Source file src/debug/dwarf/entry.go

Documentation: debug/dwarf

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // DWARF debug information entry parser.
     6  // An entry is a sequence of data items of a given format.
     7  // The first word in the entry is an index into what DWARF
     8  // calls the ``abbreviation table.''  An abbreviation is really
     9  // just a type descriptor: it's an array of attribute tag/value format pairs.
    10  
    11  package dwarf
    12  
    13  import (
    14  	"encoding/binary"
    15  	"errors"
    16  	"strconv"
    17  )
    18  
    19  // a single entry's description: a sequence of attributes
    20  type abbrev struct {
    21  	tag      Tag
    22  	children bool
    23  	field    []afield
    24  }
    25  
    26  type afield struct {
    27  	attr  Attr
    28  	fmt   format
    29  	class Class
    30  	val   int64 // for formImplicitConst
    31  }
    32  
    33  // a map from entry format ids to their descriptions
    34  type abbrevTable map[uint32]abbrev
    35  
    36  // ParseAbbrev returns the abbreviation table that starts at byte off
    37  // in the .debug_abbrev section.
    38  func (d *Data) parseAbbrev(off uint64, vers int) (abbrevTable, error) {
    39  	if m, ok := d.abbrevCache[off]; ok {
    40  		return m, nil
    41  	}
    42  
    43  	data := d.abbrev
    44  	if off > uint64(len(data)) {
    45  		data = nil
    46  	} else {
    47  		data = data[off:]
    48  	}
    49  	b := makeBuf(d, unknownFormat{}, "abbrev", 0, data)
    50  
    51  	// Error handling is simplified by the buf getters
    52  	// returning an endless stream of 0s after an error.
    53  	m := make(abbrevTable)
    54  	for {
    55  		// Table ends with id == 0.
    56  		id := uint32(b.uint())
    57  		if id == 0 {
    58  			break
    59  		}
    60  
    61  		// Walk over attributes, counting.
    62  		n := 0
    63  		b1 := b // Read from copy of b.
    64  		b1.uint()
    65  		b1.uint8()
    66  		for {
    67  			tag := b1.uint()
    68  			fmt := b1.uint()
    69  			if tag == 0 && fmt == 0 {
    70  				break
    71  			}
    72  			if format(fmt) == formImplicitConst {
    73  				b1.int()
    74  			}
    75  			n++
    76  		}
    77  		if b1.err != nil {
    78  			return nil, b1.err
    79  		}
    80  
    81  		// Walk over attributes again, this time writing them down.
    82  		var a abbrev
    83  		a.tag = Tag(b.uint())
    84  		a.children = b.uint8() != 0
    85  		a.field = make([]afield, n)
    86  		for i := range a.field {
    87  			a.field[i].attr = Attr(b.uint())
    88  			a.field[i].fmt = format(b.uint())
    89  			a.field[i].class = formToClass(a.field[i].fmt, a.field[i].attr, vers, &b)
    90  			if a.field[i].fmt == formImplicitConst {
    91  				a.field[i].val = b.int()
    92  			}
    93  		}
    94  		b.uint()
    95  		b.uint()
    96  
    97  		m[id] = a
    98  	}
    99  	if b.err != nil {
   100  		return nil, b.err
   101  	}
   102  	d.abbrevCache[off] = m
   103  	return m, nil
   104  }
   105  
   106  // attrIsExprloc indicates attributes that allow exprloc values that
   107  // are encoded as block values in DWARF 2 and 3. See DWARF 4, Figure
   108  // 20.
   109  var attrIsExprloc = map[Attr]bool{
   110  	AttrLocation:      true,
   111  	AttrByteSize:      true,
   112  	AttrBitOffset:     true,
   113  	AttrBitSize:       true,
   114  	AttrStringLength:  true,
   115  	AttrLowerBound:    true,
   116  	AttrReturnAddr:    true,
   117  	AttrStrideSize:    true,
   118  	AttrUpperBound:    true,
   119  	AttrCount:         true,
   120  	AttrDataMemberLoc: true,
   121  	AttrFrameBase:     true,
   122  	AttrSegment:       true,
   123  	AttrStaticLink:    true,
   124  	AttrUseLocation:   true,
   125  	AttrVtableElemLoc: true,
   126  	AttrAllocated:     true,
   127  	AttrAssociated:    true,
   128  	AttrDataLocation:  true,
   129  	AttrStride:        true,
   130  }
   131  
   132  // attrPtrClass indicates the *ptr class of attributes that have
   133  // encoding formSecOffset in DWARF 4 or formData* in DWARF 2 and 3.
   134  var attrPtrClass = map[Attr]Class{
   135  	AttrLocation:      ClassLocListPtr,
   136  	AttrStmtList:      ClassLinePtr,
   137  	AttrStringLength:  ClassLocListPtr,
   138  	AttrReturnAddr:    ClassLocListPtr,
   139  	AttrStartScope:    ClassRangeListPtr,
   140  	AttrDataMemberLoc: ClassLocListPtr,
   141  	AttrFrameBase:     ClassLocListPtr,
   142  	AttrMacroInfo:     ClassMacPtr,
   143  	AttrSegment:       ClassLocListPtr,
   144  	AttrStaticLink:    ClassLocListPtr,
   145  	AttrUseLocation:   ClassLocListPtr,
   146  	AttrVtableElemLoc: ClassLocListPtr,
   147  	AttrRanges:        ClassRangeListPtr,
   148  	// The following are new in DWARF 5.
   149  	AttrStrOffsetsBase: ClassStrOffsetsPtr,
   150  	AttrAddrBase:       ClassAddrPtr,
   151  	AttrRnglistsBase:   ClassRngListsPtr,
   152  	AttrLoclistsBase:   ClassLocListPtr,
   153  }
   154  
   155  // formToClass returns the DWARF 4 Class for the given form. If the
   156  // DWARF version is less then 4, it will disambiguate some forms
   157  // depending on the attribute.
   158  func formToClass(form format, attr Attr, vers int, b *buf) Class {
   159  	switch form {
   160  	default:
   161  		b.error("cannot determine class of unknown attribute form")
   162  		return 0
   163  
   164  	case formIndirect:
   165  		return ClassUnknown
   166  
   167  	case formAddr, formAddrx, formAddrx1, formAddrx2, formAddrx3, formAddrx4:
   168  		return ClassAddress
   169  
   170  	case formDwarfBlock1, formDwarfBlock2, formDwarfBlock4, formDwarfBlock:
   171  		// In DWARF 2 and 3, ClassExprLoc was encoded as a
   172  		// block. DWARF 4 distinguishes ClassBlock and
   173  		// ClassExprLoc, but there are no attributes that can
   174  		// be both, so we also promote ClassBlock values in
   175  		// DWARF 4 that should be ClassExprLoc in case
   176  		// producers get this wrong.
   177  		if attrIsExprloc[attr] {
   178  			return ClassExprLoc
   179  		}
   180  		return ClassBlock
   181  
   182  	case formData1, formData2, formData4, formData8, formSdata, formUdata, formData16, formImplicitConst:
   183  		// In DWARF 2 and 3, ClassPtr was encoded as a
   184  		// constant. Unlike ClassExprLoc/ClassBlock, some
   185  		// DWARF 4 attributes need to distinguish Class*Ptr
   186  		// from ClassConstant, so we only do this promotion
   187  		// for versions 2 and 3.
   188  		if class, ok := attrPtrClass[attr]; vers < 4 && ok {
   189  			return class
   190  		}
   191  		return ClassConstant
   192  
   193  	case formFlag, formFlagPresent:
   194  		return ClassFlag
   195  
   196  	case formRefAddr, formRef1, formRef2, formRef4, formRef8, formRefUdata, formRefSup4, formRefSup8:
   197  		return ClassReference
   198  
   199  	case formRefSig8:
   200  		return ClassReferenceSig
   201  
   202  	case formString, formStrp, formStrx, formStrpSup, formLineStrp, formStrx1, formStrx2, formStrx3, formStrx4:
   203  		return ClassString
   204  
   205  	case formSecOffset:
   206  		// DWARF 4 defines four *ptr classes, but doesn't
   207  		// distinguish them in the encoding. Disambiguate
   208  		// these classes using the attribute.
   209  		if class, ok := attrPtrClass[attr]; ok {
   210  			return class
   211  		}
   212  		return ClassUnknown
   213  
   214  	case formExprloc:
   215  		return ClassExprLoc
   216  
   217  	case formGnuRefAlt:
   218  		return ClassReferenceAlt
   219  
   220  	case formGnuStrpAlt:
   221  		return ClassStringAlt
   222  
   223  	case formLoclistx:
   224  		return ClassLocList
   225  
   226  	case formRnglistx:
   227  		return ClassRngList
   228  	}
   229  }
   230  
   231  // An entry is a sequence of attribute/value pairs.
   232  type Entry struct {
   233  	Offset   Offset // offset of Entry in DWARF info
   234  	Tag      Tag    // tag (kind of Entry)
   235  	Children bool   // whether Entry is followed by children
   236  	Field    []Field
   237  }
   238  
   239  // A Field is a single attribute/value pair in an Entry.
   240  //
   241  // A value can be one of several "attribute classes" defined by DWARF.
   242  // The Go types corresponding to each class are:
   243  //
   244  //    DWARF class       Go type        Class
   245  //    -----------       -------        -----
   246  //    address           uint64         ClassAddress
   247  //    block             []byte         ClassBlock
   248  //    constant          int64          ClassConstant
   249  //    flag              bool           ClassFlag
   250  //    reference
   251  //      to info         dwarf.Offset   ClassReference
   252  //      to type unit    uint64         ClassReferenceSig
   253  //    string            string         ClassString
   254  //    exprloc           []byte         ClassExprLoc
   255  //    lineptr           int64          ClassLinePtr
   256  //    loclistptr        int64          ClassLocListPtr
   257  //    macptr            int64          ClassMacPtr
   258  //    rangelistptr      int64          ClassRangeListPtr
   259  //
   260  // For unrecognized or vendor-defined attributes, Class may be
   261  // ClassUnknown.
   262  type Field struct {
   263  	Attr  Attr
   264  	Val   interface{}
   265  	Class Class
   266  }
   267  
   268  // A Class is the DWARF 4 class of an attribute value.
   269  //
   270  // In general, a given attribute's value may take on one of several
   271  // possible classes defined by DWARF, each of which leads to a
   272  // slightly different interpretation of the attribute.
   273  //
   274  // DWARF version 4 distinguishes attribute value classes more finely
   275  // than previous versions of DWARF. The reader will disambiguate
   276  // coarser classes from earlier versions of DWARF into the appropriate
   277  // DWARF 4 class. For example, DWARF 2 uses "constant" for constants
   278  // as well as all types of section offsets, but the reader will
   279  // canonicalize attributes in DWARF 2 files that refer to section
   280  // offsets to one of the Class*Ptr classes, even though these classes
   281  // were only defined in DWARF 3.
   282  type Class int
   283  
   284  const (
   285  	// ClassUnknown represents values of unknown DWARF class.
   286  	ClassUnknown Class = iota
   287  
   288  	// ClassAddress represents values of type uint64 that are
   289  	// addresses on the target machine.
   290  	ClassAddress
   291  
   292  	// ClassBlock represents values of type []byte whose
   293  	// interpretation depends on the attribute.
   294  	ClassBlock
   295  
   296  	// ClassConstant represents values of type int64 that are
   297  	// constants. The interpretation of this constant depends on
   298  	// the attribute.
   299  	ClassConstant
   300  
   301  	// ClassExprLoc represents values of type []byte that contain
   302  	// an encoded DWARF expression or location description.
   303  	ClassExprLoc
   304  
   305  	// ClassFlag represents values of type bool.
   306  	ClassFlag
   307  
   308  	// ClassLinePtr represents values that are an int64 offset
   309  	// into the "line" section.
   310  	ClassLinePtr
   311  
   312  	// ClassLocListPtr represents values that are an int64 offset
   313  	// into the "loclist" section.
   314  	ClassLocListPtr
   315  
   316  	// ClassMacPtr represents values that are an int64 offset into
   317  	// the "mac" section.
   318  	ClassMacPtr
   319  
   320  	// ClassMacPtr represents values that are an int64 offset into
   321  	// the "rangelist" section.
   322  	ClassRangeListPtr
   323  
   324  	// ClassReference represents values that are an Offset offset
   325  	// of an Entry in the info section (for use with Reader.Seek).
   326  	// The DWARF specification combines ClassReference and
   327  	// ClassReferenceSig into class "reference".
   328  	ClassReference
   329  
   330  	// ClassReferenceSig represents values that are a uint64 type
   331  	// signature referencing a type Entry.
   332  	ClassReferenceSig
   333  
   334  	// ClassString represents values that are strings. If the
   335  	// compilation unit specifies the AttrUseUTF8 flag (strongly
   336  	// recommended), the string value will be encoded in UTF-8.
   337  	// Otherwise, the encoding is unspecified.
   338  	ClassString
   339  
   340  	// ClassReferenceAlt represents values of type int64 that are
   341  	// an offset into the DWARF "info" section of an alternate
   342  	// object file.
   343  	ClassReferenceAlt
   344  
   345  	// ClassStringAlt represents values of type int64 that are an
   346  	// offset into the DWARF string section of an alternate object
   347  	// file.
   348  	ClassStringAlt
   349  
   350  	// ClassAddrPtr represents values that are an int64 offset
   351  	// into the "addr" section.
   352  	ClassAddrPtr
   353  
   354  	// ClassLocList represents values that are an int64 offset
   355  	// into the "loclists" section.
   356  	ClassLocList
   357  
   358  	// ClassRngList represents values that are an int64 offset
   359  	// from the base of the "rnglists" section.
   360  	ClassRngList
   361  
   362  	// ClassRngListsPtr represents values that are an int64 offset
   363  	// into the "rnglists" section. These are used as the base for
   364  	// ClassRngList values.
   365  	ClassRngListsPtr
   366  
   367  	// ClassStrOffsetsPtr represents values that are an int64
   368  	// offset into the "str_offsets" section.
   369  	ClassStrOffsetsPtr
   370  )
   371  
   372  //go:generate stringer -type=Class
   373  
   374  func (i Class) GoString() string {
   375  	return "dwarf." + i.String()
   376  }
   377  
   378  // Val returns the value associated with attribute Attr in Entry,
   379  // or nil if there is no such attribute.
   380  //
   381  // A common idiom is to merge the check for nil return with
   382  // the check that the value has the expected dynamic type, as in:
   383  //	v, ok := e.Val(AttrSibling).(int64)
   384  //
   385  func (e *Entry) Val(a Attr) interface{} {
   386  	if f := e.AttrField(a); f != nil {
   387  		return f.Val
   388  	}
   389  	return nil
   390  }
   391  
   392  // AttrField returns the Field associated with attribute Attr in
   393  // Entry, or nil if there is no such attribute.
   394  func (e *Entry) AttrField(a Attr) *Field {
   395  	for i, f := range e.Field {
   396  		if f.Attr == a {
   397  			return &e.Field[i]
   398  		}
   399  	}
   400  	return nil
   401  }
   402  
   403  // An Offset represents the location of an Entry within the DWARF info.
   404  // (See Reader.Seek.)
   405  type Offset uint32
   406  
   407  // Entry reads a single entry from buf, decoding
   408  // according to the given abbreviation table.
   409  func (b *buf) entry(cu *Entry, atab abbrevTable, ubase Offset, vers int) *Entry {
   410  	off := b.off
   411  	id := uint32(b.uint())
   412  	if id == 0 {
   413  		return &Entry{}
   414  	}
   415  	a, ok := atab[id]
   416  	if !ok {
   417  		b.error("unknown abbreviation table index")
   418  		return nil
   419  	}
   420  	e := &Entry{
   421  		Offset:   off,
   422  		Tag:      a.tag,
   423  		Children: a.children,
   424  		Field:    make([]Field, len(a.field)),
   425  	}
   426  	for i := range e.Field {
   427  		e.Field[i].Attr = a.field[i].attr
   428  		e.Field[i].Class = a.field[i].class
   429  		fmt := a.field[i].fmt
   430  		if fmt == formIndirect {
   431  			fmt = format(b.uint())
   432  			e.Field[i].Class = formToClass(fmt, a.field[i].attr, vers, b)
   433  		}
   434  		var val interface{}
   435  		switch fmt {
   436  		default:
   437  			b.error("unknown entry attr format 0x" + strconv.FormatInt(int64(fmt), 16))
   438  
   439  		// address
   440  		case formAddr:
   441  			val = b.addr()
   442  		case formAddrx, formAddrx1, formAddrx2, formAddrx3, formAddrx4:
   443  			var off uint64
   444  			switch fmt {
   445  			case formAddrx:
   446  				off = b.uint()
   447  			case formAddrx1:
   448  				off = uint64(b.uint8())
   449  			case formAddrx2:
   450  				off = uint64(b.uint16())
   451  			case formAddrx3:
   452  				off = uint64(b.uint24())
   453  			case formAddrx4:
   454  				off = uint64(b.uint32())
   455  			}
   456  			if len(b.dwarf.addr) == 0 {
   457  				b.error("DW_FORM_addrx with no .debug_addr section")
   458  			}
   459  			if b.err != nil {
   460  				return nil
   461  			}
   462  			addrsize := b.format.addrsize()
   463  			if addrsize == 0 {
   464  				b.error("unknown address size for DW_FORM_addrx")
   465  			}
   466  			off *= uint64(addrsize)
   467  
   468  			// We have to adjust by the offset of the
   469  			// compilation unit. This won't work if the
   470  			// program uses Reader.Seek to skip over the
   471  			// unit. Not much we can do about that.
   472  			if cu != nil {
   473  				cuOff, ok := cu.Val(AttrAddrBase).(int64)
   474  				if ok {
   475  					off += uint64(cuOff)
   476  				}
   477  			}
   478  
   479  			if uint64(int(off)) != off {
   480  				b.error("DW_FORM_addrx offset out of range")
   481  			}
   482  
   483  			b1 := makeBuf(b.dwarf, b.format, "addr", 0, b.dwarf.addr)
   484  			b1.skip(int(off))
   485  			val = b1.addr()
   486  			if b1.err != nil {
   487  				b.err = b1.err
   488  				return nil
   489  			}
   490  
   491  		// block
   492  		case formDwarfBlock1:
   493  			val = b.bytes(int(b.uint8()))
   494  		case formDwarfBlock2:
   495  			val = b.bytes(int(b.uint16()))
   496  		case formDwarfBlock4:
   497  			val = b.bytes(int(b.uint32()))
   498  		case formDwarfBlock:
   499  			val = b.bytes(int(b.uint()))
   500  
   501  		// constant
   502  		case formData1:
   503  			val = int64(b.uint8())
   504  		case formData2:
   505  			val = int64(b.uint16())
   506  		case formData4:
   507  			val = int64(b.uint32())
   508  		case formData8:
   509  			val = int64(b.uint64())
   510  		case formData16:
   511  			val = b.bytes(16)
   512  		case formSdata:
   513  			val = int64(b.int())
   514  		case formUdata:
   515  			val = int64(b.uint())
   516  		case formImplicitConst:
   517  			val = a.field[i].val
   518  
   519  		// flag
   520  		case formFlag:
   521  			val = b.uint8() == 1
   522  		// New in DWARF 4.
   523  		case formFlagPresent:
   524  			// The attribute is implicitly indicated as present, and no value is
   525  			// encoded in the debugging information entry itself.
   526  			val = true
   527  
   528  		// reference to other entry
   529  		case formRefAddr:
   530  			vers := b.format.version()
   531  			if vers == 0 {
   532  				b.error("unknown version for DW_FORM_ref_addr")
   533  			} else if vers == 2 {
   534  				val = Offset(b.addr())
   535  			} else {
   536  				is64, known := b.format.dwarf64()
   537  				if !known {
   538  					b.error("unknown size for DW_FORM_ref_addr")
   539  				} else if is64 {
   540  					val = Offset(b.uint64())
   541  				} else {
   542  					val = Offset(b.uint32())
   543  				}
   544  			}
   545  		case formRef1:
   546  			val = Offset(b.uint8()) + ubase
   547  		case formRef2:
   548  			val = Offset(b.uint16()) + ubase
   549  		case formRef4:
   550  			val = Offset(b.uint32()) + ubase
   551  		case formRef8:
   552  			val = Offset(b.uint64()) + ubase
   553  		case formRefUdata:
   554  			val = Offset(b.uint()) + ubase
   555  
   556  		// string
   557  		case formString:
   558  			val = b.string()
   559  		case formStrp, formLineStrp:
   560  			var off uint64 // offset into .debug_str
   561  			is64, known := b.format.dwarf64()
   562  			if !known {
   563  				b.error("unknown size for DW_FORM_strp/line_strp")
   564  			} else if is64 {
   565  				off = b.uint64()
   566  			} else {
   567  				off = uint64(b.uint32())
   568  			}
   569  			if uint64(int(off)) != off {
   570  				b.error("DW_FORM_strp/line_strp offset out of range")
   571  			}
   572  			if b.err != nil {
   573  				return nil
   574  			}
   575  			var b1 buf
   576  			if fmt == formStrp {
   577  				b1 = makeBuf(b.dwarf, b.format, "str", 0, b.dwarf.str)
   578  			} else {
   579  				if len(b.dwarf.lineStr) == 0 {
   580  					b.error("DW_FORM_line_strp with no .debug_line_str section")
   581  				}
   582  				b1 = makeBuf(b.dwarf, b.format, "line_str", 0, b.dwarf.lineStr)
   583  			}
   584  			b1.skip(int(off))
   585  			val = b1.string()
   586  			if b1.err != nil {
   587  				b.err = b1.err
   588  				return nil
   589  			}
   590  		case formStrx, formStrx1, formStrx2, formStrx3, formStrx4:
   591  			var off uint64
   592  			switch fmt {
   593  			case formStrx:
   594  				off = b.uint()
   595  			case formStrx1:
   596  				off = uint64(b.uint8())
   597  			case formStrx2:
   598  				off = uint64(b.uint16())
   599  			case formStrx3:
   600  				off = uint64(b.uint24())
   601  			case formStrx4:
   602  				off = uint64(b.uint32())
   603  			}
   604  			if len(b.dwarf.strOffsets) == 0 {
   605  				b.error("DW_FORM_strx with no .debug_str_offsets section")
   606  			}
   607  			is64, known := b.format.dwarf64()
   608  			if !known {
   609  				b.error("unknown offset size for DW_FORM_strx")
   610  			}
   611  			if b.err != nil {
   612  				return nil
   613  			}
   614  			if is64 {
   615  				off *= 8
   616  			} else {
   617  				off *= 4
   618  			}
   619  
   620  			// We have to adjust by the offset of the
   621  			// compilation unit. This won't work if the
   622  			// program uses Reader.Seek to skip over the
   623  			// unit. Not much we can do about that.
   624  			if cu != nil {
   625  				cuOff, ok := cu.Val(AttrStrOffsetsBase).(int64)
   626  				if ok {
   627  					off += uint64(cuOff)
   628  				}
   629  			}
   630  
   631  			if uint64(int(off)) != off {
   632  				b.error("DW_FORM_strx offset out of range")
   633  			}
   634  
   635  			b1 := makeBuf(b.dwarf, b.format, "str_offsets", 0, b.dwarf.strOffsets)
   636  			b1.skip(int(off))
   637  			if is64 {
   638  				off = b1.uint64()
   639  			} else {
   640  				off = uint64(b1.uint32())
   641  			}
   642  			if b1.err != nil {
   643  				b.err = b1.err
   644  				return nil
   645  			}
   646  			if uint64(int(off)) != off {
   647  				b.error("DW_FORM_strx indirect offset out of range")
   648  			}
   649  			b1 = makeBuf(b.dwarf, b.format, "str", 0, b.dwarf.str)
   650  			b1.skip(int(off))
   651  			val = b1.string()
   652  			if b1.err != nil {
   653  				b.err = b1.err
   654  				return nil
   655  			}
   656  		case formStrpSup:
   657  			is64, known := b.format.dwarf64()
   658  			if !known {
   659  				b.error("unknown size for DW_FORM_strp_sup")
   660  			} else if is64 {
   661  				val = b.uint64()
   662  			} else {
   663  				val = b.uint32()
   664  			}
   665  
   666  		// lineptr, loclistptr, macptr, rangelistptr
   667  		// New in DWARF 4, but clang can generate them with -gdwarf-2.
   668  		// Section reference, replacing use of formData4 and formData8.
   669  		case formSecOffset, formGnuRefAlt, formGnuStrpAlt:
   670  			is64, known := b.format.dwarf64()
   671  			if !known {
   672  				b.error("unknown size for form 0x" + strconv.FormatInt(int64(fmt), 16))
   673  			} else if is64 {
   674  				val = int64(b.uint64())
   675  			} else {
   676  				val = int64(b.uint32())
   677  			}
   678  
   679  		// exprloc
   680  		// New in DWARF 4.
   681  		case formExprloc:
   682  			val = b.bytes(int(b.uint()))
   683  
   684  		// reference
   685  		// New in DWARF 4.
   686  		case formRefSig8:
   687  			// 64-bit type signature.
   688  			val = b.uint64()
   689  		case formRefSup4:
   690  			val = b.uint32()
   691  		case formRefSup8:
   692  			val = b.uint64()
   693  
   694  		// loclist
   695  		case formLoclistx:
   696  			val = b.uint()
   697  
   698  		// rnglist
   699  		case formRnglistx:
   700  			val = b.uint()
   701  		}
   702  		e.Field[i].Val = val
   703  	}
   704  	if b.err != nil {
   705  		return nil
   706  	}
   707  	return e
   708  }
   709  
   710  // A Reader allows reading Entry structures from a DWARF ``info'' section.
   711  // The Entry structures are arranged in a tree. The Reader's Next function
   712  // return successive entries from a pre-order traversal of the tree.
   713  // If an entry has children, its Children field will be true, and the children
   714  // follow, terminated by an Entry with Tag 0.
   715  type Reader struct {
   716  	b            buf
   717  	d            *Data
   718  	err          error
   719  	unit         int
   720  	lastChildren bool   // .Children of last entry returned by Next
   721  	lastSibling  Offset // .Val(AttrSibling) of last entry returned by Next
   722  	cu           *Entry // current compilation unit
   723  }
   724  
   725  // Reader returns a new Reader for Data.
   726  // The reader is positioned at byte offset 0 in the DWARF ``info'' section.
   727  func (d *Data) Reader() *Reader {
   728  	r := &Reader{d: d}
   729  	r.Seek(0)
   730  	return r
   731  }
   732  
   733  // AddressSize returns the size in bytes of addresses in the current compilation
   734  // unit.
   735  func (r *Reader) AddressSize() int {
   736  	return r.d.unit[r.unit].asize
   737  }
   738  
   739  // ByteOrder returns the byte order in the current compilation unit.
   740  func (r *Reader) ByteOrder() binary.ByteOrder {
   741  	return r.b.order
   742  }
   743  
   744  // Seek positions the Reader at offset off in the encoded entry stream.
   745  // Offset 0 can be used to denote the first entry.
   746  func (r *Reader) Seek(off Offset) {
   747  	d := r.d
   748  	r.err = nil
   749  	r.lastChildren = false
   750  	if off == 0 {
   751  		if len(d.unit) == 0 {
   752  			return
   753  		}
   754  		u := &d.unit[0]
   755  		r.unit = 0
   756  		r.b = makeBuf(r.d, u, "info", u.off, u.data)
   757  		r.cu = nil
   758  		return
   759  	}
   760  
   761  	i := d.offsetToUnit(off)
   762  	if i == -1 {
   763  		r.err = errors.New("offset out of range")
   764  		return
   765  	}
   766  	if i != r.unit {
   767  		r.cu = nil
   768  	}
   769  	u := &d.unit[i]
   770  	r.unit = i
   771  	r.b = makeBuf(r.d, u, "info", off, u.data[off-u.off:])
   772  }
   773  
   774  // maybeNextUnit advances to the next unit if this one is finished.
   775  func (r *Reader) maybeNextUnit() {
   776  	for len(r.b.data) == 0 && r.unit+1 < len(r.d.unit) {
   777  		r.unit++
   778  		u := &r.d.unit[r.unit]
   779  		r.b = makeBuf(r.d, u, "info", u.off, u.data)
   780  		r.cu = nil
   781  	}
   782  }
   783  
   784  // Next reads the next entry from the encoded entry stream.
   785  // It returns nil, nil when it reaches the end of the section.
   786  // It returns an error if the current offset is invalid or the data at the
   787  // offset cannot be decoded as a valid Entry.
   788  func (r *Reader) Next() (*Entry, error) {
   789  	if r.err != nil {
   790  		return nil, r.err
   791  	}
   792  	r.maybeNextUnit()
   793  	if len(r.b.data) == 0 {
   794  		return nil, nil
   795  	}
   796  	u := &r.d.unit[r.unit]
   797  	e := r.b.entry(r.cu, u.atable, u.base, u.vers)
   798  	if r.b.err != nil {
   799  		r.err = r.b.err
   800  		return nil, r.err
   801  	}
   802  	if e != nil {
   803  		r.lastChildren = e.Children
   804  		if r.lastChildren {
   805  			r.lastSibling, _ = e.Val(AttrSibling).(Offset)
   806  		}
   807  		if e.Tag == TagCompileUnit || e.Tag == TagPartialUnit {
   808  			r.cu = e
   809  		}
   810  	} else {
   811  		r.lastChildren = false
   812  	}
   813  	return e, nil
   814  }
   815  
   816  // SkipChildren skips over the child entries associated with
   817  // the last Entry returned by Next. If that Entry did not have
   818  // children or Next has not been called, SkipChildren is a no-op.
   819  func (r *Reader) SkipChildren() {
   820  	if r.err != nil || !r.lastChildren {
   821  		return
   822  	}
   823  
   824  	// If the last entry had a sibling attribute,
   825  	// that attribute gives the offset of the next
   826  	// sibling, so we can avoid decoding the
   827  	// child subtrees.
   828  	if r.lastSibling >= r.b.off {
   829  		r.Seek(r.lastSibling)
   830  		return
   831  	}
   832  
   833  	for {
   834  		e, err := r.Next()
   835  		if err != nil || e == nil || e.Tag == 0 {
   836  			break
   837  		}
   838  		if e.Children {
   839  			r.SkipChildren()
   840  		}
   841  	}
   842  }
   843  
   844  // clone returns a copy of the reader. This is used by the typeReader
   845  // interface.
   846  func (r *Reader) clone() typeReader {
   847  	return r.d.Reader()
   848  }
   849  
   850  // offset returns the current buffer offset. This is used by the
   851  // typeReader interface.
   852  func (r *Reader) offset() Offset {
   853  	return r.b.off
   854  }
   855  
   856  // SeekPC returns the Entry for the compilation unit that includes pc,
   857  // and positions the reader to read the children of that unit.  If pc
   858  // is not covered by any unit, SeekPC returns ErrUnknownPC and the
   859  // position of the reader is undefined.
   860  //
   861  // Because compilation units can describe multiple regions of the
   862  // executable, in the worst case SeekPC must search through all the
   863  // ranges in all the compilation units. Each call to SeekPC starts the
   864  // search at the compilation unit of the last call, so in general
   865  // looking up a series of PCs will be faster if they are sorted. If
   866  // the caller wishes to do repeated fast PC lookups, it should build
   867  // an appropriate index using the Ranges method.
   868  func (r *Reader) SeekPC(pc uint64) (*Entry, error) {
   869  	unit := r.unit
   870  	for i := 0; i < len(r.d.unit); i++ {
   871  		if unit >= len(r.d.unit) {
   872  			unit = 0
   873  		}
   874  		r.err = nil
   875  		r.lastChildren = false
   876  		r.unit = unit
   877  		u := &r.d.unit[unit]
   878  		r.b = makeBuf(r.d, u, "info", u.off, u.data)
   879  		e, err := r.Next()
   880  		if err != nil {
   881  			return nil, err
   882  		}
   883  		ranges, err := r.d.Ranges(e)
   884  		if err != nil {
   885  			return nil, err
   886  		}
   887  		for _, pcs := range ranges {
   888  			if pcs[0] <= pc && pc < pcs[1] {
   889  				return e, nil
   890  			}
   891  		}
   892  		unit++
   893  	}
   894  	return nil, ErrUnknownPC
   895  }
   896  
   897  // Ranges returns the PC ranges covered by e, a slice of [low,high) pairs.
   898  // Only some entry types, such as TagCompileUnit or TagSubprogram, have PC
   899  // ranges; for others, this will return nil with no error.
   900  func (d *Data) Ranges(e *Entry) ([][2]uint64, error) {
   901  	var ret [][2]uint64
   902  
   903  	low, lowOK := e.Val(AttrLowpc).(uint64)
   904  
   905  	var high uint64
   906  	var highOK bool
   907  	highField := e.AttrField(AttrHighpc)
   908  	if highField != nil {
   909  		switch highField.Class {
   910  		case ClassAddress:
   911  			high, highOK = highField.Val.(uint64)
   912  		case ClassConstant:
   913  			off, ok := highField.Val.(int64)
   914  			if ok {
   915  				high = low + uint64(off)
   916  				highOK = true
   917  			}
   918  		}
   919  	}
   920  
   921  	if lowOK && highOK {
   922  		ret = append(ret, [2]uint64{low, high})
   923  	}
   924  
   925  	ranges, rangesOK := e.Val(AttrRanges).(int64)
   926  	if rangesOK && d.ranges != nil {
   927  		// The initial base address is the lowpc attribute
   928  		// of the enclosing compilation unit.
   929  		// Although DWARF specifies the lowpc attribute,
   930  		// comments in gdb/dwarf2read.c say that some versions
   931  		// of GCC use the entrypc attribute, so we check that too.
   932  		var cu *Entry
   933  		if e.Tag == TagCompileUnit {
   934  			cu = e
   935  		} else {
   936  			i := d.offsetToUnit(e.Offset)
   937  			if i == -1 {
   938  				return nil, errors.New("no unit for entry")
   939  			}
   940  			u := &d.unit[i]
   941  			b := makeBuf(d, u, "info", u.off, u.data)
   942  			cu = b.entry(nil, u.atable, u.base, u.vers)
   943  			if b.err != nil {
   944  				return nil, b.err
   945  			}
   946  		}
   947  
   948  		var base uint64
   949  		if cuEntry, cuEntryOK := cu.Val(AttrEntrypc).(uint64); cuEntryOK {
   950  			base = cuEntry
   951  		} else if cuLow, cuLowOK := cu.Val(AttrLowpc).(uint64); cuLowOK {
   952  			base = cuLow
   953  		}
   954  
   955  		u := &d.unit[d.offsetToUnit(e.Offset)]
   956  		buf := makeBuf(d, u, "ranges", Offset(ranges), d.ranges[ranges:])
   957  		for len(buf.data) > 0 {
   958  			low = buf.addr()
   959  			high = buf.addr()
   960  
   961  			if low == 0 && high == 0 {
   962  				break
   963  			}
   964  
   965  			if low == ^uint64(0)>>uint((8-u.addrsize())*8) {
   966  				base = high
   967  			} else {
   968  				ret = append(ret, [2]uint64{base + low, base + high})
   969  			}
   970  		}
   971  	}
   972  
   973  	return ret, nil
   974  }
   975  

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