loader.go

		if pp.objidx == 0 {
			return FuncInfo{}
		}
		r = l.objs[pp.objidx].r
		auxs = pp.auxs
	} else {
		var li uint32
		r, li = l.toLocal(i)
		auxs = r.Auxs(li)
	}
	for j := range auxs {
		a := &auxs[j]
		if a.Type() == goobj.AuxFuncInfo {
			b := r.Data(a.Sym().SymIdx)
			return FuncInfo{l, r, b, auxs, goobj.FuncInfoLengths{}}
		}
	}
	return FuncInfo{}
}

// Preload a package: adds autolib.
// Does not add defined package or non-packaged symbols to the symbol table.
// These are done in LoadSyms.
// Does not read symbol data.
// Returns the fingerprint of the object.
func (l *Loader) Preload(localSymVersion int, f *bio.Reader, lib *sym.Library, unit *sym.CompilationUnit, length int64) goobj.FingerprintType {
	roObject, readonly, err := f.Slice(uint64(length)) // TODO: no need to map blocks that are for tools only (e.g. RefName)
	if err != nil {
		log.Fatal("cannot read object file:", err)
	}
	r := goobj.NewReaderFromBytes(roObject, readonly)
	if r == nil {
		if len(roObject) >= 8 && bytes.Equal(roObject[:8], []byte("\x00go114ld")) {
			log.Fatalf("found object file %s in old format", f.File().Name())
		}
		panic("cannot read object file")
	}
	pkgprefix := objabi.PathToPrefix(lib.Pkg) + "."
	ndef := r.NSym()
	nhashed64def := r.NHashed64def()
	nhasheddef := r.NHasheddef()
	or := &oReader{
		Reader:       r,
		unit:         unit,
		version:      localSymVersion,
		flags:        r.Flags(),
		pkgprefix:    pkgprefix,
		syms:         make([]Sym, ndef+nhashed64def+nhasheddef+r.NNonpkgdef()+r.NNonpkgref()),
		ndef:         ndef,
		nhasheddef:   nhasheddef,
		nhashed64def: nhashed64def,
		objidx:       uint32(len(l.objs)),
	}

	// Autolib
	lib.Autolib = append(lib.Autolib, r.Autolib()...)

	// DWARF file table
	nfile := r.NFile()
	unit.FileTable = make([]string, nfile)
	for i := range unit.FileTable {
		unit.FileTable[i] = r.File(i)
	}

	l.addObj(lib.Pkg, or)

	// The caller expects us consuming all the data
	f.MustSeek(length, os.SEEK_CUR)

	return r.Fingerprint()
}

// Holds the loader along with temporary states for loading symbols.
type loadState struct {
	l            *Loader
	hashed64Syms map[uint64]symAndSize         // short hashed (content-addressable) symbols, keyed by content hash
	hashedSyms   map[goobj.HashType]symAndSize // hashed (content-addressable) symbols, keyed by content hash
}

// Preload symbols of given kind from an object.
func (st *loadState) preloadSyms(r *oReader, kind int) {
	l := st.l
	var start, end uint32
	switch kind {
	case pkgDef:
		start = 0
		end = uint32(r.ndef)
	case hashed64Def:
		start = uint32(r.ndef)
		end = uint32(r.ndef + r.nhashed64def)
	case hashedDef:
		start = uint32(r.ndef + r.nhashed64def)
		end = uint32(r.ndef + r.nhashed64def + r.nhasheddef)
		if l.hasUnknownPkgPath {
			// The content hash depends on symbol name expansion. If any package is
			// built without fully expanded names, the content hash is unreliable.
			// Treat them as named symbols.
			// This is rare.
			// (We don't need to do this for hashed64Def case, as there the hash
			// function is simply the identity function, which doesn't depend on
			// name expansion.)
			kind = nonPkgDef
		}
	case nonPkgDef:
		start = uint32(r.ndef + r.nhashed64def + r.nhasheddef)
		end = uint32(r.ndef + r.nhashed64def + r.nhasheddef + r.NNonpkgdef())
	default:
		panic("preloadSyms: bad kind")
	}
	l.growAttrBitmaps(len(l.objSyms) + int(end-start))
	needNameExpansion := r.NeedNameExpansion()
	loadingRuntimePkg := r.unit.Lib.Pkg == "runtime"
	for i := start; i < end; i++ {
		osym := r.Sym(i)
		var name string
		var v int
		if kind != hashed64Def && kind != hashedDef { // we don't need the name, etc. for hashed symbols
			name = osym.Name(r.Reader)
			if needNameExpansion {
				name = strings.Replace(name, "\"\".", r.pkgprefix, -1)
			}
			v = abiToVer(osym.ABI(), r.version)
		}
		gi := st.addSym(name, v, r, i, kind, osym)
		r.syms[i] = gi
		if osym.TopFrame() {
			l.SetAttrTopFrame(gi, true)
		}
		if osym.Local() {
			l.SetAttrLocal(gi, true)
		}
		if osym.UsedInIface() {
			l.SetAttrUsedInIface(gi, true)
		}
		if strings.HasPrefix(name, "runtime.") ||
			(loadingRuntimePkg && strings.HasPrefix(name, "type.")) {
			if bi := goobj.BuiltinIdx(name, v); bi != -1 {
				// This is a definition of a builtin symbol. Record where it is.
				l.builtinSyms[bi] = gi
			}
		}
		if a := int32(osym.Align()); a != 0 && a > l.SymAlign(gi) {
			l.SetSymAlign(gi, a)
		}
	}
}

// Add syms, hashed (content-addressable) symbols, non-package symbols, and
// references to external symbols (which are always named).
func (l *Loader) LoadSyms(arch *sys.Arch) {
	// Allocate space for symbols, making a guess as to how much space we need.
	// This function was determined empirically by looking at the cmd/compile on
	// Darwin, and picking factors for hashed and hashed64 syms.
	var symSize, hashedSize, hashed64Size int
	for _, o := range l.objs[goObjStart:] {
		symSize += o.r.ndef + o.r.nhasheddef/2 + o.r.nhashed64def/2 + o.r.NNonpkgdef()
		hashedSize += o.r.nhasheddef / 2
		hashed64Size += o.r.nhashed64def / 2
	}
	// Index 0 is invalid for symbols.
	l.objSyms = make([]objSym, 1, symSize)

	l.npkgsyms = l.NSym()
	st := loadState{
		l:            l,
		hashed64Syms: make(map[uint64]symAndSize, hashed64Size),
		hashedSyms:   make(map[goobj.HashType]symAndSize, hashedSize),
	}

	for _, o := range l.objs[goObjStart:] {
		st.preloadSyms(o.r, pkgDef)
	}
	for _, o := range l.objs[goObjStart:] {
		st.preloadSyms(o.r, hashed64Def)
		st.preloadSyms(o.r, hashedDef)
		st.preloadSyms(o.r, nonPkgDef)
	}
	l.nhashedsyms = len(st.hashed64Syms) + len(st.hashedSyms)
	for _, o := range l.objs[goObjStart:] {
		loadObjRefs(l, o.r, arch)
	}
	l.values = make([]int64, l.NSym(), l.NSym()+1000) // +1000 make some room for external symbols
}

func loadObjRefs(l *Loader, r *oReader, arch *sys.Arch) {
	// load non-package refs
	ndef := uint32(r.NAlldef())
	needNameExpansion := r.NeedNameExpansion()
	for i, n := uint32(0), uint32(r.NNonpkgref()); i < n; i++ {
		osym := r.Sym(ndef + i)
		name := osym.Name(r.Reader)
		if needNameExpansion {
			name = strings.Replace(name, "\"\".", r.pkgprefix, -1)
		}
		v := abiToVer(osym.ABI(), r.version)
		r.syms[ndef+i] = l.LookupOrCreateSym(name, v)
		gi := r.syms[ndef+i]
		if osym.Local() {
			l.SetAttrLocal(gi, true)
		}
		if osym.UsedInIface() {
			l.SetAttrUsedInIface(gi, true)
		}
	}

	// referenced packages
	npkg := r.NPkg()
	r.pkg = make([]uint32, npkg)
	for i := 1; i < npkg; i++ { // PkgIdx 0 is a dummy invalid package
		pkg := r.Pkg(i)
		objidx, ok := l.objByPkg[pkg]
		if !ok {
			log.Fatalf("reference of nonexisted package %s, from %v", pkg, r.unit.Lib)
		}
		r.pkg[i] = objidx
	}

	// load flags of package refs
	for i, n := 0, r.NRefFlags(); i < n; i++ {
		rf := r.RefFlags(i)
		gi := l.resolve(r, rf.Sym())
		if rf.Flag2()&goobj.SymFlagUsedInIface != 0 {
			l.SetAttrUsedInIface(gi, true)
		}
	}
}

func abiToVer(abi uint16, localSymVersion int) int {
	var v int
	if abi == goobj.SymABIstatic {
		// Static
		v = localSymVersion
	} else if abiver := sym.ABIToVersion(obj.ABI(abi)); abiver != -1 {
		// Note that data symbols are "ABI0", which maps to version 0.
		v = abiver
	} else {
		log.Fatalf("invalid symbol ABI: %d", abi)
	}
	return v
}

// ResolveABIAlias given a symbol returns the ABI alias target of that
// symbol. If the sym in question is not an alias, the sym itself is
// returned.
func (l *Loader) ResolveABIAlias(s Sym) Sym {
	if s == 0 {
		return 0
	}
	if l.SymType(s) != sym.SABIALIAS {
		return s
	}
	relocs := l.Relocs(s)
	target := relocs.At(0).Sym()
	if l.SymType(target) == sym.SABIALIAS {
		panic(fmt.Sprintf("ABI alias %s references another ABI alias %s", l.SymName(s), l.SymName(target)))
	}
	return target
}

// TopLevelSym tests a symbol (by name and kind) to determine whether
// the symbol first class sym (participating in the link) or is an
// anonymous aux or sub-symbol containing some sub-part or payload of
// another symbol.
func (l *Loader) TopLevelSym(s Sym) bool {
	return topLevelSym(l.RawSymName(s), l.SymType(s))
}

// topLevelSym tests a symbol name and kind to determine whether
// the symbol first class sym (participating in the link) or is an
// anonymous aux or sub-symbol containing some sub-part or payload of
// another symbol.
func topLevelSym(sname string, skind sym.SymKind) bool {
	if sname != "" {
		return true
	}
	switch skind {
	case sym.SDWARFFCN, sym.SDWARFABSFCN, sym.SDWARFTYPE, sym.SDWARFCONST, sym.SDWARFCUINFO, sym.SDWARFRANGE, sym.SDWARFLOC, sym.SDWARFLINES, sym.SGOFUNC:
		return true
	default:
		return false
	}
}

// cloneToExternal takes the existing object file symbol (symIdx)
// and creates a new external symbol payload that is a clone with
// respect to name, version, type, relocations, etc. The idea here
// is that if the linker decides it wants to update the contents of
// a symbol originally discovered as part of an object file, it's
// easier to do this if we make the updates to an external symbol
// payload.
func (l *Loader) cloneToExternal(symIdx Sym) {
	if l.IsExternal(symIdx) {
		panic("sym is already external, no need for clone")
	}

	// Read the particulars from object.
	r, li := l.toLocal(symIdx)
	osym := r.Sym(li)
	sname := osym.Name(r.Reader)
	if r.NeedNameExpansion() {
		sname = strings.Replace(sname, "\"\".", r.pkgprefix, -1)
	}
	sver := abiToVer(osym.ABI(), r.version)
	skind := sym.AbiSymKindToSymKind[objabi.SymKind(osym.Type())]

	// Create new symbol, update version and kind.
	pi := l.newPayload(sname, sver)
	pp := l.payloads[pi]
	pp.kind = skind
	pp.ver = sver
	pp.size = int64(osym.Siz())
	pp.objidx = r.objidx

	// If this is a def, then copy the guts. We expect this case
	// to be very rare (one case it may come up is with -X).
	if li < uint32(r.NAlldef()) {

		// Copy relocations
		relocs := l.Relocs(symIdx)
		pp.relocs = make([]goobj.Reloc, relocs.Count())
		pp.reltypes = make([]objabi.RelocType, relocs.Count())
		for i := range pp.relocs {
			// Copy the relocs slice.
			// Convert local reference to global reference.
			rel := relocs.At(i)
			pp.relocs[i].Set(rel.Off(), rel.Siz(), 0, rel.Add(), goobj.SymRef{PkgIdx: 0, SymIdx: uint32(rel.Sym())})
			pp.reltypes[i] = rel.Type()
		}

		// Copy data
		pp.data = r.Data(li)
	}

	// If we're overriding a data symbol, collect the associated
	// Gotype, so as to propagate it to the new symbol.
	auxs := r.Auxs(li)
	pp.auxs = auxs

	// Install new payload to global index space.
	// (This needs to happen at the end, as the accessors above
	// need to access the old symbol content.)
	l.objSyms[symIdx] = objSym{l.extReader.objidx, uint32(pi)}
	l.extReader.syms = append(l.extReader.syms, symIdx)
}

// Copy the payload of symbol src to dst. Both src and dst must be external
// symbols.
// The intended use case is that when building/linking against a shared library,
// where we do symbol name mangling, the Go object file may have reference to
// the original symbol name whereas the shared library provides a symbol with
// the mangled name. When we do mangling, we copy payload of mangled to original.
func (l *Loader) CopySym(src, dst Sym) {
	if !l.IsExternal(dst) {
		panic("dst is not external") //l.newExtSym(l.SymName(dst), l.SymVersion(dst))
	}
	if !l.IsExternal(src) {
		panic("src is not external") //l.cloneToExternal(src)
	}
	l.payloads[l.extIndex(dst)] = l.payloads[l.extIndex(src)]
	l.SetSymPkg(dst, l.SymPkg(src))
	// TODO: other attributes?
}

// CopyAttributes copies over all of the attributes of symbol 'src' to
// symbol 'dst'.
func (l *Loader) CopyAttributes(src Sym, dst Sym) {
	l.SetAttrReachable(dst, l.AttrReachable(src))
	l.SetAttrOnList(dst, l.AttrOnList(src))
	l.SetAttrLocal(dst, l.AttrLocal(src))
	l.SetAttrNotInSymbolTable(dst, l.AttrNotInSymbolTable(src))
	if l.IsExternal(dst) {
		l.SetAttrVisibilityHidden(dst, l.AttrVisibilityHidden(src))
		l.SetAttrDuplicateOK(dst, l.AttrDuplicateOK(src))
		l.SetAttrShared(dst, l.AttrShared(src))
		l.SetAttrExternal(dst, l.AttrExternal(src))
	} else {
		// Some attributes are modifiable only for external symbols.
		// In such cases, don't try to transfer over the attribute
		// from the source even if there is a clash. This comes up
		// when copying attributes from a dupOK ABI wrapper symbol to
		// the real target symbol (which may not be marked dupOK).
	}
	l.SetAttrTopFrame(dst, l.AttrTopFrame(src))
	l.SetAttrSpecial(dst, l.AttrSpecial(src))
	l.SetAttrCgoExportDynamic(dst, l.AttrCgoExportDynamic(src))
	l.SetAttrCgoExportStatic(dst, l.AttrCgoExportStatic(src))
	l.SetAttrReadOnly(dst, l.AttrReadOnly(src))
}

// CreateExtSym creates a new external symbol with the specified name
// without adding it to any lookup tables, returning a Sym index for it.
func (l *Loader) CreateExtSym(name string, ver int) Sym {
	return l.newExtSym(name, ver)
}

// CreateStaticSym creates a new static symbol with the specified name
// without adding it to any lookup tables, returning a Sym index for it.
func (l *Loader) CreateStaticSym(name string) Sym {
	// Assign a new unique negative version -- this is to mark the
	// symbol so that it is not included in the name lookup table.
	l.anonVersion--
	return l.newExtSym(name, l.anonVersion)
}

func (l *Loader) FreeSym(i Sym) {
	if l.IsExternal(i) {
		pp := l.getPayload(i)
		*pp = extSymPayload{}
	}
}

// relocId is essentially a <S,R> tuple identifying the Rth
// relocation of symbol S.
type relocId struct {
	sym  Sym
	ridx int
}

// SetRelocVariant sets the 'variant' property of a relocation on
// some specific symbol.
func (l *Loader) SetRelocVariant(s Sym, ri int, v sym.RelocVariant) {
	// sanity check
	if relocs := l.Relocs(s); ri >= relocs.Count() {
		panic("invalid relocation ID")
	}
	if l.relocVariant == nil {
		l.relocVariant = make(map[relocId]sym.RelocVariant)
	}
	if v != 0 {
		l.relocVariant[relocId{s, ri}] = v
	} else {
		delete(l.relocVariant, relocId{s, ri})
	}
}

// RelocVariant returns the 'variant' property of a relocation on
// some specific symbol.
func (l *Loader) RelocVariant(s Sym, ri int) sym.RelocVariant {
	return l.relocVariant[relocId{s, ri}]
}

// UndefinedRelocTargets iterates through the global symbol index
// space, looking for symbols with relocations targeting undefined
// references. The linker's loadlib method uses this to determine if
// there are unresolved references to functions in system libraries
// (for example, libgcc.a), presumably due to CGO code. Return
// value is a list of loader.Sym's corresponding to the undefined
// cross-refs. The "limit" param controls the maximum number of
// results returned; if "limit" is -1, then all undefs are returned.
func (l *Loader) UndefinedRelocTargets(limit int) []Sym {
	result := []Sym{}
	for si := Sym(1); si < Sym(len(l.objSyms)); si++ {
		relocs := l.Relocs(si)
		for ri := 0; ri < relocs.Count(); ri++ {
			r := relocs.At(ri)
			rs := r.Sym()
			if rs != 0 && l.SymType(rs) == sym.SXREF && l.RawSymName(rs) != ".got" {
				result = append(result, rs)
				if limit != -1 && len(result) >= limit {
					break
				}
			}
		}
	}
	return result
}

// AssignTextSymbolOrder populates the Textp slices within each
// library and compilation unit, insuring that packages are laid down
// in dependency order (internal first, then everything else). Return value
// is a slice of all text syms.
func (l *Loader) AssignTextSymbolOrder(libs []*sym.Library, intlibs []bool, extsyms []Sym) []Sym {

	// Library Textp lists should be empty at this point.
	for _, lib := range libs {
		if len(lib.Textp) != 0 {
			panic("expected empty Textp slice for library")
		}
		if len(lib.DupTextSyms) != 0 {
			panic("expected empty DupTextSyms slice for library")
		}
	}

	// Used to record which dupok symbol we've assigned to a unit.
	// Can't use the onlist attribute here because it will need to
	// clear for the later assignment of the sym.Symbol to a unit.
	// NB: we can convert to using onList once we no longer have to
	// call the regular addToTextp.
	assignedToUnit := MakeBitmap(l.NSym() + 1)

	// Start off textp with reachable external syms.
	textp := []Sym{}
	for _, sym := range extsyms {
		if !l.attrReachable.Has(sym) {
			continue
		}
		textp = append(textp, sym)
	}

	// Walk through all text symbols from Go object files and append
	// them to their corresponding library's textp list.
	for _, o := range l.objs[goObjStart:] {
		r := o.r
		lib := r.unit.Lib
		for i, n := uint32(0), uint32(r.NAlldef()); i < n; i++ {
			gi := l.toGlobal(r, i)
			if !l.attrReachable.Has(gi) {
				continue
			}
			osym := r.Sym(i)
			st := sym.AbiSymKindToSymKind[objabi.SymKind(osym.Type())]
			if st != sym.STEXT {
				continue
			}
			dupok := osym.Dupok()
			if r2, i2 := l.toLocal(gi); r2 != r || i2 != i {
				// A dupok text symbol is resolved to another package.
				// We still need to record its presence in the current
				// package, as the trampoline pass expects packages
				// are laid out in dependency order.
				lib.DupTextSyms = append(lib.DupTextSyms, sym.LoaderSym(gi))
				continue // symbol in different object
			}
			if dupok {
				lib.DupTextSyms = append(lib.DupTextSyms, sym.LoaderSym(gi))
				continue
			}

			lib.Textp = append(lib.Textp, sym.LoaderSym(gi))
		}
	}

	// Now assemble global textp, and assign text symbols to units.
	for _, doInternal := range [2]bool{true, false} {
		for idx, lib := range libs {
			if intlibs[idx] != doInternal {
				continue
			}
			lists := [2][]sym.LoaderSym{lib.Textp, lib.DupTextSyms}
			for i, list := range lists {
				for _, s := range list {
					sym := Sym(s)
					if l.attrReachable.Has(sym) && !assignedToUnit.Has(sym) {
						textp = append(textp, sym)
						unit := l.SymUnit(sym)
						if unit != nil {
							unit.Textp = append(unit.Textp, s)
							assignedToUnit.Set(sym)
						}
						// Dupok symbols may be defined in multiple packages; the
						// associated package for a dupok sym is chosen sort of
						// arbitrarily (the first containing package that the linker
						// loads). Canonicalizes its Pkg to the package with which
						// it will be laid down in text.
						if i == 1 /* DupTextSyms2 */ && l.SymPkg(sym) != lib.Pkg {
							l.SetSymPkg(sym, lib.Pkg)
						}
					}
				}
			}
			lib.Textp = nil
			lib.DupTextSyms = nil
		}
	}

	return textp
}

// ErrorReporter is a helper class for reporting errors.
type ErrorReporter struct {
	ldr              *Loader
	AfterErrorAction func()
}

// Errorf method logs an error message.
//
// After each error, the error actions function will be invoked; this
// will either terminate the link immediately (if -h option given)
// or it will keep a count and exit if more than 20 errors have been printed.
//
// Logging an error means that on exit cmd/link will delete any
// output file and return a non-zero error code.
//
func (reporter *ErrorReporter) Errorf(s Sym, format string, args ...interface{}) {
	if s != 0 && reporter.ldr.SymName(s) != "" {
		format = reporter.ldr.SymName(s) + ": " + format
	} else {
		format = fmt.Sprintf("sym %d: %s", s, format)
	}
	format += "\n"
	fmt.Fprintf(os.Stderr, format, args...)
	reporter.AfterErrorAction()
}

// GetErrorReporter returns the loader's associated error reporter.
func (l *Loader) GetErrorReporter() *ErrorReporter {
	return l.errorReporter
}

// Errorf method logs an error message. See ErrorReporter.Errorf for details.
func (l *Loader) Errorf(s Sym, format string, args ...interface{}) {
	l.errorReporter.Errorf(s, format, args...)
}

// Symbol statistics.
func (l *Loader) Stat() string {
	s := fmt.Sprintf("%d symbols, %d reachable\n", l.NSym(), l.NReachableSym())
	s += fmt.Sprintf("\t%d package symbols, %d hashed symbols, %d non-package symbols, %d external symbols\n",
		l.npkgsyms, l.nhashedsyms, int(l.extStart)-l.npkgsyms-l.nhashedsyms, l.NSym()-int(l.extStart))
	return s
}

// For debugging.
func (l *Loader) Dump() {
	fmt.Println("objs")
	for _, obj := range l.objs[goObjStart:] {
		if obj.r != nil {
			fmt.Println(obj.i, obj.r.unit.Lib)
		}
	}
	fmt.Println("extStart:", l.extStart)
	fmt.Println("Nsyms:", len(l.objSyms))
	fmt.Println("syms")
	for i := Sym(1); i < Sym(len(l.objSyms)); i++ {
		pi := ""
		if l.IsExternal(i) {
			pi = fmt.Sprintf("<ext %d>", l.extIndex(i))
		}
		sect := ""
		if l.SymSect(i) != nil {
			sect = l.SymSect(i).Name
		}
		fmt.Printf("%v %v %v %v %x %v\n", i, l.SymName(i), l.SymType(i), pi, l.SymValue(i), sect)
	}
	fmt.Println("symsByName")
	for name, i := range l.symsByName[0] {
		fmt.Println(i, name, 0)
	}
	for name, i := range l.symsByName[1] {
		fmt.Println(i, name, 1)
	}
	fmt.Println("payloads:")
	for i := range l.payloads {
		pp := l.payloads[i]
		fmt.Println(i, pp.name, pp.ver, pp.kind)
	}
}