writeGDBLinkerScript creates gcc linker script file in temp
// directory. writeGDBLinkerScript returns created file path.
// The script is used to work around gcc bug
// (see https://golang.org/issue/20183 for details).
func writeGDBLinkerScript() string {
	name := "fix_debug_gdb_scripts.ld"
	path := filepath.Join(*flagTmpdir, name)
	src := `SECTIONS
{
  .debug_gdb_scripts BLOCK(__section_alignment__) (NOLOAD) :
  {
    *(.debug_gdb_scripts)
  }
}
INSERT AFTER .debug_types;
`
	err := ioutil.WriteFile(path, []byte(src), 0666)
	if err != nil {
		Errorf(nil, "WriteFile %s failed: %v", name, err)
	}
	return path
}

// archive builds a .a archive from the hostobj object files.
func (ctxt *Link) archive() {
	if ctxt.BuildMode != BuildModeCArchive {
		return
	}

	exitIfErrors()

	if *flagExtar == "" {
		*flagExtar = "ar"
	}

	mayberemoveoutfile()

	// Force the buffer to flush here so that external
	// tools will see a complete file.
	if err := ctxt.Out.Close(); err != nil {
		Exitf("error closing %v", *flagOutfile)
	}

	argv := []string{*flagExtar, "-q", "-c", "-s"}
	if ctxt.HeadType == objabi.Haix {
		argv = append(argv, "-X64")
	}
	argv = append(argv, *flagOutfile)
	argv = append(argv, filepath.Join(*flagTmpdir, "go.o"))
	argv = append(argv, hostobjCopy()...)

	if ctxt.Debugvlog != 0 {
		ctxt.Logf("archive: %s\n", strings.Join(argv, " "))
	}

	// If supported, use syscall.Exec() to invoke the archive command,
	// which should be the final remaining step needed for the link.
	// This will reduce peak RSS for the link (and speed up linking of
	// large applications), since when the archive command runs we
	// won't be holding onto all of the linker's live memory.
	if syscallExecSupported && !ownTmpDir {
		runAtExitFuncs()
		ctxt.execArchive(argv)
		panic("should not get here")
	}

	// Otherwise invoke 'ar' in the usual way (fork + exec).
	if out, err := exec.Command(argv[0], argv[1:]...).CombinedOutput(); err != nil {
		Exitf("running %s failed: %v\n%s", argv[0], err, out)
	}
}

func (ctxt *Link) hostlink() {
	if ctxt.LinkMode != LinkExternal || nerrors > 0 {
		return
	}
	if ctxt.BuildMode == BuildModeCArchive {
		return
	}

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

	if *FlagS || debug_s {
		if ctxt.HeadType == objabi.Hdarwin {
			// Recent versions of macOS print
			//	ld: warning: option -s is obsolete and being ignored
			// so do not pass any arguments.
		} else {
			argv = append(argv, "-s")
		}
	}

	switch ctxt.HeadType {
	case objabi.Hdarwin:
		if machoPlatform == PLATFORM_MACOS {
			// -headerpad is incompatible with -fembed-bitcode.
			argv = append(argv, "-Wl,-headerpad,1144")
		}
		if ctxt.DynlinkingGo() && !ctxt.Arch.InFamily(sys.ARM, sys.ARM64) {
			argv = append(argv, "-Wl,-flat_namespace")
		}
	case objabi.Hopenbsd:
		argv = append(argv, "-Wl,-nopie")
	case objabi.Hwindows:
		if windowsgui {
			argv = append(argv, "-mwindows")
		} else {
			argv = append(argv, "-mconsole")
		}
		// Mark as having awareness of terminal services, to avoid
		// ancient compatibility hacks.
		argv = append(argv, "-Wl,--tsaware")

		// Enable DEP
		argv = append(argv, "-Wl,--nxcompat")

		argv = append(argv, fmt.Sprintf("-Wl,--major-os-version=%d", PeMinimumTargetMajorVersion))
		argv = append(argv, fmt.Sprintf("-Wl,--minor-os-version=%d", PeMinimumTargetMinorVersion))
		argv = append(argv, fmt.Sprintf("-Wl,--major-subsystem-version=%d", PeMinimumTargetMajorVersion))
		argv = append(argv, fmt.Sprintf("-Wl,--minor-subsystem-version=%d", PeMinimumTargetMinorVersion))
	case objabi.Haix:
		argv = append(argv, "-pthread")
		// prevent ld to reorder .text functions to keep the same
		// first/last functions for moduledata.
		argv = append(argv, "-Wl,-bnoobjreorder")
		// mcmodel=large is needed for every gcc generated files, but
		// ld still need -bbigtoc in order to allow larger TOC.
		argv = append(argv, "-mcmodel=large")
		argv = append(argv, "-Wl,-bbigtoc")
	}

	switch ctxt.BuildMode {
	case BuildModeExe:
		if ctxt.HeadType == objabi.Hdarwin {
			if machoPlatform == PLATFORM_MACOS {
				argv = append(argv, "-Wl,-no_pie")
				argv = append(argv, "-Wl,-pagezero_size,4000000")
			}
		}
	case BuildModePIE:
		switch ctxt.HeadType {
		case objabi.Hdarwin, objabi.Haix:
		case objabi.Hwindows:
			// Enable ASLR.
			argv = append(argv, "-Wl,--dynamicbase")
			// enable high-entropy ASLR on 64-bit.
			if ctxt.Arch.PtrSize >= 8 {
				argv = append(argv, "-Wl,--high-entropy-va")
			}
			// Work around binutils limitation that strips relocation table for dynamicbase.
			// See https://sourceware.org/bugzilla/show_bug.cgi?id=19011
			argv = append(argv, "-Wl,--export-all-symbols")
		default:
			// ELF.
			if ctxt.UseRelro() {
				argv = append(argv, "-Wl,-z,relro")
			}
			argv = append(argv, "-pie")
		}
	case BuildModeCShared:
		if ctxt.HeadType == objabi.Hdarwin {
			argv = append(argv, "-dynamiclib")
			if ctxt.Arch.Family != sys.AMD64 {
				argv = append(argv, "-Wl,-read_only_relocs,suppress")
			}
		} else {
			// ELF.
			argv = append(argv, "-Wl,-Bsymbolic")
			if ctxt.UseRelro() {
				argv = append(argv, "-Wl,-z,relro")
			}
			argv = append(argv, "-shared")
			if ctxt.HeadType != objabi.Hwindows {
				// Pass -z nodelete to mark the shared library as
				// non-closeable: a dlclose will do nothing.
				argv = append(argv, "-Wl,-z,nodelete")
			}
		}
	case BuildModeShared:
		if ctxt.UseRelro() {
			argv = append(argv, "-Wl,-z,relro")
		}
		argv = append(argv, "-shared")
	case BuildModePlugin:
		if ctxt.HeadType == objabi.Hdarwin {
			argv = append(argv, "-dynamiclib")
		} else {
			if ctxt.UseRelro() {
				argv = append(argv, "-Wl,-z,relro")
			}
			argv = append(argv, "-shared")
		}
	}

	var altLinker string
	if ctxt.IsELF && ctxt.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")

		// Do not let the host linker generate COPY relocations. These
		// can move symbols out of sections that rely on stable offsets
		// from the beginning of the section (like sym.STYPE).
		argv = append(argv, "-Wl,-znocopyreloc")

		if objabi.GOOS == "android" {
			// Use lld to avoid errors from default linker (issue #38838)
			altLinker = "lld"
		}

		if ctxt.Arch.InFamily(sys.ARM, sys.ARM64) && objabi.GOOS == "linux" {
			// On ARM, the GNU linker will generate COPY relocations
			// even with -znocopyreloc set.
			// https://sourceware.org/bugzilla/show_bug.cgi?id=19962
			//
			// On ARM64, the GNU linker will fail instead of
			// generating COPY relocations.
			//
			// In both cases, switch to gold.
			altLinker = "gold"

			// If gold is not installed, gcc will silently switch
			// back to ld.bfd. So we parse the version information
			// and provide a useful error if gold is missing.
			cmd := exec.Command(*flagExtld, "-fuse-ld=gold", "-Wl,--version")
			if out, err := cmd.CombinedOutput(); err == nil {
				if !bytes.Contains(out, []byte("GNU gold")) {
					log.Fatalf("ARM external linker must be gold (issue #15696), but is not: %s", out)
				}
			}
		}
	}
	if ctxt.Arch.Family == sys.ARM64 && objabi.GOOS == "freebsd" {
		// Switch to ld.bfd on freebsd/arm64.
		altLinker = "bfd"

		// Provide a useful error if ld.bfd is missing.
		cmd := exec.Command(*flagExtld, "-fuse-ld=bfd", "-Wl,--version")
		if out, err := cmd.CombinedOutput(); err == nil {
			if !bytes.Contains(out, []byte("GNU ld")) {
				log.Fatalf("ARM64 external linker must be ld.bfd (issue #35197), please install devel/binutils")
			}
		}
	}
	if altLinker != "" {
		argv = append(argv, "-fuse-ld="+altLinker)
	}

	if ctxt.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 := *flagOutfile
	if objabi.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 ctxt.IsELF {
		argv = append(argv, "-rdynamic")
	}
	if ctxt.HeadType == objabi.Haix {
		fileName := xcoffCreateExportFile(ctxt)
		argv = append(argv, "-Wl,-bE:"+fileName)
	}

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

	const compressDWARF = "-Wl,--compress-debug-sections=zlib-gnu"
	if ctxt.compressDWARF && linkerFlagSupported(argv[0], altLinker, compressDWARF) {
		argv = append(argv, compressDWARF)
	}

	argv = append(argv, filepath.Join(*flagTmpdir, "go.o"))
	argv = append(argv, hostobjCopy()...)
	if ctxt.HeadType == objabi.Haix {
		// We want to have C files after Go files to remove
		// trampolines csects made by ld.
		argv = append(argv, "-nostartfiles")
		argv = append(argv, "/lib/crt0_64.o")

		extld := ctxt.extld()
		// Get starting files.
		getPathFile := func(file string) string {
			args := []string{"-maix64", "--print-file-name=" + file}
			out, err := exec.Command(extld, args...).CombinedOutput()
			if err != nil {
				log.Fatalf("running %s failed: %v\n%s", extld, err, out)
			}
			return strings.Trim(string(out), "\n")
		}
		argv = append(argv, getPathFile("crtcxa.o"))
		argv = append(argv, getPathFile("crtdbase.o"))
	}

	if ctxt.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(ctxt, dep)
				if libpath != "" {
					addshlib(libpath)
				}
			}
		}
	}

	// 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.
	checkStatic := func(arg string) {
		if ctxt.IsELF && arg == "-static" {
			for i := range argv {
				if argv[i] == "-rdynamic" {
					argv[i] = "-static"
				}
			}
		}
	}

	for _, p := range ldflag {
		argv = append(argv, p)
		checkStatic(p)
	}

	// When building a program with the default -buildmode=exe the
	// gc compiler generates code requires DT_TEXTREL in a
	// position independent executable (PIE). On systems where the
	// toolchain creates PIEs by default, and where DT_TEXTREL
	// does not work, the resulting programs will not run. See
	// issue #17847. To avoid this problem pass -no-pie to the
	// toolchain if it is supported.
	if ctxt.BuildMode == BuildModeExe && !ctxt.linkShared {
		// GCC uses -no-pie, clang uses -nopie.
		for _, nopie := range []string{"-no-pie", "-nopie"} {
			if linkerFlagSupported(argv[0], altLinker, nopie) {
				argv = append(argv, nopie)
				break
			}
		}
	}

	for _, p := range strings.Fields(*flagExtldflags) {
		argv = append(argv, p)
		checkStatic(p)
	}
	if ctxt.HeadType == objabi.Hwindows {
		// use gcc linker script to work around gcc bug
		// (see https://golang.org/issue/20183 for details).
		p := writeGDBLinkerScript()
		argv = append(argv, "-Wl,-T,"+p)
		// libmingw32 and libmingwex have some inter-dependencies,
		// so must use linker groups.
		argv = append(argv, "-Wl,--start-group", "-lmingwex", "-lmingw32", "-Wl,--end-group")
		argv = append(argv, peimporteddlls()...)
	}

	if ctxt.Debugvlog != 0 {
		ctxt.Logf("host link:")
		for _, v := range argv {
			ctxt.Logf(" %q", v)
		}
		ctxt.Logf("\n")
	}

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

	// Filter out useless linker warnings caused by bugs outside Go.
	// See also cmd/go/internal/work/exec.go's gccld method.
	var save [][]byte
	var skipLines int
	for _, line := range bytes.SplitAfter(out, []byte("\n")) {
		// golang.org/issue/26073 - Apple Xcode bug
		if bytes.Contains(line, []byte("ld: warning: text-based stub file")) {
			continue
		}

		if skipLines > 0 {
			skipLines--
			continue
		}

		// Remove TOC overflow warning on AIX.
		if bytes.Contains(line, []byte("ld: 0711-783")) {
			skipLines = 2
			continue
		}

		save = append(save, line)
	}
	out = bytes.Join(save, nil)

	if len(out) > 0 {
		// always print external output even if the command is successful, so that we don't
		// swallow linker warnings (see https://golang.org/issue/17935).
		ctxt.Logf("%s", out)
	}

	if !*FlagS && !*FlagW && !debug_s && ctxt.HeadType == objabi.Hdarwin {
		dsym := filepath.Join(*flagTmpdir, "go.dwarf")
		if out, err := exec.Command("dsymutil", "-f", *flagOutfile, "-o", dsym).CombinedOutput(); err != 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 := *flagOutfile + "~"
		exef, err := os.Open(*flagOutfile)
		if err != nil {
			Exitf("%s: combining dwarf failed: %v", os.Args[0], err)
		}
		defer exef.Close()
		exem, err := macho.NewFile(exef)
		if err != nil {
			Exitf("%s: parsing Mach-O header failed: %v", os.Args[0], err)
		}
		// Only macOS supports unmapped segments such as our __DWARF segment.
		if machoPlatform == PLATFORM_MACOS {
			if err := machoCombineDwarf(ctxt, exef, exem, dsym, combinedOutput); err != nil {
				Exitf("%s: combining dwarf failed: %v", os.Args[0], err)
			}
			os.Remove(*flagOutfile)
			if err := os.Rename(combinedOutput, *flagOutfile); err != nil {
				Exitf("%s: %v", os.Args[0], err)
			}
		}
	}
}

var createTrivialCOnce sync.Once

func linkerFlagSupported(linker, altLinker, flag string) bool {
	createTrivialCOnce.Do(func() {
		src := filepath.Join(*flagTmpdir, "trivial.c")
		if err := ioutil.WriteFile(src, []byte("int main() { return 0; }"), 0666); err != nil {
			Errorf(nil, "WriteFile trivial.c failed: %v", err)
		}
	})

	flagsWithNextArgSkip := []string{
		"-F",
		"-l",
		"-L",
		"-framework",
		"-Wl,-framework",
		"-Wl,-rpath",
		"-Wl,-undefined",
	}
	flagsWithNextArgKeep := []string{
		"-arch",
		"-isysroot",
		"--sysroot",
		"-target",
	}
	prefixesToKeep := []string{
		"-f",
		"-m",
		"-p",
		"-Wl,",
		"-arch",
		"-isysroot",
		"--sysroot",
		"-target",
	}

	var flags []string
	keep := false
	skip := false
	extldflags := strings.Fields(*flagExtldflags)
	for _, f := range append(extldflags, ldflag...) {
		if keep {
			flags = append(flags, f)
			keep = false
		} else if skip {
			skip = false
		} else if f == "" || f[0] != '-' {
		} else if contains(flagsWithNextArgSkip, f) {
			skip = true
		} else if contains(flagsWithNextArgKeep, f) {
			flags = append(flags, f)
			keep = true
		} else {
			for _, p := range prefixesToKeep {
				if strings.HasPrefix(f, p) {
					flags = append(flags, f)
					break
				}
			}
		}
	}

	if altLinker != "" {
		flags = append(flags, "-fuse-ld="+altLinker)
	}
	flags = append(flags, flag, "trivial.c")

	cmd := exec.Command(linker, flags...)
	cmd.Dir = *flagTmpdir
	cmd.Env = append([]string{"LC_ALL=C"}, os.Environ()...)
	out, err := cmd.CombinedOutput()
	// GCC says "unrecognized command line option ‘-no-pie’"
	// clang says "unknown argument: '-no-pie'"
	return err == nil && !bytes.Contains(out, []byte("unrecognized")) && !bytes.Contains(out, []byte("unknown"))
}

// hostlinkArchArgs returns arguments to pass to the external linker
// based on the architecture.
func hostlinkArchArgs(arch *sys.Arch) []string {
	switch arch.Family {
	case sys.I386:
		return []string{"-m32"}
	case sys.AMD64, sys.S390X:
		return []string{"-m64"}
	case sys.ARM:
		return []string{"-marm"}
	case sys.ARM64:
		// nothing needed
	case sys.MIPS64:
		return []string{"-mabi=64"}
	case sys.MIPS:
		return []string{"-mabi=32"}
	case sys.PPC64:
		if objabi.GOOS == "aix" {
			return []string{"-maix64"}
		} else {
			return []string{"-m64"}
		}

	}
	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(ctxt *Link, f *bio.Reader, lib *sym.Library, length int64, pn string, file string) *Hostobj {
	pkg := objabi.PathToPrefix(lib.Pkg)

	eof := f.Offset() + length
	start := f.Offset()
	c1 := bgetc(f)
	c2 := bgetc(f)
	c3 := bgetc(f)
	c4 := bgetc(f)
	f.MustSeek(start, 0)

	unit := &sym.CompilationUnit{Lib: lib}
	lib.Units = append(lib.Units, unit)

	magic := uint32(c1)<<24 | uint32(c2)<<16 | uint32(c3)<<8 | uint32(c4)
	if magic == 0x7f454c46 { // \x7F E L F
		ldelf := func(ctxt *Link, f *bio.Reader, pkg string, length int64, pn string) {
			textp, flags, err := loadelf.Load(ctxt.loader, ctxt.Arch, ctxt.IncVersion(), f, pkg, length, pn, ehdr.flags)
			if err != nil {
				Errorf(nil, "%v", err)
				return
			}
			ehdr.flags = flags
			ctxt.Textp = append(ctxt.Textp, textp...)
		}
		return ldhostobj(ldelf, ctxt.HeadType, f, pkg, length, pn, file)
	}

	if magic&^1 == 0xfeedface || magic&^0x01000000 == 0xcefaedfe {
		ldmacho := func(ctxt *Link, f *bio.Reader, pkg string, length int64, pn string) {
			textp, err := loadmacho.Load(ctxt.loader, ctxt.Arch, ctxt.IncVersion(), f, pkg, length, pn)
			if err != nil {
				Errorf(nil, "%v", err)
				return
			}
			ctxt.Textp = append(ctxt.Textp, textp...)
		}
		return ldhostobj(ldmacho, ctxt.HeadType, f, pkg, length, pn, file)
	}

	if c1 == 0x4c && c2 == 0x01 || c1 == 0x64 && c2 == 0x86 {
		ldpe := func(ctxt *Link, f *bio.Reader, pkg string, length int64, pn string) {
			textp, rsrc, err := loadpe.Load(ctxt.loader, ctxt.Arch, ctxt.IncVersion(), f, pkg, length, pn)
			if err != nil {
				Errorf(nil, "%v", err)
				return
			}
			if rsrc != 0 {
				setpersrc(ctxt, rsrc)
			}
			ctxt.Textp = append(ctxt.Textp, textp...)
		}
		return ldhostobj(ldpe, ctxt.HeadType, f, pkg, length, pn, file)
	}

	if c1 == 0x01 && (c2 == 0xD7 || c2 == 0xF7) {
		ldxcoff := func(ctxt *Link, f *bio.Reader, pkg string, length int64, pn string) {
			textp, err := loadxcoff.Load(ctxt.loader, ctxt.Arch, ctxt.IncVersion(), f, pkg, length, pn)
			if err != nil {
				Errorf(nil, "%v", err)
				return
			}
			ctxt.Textp = append(ctxt.Textp, textp...)
		}
		return ldhostobj(ldxcoff, ctxt.HeadType, f, pkg, length, pn, file)
	}

	/* check the header */
	line, err := f.ReadString('\n')
	if err != nil {
		Errorf(nil, "truncated object file: %s: %v", pn, err)
		return nil
	}

	if !strings.HasPrefix(line, "go object ") {
		if strings.HasSuffix(pn, ".go") {
			Exitf("%s: uncompiled .go source file", pn)
			return nil
		}

		if line == ctxt.Arch.Name {
			// old header format: just $GOOS
			Errorf(nil, "%s: stale object file", pn)
			return nil
		}

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

	// First, check that the basic GOOS, GOARCH, and Version match.
	t := fmt.Sprintf("%s %s %s ", objabi.GOOS, objabi.GOARCH, objabi.Version)

	line = strings.TrimRight(line, "\n")
	if !strings.HasPrefix(line[10:]+" ", t) && !*flagF {
		Errorf(nil, "%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:] {
			Errorf(nil, "%s: object is [%s] expected [%s]", pn, line[10:], theline)
			return nil
		}
	}

	// Skip over exports and other info -- ends with \n!\n.
	//
	// Note: It's possible for "\n!\n" to appear within the binary
	// package export data format. To avoid truncating the package
	// definition prematurely (issue 21703), we keep track of
	// how many "$$" delimiters we've seen.

	import0 := f.Offset()

	c1 = '\n' // the last line ended in \n
	c2 = bgetc(f)
	c3 = bgetc(f)
	markers := 0
	for {
		if c1 == '\n' {
			if markers%2 == 0 && c2 == '!' && c3 == '\n' {
				break
			}
			if c2 == '$' && c3 == '$' {
				markers++
			}
		}

		c1 = c2
		c2 = c3
		c3 = bgetc(f)
		if c3 == -1 {
			Errorf(nil, "truncated object file: %s", pn)
			return nil
		}
	}

	import1 := f.Offset()

	f.MustSeek(import0, 0)
	ldpkg(ctxt, f, lib, import1-import0-2, pn) // -2 for !\n
	f.MustSeek(import1, 0)

	fingerprint := ctxt.loader.Preload(ctxt.IncVersion(), f, lib, unit, eof-f.Offset())
	if !fingerprint.IsZero() { // Assembly objects don't have fingerprints. Ignore them.
		// Check fingerprint, to ensure the importing and imported packages
		// have consistent view of symbol indices.
		// Normally the go command should ensure this. But in case something
		// goes wrong, it could lead to obscure bugs like run-time crash.
		// Check it here to be sure.
		if lib.Fingerprint.IsZero() { // Not yet imported. Update its fingerprint.
			lib.Fingerprint = fingerprint
		}
		checkFingerprint(lib, fingerprint, lib.Srcref, lib.Fingerprint)
	}

	addImports(ctxt, lib, pn)
	return nil
}

func checkFingerprint(lib *sym.Library, libfp goobj2.FingerprintType, src string, srcfp goobj2.FingerprintType) {
	if libfp != srcfp {
		Exitf("fingerprint mismatch: %s has %x, import from %s expecting %x", lib, libfp, src, srcfp)
	}
}

func readelfsymboldata(ctxt *Link, 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 {
		Errorf(nil, "reading %s from non-data section", sym.Name)
	}
	n, err := sect.ReadAt(data, int64(sym.Value-sect.Addr))
	if uint64(n) != sym.Size {
		Errorf(nil, "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(ctxt *Link, shlib string) string {
	if filepath.IsAbs(shlib) {
		return shlib
	}
	for _, libdir := range ctxt.Libdir {
		libpath := filepath.Join(libdir, shlib)
		if _, err := os.Stat(libpath); err == nil {
			return libpath
		}
	}
	Errorf(nil, "cannot find shared library: %s", shlib)
	return ""
}

func ldshlibsyms(ctxt *Link, shlib string) {
	var libpath string
	if filepath.IsAbs(shlib) {
		libpath = shlib
		shlib = filepath.Base(shlib)
	} else {
		libpath = findshlib(ctxt, shlib)
		if libpath == "" {
			return
		}
	}
	for _, processedlib := range ctxt.Shlibs {
		if processedlib.Path == libpath {
			return
		}
	}
	if ctxt.Debugvlog > 1 {
		ctxt.Logf("ldshlibsyms: found library with name %s at %s\n", shlib, libpath)
	}

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

	hash, err := readnote(f, ELF_NOTE_GO_NAME, ELF_NOTE_GOABIHASH_TAG)
	if err != nil {
		Errorf(nil, "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 {
		Errorf(nil, "cannot read dep list from shared library %s: %v", libpath, err)
		return
	}
	var deps []string
	for _, dep := range strings.Split(string(depsbytes), "\n") {
		if dep == "" {
			continue
		}
		if !filepath.IsAbs(dep) {
			// If the dep can be interpreted as a path relative to the shlib
			// in which it was found, do that. Otherwise, we will leave it
			// to be resolved by libdir lookup.
			abs := filepath.Join(filepath.Dir(libpath), dep)
			if _, err := os.Stat(abs); err == nil {
				dep = abs
			}
		}
		deps = append(deps, dep)
	}

	syms, err := f.DynamicSymbols()
	if err != nil {
		Errorf(nil, "cannot read symbols from shared library: %s", libpath)
		return
	}
	gcdataLocations := make(map[uint64]loader.Sym)
	for _, elfsym := range syms {
		if elf.ST_TYPE(elfsym.Info) == elf.STT_NOTYPE || elf.ST_TYPE(elfsym.Info) == elf.STT_SECTION {
			continue
		}

		// Symbols whose names start with "type." are compiler
		// generated, so make functions with that prefix internal.
		ver := 0
		if elf.ST_TYPE(elfsym.Info) == elf.STT_FUNC && strings.HasPrefix(elfsym.Name, "type.") {
			ver = sym.SymVerABIInternal
		}

		l := ctxt.loader
		s := l.LookupOrCreateSym(elfsym.Name, ver)

		// Because loadlib above loads all .a files before loading
		// any shared libraries, any non-dynimport symbols we find
		// that duplicate symbols already loaded should be ignored
		// (the symbols from the .a files "win").
		if l.SymType(s) != 0 && l.SymType(s) != sym.SDYNIMPORT {
			continue
		}
		su := l.MakeSymbolUpdater(s)
		su.SetType(sym.SDYNIMPORT)
		l.SetSymElfType(s, elf.ST_TYPE(elfsym.Info))
		su.SetSize(int64(elfsym.Size))
		if elfsym.Section != elf.SHN_UNDEF {
			// If it's not undefined, mark the symbol as reachable
			// so as to protect it from dead code elimination,
			// even if there aren't any explicit references to it.
			// Under the previous sym.Symbol based regime this
			// wasn't necessary, but for the loader-based deadcode
			// it is definitely needed.
			//
			// FIXME: have a more general/flexible mechanism for this?
			//
			l.SetAttrReachable(s, true)

			// Set .File for the library that actually defines the symbol.
			l.SetSymPkg(s, libpath)

			// The decodetype_* functions in decodetype.go need access to
			// the type data.
			sname := l.SymName(s)
			if strings.HasPrefix(sname, "type.") && !strings.HasPrefix(sname, "type..") {
				su.SetData(readelfsymboldata(ctxt, f, &elfsym))
				gcdataLocations[elfsym.Value+2*uint64(ctxt.Arch.PtrSize)+8+1*uint64(ctxt.Arch.PtrSize)] = s
			}
		}

		// For function symbols, we don't know what ABI is
		// available, so alias it under both ABIs.
		//
		// TODO(austin): This is almost certainly wrong once
		// the ABIs are actually different. We might have to
		// mangle Go function names in the .so to include the
		// ABI.
		if elf.ST_TYPE(elfsym.Info) == elf.STT_FUNC && ver == 0 {
			alias := ctxt.loader.LookupOrCreateSym(elfsym.Name, sym.SymVerABIInternal)
			if l.SymType(alias) != 0 {
				continue
			}
			su := l.MakeSymbolUpdater(alias)
			su.SetType(sym.SABIALIAS)
			su.AddReloc(loader.Reloc{Sym: s})
		}
	}
	if ctxt.Arch.Family == sys.ARM64 {
		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 {
						Errorf(nil, "reading relocation failed %v", err)
						return
					}
					t := elf.R_AARCH64(rela.Info & 0xffff)
					if t != elf.R_AARCH64_RELATIVE {
						continue
					}
				}
			}
		}
	}

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

func addsection(ldr *loader.Loader, arch *sys.Arch, seg *sym.Segment, name string, rwx int) *sym.Section {
	sect := ldr.NewSection()
	sect.Rwx = uint8(rwx)
	sect.Name = name
	sect.Seg = seg
	sect.Align = int32(arch.PtrSize) // everything is at least pointer-aligned
	seg.Sections = append(seg.Sections, sect)
	return sect
}

type chain struct {
	sym   loader.Sym
	up    *chain
	limit int // limit on entry to sym
}

func haslinkregister(ctxt *Link) bool {
	return ctxt.FixedFrameSize() != 0
}

func callsize(ctxt *Link) int {
	if haslinkregister(ctxt) {
		return 0
	}
	return ctxt.Arch.RegSize
}

type stkChk struct {
	ldr       *loader.Loader
	ctxt      *Link
	morestack loader.Sym
	done      loader.Bitmap
}

// Walk the call tree and check that there is always enough stack space
// for the call frames, especially for a chain of nosplit functions.
func (ctxt *Link) dostkcheck() {
	ldr := ctxt.loader
	sc := stkChk{
		ldr:       ldr,
		ctxt:      ctxt,
		morestack: ldr.Lookup("runtime.morestack", 0),
		done:      loader.MakeBitmap(ldr.NSym()),
	}

	// 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.
	var ch chain
	ch.limit = objabi.StackLimit - callsize(ctxt)
	if objabi.GOARCH == "arm64" {
		// need extra 8 bytes below SP to save FP
		ch.limit -= 8
	}

	// Check every function, but do the nosplit functions in a first pass,
	// to make the printed failure chains as short as possible.
	for _, s := range ctxt.Textp {
		if ldr.IsNoSplit(s) {
			ch.sym = s
			sc.check(&ch, 0)
		}
	}

	for _, s := range ctxt.Textp {
		if !ldr.IsNoSplit(s) {
			ch.sym = s
			sc.check(&ch, 0)
		}
	}
}

func (sc *stkChk) check(up *chain, depth int) int {
	limit := up.limit
	s := up.sym
	ldr := sc.ldr
	ctxt := sc.ctxt

	// Don't duplicate work: only need to consider each
	// function at top of safe zone once.
	top := limit == objabi.StackLimit-callsize(ctxt)
	if top {
		if sc.done.Has(s) {
			return 0
		}
		sc.done.Set(s)
	}

	if depth > 500 {
		sc.ctxt.Errorf(s, "nosplit stack check too deep")
		sc.broke(up, 0)
		return -1
	}

	if ldr.AttrExternal(s) {
		// external function.
		// should never be called directly.
		// onlyctxt.Diagnose the direct caller.
		// TODO(mwhudson): actually think about this.
		// TODO(khr): disabled for now. Calls to external functions can only happen on the g0 stack.
		// See the trampolines in src/runtime/sys_darwin_$ARCH.go.
		//if depth == 1 && ldr.SymType(s) != sym.SXREF && !ctxt.DynlinkingGo() &&
		//	ctxt.BuildMode != BuildModeCArchive && ctxt.BuildMode != BuildModePIE && ctxt.BuildMode != BuildModeCShared && ctxt.BuildMode != BuildModePlugin {
		//	Errorf(s, "call to external function")
		//}
		return -1
	}
	info := ldr.FuncInfo(s)
	if !info.Valid() { // external function. see above.
		return -1
	}

	if limit < 0 {
		sc.broke(up, limit)
		return -1
	}

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

	var ch chain
	ch.up = up

	if !ldr.IsNoSplit(s) {
		// Ensure we have enough stack to call morestack.
		ch.limit = limit - callsize(ctxt)
		ch.sym = sc.morestack
		if sc.check(&ch, depth+1) < 0 {
			return -1
		}
		if !top {
			return 0
		}
		// Raise limit to allow frame.
		locals := info.Locals()
		limit = objabi.StackLimit + int(locals) + int(ctxt.FixedFrameSize())
	}

	// Walk through sp adjustments in function, consuming relocs.
	relocs := ldr.Relocs(s)
	var ch1 chain
	pcsp := obj.NewPCIter(uint32(ctxt.Arch.MinLC))
	ri := 0
	for pcsp.Init(info.Pcsp()); !pcsp.Done; pcsp.Next() {
		// pcsp.value is in effect for [pcsp.pc, pcsp.nextpc).

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

		// Process calls in this span.
		for ; ri < relocs.Count(); ri++ {
			r := relocs.At2(ri)
			if uint32(r.Off()) >= pcsp.NextPC {
				break
			}
			t := r.Type()
			switch {
			case t.IsDirectCall():
				ch.limit = int(int32(limit) - pcsp.Value - int32(callsize(ctxt)))
				ch.sym = r.Sym()
				if sc.check(&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 t == objabi.R_CALLIND:
				ch.limit = int(int32(limit) - pcsp.Value - int32(callsize(ctxt)))
				ch.sym = 0
				ch1.limit = ch.limit - callsize(ctxt) // for morestack in called prologue
				ch1.up = &ch
				ch1.sym = sc.morestack
				if sc.check(&ch1, depth+2) < 0 {
					return -1
				}
			}
		}
	}

	return 0
}

func (sc *stkChk) broke(ch *chain, limit int) {
	sc.ctxt.Errorf(ch.sym, "nosplit stack overflow")
	sc.print(ch, limit)
}

func (sc *stkChk) print(ch *chain, limit int) {
	ldr := sc.ldr
	ctxt := sc.ctxt
	var name string
	if ch.sym != 0 {
		name = ldr.SymName(ch.sym)
		if ldr.IsNoSplit(ch.sym) {
			name += " (nosplit)"
		}
	} else {
		name = "function pointer"
	}

	if ch.up == nil {
		// top of chain. ch.sym != 0.
		if ldr.IsNoSplit(ch.sym) {
			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 {
		sc.print(ch.up, ch.limit+callsize(ctxt))
		if !haslinkregister(ctxt) {
			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 usage() {
	fmt.Fprintf(os.Stderr, "usage: link [options] main.o\n")
	objabi.Flagprint(os.Stderr)
	Exit(2)
}

type SymbolType int8 // TODO: after genasmsym is gone, maybe rename to plan9typeChar or something

const (
	// see also https://9p.io/magic/man2html/1/nm
	TextSym      SymbolType = 'T'
	DataSym      SymbolType = 'D'
	BSSSym       SymbolType = 'B'
	UndefinedSym SymbolType = 'U'
	TLSSym       SymbolType = 't'
	FrameSym     SymbolType = 'm'
	ParamSym     SymbolType = 'p'
	AutoSym      SymbolType = 'a'

	// Deleted auto (not a real sym, just placeholder for type)
	DeletedAutoSym = 'x'
)

func (ctxt *Link) xdefine(p string, t sym.SymKind, v int64) loader.Sym {
	ldr := ctxt.loader
	s := ldr.CreateSymForUpdate(p, 0)
	s.SetType(t)
	s.SetValue(v)
	s.SetSpecial(true)
	s.SetLocal(true)
	return s.Sym()
}

func datoff(ldr *loader.Loader, s loader.Sym, 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)
	}
	ldr.Errorf(s, "invalid datoff %#x", addr)
	return 0
}

func Entryvalue(ctxt *Link) int64 {
	a := *flagEntrySymbol
	if a[0] >= '0' && a[0] <= '9' {
		return atolwhex(a)
	}
	ldr := ctxt.loader
	s := ldr.Lookup(a, 0)
	st := ldr.SymType(s)
	if st == 0 {
		return *FlagTextAddr
	}
	if !ctxt.IsAIX() && st != sym.STEXT {
		ldr.Errorf(s, "entry not text")
	}
	return ldr.SymValue(s)
}

func (ctxt *Link) callgraph() {
	if !*FlagC {
		return
	}

	ldr := ctxt.loader
	for _, s := range ctxt.Textp {
		relocs := ldr.Relocs(s)
		for i := 0; i < relocs.Count(); i++ {
			r := relocs.At2(i)
			rs := r.Sym()
			if rs == 0 {
				continue
			}
			if r.Type().IsDirectCall() && (ldr.SymType(rs) == sym.STEXT || ldr.SymType(rs) == sym.SABIALIAS) {
				ctxt.Logf("%s calls %s\n", ldr.SymName(s), ldr.SymName(rs))
			}
		}
	}
}

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
}

func bgetc(r *bio.Reader) int {
	c, err := r.ReadByte()
	if err != nil {
		if err != io.EOF {
			log.Fatalf("reading input: %v", err)
		}
		return -1
	}
	return int(c)
}

type markKind uint8 // for postorder traversal
const (
	_ markKind = iota
	visiting
	visited
)

func postorder(libs []*sym.Library) []*sym.Library {
	order := make([]*sym.Library, 0, len(libs)) // hold the result
	mark := make(map[*sym.Library]markKind, len(libs))
	for _, lib := range libs {
		dfs(lib, mark, &order)
	}
	return order
}

func dfs(lib *sym.Library, mark map[*sym.Library]markKind, order *[]*sym.Library) {
	if mark[lib] == visited {
		return
	}
	if mark[lib] == visiting {
		panic("found import cycle while visiting " + lib.Pkg)
	}
	mark[lib] = visiting
	for _, i := range lib.Imports {
		dfs(i, mark, order)
	}
	mark[lib] = visited
	*order = append(*order, lib)
}

func ElfSymForReloc(ctxt *Link, s loader.Sym) int32 {
	// If putelfsym created a local version of this symbol, use that in all
	// relocations.
	les := ctxt.loader.SymLocalElfSym(s)
	if les != 0 {
		return les
	} else {
		return ctxt.loader.SymElfSym(s)
	}
}

func AddGotSym(target *Target, ldr *loader.Loader, syms *ArchSyms, s loader.Sym, elfRelocTyp uint32) {
	if ldr.SymGot(s) >= 0 {
		return
	}

	Adddynsym(ldr, target, syms, s)
	got := ldr.MakeSymbolUpdater(syms.GOT)
	ldr.SetGot(s, int32(got.Size()))
	got.AddUint(target.Arch, 0)

	if target.IsElf() {
		if target.Arch.PtrSize == 8 {
			rela := ldr.MakeSymbolUpdater(syms.Rela)
			rela.AddAddrPlus(target.Arch, got.Sym(), int64(ldr.SymGot(s)))
			rela.AddUint64(target.Arch, ELF64_R_INFO(uint32(ldr.SymDynid(s)), elfRelocTyp))
			rela.AddUint64(target.Arch, 0)
		} else {
			rel := ldr.MakeSymbolUpdater(syms.Rel)
			rel.AddAddrPlus(target.Arch, got.Sym(), int64(ldr.SymGot(s)))
			rel.AddUint32(target.Arch, ELF32_R_INFO(uint32(ldr.SymDynid(s)), elfRelocTyp))
		}
	} else if target.IsDarwin() {
		leg := ldr.MakeSymbolUpdater(syms.LinkEditGOT)
		leg.AddUint32(target.Arch, uint32(ldr.SymDynid(s)))
	} else {
		ldr.Errorf(s, "addgotsym: unsupported binary format")
	}
}