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/goobj"
	"cmd/internal/obj"
	"cmd/internal/objabi"
	"cmd/internal/sys"
	"cmd/link/internal/sym"
	"debug/elf"
	"fmt"
	"internal/abi"
	"io"
	"log"
	"math/bits"
	"os"
	"sort"
	"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 = sym.LoaderSym

// 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 []goobj.Reloc

	li uint32   // local index of symbol whose relocs we're examining
	r  *oReader // object reader for containing package
	l  *Loader  // loader
}

// ExtReloc contains the payload for an external relocation.
type ExtReloc struct {
	Xsym Sym
	Xadd int64
	Type objabi.RelocType
	Size uint8
}

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

func (rel Reloc) Type() objabi.RelocType     { return objabi.RelocType(rel.Reloc.Type()) &^ objabi.R_WEAK }
func (rel Reloc) Weak() bool                 { return objabi.RelocType(rel.Reloc.Type())&objabi.R_WEAK != 0 }
func (rel Reloc) SetType(t objabi.RelocType) { rel.Reloc.SetType(uint16(t)) }
func (rel Reloc) Sym() Sym                   { return rel.l.resolve(rel.r, rel.Reloc.Sym()) }
func (rel Reloc) SetSym(s Sym)               { rel.Reloc.SetSym(goobj.SymRef{PkgIdx: 0, SymIdx: uint32(s)}) }
func (rel Reloc) IsMarker() bool             { return rel.Siz() == 0 }

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

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

// oReader is a wrapper type of obj.Reader, along with some
// extra information.
type oReader struct {
	*goobj.Reader
	unit         *sym.CompilationUnit
	version      int // version of static symbol
	pkgprefix    string
	syms         []Sym    // Sym's global index, indexed by local index
	pkg          []uint32 // indices of referenced package by PkgIdx (index into loader.objs array)
	ndef         int      // cache goobj.Reader.NSym()
	nhashed64def int      // cache goobj.Reader.NHashed64Def()
	nhasheddef   int      // cache goobj.Reader.NHashedDef()
	objidx       uint32   // index of this reader in the objs slice
}

// Total number of defined symbols (package symbols, hashed symbols, and
// non-package symbols).
func (r *oReader) NAlldef() int { return r.ndef + r.nhashed64def + r.nhasheddef + r.NNonpkgdef() }

// objSym represents a symbol in an object file. It is a tuple of
// the object and the symbol's local index.
// For external symbols, objidx is the index of l.extReader (extObj),
// s is its index into the payload array.
// {0, 0} represents the nil symbol.
type objSym struct {
	objidx uint32 // index of the object (in l.objs array)
	s      uint32 // 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
}

type symAndSize struct {
	sym  Sym
	size uint32
}

// 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.
//
//   - 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 {
	objs        []*oReader
	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

	align []uint8 // symbol 2^N alignment, indexed by global index

	deferReturnTramp map[Sym]bool // whether the symbol is a trampoline of a deferreturn call

	objByPkg map[string]uint32 // map package path to the index of its Go object reader

	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 global idx
	attrUsedInIface      Bitmap // "used in interface" symbols, indexed by global idx
	attrSpecial          Bitmap // "special" frame symbols, indexed by global 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
	generatedSyms        Bitmap // symbols that generate their content, indexed by ext sym idx

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

	// Outer and Sub relations for symbols.
	outer []Sym // indexed by global index
	sub   map[Sym]Sym

	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

	// CgoExports records cgo-exported symbols by SymName.
	CgoExports map[string]Sym

	WasmExports []Sym

	flags uint32

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

	errorReporter *ErrorReporter

	npkgsyms    int // number of package symbols, for accounting
	nhashedsyms int // number of hashed symbols, for accounting
}

const (
	pkgDef = iota
	hashed64Def
	hashedDef
	nonPkgDef
	nonPkgRef
)

// objidx
const (
	nilObj = iota
	extObj
	goObjStart
)

// 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
	relocs []goobj.Reloc
	data   []byte
	auxs   []goobj.Aux
}

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

func NewLoader(flags uint32, reporter *ErrorReporter) *Loader {
	nbuiltin := goobj.NBuiltin()
	extReader := &oReader{objidx: extObj}
	ldr := &Loader{
		objs:                 []*oReader{nil, extReader}, // reserve index 0 for nil symbol, 1 for external symbols
		objSyms:              make([]objSym, 1, 1),       // This will get overwritten later.
		extReader:            extReader,
		symsByName:           [2]map[string]Sym{make(map[string]Sym, 80000), make(map[string]Sym, 50000)}, // preallocate ~2MB for ABI0 and ~1MB for ABI1 symbols
		objByPkg:             make(map[string]uint32),
		sub:                  make(map[Sym]Sym),
		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),
		attrCgoExportDynamic: make(map[Sym]struct{}),
		attrCgoExportStatic:  make(map[Sym]struct{}),
		deferReturnTramp:     make(map[Sym]bool),
		extStaticSyms:        make(map[nameVer]Sym),
		builtinSyms:          make([]Sym, nbuiltin),
		flags:                flags,
		errorReporter:        reporter,
		sects:                []*sym.Section{nil}, // reserve index 0 for nil section
	}
	reporter.ldr = ldr
	return ldr
}

// Add object file r
func (l *Loader) addObj(pkg string, r *oReader) {
	pkg = objabi.PathToPrefix(pkg) // the object file contains escaped package path
	if _, ok := l.objByPkg[pkg]; !ok {
		l.objByPkg[pkg] = r.objidx
	}
	l.objs = append(l.objs, r)
}

// Add a symbol from an object file, return the global index.
// If the symbol already exist, it returns the index of that symbol.
func (st *loadState) addSym(name string, ver int, r *oReader, li uint32, kind int, osym *goobj.Sym) Sym {
	l := st.l
	if l.extStart != 0 {
		panic("addSym called after external symbol is created")
	}
	i := Sym(len(l.objSyms))
	if int(i) != len(l.objSyms) { // overflow
		panic("too many symbols")
	}
	addToGlobal := func() {
		l.objSyms = append(l.objSyms, objSym{r.objidx, li})
	}
	if name == "" && kind != hashed64Def && kind != hashedDef {
		addToGlobal()
		return i // 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
	}
	switch kind {
	case 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
	case hashed64Def, hashedDef:
		// Hashed (content-addressable) symbol. Check the hash
		// but don't add to name lookup table, as they are not
		// referenced by name. Also no need to do overwriting
		// check, as same hash indicates same content.
		var checkHash func() (symAndSize, bool)
		var addToHashMap func(symAndSize)
		var h64 uint64        // only used for hashed64Def
		var h *goobj.HashType // only used for hashedDef
		if kind == hashed64Def {
			checkHash = func() (symAndSize, bool) {
				h64 = r.Hash64(li - uint32(r.ndef))
				s, existed := st.hashed64Syms[h64]
				return s, existed
			}
			addToHashMap = func(ss symAndSize) { st.hashed64Syms[h64] = ss }
		} else {
			checkHash = func() (symAndSize, bool) {
				h = r.Hash(li - uint32(r.ndef+r.nhashed64def))
				s, existed := st.hashedSyms[*h]
				return s, existed
			}
			addToHashMap = func(ss symAndSize) { st.hashedSyms[*h] = ss }
		}
		siz := osym.Siz()
		if s, existed := checkHash(); existed {
			// The content hash is built from symbol data and relocations. In the
			// object file, the symbol data may not always contain trailing zeros,
			// e.g. for [5]int{1,2,3} and [100]int{1,2,3}, the data is same
			// (although the size is different).
			// Also, for short symbols, the content hash is the identity function of
			// the 8 bytes, and trailing zeros doesn't change the hash value, e.g.
			// hash("A") == hash("A\0\0\0").
			// So when two symbols have the same hash, we need to use the one with
			// larger size.
			if siz > s.size {
				// New symbol has larger size, use the new one. Rewrite the index mapping.
				l.objSyms[s.sym] = objSym{r.objidx, li}
				addToHashMap(symAndSize{s.sym, siz})
			}
			return s.sym
		}
		addToHashMap(symAndSize{i, siz})
		addToGlobal()
		return i
	}

	// 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
	}
	// symbol already exists
	// Fix for issue #47185 -- given two dupok or BSS symbols with
	// different sizes, favor symbol with larger size. See also
	// issue #46653 and #72032.
	oldsz := l.SymSize(oldi)
	sz := int64(r.Sym(li).Siz())
	if osym.Dupok() {
		if l.flags&FlagStrictDups != 0 {
			l.checkdup(name, r, li, oldi)
		}
		if oldsz < sz {
			// new symbol overwrites old symbol.
			l.objSyms[oldi] = objSym{r.objidx, li}
		}
		return oldi
	}
	oldr, oldli := l.toLocal(oldi)
	oldsym := oldr.Sym(oldli)
	if oldsym.Dupok() {
		return oldi
	}
	// If one is a DATA symbol (i.e. has content, DataSize != 0)
	// and the other is BSS, the one with content wins.
	// If both are BSS, the one with larger size wins.
	// Specifically, the "overwrite" variable and the final result are
	//
	// new sym       old sym       overwrite
	// ---------------------------------------------
	// DATA          DATA          true  => ERROR
	// DATA lg/eq    BSS  sm/eq    true  => new wins
	// DATA small    BSS  large    true  => ERROR
	// BSS  large    DATA small    true  => ERROR
	// BSS  large    BSS  small    true  => new wins
	// BSS  sm/eq    D/B  lg/eq    false => old wins
	overwrite := r.DataSize(li) != 0 || oldsz < sz
	if overwrite {
		// new symbol overwrites old symbol.
		oldtyp := sym.AbiSymKindToSymKind[objabi.SymKind(oldsym.Type())]
		if !(oldtyp.IsData() && oldr.DataSize(oldli) == 0) || oldsz > sz {
			log.Fatalf("duplicated definition of symbol %s, from %s and %s", name, r.unit.Lib.Pkg, oldr.unit.Lib.Pkg)
		}
		l.objSyms[oldi] = objSym{r.objidx, li}
	} else {
		// old symbol overwrites new symbol.
		typ := sym.AbiSymKindToSymKind[objabi.SymKind(oldsym.Type())]
		if !typ.IsData() { // only allow overwriting data symbol
			log.Fatalf("duplicated definition of symbol %s, from %s and %s", name, r.unit.Lib.Pkg, oldr.unit.Lib.Pkg)
		}
	}
	return oldi
}

// 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 int(i) != len(l.objSyms) { // overflow
		panic("too many symbols")
	}
	if l.extStart == 0 {
		l.extStart = i
	}
	l.growValues(int(i) + 1)
	l.growOuter(int(i) + 1)
	l.growAttrBitmaps(int(i) + 1)
	pi := l.newPayload(name, ver)
	l.objSyms = append(l.objSyms, objSym{l.extReader.objidx, uint32(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
}

// AddCgoExport records a cgo-exported symbol in l.CgoExports.
// This table is used to identify the correct Go symbol ABI to use
// to resolve references from host objects (which don't have ABIs).
func (l *Loader) AddCgoExport(s Sym) {
	if l.CgoExports == nil {
		l.CgoExports = make(map[string]Sym)
	}
	l.CgoExports[l.SymName(s)] = s
}

// LookupOrCreateCgoExport is like LookupOrCreateSym, but if ver
// indicates a global symbol, it uses the CgoExport table to determine
// the appropriate symbol version (ABI) to use. ver must be either 0
// or a static symbol version.
func (l *Loader) LookupOrCreateCgoExport(name string, ver int) Sym {
	if ver >= sym.SymVerStatic {
		return l.LookupOrCreateSym(name, ver)
	}
	if ver != 0 {
		panic("ver must be 0 or a static version")
	}
	// Look for a cgo-exported symbol from Go.
	if s, ok := l.CgoExports[name]; ok {
		return s
	}
	// Otherwise, this must just be a symbol in the host object.
	// Create a version 0 symbol for it.
	return l.LookupOrCreateSym(name, 0)
}

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]
}

// Convert a local index to a global index.
func (l *Loader) toGlobal(r *oReader, i uint32) Sym {
	return r.syms[i]
}

// Convert a global index to a local index.
func (l *Loader) toLocal(i Sym) (*oReader, uint32) {
	return l.objs[l.objSyms[i].objidx], l.objSyms[i].s
}

// Resolve a local symbol reference. Return global index.
func (l *Loader) resolve(r *oReader, s goobj.SymRef) Sym {
	var rr *oReader
	switch p := s.PkgIdx; p {
	case goobj.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 goobj.PkgIdxHashed64:
		i := int(s.SymIdx) + r.ndef
		return r.syms[i]
	case goobj.PkgIdxHashed:
		i := int(s.SymIdx) + r.ndef + r.nhashed64def
		return r.syms[i]
	case goobj.PkgIdxNone:
		i := int(s.SymIdx) + r.ndef + r.nhashed64def + r.nhasheddef
		return r.syms[i]
	case goobj.PkgIdxBuiltin:
		if bi := l.builtinSyms[s.SymIdx]; bi != 0 {
			return bi
		}
		l.reportMissingBuiltin(int(s.SymIdx), r.unit.Lib.Pkg)
		return 0
	case goobj.PkgIdxSelf:
		rr = r
	default:
		rr = l.objs[r.pkg[p]]
	}
	return l.toGlobal(rr, s.SymIdx)
}

// reportMissingBuiltin issues an error in the case where we have a
// relocation against a runtime builtin whose definition is not found
// when the runtime package is built. The canonical example is
// "runtime.racefuncenter" -- currently if you do something like
//
//	go build -gcflags=-race myprogram.go
//
// the compiler will insert calls to the builtin runtime.racefuncenter,
// but the version of the runtime used for linkage won't actually contain
// definitions of that symbol. See issue #42396 for details.
//
// As currently implemented, this is a fatal error. This has drawbacks
// in that if there are multiple missing builtins, the error will only
// cite the first one. On the plus side, terminating the link here has
// advantages in that we won't run the risk of panics or crashes later
// on in the linker due to R_CALL relocations with 0-valued target
// symbols.
func (l *Loader) reportMissingBuiltin(bsym int, reflib string) {
	bname, _ := goobj.BuiltinName(bsym)
	log.Fatalf("reference to undefined builtin %q from package %q",
		bname, reflib)
}

// 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 uint32, dup Sym) {
	p := r.Data(li)
	rdup, ldup := l.toLocal(dup)
	pdup := rdup.Data(ldup)
	reason := "same length but different contents"
	if len(p) != len(pdup) {
		reason = fmt.Sprintf("new length %d != old length %d", len(p), len(pdup))
	} else if bytes.Equal(p, pdup) {
		// For BSS symbols, we need to check size as well, see issue 46653.
		szdup := l.SymSize(dup)
		sz := int64(r.Sym(li).Siz())
		if szdup == sz {
			return
		}
		reason = fmt.Sprintf("different sizes: new size %d != old size %d",
			sz, szdup)
	}
	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, allow 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.
	allowed := strings.HasPrefix(name, "go:info.go.interface") ||
		strings.HasPrefix(name, "go:info.go.builtin") ||
		strings.HasPrefix(name, "go:debuglines")
	if !allowed {
		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 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)
	if r == nil {
		return "?"
	}
	return r.Sym(li).Name(r.Reader)
}

// 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))
}

func (l *Loader) IsFileLocal(i Sym) bool {
	return l.SymVersion(i) >= sym.SymVerStatic
}

// IsFromAssembly returns true if this symbol is derived from an
// object file generated by the Go assembler.
func (l *Loader) IsFromAssembly(i Sym) bool {
	if l.IsExternal(i) {
		pp := l.getPayload(i)
		if pp.objidx != 0 {
			r := l.objs[pp.objidx]
			return r.FromAssembly()
		}
		return false
	}
	r, _ := l.toLocal(i)
	return r.FromAssembly()
}

// 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)
	}
}

// AttrUsedInIface returns true for a type symbol that is used in
// an interface.
func (l *Loader) AttrUsedInIface(i Sym) bool {
	return l.attrUsedInIface.Has(i)
}

func (l *Loader) SetAttrUsedInIface(i Sym, v bool) {
	if v {
		l.attrUsedInIface.Set(i)
	} else {
		l.attrUsedInIface.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.Shared()
	}
	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 a host object