loader.go

		if int(i) < len(l.Syms) && l.Syms[i] != nil {
			return l.Syms[i]
		}
		if l.IsExternal(i) {
			panic("Can't load an external symbol.")
		}
		return l.loadSymbol(name, version)
	}
	i = l.AddExtSym(name, version)
	s := l.allocSym(name, version)
	l.Syms[i] = s
	return s
}

// cloneToExternal takes the existing object file symbol (symIdx)
// and creates a new external symbol 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 a new and similarly
// named external copy of that symbol.
func (l *Loader) cloneToExternal(symIdx Sym) Sym {
	if l.IsExternal(symIdx) {
		panic("sym is already external, no need for clone")
	}

	// Read the particulars from object.
	osym := goobj2.Sym{}
	r, li := l.toLocal(symIdx)
	osym.Read(r.Reader, r.SymOff(li))
	sname := strings.Replace(osym.Name, "\"\".", r.pkgprefix, -1)
	sver := abiToVer(osym.ABI, r.version)
	skind := sym.AbiSymKindToSymKind[objabi.SymKind(osym.Type)]

	// Create new symbol, update version and kind.
	ns := l.newExtSym(sname, sver)
	pp := l.payloads[ns-l.extStart]
	pp.kind = skind
	pp.ver = sver
	pp.size = int64(osym.Siz)
	pp.objidx = uint32(l.ocache)

	// 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 < (r.NSym() + r.NNonpkgdef()) {

		// Copy relocations
		relocs := l.Relocs(symIdx)
		pp.relocs = relocs.ReadAll(nil)

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

		// Copy read-only attr
		if r.ReadOnly() {
			l.attrReadOnly[ns] = true
		}
	}

	// If we're overriding a data symbol, collect the associated
	// Gotype, so as to propagate it to the new symbol.
	naux := r.NAux(li)
	for j := 0; j < naux; j++ {
		a := goobj2.Aux{}
		a.Read(r.Reader, r.AuxOff(li, j))
		switch a.Type {
		case goobj2.AuxGotype:
			pp.gotype = l.resolve(r, a.Sym)
		default:
			log.Fatalf("internal error: cloneToExternal applied to %s symbol %s with non-gotype aux data %d", skind.String(), sname, a.Type)
		}
	}

	// Fix up the lookup tables if the symbol in question was
	// present in the lookup tables. At the moment it only makes
	// sense to do this sort of clone/update for symbols that are
	// in the symbol table (as opposed to anonymous symbols);
	// issue an error if we can't look up the original symbol.
	if sver >= sym.SymVerStatic {
		s, ok := l.extStaticSyms[nameVer{sname, sver}]
		if !ok || s != symIdx {
			panic("lookup failed for clone of non-external static symbol")
		}
		l.extStaticSyms[nameVer{sname, sver}] = ns
	} else {
		s, ok := l.symsByName[sver][sname]
		if !ok || s != symIdx {
			panic("lookup failed for clone of non-external symbol")
		}
		l.symsByName[sver][sname] = ns
	}

	// Copy over selected attributes / properties. This is
	// probably overkill for most of these attributes, but it's
	// simpler just to copy everything.
	l.copyAttributes(symIdx, ns)
	if l.SymExtname(symIdx) != "" {
		l.SetSymExtname(ns, l.SymExtname(symIdx))
	}
	if l.SymDynimplib(symIdx) != "" {
		l.SetSymDynimplib(ns, l.SymDynimplib(symIdx))
	}
	if l.SymDynimpvers(symIdx) != "" {
		l.SetSymDynimpvers(ns, l.SymDynimpvers(symIdx))
	}

	// Add an overwrite entry (in case there are relocations against
	// the old symbol).
	l.overwrite[symIdx] = ns

	return ns
}

// copyAttributes copies over all of the attributes of symbol 'src' to
// symbol 'dst'. The assumption is that 'dst' is an external symbol.
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))
	l.SetAttrVisibilityHidden(dst, l.AttrVisibilityHidden(src))
	l.SetAttrDuplicateOK(dst, l.AttrDuplicateOK(src))
	l.SetAttrShared(dst, l.AttrShared(src))
	l.SetAttrExternal(dst, l.AttrExternal(src))
	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))
}

// migrateAttributes copies over all of the attributes of symbol 'src' to
// sym.Symbol 'dst'.
func (l *Loader) migrateAttributes(src Sym, dst *sym.Symbol) {
	src = l.getOverwrite(src)
	dst.Attr.Set(sym.AttrReachable, l.AttrReachable(src))
	dst.Attr.Set(sym.AttrOnList, l.AttrOnList(src))
	dst.Attr.Set(sym.AttrLocal, l.AttrLocal(src))
	dst.Attr.Set(sym.AttrNotInSymbolTable, l.AttrNotInSymbolTable(src))
	dst.Attr.Set(sym.AttrVisibilityHidden, l.AttrVisibilityHidden(src))
	dst.Attr.Set(sym.AttrDuplicateOK, l.AttrDuplicateOK(src))
	dst.Attr.Set(sym.AttrShared, l.AttrShared(src))
	dst.Attr.Set(sym.AttrExternal, l.AttrExternal(src))
	dst.Attr.Set(sym.AttrTopFrame, l.AttrTopFrame(src))
	dst.Attr.Set(sym.AttrSpecial, l.AttrSpecial(src))
	dst.Attr.Set(sym.AttrCgoExportDynamic, l.AttrCgoExportDynamic(src))
	dst.Attr.Set(sym.AttrCgoExportStatic, l.AttrCgoExportStatic(src))
	dst.Attr.Set(sym.AttrReadOnly, l.AttrReadOnly(src))

	// Convert outer/sub relationships
	if outer, ok := l.outer[src]; ok {
		dst.Outer = l.Syms[outer]
	}
	if sub, ok := l.sub[src]; ok {
		dst.Sub = l.Syms[sub]
	}

	// Set sub-symbol attribute. FIXME: would be better to do away
	// with this and just use l.OuterSymbol() != 0 elsewhere within
	// the linker.
	dst.Attr.Set(sym.AttrSubSymbol, dst.Outer != nil)

	// Copy over dynimplib, dynimpvers, extname.
	if l.SymExtname(src) != "" {
		dst.SetExtname(l.SymExtname(src))
	}
	if l.SymDynimplib(src) != "" {
		dst.SetDynimplib(l.SymDynimplib(src))
	}
	if l.SymDynimpvers(src) != "" {
		dst.SetDynimpvers(l.SymDynimpvers(src))
	}

	// Copy ELF type if set.
	if et, ok := l.elfType[src]; ok {
		dst.SetElfType(et)
	}
}

// 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) Sym {
	// Assign a new unique negative version -- this is to mark the
	// symbol so that it can be skipped when ExtractSymbols is adding
	// ext syms to the sym.Symbols hash.
	l.anonVersion--
	return l.newExtSym(name, l.anonVersion)
}

// Create creates a symbol with the specified name, returning a
// sym.Symbol object for it. This method is intended for static/hidden
// symbols discovered while loading host objects. We can see more than
// one instance of a given static symbol with the same name/version,
// so we can't add them to the lookup tables "as is". Instead assign
// them fictitious (unique) versions, starting at -1 and decreasing by
// one for each newly created symbol, and record them in the
// extStaticSyms hash.
func (l *Loader) Create(name string) *sym.Symbol {
	i := l.max + 1
	l.max++
	if l.extStart == 0 {
		l.extStart = i
	}

	// Assign a new unique negative version -- this is to mark the
	// symbol so that it can be skipped when ExtractSymbols is adding
	// ext syms to the sym.Symbols hash.
	l.anonVersion--
	ver := l.anonVersion
	l.growSyms(int(i))
	s := l.allocSym(name, ver)
	l.installSym(i, s)
	l.extStaticSyms[nameVer{name, ver}] = i

	return s
}

func loadObjFull(l *Loader, r *oReader) {
	lib := r.unit.Lib
	istart := l.startIndex(r)

	resolveSymRef := func(s goobj2.SymRef) *sym.Symbol {
		i := l.resolve(r, s)
		return l.Syms[i]
	}

	funcs := []funcInfoSym{}
	fdsyms := []*sym.Symbol{}
	var funcAllocCounts funcAllocInfo
	pcdataBase := r.PcdataBase()
	rslice := []Reloc{}
	for i, n := 0, r.NSym()+r.NNonpkgdef(); i < n; i++ {
		osym := goobj2.Sym{}
		osym.Read(r.Reader, r.SymOff(i))
		name := strings.Replace(osym.Name, "\"\".", r.pkgprefix, -1)
		if name == "" {
			continue
		}
		ver := abiToVer(osym.ABI, r.version)
		dupok := osym.Dupok()
		if dupok {
			if dupsym := l.symsByName[ver][name]; dupsym != istart+Sym(i) {
				if l.attrReachable.has(dupsym) {
					// A dupok 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.
					s := l.Syms[dupsym]
					if s.Type == sym.STEXT {
						lib.DupTextSyms = append(lib.DupTextSyms, s)
						lib.DupTextSyms2 = append(lib.DupTextSyms2, sym.LoaderSym(dupsym))
					}
				}
				continue
			}
		}

		s := l.Syms[istart+Sym(i)]
		if s == nil {
			continue
		}
		if s.Name != name { // Sanity check. We can remove it in the final version.
			fmt.Println("name mismatch:", lib, i, s.Name, name)
			panic("name mismatch")
		}

		local := osym.Local()
		makeTypelink := osym.Typelink()
		size := osym.Siz

		// Symbol data
		s.P = r.Data(i)
		s.Attr.Set(sym.AttrReadOnly, r.ReadOnly())

		// Relocs
		relocs := l.relocs(r, i)
		rslice = relocs.ReadAll(rslice)
		batch := l.relocBatch
		s.R = batch[:relocs.Count:relocs.Count]
		l.relocBatch = batch[relocs.Count:]
		l.convertRelocations(rslice, s)

		// Aux symbol info
		isym := -1
		naux := r.NAux(i)
		for j := 0; j < naux; j++ {
			a := goobj2.Aux{}
			a.Read(r.Reader, r.AuxOff(i, j))
			switch a.Type {
			case goobj2.AuxGotype:
				typ := resolveSymRef(a.Sym)
				if typ != nil {
					s.Gotype = typ
				}
			case goobj2.AuxFuncdata:
				fdsyms = append(fdsyms, resolveSymRef(a.Sym))
			case goobj2.AuxFuncInfo:
				if a.Sym.PkgIdx != goobj2.PkgIdxSelf {
					panic("funcinfo symbol not defined in current package")
				}
				isym = int(a.Sym.SymIdx)
			case goobj2.AuxDwarfInfo, goobj2.AuxDwarfLoc, goobj2.AuxDwarfRanges, goobj2.AuxDwarfLines:
				// ignored for now
			default:
				panic("unknown aux type")
			}
		}

		s.File = r.pkgprefix[:len(r.pkgprefix)-1]
		if dupok {
			s.Attr |= sym.AttrDuplicateOK
		}
		if s.Size < int64(size) {
			s.Size = int64(size)
		}
		s.Attr.Set(sym.AttrLocal, local)
		s.Attr.Set(sym.AttrMakeTypelink, makeTypelink)

		if s.Type == sym.SDWARFINFO {
			// For DWARF symbols, replace `"".` to actual package prefix
			// in the symbol content.
			// TODO: maybe we should do this in the compiler and get rid
			// of this.
			patchDWARFName(s, r)
		}

		if s.Type != sym.STEXT {
			continue
		}

		if isym == -1 {
			continue
		}

		// Record function sym and associated info for additional
		// processing in the loop below.
		fwis := funcInfoSym{s: s, isym: isym, osym: osym}
		funcs = append(funcs, fwis)

		// Read the goobj2.FuncInfo for this text symbol so that we can
		// collect allocation counts. We'll read it again in the loop
		// below.
		b := r.Data(isym)
		info := goobj2.FuncInfo{}
		info.Read(b)
		funcAllocCounts.symPtr += uint32(len(info.File))
		funcAllocCounts.pcData += uint32(len(info.Pcdata))
		funcAllocCounts.inlCall += uint32(len(info.InlTree))
		funcAllocCounts.fdOff += uint32(len(info.Funcdataoff))
	}

	// At this point we can do batch allocation of the sym.FuncInfo's,
	// along with the slices of sub-objects they use.
	fiBatch := make([]sym.FuncInfo, len(funcs))
	inlCallBatch := make([]sym.InlinedCall, funcAllocCounts.inlCall)
	symPtrBatch := make([]*sym.Symbol, funcAllocCounts.symPtr)
	pcDataBatch := make([]sym.Pcdata, funcAllocCounts.pcData)
	fdOffBatch := make([]int64, funcAllocCounts.fdOff)

	// Populate FuncInfo contents for func symbols.
	for fi := 0; fi < len(funcs); fi++ {
		s := funcs[fi].s
		isym := funcs[fi].isym
		osym := funcs[fi].osym

		s.FuncInfo = &fiBatch[0]
		fiBatch = fiBatch[1:]

		b := r.Data(isym)
		info := goobj2.FuncInfo{}
		info.Read(b)

		if info.NoSplit != 0 {
			s.Attr |= sym.AttrNoSplit
		}
		if osym.ReflectMethod() {
			s.Attr |= sym.AttrReflectMethod
		}
		if r.Flags()&goobj2.ObjFlagShared != 0 {
			s.Attr |= sym.AttrShared
		}
		if osym.TopFrame() {
			s.Attr |= sym.AttrTopFrame
		}

		pc := s.FuncInfo

		if len(info.Funcdataoff) != 0 {
			nfd := len(info.Funcdataoff)
			pc.Funcdata = fdsyms[:nfd:nfd]
			fdsyms = fdsyms[nfd:]
		}

		info.Pcdata = append(info.Pcdata, info.PcdataEnd) // for the ease of knowing where it ends
		pc.Args = int32(info.Args)
		pc.Locals = int32(info.Locals)

		npc := len(info.Pcdata) - 1 // -1 as we appended one above
		pc.Pcdata = pcDataBatch[:npc:npc]
		pcDataBatch = pcDataBatch[npc:]

		nfd := len(info.Funcdataoff)
		pc.Funcdataoff = fdOffBatch[:nfd:nfd]
		fdOffBatch = fdOffBatch[nfd:]

		nsp := len(info.File)
		pc.File = symPtrBatch[:nsp:nsp]
		symPtrBatch = symPtrBatch[nsp:]

		nic := len(info.InlTree)
		pc.InlTree = inlCallBatch[:nic:nic]
		inlCallBatch = inlCallBatch[nic:]

		pc.Pcsp.P = r.BytesAt(pcdataBase+info.Pcsp, int(info.Pcfile-info.Pcsp))
		pc.Pcfile.P = r.BytesAt(pcdataBase+info.Pcfile, int(info.Pcline-info.Pcfile))
		pc.Pcline.P = r.BytesAt(pcdataBase+info.Pcline, int(info.Pcinline-info.Pcline))
		pc.Pcinline.P = r.BytesAt(pcdataBase+info.Pcinline, int(info.Pcdata[0]-info.Pcinline))
		for k := range pc.Pcdata {
			pc.Pcdata[k].P = r.BytesAt(pcdataBase+info.Pcdata[k], int(info.Pcdata[k+1]-info.Pcdata[k]))
		}
		for k := range pc.Funcdataoff {
			pc.Funcdataoff[k] = int64(info.Funcdataoff[k])
		}
		for k := range pc.File {
			pc.File[k] = resolveSymRef(info.File[k])
		}
		for k := range pc.InlTree {
			inl := &info.InlTree[k]
			pc.InlTree[k] = sym.InlinedCall{
				Parent:   inl.Parent,
				File:     resolveSymRef(inl.File),
				Line:     inl.Line,
				Func:     l.SymName(l.resolve(r, inl.Func)),
				ParentPC: inl.ParentPC,
			}
		}

		dupok := osym.Dupok()
		if !dupok {
			if s.Attr.OnList() {
				log.Fatalf("symbol %s listed multiple times", s.Name)
			}
			s.Attr.Set(sym.AttrOnList, true)
			lib.Textp = append(lib.Textp, s)
			lib.Textp2 = append(lib.Textp2, sym.LoaderSym(isym))
		} else {
			// there may be a dup in another package
			// put into a temp list and add to text later
			lib.DupTextSyms = append(lib.DupTextSyms, s)
			lib.DupTextSyms2 = append(lib.DupTextSyms2, sym.LoaderSym(isym))
		}
	}
}

// convertRelocations takes a vector of loader.Reloc relocations and
// translates them into an equivalent set of sym.Reloc relocations on
// the symbol "dst", performing fixups along the way for ABI aliases,
// etc. It is assumed that the called has pre-allocated the dst symbol
// relocations slice.
func (l *Loader) convertRelocations(src []Reloc, dst *sym.Symbol) {
	for j := range dst.R {
		r := src[j]
		rs := r.Sym
		sz := r.Size
		rt := r.Type
		if rt == objabi.R_METHODOFF {
			if l.attrReachable.has(rs) {
				rt = objabi.R_ADDROFF
			} else {
				sz = 0
				rs = 0
			}
		}
		if rt == objabi.R_WEAKADDROFF && !l.attrReachable.has(rs) {
			rs = 0
			sz = 0
		}
		if rs != 0 && l.Syms[rs] != nil && l.Syms[rs].Type == sym.SABIALIAS {
			rsrelocs := l.Relocs(rs)
			rs = rsrelocs.At(0).Sym
		}
		dst.R[j] = sym.Reloc{
			Off:  r.Off,
			Siz:  sz,
			Type: rt,
			Add:  r.Add,
			Sym:  l.Syms[rs],
		}
	}
}

var emptyPkg = []byte(`"".`)

func patchDWARFName1(p []byte, r *oReader) ([]byte, int) {
	// This is kind of ugly. Really the package name should not
	// even be included here.
	if len(p) < 1 || p[0] != dwarf.DW_ABRV_FUNCTION {
		return p, -1
	}
	e := bytes.IndexByte(p, 0)
	if e == -1 {
		return p, -1
	}
	if !bytes.Contains(p[:e], emptyPkg) {
		return p, -1
	}
	pkgprefix := []byte(r.pkgprefix)
	patched := bytes.Replace(p[:e], emptyPkg, pkgprefix, -1)
	return append(patched, p[e:]...), e
}

func patchDWARFName(s *sym.Symbol, r *oReader) {
	patched, e := patchDWARFName1(s.P, r)
	if e == -1 {
		return
	}
	s.P = patched
	s.Attr.Set(sym.AttrReadOnly, false)
	delta := int64(len(s.P)) - s.Size
	s.Size = int64(len(s.P))
	for i := range s.R {
		r := &s.R[i]
		if r.Off > int32(e) {
			r.Off += int32(delta)
		}
	}
}

// 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{}
	rslice := []Reloc{}
	for si := Sym(1); si <= l.max; si++ {
		if _, ok := l.overwrite[si]; ok {
			continue
		}
		relocs := l.Relocs(si)
		rslice = relocs.ReadAll(rslice)
		for ri := 0; ri < relocs.Count; ri++ {
			r := &rslice[ri]
			if r.Sym != 0 && l.SymType(r.Sym) == sym.SXREF && l.RawSymName(r.Sym) != ".got" {
				result = append(result, r.Sym)
				if limit != -1 && len(result) >= limit {
					break
				}
			}
		}
	}
	return result
}

// For debugging.
func (l *Loader) Dump() {
	fmt.Println("objs")
	for _, obj := range l.objs {
		if obj.r != nil {
			fmt.Println(obj.i, obj.r.unit.Lib)
		}
	}
	fmt.Println("extStart:", l.extStart)
	fmt.Println("max:", l.max)
	fmt.Println("syms")
	for i, s := range l.Syms {
		if i == 0 {
			continue
		}
		if s != nil {
			fmt.Println(i, s, s.Type)
		} else {
			otag := ""
			si := Sym(i)
			if _, ok := l.overwrite[si]; ok {
				si = l.getOverwrite(si)
				otag = fmt.Sprintf(" <overwritten to %d>", si)
			}
			fmt.Println(i, l.SymName(si), "<not loaded>", otag)
		}
	}
	fmt.Println("overwrite:", l.overwrite)
	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)
	}
}