loader.go

// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package loader

import (
	"bytes"
	"cmd/internal/bio"
	"cmd/internal/goobj2"
	"cmd/internal/obj"
	"cmd/internal/objabi"
	"cmd/internal/sys"
	"cmd/link/internal/sym"
	"debug/elf"
	"fmt"
	"log"
	"math/bits"
	"os"
	"sort"
	"strconv"
	"strings"
)

var _ = fmt.Print

// Sym encapsulates a global symbol index, used to identify a specific
// Go symbol. The 0-valued Sym is corresponds to an invalid symbol.
type Sym int

// Relocs encapsulates the set of relocations on a given symbol; an
// instance of this type is returned by the Loader Relocs() method.
type Relocs struct {
	rs []goobj2.Reloc

	li int      // local index of symbol whose relocs we're examining
	r  *oReader // object reader for containing package
	l  *Loader  // loader
}

// Reloc contains the payload for a specific relocation.
// TODO: replace this with sym.Reloc, once we change the
// relocation target from "*sym.Symbol" to "loader.Sym" in sym.Reloc.
type Reloc struct {
	Off  int32            // offset to rewrite
	Size uint8            // number of bytes to rewrite: 0, 1, 2, or 4
	Type objabi.RelocType // the relocation type
	Add  int64            // addend
	Sym  Sym              // global index of symbol the reloc addresses
}

// ExtReloc contains the payload for an external relocation.
type ExtReloc struct {
	Idx  int // index of the original relocation
	Xsym Sym
	Xadd int64
}

// Reloc2 holds a "handle" to access a relocation record from an
// object file.
type Reloc2 struct {
	*goobj2.Reloc
	r *oReader
	l *Loader

	// External reloc types may not fit into a uint8 which the Go object file uses.
	// Store it here, instead of in the byte of goobj2.Reloc2.
	// For Go symbols this will always be zero.
	// goobj2.Reloc2.Type() + typ is always the right type, for both Go and external
	// symbols.
	typ objabi.RelocType
}

func (rel Reloc2) Type() objabi.RelocType { return objabi.RelocType(rel.Reloc.Type()) + rel.typ }
func (rel Reloc2) Sym() Sym               { return rel.l.resolve(rel.r, rel.Reloc.Sym()) }
func (rel Reloc2) SetSym(s Sym)           { rel.Reloc.SetSym(goobj2.SymRef{PkgIdx: 0, SymIdx: uint32(s)}) }

func (rel Reloc2) SetType(t objabi.RelocType) {
	if t != objabi.RelocType(uint8(t)) {
		panic("SetType: type doesn't fit into Reloc2")
	}
	rel.Reloc.SetType(uint8(t))
	if rel.typ != 0 {
		// should use SymbolBuilder.SetRelocType
		panic("wrong method to set reloc type")
	}
}

// Aux2 holds a "handle" to access an aux symbol record from an
// object file.
type Aux2 struct {
	*goobj2.Aux
	r *oReader
	l *Loader
}

func (a Aux2) Sym() Sym { return a.l.resolve(a.r, a.Aux.Sym()) }

// oReader is a wrapper type of obj.Reader, along with some
// extra information.
// TODO: rename to objReader once the old one is gone?
type oReader struct {
	*goobj2.Reader
	unit      *sym.CompilationUnit
	version   int    // version of static symbol
	flags     uint32 // read from object file
	pkgprefix string
	syms      []Sym  // Sym's global index, indexed by local index
	ndef      int    // cache goobj2.Reader.NSym()
	objidx    uint32 // index of this reader in the objs slice
}

type objIdx struct {
	r *oReader
	i Sym // start index
}

// objSym represents a symbol in an object file. It is a tuple of
// the object and the symbol's local index.
// For external symbols, r is l.extReader, s is its index into the
// payload array.
// {nil, 0} represents the nil symbol.
type objSym struct {
	r *oReader
	s int // local index
}

type nameVer struct {
	name string
	v    int
}

type Bitmap []uint32

// set the i-th bit.
func (bm Bitmap) Set(i Sym) {
	n, r := uint(i)/32, uint(i)%32
	bm[n] |= 1 << r
}

// unset the i-th bit.
func (bm Bitmap) Unset(i Sym) {
	n, r := uint(i)/32, uint(i)%32
	bm[n] &^= (1 << r)
}

// whether the i-th bit is set.
func (bm Bitmap) Has(i Sym) bool {
	n, r := uint(i)/32, uint(i)%32
	return bm[n]&(1<<r) != 0
}

// return current length of bitmap in bits.
func (bm Bitmap) Len() int {
	return len(bm) * 32
}

// return the number of bits set.
func (bm Bitmap) Count() int {
	s := 0
	for _, x := range bm {
		s += bits.OnesCount32(x)
	}
	return s
}

func MakeBitmap(n int) Bitmap {
	return make(Bitmap, (n+31)/32)
}

// growBitmap insures that the specified bitmap has enough capacity,
// reallocating (doubling the size) if needed.
func growBitmap(reqLen int, b Bitmap) Bitmap {
	curLen := b.Len()
	if reqLen > curLen {
		b = append(b, MakeBitmap(reqLen+1-curLen)...)
	}
	return b
}

// A Loader loads new object files and resolves indexed symbol references.
//
// Notes on the layout of global symbol index space:
//
// - Go object files are read before host object files; each Go object
//   read adds its defined package symbols to the global index space.
//   Nonpackage symbols are not yet added.
//
// - In loader.LoadNonpkgSyms, add non-package defined symbols and
//   references in all object files to the global index space.
//
// - Host object file loading happens; the host object loader does a
//   name/version lookup for each symbol it finds; this can wind up
//   extending the external symbol index space range. The host object
//   loader stores symbol payloads in loader.payloads using SymbolBuilder.
//
// - For now, in loader.LoadFull we convert all symbols (Go + external)
//   to sym.Symbols.
//
// - At some point (when the wayfront is pushed through all of the
//   linker), all external symbols will be payload-based, and we can
//   get rid of the loader.Syms array.
//
// - Each symbol gets a unique global index. For duplicated and
//   overwriting/overwritten symbols, the second (or later) appearance
//   of the symbol gets the same global index as the first appearance.
type Loader struct {
	start       map[*oReader]Sym // map from object file to its start index
	objs        []objIdx         // sorted by start index (i.e. objIdx.i)
	extStart    Sym              // from this index on, the symbols are externally defined
	builtinSyms []Sym            // global index of builtin symbols

	objSyms []objSym // global index mapping to local index

	symsByName    [2]map[string]Sym // map symbol name to index, two maps are for ABI0 and ABIInternal
	extStaticSyms map[nameVer]Sym   // externally defined static symbols, keyed by name

	extReader    *oReader // a dummy oReader, for external symbols
	payloadBatch []extSymPayload
	payloads     []*extSymPayload // contents of linker-materialized external syms
	values       []int64          // symbol values, indexed by global sym index

	sects    []*sym.Section // sections
	symSects []uint16       // symbol's section, index to sects array

	outdata   [][]byte     // symbol's data in the output buffer
	extRelocs [][]ExtReloc // symbol's external relocations

	itablink map[Sym]struct{} // itablink[j] defined if j is go.itablink.*

	objByPkg map[string]*oReader // map package path to its Go object reader

	Syms     []*sym.Symbol // indexed symbols. XXX we still make sym.Symbol for now.
	symBatch []sym.Symbol  // batch of symbols.

	anonVersion int // most recently assigned ext static sym pseudo-version

	// Bitmaps and other side structures used to store data used to store
	// symbol flags/attributes; these are to be accessed via the
	// corresponding loader "AttrXXX" and "SetAttrXXX" methods. Please
	// visit the comments on these methods for more details on the
	// semantics / interpretation of the specific flags or attribute.
	attrReachable        Bitmap // reachable symbols, indexed by global index
	attrOnList           Bitmap // "on list" symbols, indexed by global index
	attrLocal            Bitmap // "local" symbols, indexed by global index
	attrNotInSymbolTable Bitmap // "not in symtab" symbols, indexed by glob idx
	attrVisibilityHidden Bitmap // hidden symbols, indexed by ext sym index
	attrDuplicateOK      Bitmap // dupOK symbols, indexed by ext sym index
	attrShared           Bitmap // shared symbols, indexed by ext sym index
	attrExternal         Bitmap // external symbols, indexed by ext sym index

	attrReadOnly         map[Sym]bool     // readonly data for this sym
	attrTopFrame         map[Sym]struct{} // top frame symbols
	attrSpecial          map[Sym]struct{} // "special" frame symbols
	attrCgoExportDynamic map[Sym]struct{} // "cgo_export_dynamic" symbols
	attrCgoExportStatic  map[Sym]struct{} // "cgo_export_static" symbols

	// Outer and Sub relations for symbols.
	// TODO: figure out whether it's more efficient to just have these
	// as fields on extSymPayload (note that this won't be a viable
	// strategy if somewhere in the linker we set sub/outer for a
	// non-external sym).
	outer map[Sym]Sym
	sub   map[Sym]Sym

	align map[Sym]int32 // stores alignment for symbols

	dynimplib   map[Sym]string      // stores Dynimplib symbol attribute
	dynimpvers  map[Sym]string      // stores Dynimpvers symbol attribute
	localentry  map[Sym]uint8       // stores Localentry symbol attribute
	extname     map[Sym]string      // stores Extname symbol attribute
	elfType     map[Sym]elf.SymType // stores elf type symbol property
	elfSym      map[Sym]int32       // stores elf sym symbol property
	localElfSym map[Sym]int32       // stores "local" elf sym symbol property
	symPkg      map[Sym]string      // stores package for symbol, or library for shlib-derived syms
	plt         map[Sym]int32       // stores dynimport for pe objects
	got         map[Sym]int32       // stores got for pe objects
	dynid       map[Sym]int32       // stores Dynid for symbol

	relocVariant map[relocId]sym.RelocVariant // stores variant relocs

	// Used to implement field tracking; created during deadcode if
	// field tracking is enabled. Reachparent[K] contains the index of
	// the symbol that triggered the marking of symbol K as live.
	Reachparent []Sym

	relocBatch []sym.Reloc // for bulk allocation of relocations

	flags uint32

	strictDupMsgs int // number of strict-dup warning/errors, when FlagStrictDups is enabled

	elfsetstring elfsetstringFunc

	errorReporter *ErrorReporter

	SymLookup func(name string, ver int) *sym.Symbol
}

const (
	pkgDef = iota
	nonPkgDef
	nonPkgRef
)

type elfsetstringFunc func(s *sym.Symbol, str string, off int)

// extSymPayload holds the payload (data + relocations) for linker-synthesized
// external symbols (note that symbol value is stored in a separate slice).
type extSymPayload struct {
	name     string // TODO: would this be better as offset into str table?
	size     int64
	ver      int
	kind     sym.SymKind
	objidx   uint32 // index of original object if sym made by cloneToExternal
	gotype   Sym    // Gotype (0 if not present)
	relocs   []goobj2.Reloc
	reltypes []objabi.RelocType // relocation types
	data     []byte
	auxs     []goobj2.Aux
}

const (
	// Loader.flags
	FlagStrictDups = 1 << iota
)

func NewLoader(flags uint32, elfsetstring elfsetstringFunc, reporter *ErrorReporter) *Loader {
	nbuiltin := goobj2.NBuiltin()
	ldr := &Loader{
		start:                make(map[*oReader]Sym),
		objs:                 []objIdx{{}}, // reserve index 0 for nil symbol
		objSyms:              []objSym{{}}, // reserve index 0 for nil symbol
		extReader:            &oReader{},
		symsByName:           [2]map[string]Sym{make(map[string]Sym, 100000), make(map[string]Sym, 50000)}, // preallocate ~2MB for ABI0 and ~1MB for ABI1 symbols
		objByPkg:             make(map[string]*oReader),
		outer:                make(map[Sym]Sym),
		sub:                  make(map[Sym]Sym),
		align:                make(map[Sym]int32),
		dynimplib:            make(map[Sym]string),
		dynimpvers:           make(map[Sym]string),
		localentry:           make(map[Sym]uint8),
		extname:              make(map[Sym]string),
		attrReadOnly:         make(map[Sym]bool),
		elfType:              make(map[Sym]elf.SymType),
		elfSym:               make(map[Sym]int32),
		localElfSym:          make(map[Sym]int32),
		symPkg:               make(map[Sym]string),
		plt:                  make(map[Sym]int32),
		got:                  make(map[Sym]int32),
		dynid:                make(map[Sym]int32),
		attrTopFrame:         make(map[Sym]struct{}),
		attrSpecial:          make(map[Sym]struct{}),
		attrCgoExportDynamic: make(map[Sym]struct{}),
		attrCgoExportStatic:  make(map[Sym]struct{}),
		itablink:             make(map[Sym]struct{}),
		extStaticSyms:        make(map[nameVer]Sym),
		builtinSyms:          make([]Sym, nbuiltin),
		flags:                flags,
		elfsetstring:         elfsetstring,
		errorReporter:        reporter,
		sects:                []*sym.Section{nil}, // reserve index 0 for nil section
	}
	reporter.ldr = ldr
	return ldr
}

// Add object file r, return the start index.
func (l *Loader) addObj(pkg string, r *oReader) Sym {
	if _, ok := l.start[r]; ok {
		panic("already added")
	}
	pkg = objabi.PathToPrefix(pkg) // the object file contains escaped package path
	if _, ok := l.objByPkg[pkg]; !ok {
		l.objByPkg[pkg] = r
	}
	i := Sym(len(l.objSyms))
	l.start[r] = i
	l.objs = append(l.objs, objIdx{r, i})
	return i
}

// Add a symbol from an object file, return the global index and whether it is added.
// If the symbol already exist, it returns the index of that symbol.
func (l *Loader) AddSym(name string, ver int, r *oReader, li int, kind int, dupok bool, typ sym.SymKind) (Sym, bool) {
	if l.extStart != 0 {
		panic("AddSym called after external symbol is created")
	}
	i := Sym(len(l.objSyms))
	addToGlobal := func() {
		l.objSyms = append(l.objSyms, objSym{r, li})
	}
	if name == "" {
		addToGlobal()
		return i, true // unnamed aux symbol
	}
	if ver == r.version {
		// Static symbol. Add its global index but don't
		// add to name lookup table, as it cannot be
		// referenced by name.
		addToGlobal()
		return i, true
	}
	if kind == pkgDef {
		// Defined package symbols cannot be dup to each other.
		// We load all the package symbols first, so we don't need
		// to check dup here.
		// We still add it to the lookup table, as it may still be
		// referenced by name (e.g. through linkname).
		l.symsByName[ver][name] = i
		addToGlobal()
		return i, true
	}

	// Non-package (named) symbol. Check if it already exists.
	oldi, existed := l.symsByName[ver][name]
	if !existed {
		l.symsByName[ver][name] = i
		addToGlobal()
		return i, true
	}
	// symbol already exists
	if dupok {
		if l.flags&FlagStrictDups != 0 {
			l.checkdup(name, r, li, oldi)
		}
		return oldi, false
	}
	oldr, oldli := l.toLocal(oldi)
	oldsym := oldr.Sym(oldli)
	if oldsym.Dupok() {
		return oldi, false
	}
	overwrite := r.DataSize(li) != 0
	if overwrite {
		// new symbol overwrites old symbol.
		oldtyp := sym.AbiSymKindToSymKind[objabi.SymKind(oldsym.Type())]
		if !(oldtyp.IsData() && oldr.DataSize(oldli) == 0) {
			log.Fatalf("duplicated definition of symbol " + name)
		}
		l.objSyms[oldi] = objSym{r, li}
	} else {
		// old symbol overwrites new symbol.
		if !typ.IsData() { // only allow overwriting data symbol
			log.Fatalf("duplicated definition of symbol " + name)
		}
	}
	return oldi, true
}

// newExtSym creates a new external sym with the specified
// name/version.
func (l *Loader) newExtSym(name string, ver int) Sym {
	i := Sym(len(l.objSyms))
	if l.extStart == 0 {
		l.extStart = i
	}
	l.growSyms(int(i))
	pi := l.newPayload(name, ver)
	l.objSyms = append(l.objSyms, objSym{l.extReader, int(pi)})
	l.extReader.syms = append(l.extReader.syms, i)
	return i
}

// LookupOrCreateSym looks up the symbol with the specified name/version,
// returning its Sym index if found. If the lookup fails, a new external
// Sym will be created, entered into the lookup tables, and returned.
func (l *Loader) LookupOrCreateSym(name string, ver int) Sym {
	i := l.Lookup(name, ver)
	if i != 0 {
		return i
	}
	i = l.newExtSym(name, ver)
	static := ver >= sym.SymVerStatic || ver < 0
	if static {
		l.extStaticSyms[nameVer{name, ver}] = i
	} else {
		l.symsByName[ver][name] = i
	}
	return i
}

func (l *Loader) IsExternal(i Sym) bool {
	r, _ := l.toLocal(i)
	return l.isExtReader(r)
}

func (l *Loader) isExtReader(r *oReader) bool {
	return r == l.extReader
}

// For external symbol, return its index in the payloads array.
// XXX result is actually not a global index. We (ab)use the Sym type
// so we don't need conversion for accessing bitmaps.
func (l *Loader) extIndex(i Sym) Sym {
	_, li := l.toLocal(i)
	return Sym(li)
}

// Get a new payload for external symbol, return its index in
// the payloads array.
func (l *Loader) newPayload(name string, ver int) int {
	pi := len(l.payloads)
	pp := l.allocPayload()
	pp.name = name
	pp.ver = ver
	l.payloads = append(l.payloads, pp)
	l.growExtAttrBitmaps()
	return pi
}

// getPayload returns a pointer to the extSymPayload struct for an
// external symbol if the symbol has a payload. Will panic if the
// symbol in question is bogus (zero or not an external sym).
func (l *Loader) getPayload(i Sym) *extSymPayload {
	if !l.IsExternal(i) {
		panic(fmt.Sprintf("bogus symbol index %d in getPayload", i))
	}
	pi := l.extIndex(i)
	return l.payloads[pi]
}

// allocPayload allocates a new payload.
func (l *Loader) allocPayload() *extSymPayload {
	batch := l.payloadBatch
	if len(batch) == 0 {
		batch = make([]extSymPayload, 1000)
	}
	p := &batch[0]
	l.payloadBatch = batch[1:]
	return p
}

func (ms *extSymPayload) Grow(siz int64) {
	if int64(int(siz)) != siz {
		log.Fatalf("symgrow size %d too long", siz)
	}
	if int64(len(ms.data)) >= siz {
		return
	}
	if cap(ms.data) < int(siz) {
		cl := len(ms.data)
		ms.data = append(ms.data, make([]byte, int(siz)+1-cl)...)
		ms.data = ms.data[0:cl]
	}
	ms.data = ms.data[:siz]
}

// Ensure Syms slice has enough space.
func (l *Loader) growSyms(i int) {
	n := len(l.Syms)
	if n > i {
		return
	}
	l.Syms = append(l.Syms, make([]*sym.Symbol, i+1-n)...)
	l.growValues(int(i) + 1)
	l.growAttrBitmaps(int(i) + 1)
}

// Convert a local index to a global index.
func (l *Loader) toGlobal(r *oReader, i int) Sym {
	return r.syms[i]
}

// Convert a global index to a local index.
func (l *Loader) toLocal(i Sym) (*oReader, int) {
	return l.objSyms[i].r, int(l.objSyms[i].s)
}

// Resolve a local symbol reference. Return global index.
func (l *Loader) resolve(r *oReader, s goobj2.SymRef) Sym {
	var rr *oReader
	switch p := s.PkgIdx; p {
	case goobj2.PkgIdxInvalid:
		// {0, X} with non-zero X is never a valid sym reference from a Go object.
		// We steal this space for symbol references from external objects.
		// In this case, X is just the global index.
		if l.isExtReader(r) {
			return Sym(s.SymIdx)
		}
		if s.SymIdx != 0 {
			panic("bad sym ref")
		}
		return 0
	case goobj2.PkgIdxNone:
		i := int(s.SymIdx) + r.ndef
		return r.syms[i]
	case goobj2.PkgIdxBuiltin:
		return l.builtinSyms[s.SymIdx]
	case goobj2.PkgIdxSelf:
		rr = r
	default:
		pkg := r.Pkg(int(p))
		var ok bool
		rr, ok = l.objByPkg[pkg]
		if !ok {
			log.Fatalf("reference of nonexisted package %s, from %v", pkg, r.unit.Lib)
		}
	}
	return l.toGlobal(rr, int(s.SymIdx))
}

// Look up a symbol by name, return global index, or 0 if not found.
// This is more like Syms.ROLookup than Lookup -- it doesn't create
// new symbol.
func (l *Loader) Lookup(name string, ver int) Sym {
	if ver >= sym.SymVerStatic || ver < 0 {
		return l.extStaticSyms[nameVer{name, ver}]
	}
	return l.symsByName[ver][name]
}

// Check that duplicate symbols have same contents.
func (l *Loader) checkdup(name string, r *oReader, li int, dup Sym) {
	p := r.Data(li)
	rdup, ldup := l.toLocal(dup)
	pdup := rdup.Data(ldup)
	if bytes.Equal(p, pdup) {
		return
	}
	reason := "same length but different contents"
	if len(p) != len(pdup) {
		reason = fmt.Sprintf("new length %d != old length %d", len(p), len(pdup))
	}
	fmt.Fprintf(os.Stderr, "cmd/link: while reading object for '%v': duplicate symbol '%s', previous def at '%v', with mismatched payload: %s\n", r.unit.Lib, name, rdup.unit.Lib, reason)

	// For the moment, whitelist DWARF subprogram DIEs for
	// auto-generated wrapper functions. What seems to happen
	// here is that we get different line numbers on formal
	// params; I am guessing that the pos is being inherited
	// from the spot where the wrapper is needed.
	whitelist := strings.HasPrefix(name, "go.info.go.interface") ||
		strings.HasPrefix(name, "go.info.go.builtin") ||
		strings.HasPrefix(name, "go.debuglines")
	if !whitelist {
		l.strictDupMsgs++
	}
}

func (l *Loader) NStrictDupMsgs() int { return l.strictDupMsgs }

// Number of total symbols.
func (l *Loader) NSym() int {
	return len(l.objSyms)
}

// Number of defined Go symbols.
func (l *Loader) NDef() int {
	return int(l.extStart)
}

// Number of reachable symbols.
func (l *Loader) NReachableSym() int {
	return l.attrReachable.Count()
}

// Returns the raw (unpatched) name of the i-th symbol.
func (l *Loader) RawSymName(i Sym) string {
	if l.IsExternal(i) {
		pp := l.getPayload(i)
		return pp.name
	}
	r, li := l.toLocal(i)
	return r.Sym(li).Name(r.Reader)
}

// Returns the (patched) name of the i-th symbol.
func (l *Loader) SymName(i Sym) string {
	if l.IsExternal(i) {
		pp := l.getPayload(i)
		return pp.name
	}
	r, li := l.toLocal(i)
	return strings.Replace(r.Sym(li).Name(r.Reader), "\"\".", r.pkgprefix, -1)
}

// Returns the version of the i-th symbol.
func (l *Loader) SymVersion(i Sym) int {
	if l.IsExternal(i) {
		pp := l.getPayload(i)
		return pp.ver
	}
	r, li := l.toLocal(i)
	return int(abiToVer(r.Sym(li).ABI(), r.version))
}

// Returns the type of the i-th symbol.
func (l *Loader) SymType(i Sym) sym.SymKind {
	if l.IsExternal(i) {
		pp := l.getPayload(i)
		if pp != nil {
			return pp.kind
		}
		return 0
	}
	r, li := l.toLocal(i)
	return sym.AbiSymKindToSymKind[objabi.SymKind(r.Sym(li).Type())]
}

// Returns the attributes of the i-th symbol.
func (l *Loader) SymAttr(i Sym) uint8 {
	if l.IsExternal(i) {
		// TODO: do something? External symbols have different representation of attributes.
		// For now, ReflectMethod, NoSplit, GoType, and Typelink are used and they cannot be
		// set by external symbol.
		return 0
	}
	r, li := l.toLocal(i)
	return r.Sym(li).Flag()
}

// Returns the size of the i-th symbol.
func (l *Loader) SymSize(i Sym) int64 {
	if l.IsExternal(i) {
		pp := l.getPayload(i)
		return pp.size
	}
	r, li := l.toLocal(i)
	return int64(r.Sym(li).Siz())
}

// AttrReachable returns true for symbols that are transitively
// referenced from the entry points. Unreachable symbols are not
// written to the output.
func (l *Loader) AttrReachable(i Sym) bool {
	return l.attrReachable.Has(i)
}

// SetAttrReachable sets the reachability property for a symbol (see
// AttrReachable).
func (l *Loader) SetAttrReachable(i Sym, v bool) {
	if v {
		l.attrReachable.Set(i)
	} else {
		l.attrReachable.Unset(i)
	}
}

// AttrOnList returns true for symbols that are on some list (such as
// the list of all text symbols, or one of the lists of data symbols)
// and is consulted to avoid bugs where a symbol is put on a list
// twice.
func (l *Loader) AttrOnList(i Sym) bool {
	return l.attrOnList.Has(i)
}

// SetAttrOnList sets the "on list" property for a symbol (see
// AttrOnList).
func (l *Loader) SetAttrOnList(i Sym, v bool) {
	if v {
		l.attrOnList.Set(i)
	} else {
		l.attrOnList.Unset(i)
	}
}

// AttrLocal returns true for symbols that are only visible within the
// module (executable or shared library) being linked. This attribute
// is applied to thunks and certain other linker-generated symbols.
func (l *Loader) AttrLocal(i Sym) bool {
	return l.attrLocal.Has(i)
}

// SetAttrLocal the "local" property for a symbol (see AttrLocal above).
func (l *Loader) SetAttrLocal(i Sym, v bool) {
	if v {
		l.attrLocal.Set(i)
	} else {
		l.attrLocal.Unset(i)
	}
}

// SymAddr checks that a symbol is reachable, and returns its value.
func (l *Loader) SymAddr(i Sym) int64 {
	if !l.AttrReachable(i) {
		panic("unreachable symbol in symaddr")
	}
	return l.values[i]
}

// AttrNotInSymbolTable returns true for symbols that should not be
// added to the symbol table of the final generated load module.
func (l *Loader) AttrNotInSymbolTable(i Sym) bool {
	return l.attrNotInSymbolTable.Has(i)
}

// SetAttrNotInSymbolTable the "not in symtab" property for a symbol
// (see AttrNotInSymbolTable above).
func (l *Loader) SetAttrNotInSymbolTable(i Sym, v bool) {
	if v {
		l.attrNotInSymbolTable.Set(i)
	} else {
		l.attrNotInSymbolTable.Unset(i)
	}
}

// AttrVisibilityHidden symbols returns true for ELF symbols with
// visibility set to STV_HIDDEN. They become local symbols in
// the final executable. Only relevant when internally linking
// on an ELF platform.
func (l *Loader) AttrVisibilityHidden(i Sym) bool {
	if !l.IsExternal(i) {
		return false
	}
	return l.attrVisibilityHidden.Has(l.extIndex(i))
}

// SetAttrVisibilityHidden sets the "hidden visibility" property for a
// symbol (see AttrVisibilityHidden).
func (l *Loader) SetAttrVisibilityHidden(i Sym, v bool) {
	if !l.IsExternal(i) {
		panic("tried to set visibility attr on non-external symbol")
	}
	if v {
		l.attrVisibilityHidden.Set(l.extIndex(i))
	} else {
		l.attrVisibilityHidden.Unset(l.extIndex(i))
	}
}

// AttrDuplicateOK returns true for a symbol that can be present in
// multiple object files.
func (l *Loader) AttrDuplicateOK(i Sym) bool {
	if !l.IsExternal(i) {
		// TODO: if this path winds up being taken frequently, it
		// might make more sense to copy the flag value out of the object
		// into a larger bitmap during preload.
		r, li := l.toLocal(i)
		return r.Sym(li).Dupok()
	}
	return l.attrDuplicateOK.Has(l.extIndex(i))
}

// SetAttrDuplicateOK sets the "duplicate OK" property for an external
// symbol (see AttrDuplicateOK).
func (l *Loader) SetAttrDuplicateOK(i Sym, v bool) {
	if !l.IsExternal(i) {
		panic("tried to set dupok attr on non-external symbol")
	}
	if v {
		l.attrDuplicateOK.Set(l.extIndex(i))
	} else {
		l.attrDuplicateOK.Unset(l.extIndex(i))
	}
}

// AttrShared returns true for symbols compiled with the -shared option.
func (l *Loader) AttrShared(i Sym) bool {
	if !l.IsExternal(i) {
		// TODO: if this path winds up being taken frequently, it
		// might make more sense to copy the flag value out of the
		// object into a larger bitmap during preload.
		r, _ := l.toLocal(i)
		return (r.Flags() & goobj2.ObjFlagShared) != 0
	}
	return l.attrShared.Has(l.extIndex(i))
}

// SetAttrShared sets the "shared" property for an external
// symbol (see AttrShared).
func (l *Loader) SetAttrShared(i Sym, v bool) {
	if !l.IsExternal(i) {
		panic(fmt.Sprintf("tried to set shared attr on non-external symbol %d %s", i, l.SymName(i)))
	}
	if v {
		l.attrShared.Set(l.extIndex(i))
	} else {
		l.attrShared.Unset(l.extIndex(i))
	}
}

// AttrExternal returns true for function symbols loaded from host
// object files.
func (l *Loader) AttrExternal(i Sym) bool {
	if !l.IsExternal(i) {
		return false
	}
	return l.attrExternal.Has(l.extIndex(i))
}

// SetAttrExternal sets the "external" property for an host object
// symbol (see AttrExternal).
func (l *Loader) SetAttrExternal(i Sym, v bool) {
	if !l.IsExternal(i) {
		panic(fmt.Sprintf("tried to set external attr on non-external symbol %q", l.RawSymName(i)))
	}
	if v {
		l.attrExternal.Set(l.extIndex(i))
	} else {
		l.attrExternal.Unset(l.extIndex(i))
	}
}

// AttrTopFrame returns true for a function symbol that is an entry
// point, meaning that unwinders should stop when they hit this
// function.
func (l *Loader) AttrTopFrame(i Sym) bool {
	_, ok := l.attrTopFrame[i]
	return ok
}

// SetAttrTopFrame sets the "top frame" property for a symbol (see
// AttrTopFrame).
func (l *Loader) SetAttrTopFrame(i Sym, v bool) {
	if v {
		l.attrTopFrame[i] = struct{}{}
	} else {
		delete(l.attrTopFrame, i)
	}
}

// AttrSpecial returns true for a symbols that do not have their
// address (i.e. Value) computed by the usual mechanism of
// data.go:dodata() & data.go:address().
func (l *Loader) AttrSpecial(i Sym) bool {
	_, ok := l.attrSpecial[i]
	return ok
}

// SetAttrSpecial sets the "special" property for a symbol (see
// AttrSpecial).
func (l *Loader) SetAttrSpecial(i Sym, v bool) {
	if v {
		l.attrSpecial[i] = struct{}{}
	} else {
		delete(l.attrSpecial, i)
	}
}

// AttrCgoExportDynamic returns true for a symbol that has been
// specially marked via the "cgo_export_dynamic" compiler directive
// written by cgo (in response to //export directives in the source).
func (l *Loader) AttrCgoExportDynamic(i Sym) bool {
	_, ok := l.attrCgoExportDynamic[i]
	return ok
}

// SetAttrCgoExportDynamic sets the "cgo_export_dynamic" for a symbol
// (see AttrCgoExportDynamic).
func (l *Loader) SetAttrCgoExportDynamic(i Sym, v bool) {
	if v {
		l.attrCgoExportDynamic[i] = struct{}{}
	} else {
		delete(l.attrCgoExportDynamic, i)
	}
}

// AttrCgoExportStatic returns true for a symbol that has been
// specially marked via the "cgo_export_static" directive
// written by cgo.
func (l *Loader) AttrCgoExportStatic(i Sym) bool {
	_, ok := l.attrCgoExportStatic[i]
	return ok
}

// SetAttrCgoExportStatic sets the "cgo_export_static" for a symbol
// (see AttrCgoExportStatic).
func (l *Loader) SetAttrCgoExportStatic(i Sym, v bool) {
	if v {
		l.attrCgoExportStatic[i] = struct{}{}
	} else {
		delete(l.attrCgoExportStatic, i)
	}
}

func (l *Loader) AttrCgoExport(i Sym) bool {
	return l.AttrCgoExportDynamic(i) || l.AttrCgoExportStatic(i)
}

// AttrReadOnly returns true for a symbol whose underlying data
// is stored via a read-only mmap.
func (l *Loader) AttrReadOnly(i Sym) bool {
	if v, ok := l.attrReadOnly[i]; ok {
		return v
	}
	if l.IsExternal(i) {
		pp := l.getPayload(i)
		if pp.objidx != 0 {
			return l.objs[pp.objidx].r.ReadOnly()
		}
		return false
	}
	r, _ := l.toLocal(i)
	return r.ReadOnly()
}

// SetAttrReadOnly sets the "data is read only" property for a symbol
// (see AttrReadOnly).
func (l *Loader) SetAttrReadOnly(i Sym, v bool) {
	l.attrReadOnly[i] = v
}

// AttrSubSymbol returns true for symbols that are listed as a
// sub-symbol of some other outer symbol. The sub/outer mechanism is
// used when loading host objects (sections from the host object
// become regular linker symbols and symbols go on the Sub list of
// their section) and for constructing the global offset table when
// internally linking a dynamic executable.
//
// Note that in later stages of the linker, we set Outer(S) to some