cmd/link: start at address 0 when external linking

[gostls13.git] / 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/goobj"
        "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"
        "internal/buildcfg"
        exec "internal/execabs"
        "io"
        "io/ioutil"
        "log"
        "os"
        "path/filepath"
        "runtime"
        "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

        unreachableMethod 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)
        ctxt.loader.SetAttrReachable(*ls, true)
}

// 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)
        ctxt.loader.SetAttrReachable(ls[ver], true)
}

// 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)
        ctxt.mkArchSym("runtime.unreachableMethod", sym.SymVerABIInternal, &ctxt.unreachableMethod)

        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

        // Threshold of total text size, used for trampoline insertion. If the total
        // text size is smaller than TrampLimit, we won't need to insert trampolines.
        // It is pretty close to the offset range of a direct CALL machine instruction.
        // We leave some room for extra stuff like PLT stubs.
        TrampLimit uint64

        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

        // Plan 9 variables.
        Plan9Magic  uint32
        Plan9_64Bit bool

        Adddynrel func(*Target, *loader.Loader, *ArchSyms, loader.Sym, loader.Reloc, 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), number of external _host_ relocations needed (i.e.
        // ELF/Mach-O/etc. relocations, not Go relocations, this must match Elfreloc1,
        // etc.), and a boolean indicating success/failure (a failing value indicates
        // a fatal error).
        Archreloc func(*Target, *loader.Loader, *ArchSyms, loader.Reloc, loader.Sym,
                int64) (relocatedOffset int64, nExtReloc int, 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.Reloc,
                rv sym.RelocVariant, sym loader.Sym, offset int64, data []byte) (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)

        // Extreloc is an arch-specific hook that converts a Go relocation to an
        // external relocation. Return the external relocation and whether it is
        // needed.
        Extreloc func(*Target, *loader.Loader, loader.Reloc, loader.Sym) (loader.ExtReloc, bool)

        Elfreloc1      func(*Link, *OutBuf, *loader.Loader, loader.Sym, loader.ExtReloc, int, int64) bool
        ElfrelocSize   uint32 // size of an ELF relocation record, must match Elfreloc1.
        Elfsetupplt    func(ctxt *Link, plt, gotplt *loader.SymbolBuilder, dynamic loader.Sym)
        Gentext        func(*Link, *loader.Loader) // Generate text before addressing has been performed.
        Machoreloc1    func(*sys.Arch, *OutBuf, *loader.Loader, loader.Sym, loader.ExtReloc, int64) bool
        MachorelocSize uint32 // size of an Mach-O relocation record, must match Machoreloc1.
        PEreloc1       func(*sys.Arch, *OutBuf, *loader.Loader, loader.Sym, loader.ExtReloc, int64) bool
        Xcoffreloc1    func(*sys.Arch, *OutBuf, *loader.Loader, loader.Sym, loader.ExtReloc, int64) bool

        // Generate additional symbols for the native symbol table just prior to
        // code generation.
        GenSymsLate func(*Link, *loader.Loader)

        // 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
}

// NeedCodeSign reports whether we need to code-sign the output binary.
func (ctxt *Link) NeedCodeSign() bool {
        return ctxt.IsDarwin() && ctxt.IsARM64()
}

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 (
        // externalobj is set to true 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

        // unknownObjFormat is set to true if we see an object whose
        // format we don't recognize.
        unknownObjFormat = 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(buildcfg.GOROOT, "pkg", fmt.Sprintf("%s_%s%s%s", buildcfg.GOOS, buildcfg.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", buildcfg.GOARCH, buildcfg.GOOS)
                case BuildModeExe, BuildModePIE:
                        *flagEntrySymbol = fmt.Sprintf("_rt0_%s_%s", buildcfg.GOARCH, buildcfg.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 := goobj.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)
        }
        if !buildcfg.Experiment.RegabiWrappers {
                // Use ABI aliases if ABI wrappers are not used.
                flags |= loader.FlagUseABIAlias
        }
        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.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

        // Plugins a require cgo support to function. Similarly, plugins may require additional
        // internal linker support on some platforms which may not be implemented.
        ctxt.canUsePlugins = ctxt.LibraryByPkg["plugin"] != nil && iscgo

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

        if ctxt.LinkMode == LinkExternal && !iscgo && !(buildcfg.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")
                        }
                        for ; i < len(ctxt.Library); i++ {
                                lib := ctxt.Library[i]
                                if lib.Shlib == "" {
                                        loadobjfile(ctxt, lib)
                                }
                        }
                }
        }

        // Add non-package symbols and references of externally defined symbols.
        ctxt.loader.LoadSyms(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 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
                                */
                        }
                        if *flagLibGCC != "none" {
                                hostArchive(ctxt, *flagLibGCC)
                        }
                }
        }

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

        importcycles()

        strictDupMsgCount = ctxt.loader.NStrictDupMsgs()
}

// 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, 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.LinkMode == LinkExternal {
                // When external linking, we are creating an object file. The
                // absolute address is irrelevant.
                *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 && buildcfg.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 buildcfg.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(buildcfg.GOARM))
                }

                // Set runtime.disableMemoryProfiling bool if
                // runtime.MemProfile is not retained in the binary after
                // deadcode (and we're not dynamically linking).
                memProfile := ctxt.loader.Lookup("runtime.MemProfile", sym.SymVerABIInternal)
                if memProfile != 0 && !ctxt.loader.AttrReachable(memProfile) && !ctxt.DynlinkingGo() {
                        memProfSym := ctxt.loader.LookupOrCreateSym("runtime.disableMemoryProfiling", 0)
                        sb := ctxt.loader.MakeSymbolUpdater(memProfSym)
                        sb.SetType(sym.SDATA)
                        sb.SetSize(0)
                        sb.AddUint8(1) // true bool
                }
        } 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) && !ctxt.linkShared {
                        // If -linkshared, the GCProg generation code may need to reach
                        // out to the shared library for the type descriptor's data, even
                        // the type descriptor itself is not actually needed at run time
                        // (therefore not reachable). We still need to mangle its name,
                        // so it is consistent with the one stored in the shared library.
                        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)
                if h.ld == nil {
                        Errorf(nil, "%s: unrecognized object file format", h.pn)
                        continue
                }
                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")
                }
        }

        // On darwin, whether to combine DWARF into executable.
        // Only macOS supports unmapped segments such as our __DWARF segment.
        combineDwarf := ctxt.IsDarwin() && !*FlagS && !*FlagW && !debug_s && machoPlatform == PLATFORM_MACOS

        switch ctxt.HeadType {
        case objabi.Hdarwin:
                if combineDwarf {
                        // Leave room for DWARF combining.
                        // -headerpad is incompatible with -fembed-bitcode.
                        argv = append(argv, "-Wl,-headerpad,1144")
                }
                if ctxt.DynlinkingGo() && buildcfg.GOOS != "ios" {
                        // -flat_namespace is deprecated on iOS.
                        // It is useful for supporting plugins. We don't support plugins on iOS.
                        argv = append(argv, "-Wl,-flat_namespace")
                }
                if !combineDwarf {
                        argv = append(argv, "-Wl,-S") // suppress STAB (symbolic debugging) symbols
                }
        case objabi.Hopenbsd:
                argv = append(argv, "-Wl,-nopie")
                argv = append(argv, "-pthread")
        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")
        }

        // Enable ASLR on Windows.
        addASLRargs := func(argv []string) []string {
                // 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")
                }
                return argv
        }

        switch ctxt.BuildMode {
        case BuildModeExe:
                if ctxt.HeadType == objabi.Hdarwin {
                        if machoPlatform == PLATFORM_MACOS && ctxt.IsAMD64() {
                                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:
                        argv = addASLRargs(argv)
                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")
                } else {
                        if ctxt.UseRelro() {
                                argv = append(argv, "-Wl,-z,relro")
                        }
                        argv = append(argv, "-shared")
                        if ctxt.HeadType == objabi.Hwindows {
                                if *flagAslr {
                                        argv = addASLRargs(argv)
                                }
                        } else {
                                // Pass -z nodelete to mark the shared library as
                                // non-closeable: a dlclose will do nothing.
                                argv = append(argv, "-Wl,-z,nodelete")
                                // Only pass Bsymbolic on non-Windows.
                                argv = append(argv, "-Wl,-Bsymbolic")
                        }
                }
        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 buildcfg.GOOS == "android" {
                        // Use lld to avoid errors from default linker (issue #38838)
                        altLinker = "lld"
                }

                if ctxt.Arch.InFamily(sys.ARM, sys.ARM64) && buildcfg.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 && buildcfg.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 buildcfg.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))
        }

        if *flagInterpreter != "" {
                // Many linkers support both -I and the --dynamic-linker flags
                // to set the ELF interpreter, but lld only supports
                // --dynamic-linker so prefer that (ld on very old Solaris only
                // supports -I but that seems less important).
                argv = append(argv, fmt.Sprintf("-Wl,--dynamic-linker,%s", *flagInterpreter))
        }

        // 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)
        }

        const unusedArguments = "-Qunused-arguments"
        if linkerFlagSupported(ctxt.Arch, argv[0], altLinker, unusedArguments) {
                argv = append(argv, unusedArguments)
        }

        const compressDWARF = "-Wl,--compress-debug-sections=zlib-gnu"
        if ctxt.compressDWARF && linkerFlagSupported(ctxt.Arch, 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.
        // Similarly for -Wl,--dynamic-linker.
        checkStatic := func(arg string) {
                if ctxt.IsELF && arg == "-static" {
                        for i := range argv {
                                if argv[i] == "-rdynamic" || strings.HasPrefix(argv[i], "-Wl,--dynamic-linker,") {
                                        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 && !(ctxt.IsDarwin() && ctxt.IsARM64()) {
                // GCC uses -no-pie, clang uses -nopie.
                for _, nopie := range []string{"-no-pie", "-nopie"} {
                        if linkerFlagSupported(ctxt.Arch, 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 {
                // Determine which linker we're using. Add in the extldflags in
                // case used has specified "-fuse-ld=...".
                cmd := exec.Command(*flagExtld, *flagExtldflags, "-Wl,--version")
                usingLLD := false
                if out, err := cmd.CombinedOutput(); err == nil {
                        if bytes.Contains(out, []byte("LLD ")) {
                                usingLLD = true
                        }
                }

                // use gcc linker script to work around gcc bug
                // (see https://golang.org/issue/20183 for details).
                if !usingLLD {
                        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 combineDwarf {
                dsym := filepath.Join(*flagTmpdir, "go.dwarf")
                if out, err := exec.Command("xcrun", "dsymutil", "-f", *flagOutfile, "-o", dsym).CombinedOutput(); err != nil {
                        Exitf("%s: running dsymutil failed: %v\n%s", os.Args[0], err, out)
                }
                // Remove STAB (symbolic debugging) symbols after we are done with them (by dsymutil).
                // They contain temporary file paths and make the build not reproducible.
                if out, err := exec.Command("xcrun", "strip", "-S", *flagOutfile).CombinedOutput(); err != nil {
                        Exitf("%s: running strip 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)
                }
                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)
                }
        }
        if ctxt.NeedCodeSign() {
                err := machoCodeSign(ctxt, *flagOutfile)
                if err != nil {
                        Exitf("%s: code signing failed: %v", os.Args[0], err)
                }
        }
}

var createTrivialCOnce sync.Once

func linkerFlagSupported(arch *sys.Arch, 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",
        }

        flags := hostlinkArchArgs(arch)
        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:
                if buildcfg.GOOS == "darwin" {
                        return []string{"-arch", "x86_64", "-m64"}
                }
                return []string{"-m64"}
        case sys.S390X:
                return []string{"-m64"}
        case sys.ARM:
                return []string{"-marm"}
        case sys.ARM64:
                if buildcfg.GOOS == "darwin" {
                        return []string{"-arch", "arm64"}
                }
        case sys.MIPS64:
                return []string{"-mabi=64"}
        case sys.MIPS:
                return []string{"-mabi=32"}
        case sys.PPC64:
                if buildcfg.GOOS == "aix" {
                        return []string{"-maix64"}
                } else {
                        return []string{"-m64"}
                }

        }
        return nil
}

var wantHdr = objabi.HeaderString()

// 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)
        }

        switch c1<<8 | c2 {
        case 0x4c01, // 386
                0x6486, // amd64
                0xc401, // arm
                0x64aa: // arm64
                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 len(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)
        }

        if c1 != 'g' || c2 != 'o' || c3 != ' ' || c4 != 'o' {
                // An unrecognized object is just passed to the external linker.
                // If we try to read symbols from this object, we will
                // report an error at that time.
                unknownObjFormat = true
                return ldhostobj(nil, 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: @%d %q", pn, start, line)
                return nil
        }

        // First, check that the basic GOOS, GOARCH, and Version match.
        if line != wantHdr {
                Errorf(nil, "%s: linked object header mismatch:\nhave %q\nwant %q\n", pn, line, wantHdr)
        }

        // 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 goobj.FingerprintType, src string, srcfp goobj.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
        }
        // Keep the file open as decodetypeGcprog needs to read from it.
        // TODO: fix. Maybe mmap the file.
        //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
        }

        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
                symname := elfsym.Name // (unmangled) symbol name
                if elf.ST_TYPE(elfsym.Info) == elf.STT_FUNC && strings.HasPrefix(elfsym.Name, "type.") {
                        ver = sym.SymVerABIInternal
                } else if buildcfg.Experiment.RegabiWrappers && elf.ST_TYPE(elfsym.Info) == elf.STT_FUNC {
                        // Demangle the ABI name. Keep in sync with symtab.go:mangleABIName.
                        if strings.HasSuffix(elfsym.Name, ".abiinternal") {
                                ver = sym.SymVerABIInternal
                                symname = strings.TrimSuffix(elfsym.Name, ".abiinternal")
                        } else if strings.HasSuffix(elfsym.Name, ".abi0") {
                                ver = 0
                                symname = strings.TrimSuffix(elfsym.Name, ".abi0")
                        }
                }

                l := ctxt.loader
                s := l.LookupOrCreateSym(symname, 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 {
                        // 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))
                        }
                }

                if symname != elfsym.Name {
                        l.SetSymExtname(s, elfsym.Name)
                }

                // For function symbols, if ABI wrappers are not used, we don't
                // know what ABI is available, so alias it under both ABIs.
                if !buildcfg.Experiment.RegabiWrappers && elf.ST_TYPE(elfsym.Info) == elf.STT_FUNC && ver == 0 {
                        alias := ctxt.loader.LookupOrCreateSym(symname, sym.SymVerABIInternal)
                        if l.SymType(alias) != 0 {
                                continue
                        }
                        su := l.MakeSymbolUpdater(alias)
                        su.SetType(sym.SABIALIAS)
                        r, _ := su.AddRel(0) // type doesn't matter
                        r.SetSym(s)
                }
        }
        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 buildcfg.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(ldr.Data(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.At(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 = fmt.Sprintf("%s<%d>", ldr.SymName(ch.sym), ldr.SymVersion(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'
)

// defineInternal defines a symbol used internally by the go runtime.
func (ctxt *Link) defineInternal(p string, t sym.SymKind) loader.Sym {
        s := ctxt.loader.CreateSymForUpdate(p, 0)
        s.SetType(t)
        s.SetSpecial(true)
        s.SetLocal(true)
        return s.Sym()
}

func (ctxt *Link) xdefine(p string, t sym.SymKind, v int64) loader.Sym {
        s := ctxt.defineInternal(p, t)
        ctxt.loader.SetSymValue(s, v)
        return s
}

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.At(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, elf.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, elf.R_INFO32(uint32(ldr.SymDynid(s)), elfRelocTyp))
                }
        } else if target.IsDarwin() {
                leg := ldr.MakeSymbolUpdater(syms.LinkEditGOT)
                leg.AddUint32(target.Arch, uint32(ldr.SymDynid(s)))
                if target.IsPIE() && target.IsInternal() {
                        // Mach-O relocations are a royal pain to lay out.
                        // They use a compact stateful bytecode representation.
                        // Here we record what are needed and encode them later.
                        MachoAddBind(int64(ldr.SymGot(s)), s)
                }
        } else {
                ldr.Errorf(s, "addgotsym: unsupported binary format")
        }
}