src/cmd/link/internal/ld/go.go - go - Git at Google

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // go-specific code shared across loaders (5l, 6l, 8l).

 package ld

 import (
 	"cmd/internal/bio"
 	"cmd/internal/obj"
 	"cmd/internal/objabi"
 	"cmd/internal/sys"
 	"cmd/link/internal/loader"
 	"cmd/link/internal/sym"
 	"debug/elf"
 	"encoding/json"
 	"fmt"
 	"io"
 	"os"
 	"sort"
 	"strconv"
 	"strings"
 )

 // go-specific code shared across loaders (5l, 6l, 8l).

 // TODO:
 //	generate debugging section in binary.
 //	once the dust settles, try to move some code to
 //		libmach, so that other linkers and ar can share.

 func ldpkg(ctxt *Link, f *bio.Reader, lib *sym.Library, length int64, filename string) {
 	if *flagG {
 		return
 	}

 	if int64(int(length)) != length {
 		fmt.Fprintf(os.Stderr, "%s: too much pkg data in %s\n", os.Args[0], filename)
 		return
 	}

 	bdata := make([]byte, length)
 	if _, err := io.ReadFull(f, bdata); err != nil {
 		fmt.Fprintf(os.Stderr, "%s: short pkg read %s\n", os.Args[0], filename)
 		return
 	}
 	data := string(bdata)

 	// process header lines
 	for data != "" {
 		var line string
 		if i := strings.Index(data, "\n"); i >= 0 {
 			line, data = data[:i], data[i+1:]
 		} else {
 			line, data = data, ""
 		}
 		if line == "main" {
 			lib.Main = true
 		}
 		if line == "" {
 			break
 		}
 	}

 	// look for cgo section
 	p0 := strings.Index(data, "\n$$  // cgo")
 	var p1 int
 	if p0 >= 0 {
 		p0 += p1
 		i := strings.IndexByte(data[p0+1:], '\n')
 		if i < 0 {
 			fmt.Fprintf(os.Stderr, "%s: found $$ // cgo but no newline in %s\n", os.Args[0], filename)
 			return
 		}
 		p0 += 1 + i

 		p1 = strings.Index(data[p0:], "\n$$")
 		if p1 < 0 {
 			p1 = strings.Index(data[p0:], "\n!\n")
 		}
 		if p1 < 0 {
 			fmt.Fprintf(os.Stderr, "%s: cannot find end of // cgo section in %s\n", os.Args[0], filename)
 			return
 		}
 		p1 += p0
 		loadcgo(ctxt, filename, objabi.PathToPrefix(lib.Pkg), data[p0:p1])
 	}
 }

 func loadcgo(ctxt *Link, file string, pkg string, p string) {
 	var directives [][]string
 	if err := json.NewDecoder(strings.NewReader(p)).Decode(&directives); err != nil {
 		fmt.Fprintf(os.Stderr, "%s: %s: failed decoding cgo directives: %v\n", os.Args[0], file, err)
 		nerrors++
 		return
 	}

 	// Record the directives. We'll process them later after Symbols are created.
 	ctxt.cgodata = append(ctxt.cgodata, cgodata{file, pkg, directives})
 }

 // Set symbol attributes or flags based on cgo directives.
 // Any newly discovered HOSTOBJ syms are added to 'hostObjSyms'.
 func setCgoAttr(ctxt *Link, file string, pkg string, directives [][]string, hostObjSyms map[loader.Sym]struct{}) {
 	l := ctxt.loader
 	for _, f := range directives {
 		switch f[0] {
 		case "cgo_import_dynamic":
 			if len(f) < 2 || len(f) > 4 {
 				break
 			}

 			local := f[1]
 			remote := local
 			if len(f) > 2 {
 				remote = f[2]
 			}
 			lib := ""
 			if len(f) > 3 {
 				lib = f[3]
 			}

 			if *FlagD {
 				fmt.Fprintf(os.Stderr, "%s: %s: cannot use dynamic imports with -d flag\n", os.Args[0], file)
 				nerrors++
 				return
 			}

 			if local == "_" && remote == "_" {
 				// allow #pragma dynimport _ _ "foo.so"
 				// to force a link of foo.so.
 				havedynamic = 1

 				if ctxt.HeadType == objabi.Hdarwin {
 					machoadddynlib(lib, ctxt.LinkMode)
 				} else {
 					dynlib = append(dynlib, lib)
 				}
 				continue
 			}

 			q := ""
 			if i := strings.Index(remote, "#"); i >= 0 {
 				remote, q = remote[:i], remote[i+1:]
 			}
 			s := l.LookupOrCreateSym(local, 0)
 			st := l.SymType(s)
 			if st == 0 || st == sym.SXREF || st == sym.SBSS || st == sym.SNOPTRBSS || st == sym.SHOSTOBJ {
 				l.SetSymDynimplib(s, lib)
 				l.SetSymExtname(s, remote)
 				l.SetSymDynimpvers(s, q)
 				if st != sym.SHOSTOBJ {
 					su := l.MakeSymbolUpdater(s)
 					su.SetType(sym.SDYNIMPORT)
 				} else {
 					hostObjSyms[s] = struct{}{}
 				}
 				havedynamic = 1
 				if lib != "" && ctxt.IsDarwin() {
 					machoadddynlib(lib, ctxt.LinkMode)
 				}
 			}

 			continue

 		case "cgo_import_static":
 			if len(f) != 2 {
 				break
 			}
 			local := f[1]

 			s := l.LookupOrCreateSym(local, 0)
 			su := l.MakeSymbolUpdater(s)
 			su.SetType(sym.SHOSTOBJ)
 			su.SetSize(0)
 			hostObjSyms[s] = struct{}{}
 			continue

 		case "cgo_export_static", "cgo_export_dynamic":
 			if len(f) < 2 || len(f) > 4 {
 				break
 			}
 			local := f[1]
 			remote := local
 			if len(f) > 2 {
 				remote = f[2]
 			}
 			// The compiler adds a fourth argument giving
 			// the definition ABI of function symbols.
 			abi := obj.ABI0
 			if len(f) > 3 {
 				var ok bool
 				abi, ok = obj.ParseABI(f[3])
 				if !ok {
 					fmt.Fprintf(os.Stderr, "%s: bad ABI in cgo_export directive %s\n", os.Args[0], f)
 					nerrors++
 					return
 				}
 			}

 			s := l.LookupOrCreateSym(local, sym.ABIToVersion(abi))

 			if l.SymType(s) == sym.SHOSTOBJ {
 				hostObjSyms[s] = struct{}{}
 			}

 			switch ctxt.BuildMode {
 			case BuildModeCShared, BuildModeCArchive, BuildModePlugin:
 				if s == l.Lookup("main", 0) {
 					continue
 				}
 			}

 			// export overrides import, for openbsd/cgo.
 			// see issue 4878.
 			if l.SymDynimplib(s) != "" {
 				l.SetSymDynimplib(s, "")
 				l.SetSymDynimpvers(s, "")
 				l.SetSymExtname(s, "")
 				var su *loader.SymbolBuilder
 				su = l.MakeSymbolUpdater(s)
 				su.SetType(0)
 			}

 			if !(l.AttrCgoExportStatic(s) || l.AttrCgoExportDynamic(s)) {
 				l.SetSymExtname(s, remote)
 			} else if l.SymExtname(s) != remote {
 				fmt.Fprintf(os.Stderr, "%s: conflicting cgo_export directives: %s as %s and %s\n", os.Args[0], l.SymName(s), l.SymExtname(s), remote)
 				nerrors++
 				return
 			}

 			// Mark exported symbols and also add them to
 			// the lists used for roots in the deadcode pass.
 			if f[0] == "cgo_export_static" {
 				if ctxt.LinkMode == LinkExternal && !l.AttrCgoExportStatic(s) {
 					// Static cgo exports appear
 					// in the exported symbol table.
 					ctxt.dynexp = append(ctxt.dynexp, s)
 				}
 				if ctxt.LinkMode == LinkInternal {
 					// For internal linking, we're
 					// responsible for resolving
 					// relocations from host objects.
 					// Record the right Go symbol
 					// version to use.
 					l.AddCgoExport(s)
 				}
 				l.SetAttrCgoExportStatic(s, true)
 			} else {
 				if ctxt.LinkMode == LinkInternal && !l.AttrCgoExportDynamic(s) {
 					// Dynamic cgo exports appear
 					// in the exported symbol table.
 					ctxt.dynexp = append(ctxt.dynexp, s)
 				}
 				l.SetAttrCgoExportDynamic(s, true)
 			}

 			continue

 		case "cgo_dynamic_linker":
 			if len(f) != 2 {
 				break
 			}

 			if *flagInterpreter == "" {
 				if interpreter != "" && interpreter != f[1] {
 					fmt.Fprintf(os.Stderr, "%s: conflict dynlinker: %s and %s\n", os.Args[0], interpreter, f[1])
 					nerrors++
 					return
 				}

 				interpreter = f[1]
 			}
 			continue

 		case "cgo_ldflag":
 			if len(f) != 2 {
 				break
 			}
 			ldflag = append(ldflag, f[1])
 			continue
 		}

 		fmt.Fprintf(os.Stderr, "%s: %s: invalid cgo directive: %q\n", os.Args[0], file, f)
 		nerrors++
 	}
 	return
 }

 // openbsdTrimLibVersion indicates whether a shared library is
 // versioned and if it is, returns the unversioned name. The
 // OpenBSD library naming scheme is lib<name>.so.<major>.<minor>
 func openbsdTrimLibVersion(lib string) (string, bool) {
 	parts := strings.Split(lib, ".")
 	if len(parts) != 4 {
 		return "", false
 	}
 	if parts[1] != "so" {
 		return "", false
 	}
 	if _, err := strconv.Atoi(parts[2]); err != nil {
 		return "", false
 	}
 	if _, err := strconv.Atoi(parts[3]); err != nil {
 		return "", false
 	}
 	return fmt.Sprintf("%s.%s", parts[0], parts[1]), true
 }

 // dedupLibrariesOpenBSD dedups a list of shared libraries, treating versioned
 // and unversioned libraries as equivalents. Versioned libraries are preferred
 // and retained over unversioned libraries. This avoids the situation where
 // the use of cgo results in a DT_NEEDED for a versioned library (for example,
 // libc.so.96.1), while a dynamic import specifies an unversioned library (for
 // example, libc.so) - this would otherwise result in two DT_NEEDED entries
 // for the same library, resulting in a failure when ld.so attempts to load
 // the Go binary.
 func dedupLibrariesOpenBSD(ctxt *Link, libs []string) []string {
 	libraries := make(map[string]string)
 	for _, lib := range libs {
 		if name, ok := openbsdTrimLibVersion(lib); ok {
 			// Record unversioned name as seen.
 			seenlib[name] = true
 			libraries[name] = lib
 		} else if _, ok := libraries[lib]; !ok {
 			libraries[lib] = lib
 		}
 	}

 	libs = nil
 	for _, lib := range libraries {
 		libs = append(libs, lib)
 	}
 	sort.Strings(libs)

 	return libs
 }

 func dedupLibraries(ctxt *Link, libs []string) []string {
 	if ctxt.Target.IsOpenbsd() {
 		return dedupLibrariesOpenBSD(ctxt, libs)
 	}
 	return libs
 }

 var seenlib = make(map[string]bool)

 func adddynlib(ctxt *Link, lib string) {
 	if seenlib[lib] || ctxt.LinkMode == LinkExternal {
 		return
 	}
 	seenlib[lib] = true

 	if ctxt.IsELF {
 		dsu := ctxt.loader.MakeSymbolUpdater(ctxt.DynStr)
 		if dsu.Size() == 0 {
 			dsu.Addstring("")
 		}
 		du := ctxt.loader.MakeSymbolUpdater(ctxt.Dynamic)
 		Elfwritedynent(ctxt.Arch, du, elf.DT_NEEDED, uint64(dsu.Addstring(lib)))
 	} else {
 		Errorf(nil, "adddynlib: unsupported binary format")
 	}
 }

 func Adddynsym(ldr *loader.Loader, target *Target, syms *ArchSyms, s loader.Sym) {
 	if ldr.SymDynid(s) >= 0 || target.LinkMode == LinkExternal {
 		return
 	}

 	if target.IsELF {
 		elfadddynsym(ldr, target, syms, s)
 	} else if target.HeadType == objabi.Hdarwin {
 		ldr.Errorf(s, "adddynsym: missed symbol (Extname=%s)", ldr.SymExtname(s))
 	} else if target.HeadType == objabi.Hwindows {
 		// already taken care of
 	} else {
 		ldr.Errorf(s, "adddynsym: unsupported binary format")
 	}
 }

 func fieldtrack(arch *sys.Arch, l *loader.Loader) {
 	var buf strings.Builder
 	for i := loader.Sym(1); i < loader.Sym(l.NSym()); i++ {
 		if name := l.SymName(i); strings.HasPrefix(name, "go:track.") {
 			if l.AttrReachable(i) {
 				l.SetAttrSpecial(i, true)
 				l.SetAttrNotInSymbolTable(i, true)
 				buf.WriteString(name[9:])
 				for p := l.Reachparent[i]; p != 0; p = l.Reachparent[p] {
 					buf.WriteString("\t")
 					buf.WriteString(l.SymName(p))
 				}
 				buf.WriteString("\n")
 			}
 		}
 	}
 	l.Reachparent = nil // we are done with it
 	if *flagFieldTrack == "" {
 		return
 	}
 	s := l.Lookup(*flagFieldTrack, 0)
 	if s == 0 || !l.AttrReachable(s) {
 		return
 	}
 	bld := l.MakeSymbolUpdater(s)
 	bld.SetType(sym.SDATA)
 	addstrdata(arch, l, *flagFieldTrack, buf.String())
 }

 func (ctxt *Link) addexport() {
 	// Track undefined external symbols during external link.
 	if ctxt.LinkMode == LinkExternal {
 		for _, s := range ctxt.Textp {
 			if ctxt.loader.AttrSpecial(s) || ctxt.loader.AttrSubSymbol(s) {
 				continue
 			}
 			relocs := ctxt.loader.Relocs(s)
 			for i := 0; i < relocs.Count(); i++ {
 				if rs := relocs.At(i).Sym(); rs != 0 {
 					if ctxt.loader.SymType(rs) == sym.Sxxx && !ctxt.loader.AttrLocal(rs) {
 						// sanity check
 						if len(ctxt.loader.Data(rs)) != 0 {
 							panic("expected no data on undef symbol")
 						}
 						su := ctxt.loader.MakeSymbolUpdater(rs)
 						su.SetType(sym.SUNDEFEXT)
 					}
 				}
 			}
 		}
 	}

 	// TODO(aix)
 	if ctxt.HeadType == objabi.Hdarwin || ctxt.HeadType == objabi.Haix {
 		return
 	}

 	// Add dynamic symbols.
 	for _, s := range ctxt.dynexp {
 		// Consistency check.
 		if !ctxt.loader.AttrReachable(s) {
 			panic("dynexp entry not reachable")
 		}

 		Adddynsym(ctxt.loader, &ctxt.Target, &ctxt.ArchSyms, s)
 	}

 	for _, lib := range dedupLibraries(ctxt, dynlib) {
 		adddynlib(ctxt, lib)
 	}
 }
	// Copyright 2009 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// go-specific code shared across loaders (5l, 6l, 8l).

	package ld

	import (
	"cmd/internal/bio"
	"cmd/internal/obj"
	"cmd/internal/objabi"
	"cmd/internal/sys"
	"cmd/link/internal/loader"
	"cmd/link/internal/sym"
	"debug/elf"
	"encoding/json"
	"fmt"
	"io"
	"os"
	"sort"
	"strconv"
	"strings"
	)

	// go-specific code shared across loaders (5l, 6l, 8l).

	// TODO:
	// generate debugging section in binary.
	// once the dust settles, try to move some code to
	// libmach, so that other linkers and ar can share.

	func ldpkg(ctxt Link, f bio.Reader, lib *sym.Library, length int64, filename string) {
	if *flagG {
	return
	}

	if int64(int(length)) != length {
	fmt.Fprintf(os.Stderr, "%s: too much pkg data in %s\n", os.Args[0], filename)
	return
	}

	bdata := make([]byte, length)
	if _, err := io.ReadFull(f, bdata); err != nil {
	fmt.Fprintf(os.Stderr, "%s: short pkg read %s\n", os.Args[0], filename)
	return
	}
	data := string(bdata)

	// process header lines
	for data != "" {
	var line string
	if i := strings.Index(data, "\n"); i >= 0 {
	line, data = data[:i], data[i+1:]
	} else {
	line, data = data, ""
	}
	if line == "main" {
	lib.Main = true
	}
	if line == "" {
	break
	}
	}

	// look for cgo section
	p0 := strings.Index(data, "\n$$ // cgo")
	var p1 int
	if p0 >= 0 {
	p0 += p1
	i := strings.IndexByte(data[p0+1:], '\n')
	if i < 0 {
	fmt.Fprintf(os.Stderr, "%s: found $$ // cgo but no newline in %s\n", os.Args[0], filename)
	return
	}
	p0 += 1 + i

	p1 = strings.Index(data[p0:], "\n$$")
	if p1 < 0 {
	p1 = strings.Index(data[p0:], "\n!\n")
	}
	if p1 < 0 {
	fmt.Fprintf(os.Stderr, "%s: cannot find end of // cgo section in %s\n", os.Args[0], filename)
	return
	}
	p1 += p0
	loadcgo(ctxt, filename, objabi.PathToPrefix(lib.Pkg), data[p0:p1])
	}
	}

	func loadcgo(ctxt *Link, file string, pkg string, p string) {
	var directives [][]string
	if err := json.NewDecoder(strings.NewReader(p)).Decode(&directives); err != nil {
	fmt.Fprintf(os.Stderr, "%s: %s: failed decoding cgo directives: %v\n", os.Args[0], file, err)
	nerrors++
	return
	}

	// Record the directives. We'll process them later after Symbols are created.
	ctxt.cgodata = append(ctxt.cgodata, cgodata{file, pkg, directives})
	}

	// Set symbol attributes or flags based on cgo directives.
	// Any newly discovered HOSTOBJ syms are added to 'hostObjSyms'.
	func setCgoAttr(ctxt *Link, file string, pkg string, directives [][]string, hostObjSyms map[loader.Sym]struct{}) {
	l := ctxt.loader
	for _, f := range directives {
	switch f[0] {
	case "cgo_import_dynamic":
	if len(f) < 2 \|\| len(f) > 4 {
	break
	}

	local := f[1]
	remote := local
	if len(f) > 2 {
	remote = f[2]
	}
	lib := ""
	if len(f) > 3 {
	lib = f[3]
	}

	if *FlagD {
	fmt.Fprintf(os.Stderr, "%s: %s: cannot use dynamic imports with -d flag\n", os.Args[0], file)
	nerrors++
	return
	}

	if local == "_" && remote == "_" {
	// allow #pragma dynimport _ _ "foo.so"
	// to force a link of foo.so.
	havedynamic = 1

	if ctxt.HeadType == objabi.Hdarwin {
	machoadddynlib(lib, ctxt.LinkMode)
	} else {
	dynlib = append(dynlib, lib)
	}
	continue
	}

	q := ""
	if i := strings.Index(remote, "#"); i >= 0 {
	remote, q = remote[:i], remote[i+1:]
	}
	s := l.LookupOrCreateSym(local, 0)
	st := l.SymType(s)
	if st == 0 \|\| st == sym.SXREF \|\| st == sym.SBSS \|\| st == sym.SNOPTRBSS \|\| st == sym.SHOSTOBJ {
	l.SetSymDynimplib(s, lib)
	l.SetSymExtname(s, remote)
	l.SetSymDynimpvers(s, q)
	if st != sym.SHOSTOBJ {
	su := l.MakeSymbolUpdater(s)
	su.SetType(sym.SDYNIMPORT)
	} else {
	hostObjSyms[s] = struct{}{}
	}
	havedynamic = 1
	if lib != "" && ctxt.IsDarwin() {
	machoadddynlib(lib, ctxt.LinkMode)
	}
	}

	continue

	case "cgo_import_static":
	if len(f) != 2 {
	break
	}
	local := f[1]

	s := l.LookupOrCreateSym(local, 0)
	su := l.MakeSymbolUpdater(s)
	su.SetType(sym.SHOSTOBJ)
	su.SetSize(0)
	hostObjSyms[s] = struct{}{}
	continue

	case "cgo_export_static", "cgo_export_dynamic":
	if len(f) < 2 \|\| len(f) > 4 {
	break
	}
	local := f[1]
	remote := local
	if len(f) > 2 {
	remote = f[2]
	}
	// The compiler adds a fourth argument giving
	// the definition ABI of function symbols.
	abi := obj.ABI0
	if len(f) > 3 {
	var ok bool
	abi, ok = obj.ParseABI(f[3])
	if !ok {
	fmt.Fprintf(os.Stderr, "%s: bad ABI in cgo_export directive %s\n", os.Args[0], f)
	nerrors++
	return
	}
	}

	s := l.LookupOrCreateSym(local, sym.ABIToVersion(abi))

	if l.SymType(s) == sym.SHOSTOBJ {
	hostObjSyms[s] = struct{}{}
	}

	switch ctxt.BuildMode {
	case BuildModeCShared, BuildModeCArchive, BuildModePlugin:
	if s == l.Lookup("main", 0) {
	continue
	}
	}

	// export overrides import, for openbsd/cgo.
	// see issue 4878.
	if l.SymDynimplib(s) != "" {
	l.SetSymDynimplib(s, "")
	l.SetSymDynimpvers(s, "")
	l.SetSymExtname(s, "")
	var su *loader.SymbolBuilder
	su = l.MakeSymbolUpdater(s)
	su.SetType(0)
	}

	if !(l.AttrCgoExportStatic(s) \|\| l.AttrCgoExportDynamic(s)) {
	l.SetSymExtname(s, remote)
	} else if l.SymExtname(s) != remote {
	fmt.Fprintf(os.Stderr, "%s: conflicting cgo_export directives: %s as %s and %s\n", os.Args[0], l.SymName(s), l.SymExtname(s), remote)
	nerrors++
	return
	}

	// Mark exported symbols and also add them to
	// the lists used for roots in the deadcode pass.
	if f[0] == "cgo_export_static" {
	if ctxt.LinkMode == LinkExternal && !l.AttrCgoExportStatic(s) {
	// Static cgo exports appear
	// in the exported symbol table.
	ctxt.dynexp = append(ctxt.dynexp, s)
	}
	if ctxt.LinkMode == LinkInternal {
	// For internal linking, we're
	// responsible for resolving
	// relocations from host objects.
	// Record the right Go symbol
	// version to use.
	l.AddCgoExport(s)
	}
	l.SetAttrCgoExportStatic(s, true)
	} else {
	if ctxt.LinkMode == LinkInternal && !l.AttrCgoExportDynamic(s) {
	// Dynamic cgo exports appear
	// in the exported symbol table.
	ctxt.dynexp = append(ctxt.dynexp, s)
	}
	l.SetAttrCgoExportDynamic(s, true)
	}

	continue

	case "cgo_dynamic_linker":
	if len(f) != 2 {
	break
	}

	if *flagInterpreter == "" {
	if interpreter != "" && interpreter != f[1] {
	fmt.Fprintf(os.Stderr, "%s: conflict dynlinker: %s and %s\n", os.Args[0], interpreter, f[1])
	nerrors++
	return
	}

	interpreter = f[1]
	}
	continue

	case "cgo_ldflag":
	if len(f) != 2 {
	break
	}
	ldflag = append(ldflag, f[1])
	continue
	}

	fmt.Fprintf(os.Stderr, "%s: %s: invalid cgo directive: %q\n", os.Args[0], file, f)
	nerrors++
	}
	return
	}

	// openbsdTrimLibVersion indicates whether a shared library is
	// versioned and if it is, returns the unversioned name. The
	// OpenBSD library naming scheme is lib<name>.so.<major>.<minor>
	func openbsdTrimLibVersion(lib string) (string, bool) {
	parts := strings.Split(lib, ".")
	if len(parts) != 4 {
	return "", false
	}
	if parts[1] != "so" {
	return "", false
	}
	if _, err := strconv.Atoi(parts[2]); err != nil {
	return "", false
	}
	if _, err := strconv.Atoi(parts[3]); err != nil {
	return "", false
	}
	return fmt.Sprintf("%s.%s", parts[0], parts[1]), true
	}

	// dedupLibrariesOpenBSD dedups a list of shared libraries, treating versioned
	// and unversioned libraries as equivalents. Versioned libraries are preferred
	// and retained over unversioned libraries. This avoids the situation where
	// the use of cgo results in a DT_NEEDED for a versioned library (for example,
	// libc.so.96.1), while a dynamic import specifies an unversioned library (for
	// example, libc.so) - this would otherwise result in two DT_NEEDED entries
	// for the same library, resulting in a failure when ld.so attempts to load
	// the Go binary.
	func dedupLibrariesOpenBSD(ctxt *Link, libs []string) []string {
	libraries := make(map[string]string)
	for _, lib := range libs {
	if name, ok := openbsdTrimLibVersion(lib); ok {
	// Record unversioned name as seen.
	seenlib[name] = true
	libraries[name] = lib
	} else if _, ok := libraries[lib]; !ok {
	libraries[lib] = lib
	}
	}

	libs = nil
	for _, lib := range libraries {
	libs = append(libs, lib)
	}
	sort.Strings(libs)

	return libs
	}

	func dedupLibraries(ctxt *Link, libs []string) []string {
	if ctxt.Target.IsOpenbsd() {
	return dedupLibrariesOpenBSD(ctxt, libs)
	}
	return libs
	}

	var seenlib = make(map[string]bool)

	func adddynlib(ctxt *Link, lib string) {
	if seenlib[lib] \|\| ctxt.LinkMode == LinkExternal {
	return
	}
	seenlib[lib] = true

	if ctxt.IsELF {
	dsu := ctxt.loader.MakeSymbolUpdater(ctxt.DynStr)
	if dsu.Size() == 0 {
	dsu.Addstring("")
	}
	du := ctxt.loader.MakeSymbolUpdater(ctxt.Dynamic)
	Elfwritedynent(ctxt.Arch, du, elf.DT_NEEDED, uint64(dsu.Addstring(lib)))
	} else {
	Errorf(nil, "adddynlib: unsupported binary format")
	}
	}

	func Adddynsym(ldr loader.Loader, target Target, syms *ArchSyms, s loader.Sym) {
	if ldr.SymDynid(s) >= 0 \|\| target.LinkMode == LinkExternal {
	return
	}

	if target.IsELF {
	elfadddynsym(ldr, target, syms, s)
	} else if target.HeadType == objabi.Hdarwin {
	ldr.Errorf(s, "adddynsym: missed symbol (Extname=%s)", ldr.SymExtname(s))
	} else if target.HeadType == objabi.Hwindows {
	// already taken care of
	} else {
	ldr.Errorf(s, "adddynsym: unsupported binary format")
	}
	}

	func fieldtrack(arch sys.Arch, l loader.Loader) {
	var buf strings.Builder
	for i := loader.Sym(1); i < loader.Sym(l.NSym()); i++ {
	if name := l.SymName(i); strings.HasPrefix(name, "go:track.") {
	if l.AttrReachable(i) {
	l.SetAttrSpecial(i, true)
	l.SetAttrNotInSymbolTable(i, true)
	buf.WriteString(name[9:])
	for p := l.Reachparent[i]; p != 0; p = l.Reachparent[p] {
	buf.WriteString("\t")
	buf.WriteString(l.SymName(p))
	}
	buf.WriteString("\n")
	}
	}
	}
	l.Reachparent = nil // we are done with it
	if *flagFieldTrack == "" {
	return
	}
	s := l.Lookup(*flagFieldTrack, 0)
	if s == 0 \|\| !l.AttrReachable(s) {
	return
	}
	bld := l.MakeSymbolUpdater(s)
	bld.SetType(sym.SDATA)
	addstrdata(arch, l, *flagFieldTrack, buf.String())
	}

	func (ctxt *Link) addexport() {
	// Track undefined external symbols during external link.
	if ctxt.LinkMode == LinkExternal {
	for _, s := range ctxt.Textp {
	if ctxt.loader.AttrSpecial(s) \|\| ctxt.loader.AttrSubSymbol(s) {
	continue
	}
	relocs := ctxt.loader.Relocs(s)
	for i := 0; i < relocs.Count(); i++ {
	if rs := relocs.At(i).Sym(); rs != 0 {
	if ctxt.loader.SymType(rs) == sym.Sxxx && !ctxt.loader.AttrLocal(rs) {
	// sanity check
	if len(ctxt.loader.Data(rs)) != 0 {
	panic("expected no data on undef symbol")
	}
	su := ctxt.loader.MakeSymbolUpdater(rs)
	su.SetType(sym.SUNDEFEXT)
	}
	}
	}
	}
	}

	// TODO(aix)
	if ctxt.HeadType == objabi.Hdarwin \|\| ctxt.HeadType == objabi.Haix {
	return
	}

	// Add dynamic symbols.
	for _, s := range ctxt.dynexp {
	// Consistency check.
	if !ctxt.loader.AttrReachable(s) {
	panic("dynexp entry not reachable")
	}

	Adddynsym(ctxt.loader, &ctxt.Target, &ctxt.ArchSyms, s)
	}

	for _, lib := range dedupLibraries(ctxt, dynlib) {
	adddynlib(ctxt, lib)
	}
	}