src/cmd/internal/goobj/read.go - go - Git at Google

 // Copyright 2013 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 goobj implements reading of Go object files and archives.
 //
 // TODO(rsc): Decide where this package should live. (golang.org/issue/6932)
 // TODO(rsc): Decide the appropriate integer types for various fields.
 package goobj

 import (
 	"bufio"
 	"bytes"
 	"cmd/internal/objabi"
 	"errors"
 	"fmt"
 	"io"
 	"os"
 	"strconv"
 	"strings"
 )

 // A Sym is a named symbol in an object file.
 type Sym struct {
 	SymID                // symbol identifier (name and version)
 	Kind  objabi.SymKind // kind of symbol
 	DupOK bool           // are duplicate definitions okay?
 	Size  int64          // size of corresponding data
 	Type  SymID          // symbol for Go type information
 	Data  Data           // memory image of symbol
 	Reloc []Reloc        // relocations to apply to Data
 	Func  *Func          // additional data for functions
 }

 // A SymID - the combination of Name and Version - uniquely identifies
 // a symbol within a package.
 type SymID struct {
 	// Name is the name of a symbol.
 	Name string

 	// Version is zero for symbols with global visibility.
 	// Symbols with only file visibility (such as file-level static
 	// declarations in C) have a non-zero version distinguishing
 	// a symbol in one file from a symbol of the same name
 	// in another file
 	Version int64
 }

 func (s SymID) String() string {
 	if s.Version == 0 {
 		return s.Name
 	}
 	return fmt.Sprintf("%s<%d>", s.Name, s.Version)
 }

 // A Data is a reference to data stored in an object file.
 // It records the offset and size of the data, so that a client can
 // read the data only if necessary.
 type Data struct {
 	Offset int64
 	Size   int64
 }

 // A Reloc describes a relocation applied to a memory image to refer
 // to an address within a particular symbol.
 type Reloc struct {
 	// The bytes at [Offset, Offset+Size) within the containing Sym
 	// should be updated to refer to the address Add bytes after the start
 	// of the symbol Sym.
 	Offset int64
 	Size   int64
 	Sym    SymID
 	Add    int64

 	// The Type records the form of address expected in the bytes
 	// described by the previous fields: absolute, PC-relative, and so on.
 	// TODO(rsc): The interpretation of Type is not exposed by this package.
 	Type objabi.RelocType
 }

 // A Var describes a variable in a function stack frame: a declared
 // local variable, an input argument, or an output result.
 type Var struct {
 	// The combination of Name, Kind, and Offset uniquely
 	// identifies a variable in a function stack frame.
 	// Using fewer of these - in particular, using only Name - does not.
 	Name   string // Name of variable.
 	Kind   int64  // TODO(rsc): Define meaning.
 	Offset int64  // Frame offset. TODO(rsc): Define meaning.

 	Type SymID // Go type for variable.
 }

 // Func contains additional per-symbol information specific to functions.
 type Func struct {
 	Args     int64      // size in bytes of argument frame: inputs and outputs
 	Frame    int64      // size in bytes of local variable frame
 	Leaf     bool       // function omits save of link register (ARM)
 	NoSplit  bool       // function omits stack split prologue
 	TopFrame bool       // function is the top of the call stack
 	Var      []Var      // detail about local variables
 	PCSP     Data       // PC → SP offset map
 	PCFile   Data       // PC → file number map (index into File)
 	PCLine   Data       // PC → line number map
 	PCInline Data       // PC → inline tree index map
 	PCData   []Data     // PC → runtime support data map
 	FuncData []FuncData // non-PC-specific runtime support data
 	File     []string   // paths indexed by PCFile
 	InlTree  []InlinedCall
 }

 // TODO: Add PCData []byte and PCDataIter (similar to liblink).

 // A FuncData is a single function-specific data value.
 type FuncData struct {
 	Sym    SymID // symbol holding data
 	Offset int64 // offset into symbol for funcdata pointer
 }

 // An InlinedCall is a node in an InlTree.
 // See cmd/internal/obj.InlTree for details.
 type InlinedCall struct {
 	Parent   int64
 	File     string
 	Line     int64
 	Func     SymID
 	ParentPC int64
 }

 // A Package is a parsed Go object file or archive defining a Go package.
 type Package struct {
 	ImportPath    string          // import path denoting this package
 	Imports       []string        // packages imported by this package
 	SymRefs       []SymID         // list of symbol names and versions referred to by this pack
 	Syms          []*Sym          // symbols defined by this package
 	MaxVersion    int64           // maximum Version in any SymID in Syms
 	Arch          string          // architecture
 	Native        []*NativeReader // native object data (e.g. ELF)
 	DWARFFileList []string        // List of files for the DWARF .debug_lines section
 }

 type NativeReader struct {
 	Name string
 	io.ReaderAt
 }

 var (
 	archiveHeader = []byte("!<arch>\n")
 	archiveMagic  = []byte("`\n")
 	goobjHeader   = []byte("go objec") // truncated to size of archiveHeader

 	errCorruptArchive   = errors.New("corrupt archive")
 	errTruncatedArchive = errors.New("truncated archive")
 	errCorruptObject    = errors.New("corrupt object file")
 	errNotObject        = errors.New("unrecognized object file format")
 )

 // An objReader is an object file reader.
 type objReader struct {
 	p          *Package
 	b          *bufio.Reader
 	f          *os.File
 	err        error
 	offset     int64
 	dataOffset int64
 	limit      int64
 	tmp        [256]byte
 	pkgprefix  string
 }

 // init initializes r to read package p from f.
 func (r *objReader) init(f *os.File, p *Package) {
 	r.f = f
 	r.p = p
 	r.offset, _ = f.Seek(0, io.SeekCurrent)
 	r.limit, _ = f.Seek(0, io.SeekEnd)
 	f.Seek(r.offset, io.SeekStart)
 	r.b = bufio.NewReader(f)
 	r.pkgprefix = objabi.PathToPrefix(p.ImportPath) + "."
 }

 // error records that an error occurred.
 // It returns only the first error, so that an error
 // caused by an earlier error does not discard information
 // about the earlier error.
 func (r *objReader) error(err error) error {
 	if r.err == nil {
 		if err == io.EOF {
 			err = io.ErrUnexpectedEOF
 		}
 		r.err = err
 	}
 	// panic("corrupt") // useful for debugging
 	return r.err
 }

 // peek returns the next n bytes without advancing the reader.
 func (r *objReader) peek(n int) ([]byte, error) {
 	if r.err != nil {
 		return nil, r.err
 	}
 	if r.offset >= r.limit {
 		r.error(io.ErrUnexpectedEOF)
 		return nil, r.err
 	}
 	b, err := r.b.Peek(n)
 	if err != nil {
 		if err != bufio.ErrBufferFull {
 			r.error(err)
 		}
 	}
 	return b, err
 }

 // readByte reads and returns a byte from the input file.
 // On I/O error or EOF, it records the error but returns byte 0.
 // A sequence of 0 bytes will eventually terminate any
 // parsing state in the object file. In particular, it ends the
 // reading of a varint.
 func (r *objReader) readByte() byte {
 	if r.err != nil {
 		return 0
 	}
 	if r.offset >= r.limit {
 		r.error(io.ErrUnexpectedEOF)
 		return 0
 	}
 	b, err := r.b.ReadByte()
 	if err != nil {
 		if err == io.EOF {
 			err = io.ErrUnexpectedEOF
 		}
 		r.error(err)
 		b = 0
 	} else {
 		r.offset++
 	}
 	return b
 }

 // read reads exactly len(b) bytes from the input file.
 // If an error occurs, read returns the error but also
 // records it, so it is safe for callers to ignore the result
 // as long as delaying the report is not a problem.
 func (r *objReader) readFull(b []byte) error {
 	if r.err != nil {
 		return r.err
 	}
 	if r.offset+int64(len(b)) > r.limit {
 		return r.error(io.ErrUnexpectedEOF)
 	}
 	n, err := io.ReadFull(r.b, b)
 	r.offset += int64(n)
 	if err != nil {
 		return r.error(err)
 	}
 	return nil
 }

 // readInt reads a zigzag varint from the input file.
 func (r *objReader) readInt() int64 {
 	var u uint64

 	for shift := uint(0); ; shift += 7 {
 		if shift >= 64 {
 			r.error(errCorruptObject)
 			return 0
 		}
 		c := r.readByte()
 		u |= uint64(c&0x7F) << shift
 		if c&0x80 == 0 {
 			break
 		}
 	}

 	return int64(u>>1) ^ (int64(u) << 63 >> 63)
 }

 // readString reads a length-delimited string from the input file.
 func (r *objReader) readString() string {
 	n := r.readInt()
 	buf := make([]byte, n)
 	r.readFull(buf)
 	return string(buf)
 }

 // readSymID reads a SymID from the input file.
 func (r *objReader) readSymID() SymID {
 	i := r.readInt()
 	return r.p.SymRefs[i]
 }

 func (r *objReader) readRef() {
 	name, abiOrStatic := r.readString(), r.readInt()

 	// In a symbol name in an object file, "". denotes the
 	// prefix for the package in which the object file has been found.
 	// Expand it.
 	name = strings.ReplaceAll(name, `"".`, r.pkgprefix)

 	// The ABI field records either the ABI or -1 for static symbols.
 	//
 	// To distinguish different static symbols with the same name,
 	// we use the symbol "version". Version 0 corresponds to
 	// global symbols, and each file has a unique version > 0 for
 	// all of its static symbols. The version is incremented on
 	// each call to parseObject.
 	//
 	// For global symbols, we currently ignore the ABI.
 	//
 	// TODO(austin): Record the ABI in SymID. Since this is a
 	// public API, we'll have to keep Version as 0 and record the
 	// ABI in a new field (which differs from how the linker does
 	// this, but that's okay). Show the ABI in things like
 	// objdump.
 	var vers int64
 	if abiOrStatic == -1 {
 		// Static symbol
 		vers = r.p.MaxVersion
 	}
 	r.p.SymRefs = append(r.p.SymRefs, SymID{name, vers})
 }

 // readData reads a data reference from the input file.
 func (r *objReader) readData() Data {
 	n := r.readInt()
 	d := Data{Offset: r.dataOffset, Size: n}
 	r.dataOffset += n
 	return d
 }

 // skip skips n bytes in the input.
 func (r *objReader) skip(n int64) {
 	if n < 0 {
 		r.error(fmt.Errorf("debug/goobj: internal error: misuse of skip"))
 	}
 	if n < int64(len(r.tmp)) {
 		// Since the data is so small, a just reading from the buffered
 		// reader is better than flushing the buffer and seeking.
 		r.readFull(r.tmp[:n])
 	} else if n <= int64(r.b.Buffered()) {
 		// Even though the data is not small, it has already been read.
 		// Advance the buffer instead of seeking.
 		for n > int64(len(r.tmp)) {
 			r.readFull(r.tmp[:])
 			n -= int64(len(r.tmp))
 		}
 		r.readFull(r.tmp[:n])
 	} else {
 		// Seek, giving up buffered data.
 		_, err := r.f.Seek(r.offset+n, io.SeekStart)
 		if err != nil {
 			r.error(err)
 		}
 		r.offset += n
 		r.b.Reset(r.f)
 	}
 }

 // Parse parses an object file or archive from f,
 // assuming that its import path is pkgpath.
 func Parse(f *os.File, pkgpath string) (*Package, error) {
 	if pkgpath == "" {
 		pkgpath = `""`
 	}
 	p := new(Package)
 	p.ImportPath = pkgpath

 	var rd objReader
 	rd.init(f, p)
 	err := rd.readFull(rd.tmp[:8])
 	if err != nil {
 		if err == io.EOF {
 			err = io.ErrUnexpectedEOF
 		}
 		return nil, err
 	}

 	switch {
 	default:
 		return nil, errNotObject

 	case bytes.Equal(rd.tmp[:8], archiveHeader):
 		if err := rd.parseArchive(); err != nil {
 			return nil, err
 		}
 	case bytes.Equal(rd.tmp[:8], goobjHeader):
 		if err := rd.parseObject(goobjHeader); err != nil {
 			return nil, err
 		}
 	}

 	return p, nil
 }

 // trimSpace removes trailing spaces from b and returns the corresponding string.
 // This effectively parses the form used in archive headers.
 func trimSpace(b []byte) string {
 	return string(bytes.TrimRight(b, " "))
 }

 // parseArchive parses a Unix archive of Go object files.
 func (r *objReader) parseArchive() error {
 	for r.offset < r.limit {
 		if err := r.readFull(r.tmp[:60]); err != nil {
 			return err
 		}
 		data := r.tmp[:60]

 		// Each file is preceded by this text header (slice indices in first column):
 		//	 0:16	name
 		//	16:28 date
 		//	28:34 uid
 		//	34:40 gid
 		//	40:48 mode
 		//	48:58 size
 		//	58:60 magic - `\n
 		// We only care about name, size, and magic.
 		// The fields are space-padded on the right.
 		// The size is in decimal.
 		// The file data - size bytes - follows the header.
 		// Headers are 2-byte aligned, so if size is odd, an extra padding
 		// byte sits between the file data and the next header.
 		// The file data that follows is padded to an even number of bytes:
 		// if size is odd, an extra padding byte is inserted betw the next header.
 		if len(data) < 60 {
 			return errTruncatedArchive
 		}
 		if !bytes.Equal(data[58:60], archiveMagic) {
 			return errCorruptArchive
 		}
 		name := trimSpace(data[0:16])
 		size, err := strconv.ParseInt(trimSpace(data[48:58]), 10, 64)
 		if err != nil {
 			return errCorruptArchive
 		}
 		data = data[60:]
 		fsize := size + size&1
 		if fsize < 0 || fsize < size {
 			return errCorruptArchive
 		}
 		switch name {
 		case "__.PKGDEF":
 			r.skip(size)
 		default:
 			oldLimit := r.limit
 			r.limit = r.offset + size

 			p, err := r.peek(8)
 			if err != nil {
 				return err
 			}
 			if bytes.Equal(p, goobjHeader) {
 				if err := r.parseObject(nil); err != nil {
 					return fmt.Errorf("parsing archive member %q: %v", name, err)
 				}
 			} else {
 				r.p.Native = append(r.p.Native, &NativeReader{
 					Name:     name,
 					ReaderAt: io.NewSectionReader(r.f, r.offset, size),
 				})
 			}

 			r.skip(r.limit - r.offset)
 			r.limit = oldLimit
 		}
 		if size&1 != 0 {
 			r.skip(1)
 		}
 	}
 	return nil
 }

 // parseObject parses a single Go object file.
 // The prefix is the bytes already read from the file,
 // typically in order to detect that this is an object file.
 // The object file consists of a textual header ending in "\n!\n"
 // and then the part we want to parse begins.
 // The format of that part is defined in a comment at the top
 // of src/liblink/objfile.c.
 func (r *objReader) parseObject(prefix []byte) error {
 	r.p.MaxVersion++
 	h := make([]byte, 0, 256)
 	h = append(h, prefix...)
 	var c1, c2, c3 byte
 	for {
 		c1, c2, c3 = c2, c3, r.readByte()
 		h = append(h, c3)
 		// The new export format can contain 0 bytes.
 		// Don't consider them errors, only look for r.err != nil.
 		if r.err != nil {
 			return errCorruptObject
 		}
 		if c1 == '\n' && c2 == '!' && c3 == '\n' {
 			break
 		}
 	}

 	hs := strings.Fields(string(h))
 	if len(hs) >= 4 {
 		r.p.Arch = hs[3]
 	}
 	// TODO: extract OS + build ID if/when we need it

 	p, err := r.peek(8)
 	if err != nil {
 		return err
 	}
 	if bytes.Equal(p, []byte("\x00go114LD")) {
 		r.readNew()
 		return nil
 	}
 	r.readFull(r.tmp[:8])
 	if !bytes.Equal(r.tmp[:8], []byte("\x00go114ld")) {
 		return r.error(errCorruptObject)
 	}

 	b := r.readByte()
 	if b != 1 {
 		return r.error(errCorruptObject)
 	}

 	// Direct package dependencies.
 	for {
 		s := r.readString()
 		if s == "" {
 			break
 		}
 		r.p.Imports = append(r.p.Imports, s)
 	}

 	// Read filenames for dwarf info.
 	count := r.readInt()
 	for i := int64(0); i < count; i++ {
 		r.p.DWARFFileList = append(r.p.DWARFFileList, r.readString())
 	}

 	r.p.SymRefs = []SymID{{"", 0}}
 	for {
 		if b := r.readByte(); b != 0xfe {
 			if b != 0xff {
 				return r.error(errCorruptObject)
 			}
 			break
 		}

 		r.readRef()
 	}

 	dataLength := r.readInt()
 	r.readInt() // n relocations - ignore
 	r.readInt() // n pcdata - ignore
 	r.readInt() // n autom - ignore
 	r.readInt() // n funcdata - ignore
 	r.readInt() // n files - ignore

 	r.dataOffset = r.offset
 	r.skip(dataLength)

 	// Symbols.
 	for {
 		if b := r.readByte(); b != 0xfe {
 			if b != 0xff {
 				return r.error(errCorruptObject)
 			}
 			break
 		}

 		typ := r.readByte()
 		s := &Sym{SymID: r.readSymID()}
 		r.p.Syms = append(r.p.Syms, s)
 		s.Kind = objabi.SymKind(typ)
 		flags := r.readInt()
 		s.DupOK = flags&1 != 0
 		s.Size = r.readInt()
 		s.Type = r.readSymID()
 		s.Data = r.readData()
 		s.Reloc = make([]Reloc, r.readInt())
 		for i := range s.Reloc {
 			rel := &s.Reloc[i]
 			rel.Offset = r.readInt()
 			rel.Size = r.readInt()
 			rel.Type = objabi.RelocType(r.readInt())
 			rel.Add = r.readInt()
 			rel.Sym = r.readSymID()
 		}

 		if s.Kind == objabi.STEXT {
 			f := new(Func)
 			s.Func = f
 			f.Args = r.readInt()
 			f.Frame = r.readInt()
 			flags := r.readInt()
 			f.Leaf = flags&(1<<0) != 0
 			f.TopFrame = flags&(1<<4) != 0
 			f.NoSplit = r.readInt() != 0
 			f.Var = make([]Var, r.readInt())
 			for i := range f.Var {
 				v := &f.Var[i]
 				v.Name = r.readSymID().Name
 				v.Offset = r.readInt()
 				v.Kind = r.readInt()
 				v.Type = r.readSymID()
 			}

 			f.PCSP = r.readData()
 			f.PCFile = r.readData()
 			f.PCLine = r.readData()
 			f.PCInline = r.readData()
 			f.PCData = make([]Data, r.readInt())
 			for i := range f.PCData {
 				f.PCData[i] = r.readData()
 			}
 			f.FuncData = make([]FuncData, r.readInt())
 			for i := range f.FuncData {
 				f.FuncData[i].Sym = r.readSymID()
 			}
 			for i := range f.FuncData {
 				f.FuncData[i].Offset = r.readInt() // TODO
 			}
 			f.File = make([]string, r.readInt())
 			for i := range f.File {
 				f.File[i] = r.readSymID().Name
 			}
 			f.InlTree = make([]InlinedCall, r.readInt())
 			for i := range f.InlTree {
 				f.InlTree[i].Parent = r.readInt()
 				f.InlTree[i].File = r.readSymID().Name
 				f.InlTree[i].Line = r.readInt()
 				f.InlTree[i].Func = r.readSymID()
 				f.InlTree[i].ParentPC = r.readInt()
 			}
 		}
 	}

 	r.readFull(r.tmp[:7])
 	if !bytes.Equal(r.tmp[:7], []byte("go114ld")) {
 		return r.error(errCorruptObject)
 	}

 	return nil
 }

 func (r *Reloc) String(insnOffset uint64) string {
 	delta := r.Offset - int64(insnOffset)
 	s := fmt.Sprintf("[%d:%d]%s", delta, delta+r.Size, r.Type)
 	if r.Sym.Name != "" {
 		if r.Add != 0 {
 			return fmt.Sprintf("%s:%s+%d", s, r.Sym.Name, r.Add)
 		}
 		return fmt.Sprintf("%s:%s", s, r.Sym.Name)
 	}
 	if r.Add != 0 {
 		return fmt.Sprintf("%s:%d", s, r.Add)
 	}
 	return s
 }
	// Copyright 2013 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 goobj implements reading of Go object files and archives.
	//
	// TODO(rsc): Decide where this package should live. (golang.org/issue/6932)
	// TODO(rsc): Decide the appropriate integer types for various fields.
	package goobj

	import (
	"bufio"
	"bytes"
	"cmd/internal/objabi"
	"errors"
	"fmt"
	"io"
	"os"
	"strconv"
	"strings"
	)

	// A Sym is a named symbol in an object file.
	type Sym struct {
	SymID // symbol identifier (name and version)
	Kind objabi.SymKind // kind of symbol
	DupOK bool // are duplicate definitions okay?
	Size int64 // size of corresponding data
	Type SymID // symbol for Go type information
	Data Data // memory image of symbol
	Reloc []Reloc // relocations to apply to Data
	Func *Func // additional data for functions
	}

	// A SymID - the combination of Name and Version - uniquely identifies
	// a symbol within a package.
	type SymID struct {
	// Name is the name of a symbol.
	Name string

	// Version is zero for symbols with global visibility.
	// Symbols with only file visibility (such as file-level static
	// declarations in C) have a non-zero version distinguishing
	// a symbol in one file from a symbol of the same name
	// in another file
	Version int64
	}

	func (s SymID) String() string {
	if s.Version == 0 {
	return s.Name
	}
	return fmt.Sprintf("%s<%d>", s.Name, s.Version)
	}

	// A Data is a reference to data stored in an object file.
	// It records the offset and size of the data, so that a client can
	// read the data only if necessary.
	type Data struct {
	Offset int64
	Size int64
	}

	// A Reloc describes a relocation applied to a memory image to refer
	// to an address within a particular symbol.
	type Reloc struct {
	// The bytes at [Offset, Offset+Size) within the containing Sym
	// should be updated to refer to the address Add bytes after the start
	// of the symbol Sym.
	Offset int64
	Size int64
	Sym SymID
	Add int64

	// The Type records the form of address expected in the bytes
	// described by the previous fields: absolute, PC-relative, and so on.
	// TODO(rsc): The interpretation of Type is not exposed by this package.
	Type objabi.RelocType
	}

	// A Var describes a variable in a function stack frame: a declared
	// local variable, an input argument, or an output result.
	type Var struct {
	// The combination of Name, Kind, and Offset uniquely
	// identifies a variable in a function stack frame.
	// Using fewer of these - in particular, using only Name - does not.
	Name string // Name of variable.
	Kind int64 // TODO(rsc): Define meaning.
	Offset int64 // Frame offset. TODO(rsc): Define meaning.

	Type SymID // Go type for variable.
	}

	// Func contains additional per-symbol information specific to functions.
	type Func struct {
	Args int64 // size in bytes of argument frame: inputs and outputs
	Frame int64 // size in bytes of local variable frame
	Leaf bool // function omits save of link register (ARM)
	NoSplit bool // function omits stack split prologue
	TopFrame bool // function is the top of the call stack
	Var []Var // detail about local variables
	PCSP Data // PC → SP offset map
	PCFile Data // PC → file number map (index into File)
	PCLine Data // PC → line number map
	PCInline Data // PC → inline tree index map
	PCData []Data // PC → runtime support data map
	FuncData []FuncData // non-PC-specific runtime support data
	File []string // paths indexed by PCFile
	InlTree []InlinedCall
	}

	// TODO: Add PCData []byte and PCDataIter (similar to liblink).

	// A FuncData is a single function-specific data value.
	type FuncData struct {
	Sym SymID // symbol holding data
	Offset int64 // offset into symbol for funcdata pointer
	}

	// An InlinedCall is a node in an InlTree.
	// See cmd/internal/obj.InlTree for details.
	type InlinedCall struct {
	Parent int64
	File string
	Line int64
	Func SymID
	ParentPC int64
	}

	// A Package is a parsed Go object file or archive defining a Go package.
	type Package struct {
	ImportPath string // import path denoting this package
	Imports []string // packages imported by this package
	SymRefs []SymID // list of symbol names and versions referred to by this pack
	Syms []*Sym // symbols defined by this package
	MaxVersion int64 // maximum Version in any SymID in Syms
	Arch string // architecture
	Native []*NativeReader // native object data (e.g. ELF)
	DWARFFileList []string // List of files for the DWARF .debug_lines section
	}

	type NativeReader struct {
	Name string
	io.ReaderAt
	}

	var (
	archiveHeader = []byte("!<arch>\n")
	archiveMagic = []byte("`\n")
	goobjHeader = []byte("go objec") // truncated to size of archiveHeader

	errCorruptArchive = errors.New("corrupt archive")
	errTruncatedArchive = errors.New("truncated archive")
	errCorruptObject = errors.New("corrupt object file")
	errNotObject = errors.New("unrecognized object file format")
	)

	// An objReader is an object file reader.
	type objReader struct {
	p *Package
	b *bufio.Reader
	f *os.File
	err error
	offset int64
	dataOffset int64
	limit int64
	tmp [256]byte
	pkgprefix string
	}

	// init initializes r to read package p from f.
	func (r objReader) init(f os.File, p *Package) {
	r.f = f
	r.p = p
	r.offset, _ = f.Seek(0, io.SeekCurrent)
	r.limit, _ = f.Seek(0, io.SeekEnd)
	f.Seek(r.offset, io.SeekStart)
	r.b = bufio.NewReader(f)
	r.pkgprefix = objabi.PathToPrefix(p.ImportPath) + "."
	}

	// error records that an error occurred.
	// It returns only the first error, so that an error
	// caused by an earlier error does not discard information
	// about the earlier error.
	func (r *objReader) error(err error) error {
	if r.err == nil {
	if err == io.EOF {
	err = io.ErrUnexpectedEOF
	}
	r.err = err
	}
	// panic("corrupt") // useful for debugging
	return r.err
	}

	// peek returns the next n bytes without advancing the reader.
	func (r *objReader) peek(n int) ([]byte, error) {
	if r.err != nil {
	return nil, r.err
	}
	if r.offset >= r.limit {
	r.error(io.ErrUnexpectedEOF)
	return nil, r.err
	}
	b, err := r.b.Peek(n)
	if err != nil {
	if err != bufio.ErrBufferFull {
	r.error(err)
	}
	}
	return b, err
	}

	// readByte reads and returns a byte from the input file.
	// On I/O error or EOF, it records the error but returns byte 0.
	// A sequence of 0 bytes will eventually terminate any
	// parsing state in the object file. In particular, it ends the
	// reading of a varint.
	func (r *objReader) readByte() byte {
	if r.err != nil {
	return 0
	}
	if r.offset >= r.limit {
	r.error(io.ErrUnexpectedEOF)
	return 0
	}
	b, err := r.b.ReadByte()
	if err != nil {
	if err == io.EOF {
	err = io.ErrUnexpectedEOF
	}
	r.error(err)
	b = 0
	} else {
	r.offset++
	}
	return b
	}

	// read reads exactly len(b) bytes from the input file.
	// If an error occurs, read returns the error but also
	// records it, so it is safe for callers to ignore the result
	// as long as delaying the report is not a problem.
	func (r *objReader) readFull(b []byte) error {
	if r.err != nil {
	return r.err
	}
	if r.offset+int64(len(b)) > r.limit {
	return r.error(io.ErrUnexpectedEOF)
	}
	n, err := io.ReadFull(r.b, b)
	r.offset += int64(n)
	if err != nil {
	return r.error(err)
	}
	return nil
	}

	// readInt reads a zigzag varint from the input file.
	func (r *objReader) readInt() int64 {
	var u uint64

	for shift := uint(0); ; shift += 7 {
	if shift >= 64 {
	r.error(errCorruptObject)
	return 0
	}
	c := r.readByte()
	u \|= uint64(c&0x7F) << shift
	if c&0x80 == 0 {
	break
	}
	}

	return int64(u>>1) ^ (int64(u) << 63 >> 63)
	}

	// readString reads a length-delimited string from the input file.
	func (r *objReader) readString() string {
	n := r.readInt()
	buf := make([]byte, n)
	r.readFull(buf)
	return string(buf)
	}

	// readSymID reads a SymID from the input file.
	func (r *objReader) readSymID() SymID {
	i := r.readInt()
	return r.p.SymRefs[i]
	}

	func (r *objReader) readRef() {
	name, abiOrStatic := r.readString(), r.readInt()

	// In a symbol name in an object file, "". denotes the
	// prefix for the package in which the object file has been found.
	// Expand it.
	name = strings.ReplaceAll(name, `"".`, r.pkgprefix)

	// The ABI field records either the ABI or -1 for static symbols.
	//
	// To distinguish different static symbols with the same name,
	// we use the symbol "version". Version 0 corresponds to
	// global symbols, and each file has a unique version > 0 for
	// all of its static symbols. The version is incremented on
	// each call to parseObject.
	//
	// For global symbols, we currently ignore the ABI.
	//
	// TODO(austin): Record the ABI in SymID. Since this is a
	// public API, we'll have to keep Version as 0 and record the
	// ABI in a new field (which differs from how the linker does
	// this, but that's okay). Show the ABI in things like
	// objdump.
	var vers int64
	if abiOrStatic == -1 {
	// Static symbol
	vers = r.p.MaxVersion
	}
	r.p.SymRefs = append(r.p.SymRefs, SymID{name, vers})
	}

	// readData reads a data reference from the input file.
	func (r *objReader) readData() Data {
	n := r.readInt()
	d := Data{Offset: r.dataOffset, Size: n}
	r.dataOffset += n
	return d
	}

	// skip skips n bytes in the input.
	func (r *objReader) skip(n int64) {
	if n < 0 {
	r.error(fmt.Errorf("debug/goobj: internal error: misuse of skip"))
	}
	if n < int64(len(r.tmp)) {
	// Since the data is so small, a just reading from the buffered
	// reader is better than flushing the buffer and seeking.
	r.readFull(r.tmp[:n])
	} else if n <= int64(r.b.Buffered()) {
	// Even though the data is not small, it has already been read.
	// Advance the buffer instead of seeking.
	for n > int64(len(r.tmp)) {
	r.readFull(r.tmp[:])
	n -= int64(len(r.tmp))
	}
	r.readFull(r.tmp[:n])
	} else {
	// Seek, giving up buffered data.
	_, err := r.f.Seek(r.offset+n, io.SeekStart)
	if err != nil {
	r.error(err)
	}
	r.offset += n
	r.b.Reset(r.f)
	}
	}

	// Parse parses an object file or archive from f,
	// assuming that its import path is pkgpath.
	func Parse(f os.File, pkgpath string) (Package, error) {
	if pkgpath == "" {
	pkgpath = `""`
	}
	p := new(Package)
	p.ImportPath = pkgpath

	var rd objReader
	rd.init(f, p)
	err := rd.readFull(rd.tmp[:8])
	if err != nil {
	if err == io.EOF {
	err = io.ErrUnexpectedEOF
	}
	return nil, err
	}

	switch {
	default:
	return nil, errNotObject

	case bytes.Equal(rd.tmp[:8], archiveHeader):
	if err := rd.parseArchive(); err != nil {
	return nil, err
	}
	case bytes.Equal(rd.tmp[:8], goobjHeader):
	if err := rd.parseObject(goobjHeader); err != nil {
	return nil, err
	}
	}

	return p, nil
	}

	// trimSpace removes trailing spaces from b and returns the corresponding string.
	// This effectively parses the form used in archive headers.
	func trimSpace(b []byte) string {
	return string(bytes.TrimRight(b, " "))
	}

	// parseArchive parses a Unix archive of Go object files.
	func (r *objReader) parseArchive() error {
	for r.offset < r.limit {
	if err := r.readFull(r.tmp[:60]); err != nil {
	return err
	}
	data := r.tmp[:60]

	// Each file is preceded by this text header (slice indices in first column):
	// 0:16 name
	// 16:28 date
	// 28:34 uid
	// 34:40 gid
	// 40:48 mode
	// 48:58 size
	// 58:60 magic - `\n
	// We only care about name, size, and magic.
	// The fields are space-padded on the right.
	// The size is in decimal.
	// The file data - size bytes - follows the header.
	// Headers are 2-byte aligned, so if size is odd, an extra padding
	// byte sits between the file data and the next header.
	// The file data that follows is padded to an even number of bytes:
	// if size is odd, an extra padding byte is inserted betw the next header.
	if len(data) < 60 {
	return errTruncatedArchive
	}
	if !bytes.Equal(data[58:60], archiveMagic) {
	return errCorruptArchive
	}
	name := trimSpace(data[0:16])
	size, err := strconv.ParseInt(trimSpace(data[48:58]), 10, 64)
	if err != nil {
	return errCorruptArchive
	}
	data = data[60:]
	fsize := size + size&1
	if fsize < 0 \|\| fsize < size {
	return errCorruptArchive
	}
	switch name {
	case "__.PKGDEF":
	r.skip(size)
	default:
	oldLimit := r.limit
	r.limit = r.offset + size

	p, err := r.peek(8)
	if err != nil {
	return err
	}
	if bytes.Equal(p, goobjHeader) {
	if err := r.parseObject(nil); err != nil {
	return fmt.Errorf("parsing archive member %q: %v", name, err)
	}
	} else {
	r.p.Native = append(r.p.Native, &NativeReader{
	Name: name,
	ReaderAt: io.NewSectionReader(r.f, r.offset, size),
	})
	}

	r.skip(r.limit - r.offset)
	r.limit = oldLimit
	}
	if size&1 != 0 {
	r.skip(1)
	}
	}
	return nil
	}

	// parseObject parses a single Go object file.
	// The prefix is the bytes already read from the file,
	// typically in order to detect that this is an object file.
	// The object file consists of a textual header ending in "\n!\n"
	// and then the part we want to parse begins.
	// The format of that part is defined in a comment at the top
	// of src/liblink/objfile.c.
	func (r *objReader) parseObject(prefix []byte) error {
	r.p.MaxVersion++
	h := make([]byte, 0, 256)
	h = append(h, prefix...)
	var c1, c2, c3 byte
	for {
	c1, c2, c3 = c2, c3, r.readByte()
	h = append(h, c3)
	// The new export format can contain 0 bytes.
	// Don't consider them errors, only look for r.err != nil.
	if r.err != nil {
	return errCorruptObject
	}
	if c1 == '\n' && c2 == '!' && c3 == '\n' {
	break
	}
	}

	hs := strings.Fields(string(h))
	if len(hs) >= 4 {
	r.p.Arch = hs[3]
	}
	// TODO: extract OS + build ID if/when we need it

	p, err := r.peek(8)
	if err != nil {
	return err
	}
	if bytes.Equal(p, []byte("\x00go114LD")) {
	r.readNew()
	return nil
	}
	r.readFull(r.tmp[:8])
	if !bytes.Equal(r.tmp[:8], []byte("\x00go114ld")) {
	return r.error(errCorruptObject)
	}

	b := r.readByte()
	if b != 1 {
	return r.error(errCorruptObject)
	}

	// Direct package dependencies.
	for {
	s := r.readString()
	if s == "" {
	break
	}
	r.p.Imports = append(r.p.Imports, s)
	}

	// Read filenames for dwarf info.
	count := r.readInt()
	for i := int64(0); i < count; i++ {
	r.p.DWARFFileList = append(r.p.DWARFFileList, r.readString())
	}

	r.p.SymRefs = []SymID{{"", 0}}
	for {
	if b := r.readByte(); b != 0xfe {
	if b != 0xff {
	return r.error(errCorruptObject)
	}
	break
	}

	r.readRef()
	}

	dataLength := r.readInt()
	r.readInt() // n relocations - ignore
	r.readInt() // n pcdata - ignore
	r.readInt() // n autom - ignore
	r.readInt() // n funcdata - ignore
	r.readInt() // n files - ignore

	r.dataOffset = r.offset
	r.skip(dataLength)

	// Symbols.
	for {
	if b := r.readByte(); b != 0xfe {
	if b != 0xff {
	return r.error(errCorruptObject)
	}
	break
	}

	typ := r.readByte()
	s := &Sym{SymID: r.readSymID()}
	r.p.Syms = append(r.p.Syms, s)
	s.Kind = objabi.SymKind(typ)
	flags := r.readInt()
	s.DupOK = flags&1 != 0
	s.Size = r.readInt()
	s.Type = r.readSymID()
	s.Data = r.readData()
	s.Reloc = make([]Reloc, r.readInt())
	for i := range s.Reloc {
	rel := &s.Reloc[i]
	rel.Offset = r.readInt()
	rel.Size = r.readInt()
	rel.Type = objabi.RelocType(r.readInt())
	rel.Add = r.readInt()
	rel.Sym = r.readSymID()
	}

	if s.Kind == objabi.STEXT {
	f := new(Func)
	s.Func = f
	f.Args = r.readInt()
	f.Frame = r.readInt()
	flags := r.readInt()
	f.Leaf = flags&(1<<0) != 0
	f.TopFrame = flags&(1<<4) != 0
	f.NoSplit = r.readInt() != 0
	f.Var = make([]Var, r.readInt())
	for i := range f.Var {
	v := &f.Var[i]
	v.Name = r.readSymID().Name
	v.Offset = r.readInt()
	v.Kind = r.readInt()
	v.Type = r.readSymID()
	}

	f.PCSP = r.readData()
	f.PCFile = r.readData()
	f.PCLine = r.readData()
	f.PCInline = r.readData()
	f.PCData = make([]Data, r.readInt())
	for i := range f.PCData {
	f.PCData[i] = r.readData()
	}
	f.FuncData = make([]FuncData, r.readInt())
	for i := range f.FuncData {
	f.FuncData[i].Sym = r.readSymID()
	}
	for i := range f.FuncData {
	f.FuncData[i].Offset = r.readInt() // TODO
	}
	f.File = make([]string, r.readInt())
	for i := range f.File {
	f.File[i] = r.readSymID().Name
	}
	f.InlTree = make([]InlinedCall, r.readInt())
	for i := range f.InlTree {
	f.InlTree[i].Parent = r.readInt()
	f.InlTree[i].File = r.readSymID().Name
	f.InlTree[i].Line = r.readInt()
	f.InlTree[i].Func = r.readSymID()
	f.InlTree[i].ParentPC = r.readInt()
	}
	}
	}

	r.readFull(r.tmp[:7])
	if !bytes.Equal(r.tmp[:7], []byte("go114ld")) {
	return r.error(errCorruptObject)
	}

	return nil
	}

	func (r *Reloc) String(insnOffset uint64) string {
	delta := r.Offset - int64(insnOffset)
	s := fmt.Sprintf("[%d:%d]%s", delta, delta+r.Size, r.Type)
	if r.Sym.Name != "" {
	if r.Add != 0 {
	return fmt.Sprintf("%s:%s+%d", s, r.Sym.Name, r.Add)
	}
	return fmt.Sprintf("%s:%s", s, r.Sym.Name)
	}
	if r.Add != 0 {
	return fmt.Sprintf("%s:%d", s, r.Add)
	}
	return s
	}