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Source file src/debug/gosym/pclntab.go

Documentation: debug/gosym

     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  /*
     6   * Line tables
     7   */
     8  
     9  package gosym
    10  
    11  import (
    12  	"encoding/binary"
    13  	"sync"
    14  )
    15  
    16  // A LineTable is a data structure mapping program counters to line numbers.
    17  //
    18  // In Go 1.1 and earlier, each function (represented by a Func) had its own LineTable,
    19  // and the line number corresponded to a numbering of all source lines in the
    20  // program, across all files. That absolute line number would then have to be
    21  // converted separately to a file name and line number within the file.
    22  //
    23  // In Go 1.2, the format of the data changed so that there is a single LineTable
    24  // for the entire program, shared by all Funcs, and there are no absolute line
    25  // numbers, just line numbers within specific files.
    26  //
    27  // For the most part, LineTable's methods should be treated as an internal
    28  // detail of the package; callers should use the methods on Table instead.
    29  type LineTable struct {
    30  	Data []byte
    31  	PC   uint64
    32  	Line int
    33  
    34  	// Go 1.2 state
    35  	mu       sync.Mutex
    36  	go12     int // is this in Go 1.2 format? -1 no, 0 unknown, 1 yes
    37  	binary   binary.ByteOrder
    38  	quantum  uint32
    39  	ptrsize  uint32
    40  	functab  []byte
    41  	nfunctab uint32
    42  	filetab  []byte
    43  	nfiletab uint32
    44  	fileMap  map[string]uint32
    45  }
    46  
    47  // NOTE(rsc): This is wrong for GOARCH=arm, which uses a quantum of 4,
    48  // but we have no idea whether we're using arm or not. This only
    49  // matters in the old (pre-Go 1.2) symbol table format, so it's not worth
    50  // fixing.
    51  const oldQuantum = 1
    52  
    53  func (t *LineTable) parse(targetPC uint64, targetLine int) (b []byte, pc uint64, line int) {
    54  	// The PC/line table can be thought of as a sequence of
    55  	//  <pc update>* <line update>
    56  	// batches. Each update batch results in a (pc, line) pair,
    57  	// where line applies to every PC from pc up to but not
    58  	// including the pc of the next pair.
    59  	//
    60  	// Here we process each update individually, which simplifies
    61  	// the code, but makes the corner cases more confusing.
    62  	b, pc, line = t.Data, t.PC, t.Line
    63  	for pc <= targetPC && line != targetLine && len(b) > 0 {
    64  		code := b[0]
    65  		b = b[1:]
    66  		switch {
    67  		case code == 0:
    68  			if len(b) < 4 {
    69  				b = b[0:0]
    70  				break
    71  			}
    72  			val := binary.BigEndian.Uint32(b)
    73  			b = b[4:]
    74  			line += int(val)
    75  		case code <= 64:
    76  			line += int(code)
    77  		case code <= 128:
    78  			line -= int(code - 64)
    79  		default:
    80  			pc += oldQuantum * uint64(code-128)
    81  			continue
    82  		}
    83  		pc += oldQuantum
    84  	}
    85  	return b, pc, line
    86  }
    87  
    88  func (t *LineTable) slice(pc uint64) *LineTable {
    89  	data, pc, line := t.parse(pc, -1)
    90  	return &LineTable{Data: data, PC: pc, Line: line}
    91  }
    92  
    93  // PCToLine returns the line number for the given program counter.
    94  // Callers should use Table's PCToLine method instead.
    95  func (t *LineTable) PCToLine(pc uint64) int {
    96  	if t.isGo12() {
    97  		return t.go12PCToLine(pc)
    98  	}
    99  	_, _, line := t.parse(pc, -1)
   100  	return line
   101  }
   102  
   103  // LineToPC returns the program counter for the given line number,
   104  // considering only program counters before maxpc.
   105  // Callers should use Table's LineToPC method instead.
   106  func (t *LineTable) LineToPC(line int, maxpc uint64) uint64 {
   107  	if t.isGo12() {
   108  		return 0
   109  	}
   110  	_, pc, line1 := t.parse(maxpc, line)
   111  	if line1 != line {
   112  		return 0
   113  	}
   114  	// Subtract quantum from PC to account for post-line increment
   115  	return pc - oldQuantum
   116  }
   117  
   118  // NewLineTable returns a new PC/line table
   119  // corresponding to the encoded data.
   120  // Text must be the start address of the
   121  // corresponding text segment.
   122  func NewLineTable(data []byte, text uint64) *LineTable {
   123  	return &LineTable{Data: data, PC: text, Line: 0}
   124  }
   125  
   126  // Go 1.2 symbol table format.
   127  // See golang.org/s/go12symtab.
   128  //
   129  // A general note about the methods here: rather than try to avoid
   130  // index out of bounds errors, we trust Go to detect them, and then
   131  // we recover from the panics and treat them as indicative of a malformed
   132  // or incomplete table.
   133  //
   134  // The methods called by symtab.go, which begin with "go12" prefixes,
   135  // are expected to have that recovery logic.
   136  
   137  // isGo12 reports whether this is a Go 1.2 (or later) symbol table.
   138  func (t *LineTable) isGo12() bool {
   139  	t.go12Init()
   140  	return t.go12 == 1
   141  }
   142  
   143  const go12magic = 0xfffffffb
   144  
   145  // uintptr returns the pointer-sized value encoded at b.
   146  // The pointer size is dictated by the table being read.
   147  func (t *LineTable) uintptr(b []byte) uint64 {
   148  	if t.ptrsize == 4 {
   149  		return uint64(t.binary.Uint32(b))
   150  	}
   151  	return t.binary.Uint64(b)
   152  }
   153  
   154  // go12init initializes the Go 1.2 metadata if t is a Go 1.2 symbol table.
   155  func (t *LineTable) go12Init() {
   156  	t.mu.Lock()
   157  	defer t.mu.Unlock()
   158  	if t.go12 != 0 {
   159  		return
   160  	}
   161  
   162  	defer func() {
   163  		// If we panic parsing, assume it's not a Go 1.2 symbol table.
   164  		recover()
   165  	}()
   166  
   167  	// Check header: 4-byte magic, two zeros, pc quantum, pointer size.
   168  	t.go12 = -1 // not Go 1.2 until proven otherwise
   169  	if len(t.Data) < 16 || t.Data[4] != 0 || t.Data[5] != 0 ||
   170  		(t.Data[6] != 1 && t.Data[6] != 2 && t.Data[6] != 4) || // pc quantum
   171  		(t.Data[7] != 4 && t.Data[7] != 8) { // pointer size
   172  		return
   173  	}
   174  
   175  	switch uint32(go12magic) {
   176  	case binary.LittleEndian.Uint32(t.Data):
   177  		t.binary = binary.LittleEndian
   178  	case binary.BigEndian.Uint32(t.Data):
   179  		t.binary = binary.BigEndian
   180  	default:
   181  		return
   182  	}
   183  
   184  	t.quantum = uint32(t.Data[6])
   185  	t.ptrsize = uint32(t.Data[7])
   186  
   187  	t.nfunctab = uint32(t.uintptr(t.Data[8:]))
   188  	t.functab = t.Data[8+t.ptrsize:]
   189  	functabsize := t.nfunctab*2*t.ptrsize + t.ptrsize
   190  	fileoff := t.binary.Uint32(t.functab[functabsize:])
   191  	t.functab = t.functab[:functabsize]
   192  	t.filetab = t.Data[fileoff:]
   193  	t.nfiletab = t.binary.Uint32(t.filetab)
   194  	t.filetab = t.filetab[:t.nfiletab*4]
   195  
   196  	t.go12 = 1 // so far so good
   197  }
   198  
   199  // go12Funcs returns a slice of Funcs derived from the Go 1.2 pcln table.
   200  func (t *LineTable) go12Funcs() []Func {
   201  	// Assume it is malformed and return nil on error.
   202  	defer func() {
   203  		recover()
   204  	}()
   205  
   206  	n := len(t.functab) / int(t.ptrsize) / 2
   207  	funcs := make([]Func, n)
   208  	for i := range funcs {
   209  		f := &funcs[i]
   210  		f.Entry = t.uintptr(t.functab[2*i*int(t.ptrsize):])
   211  		f.End = t.uintptr(t.functab[(2*i+2)*int(t.ptrsize):])
   212  		info := t.Data[t.uintptr(t.functab[(2*i+1)*int(t.ptrsize):]):]
   213  		f.LineTable = t
   214  		f.FrameSize = int(t.binary.Uint32(info[t.ptrsize+2*4:]))
   215  		f.Sym = &Sym{
   216  			Value:  f.Entry,
   217  			Type:   'T',
   218  			Name:   t.string(t.binary.Uint32(info[t.ptrsize:])),
   219  			GoType: 0,
   220  			Func:   f,
   221  		}
   222  	}
   223  	return funcs
   224  }
   225  
   226  // findFunc returns the func corresponding to the given program counter.
   227  func (t *LineTable) findFunc(pc uint64) []byte {
   228  	if pc < t.uintptr(t.functab) || pc >= t.uintptr(t.functab[len(t.functab)-int(t.ptrsize):]) {
   229  		return nil
   230  	}
   231  
   232  	// The function table is a list of 2*nfunctab+1 uintptrs,
   233  	// alternating program counters and offsets to func structures.
   234  	f := t.functab
   235  	nf := t.nfunctab
   236  	for nf > 0 {
   237  		m := nf / 2
   238  		fm := f[2*t.ptrsize*m:]
   239  		if t.uintptr(fm) <= pc && pc < t.uintptr(fm[2*t.ptrsize:]) {
   240  			return t.Data[t.uintptr(fm[t.ptrsize:]):]
   241  		} else if pc < t.uintptr(fm) {
   242  			nf = m
   243  		} else {
   244  			f = f[(m+1)*2*t.ptrsize:]
   245  			nf -= m + 1
   246  		}
   247  	}
   248  	return nil
   249  }
   250  
   251  // readvarint reads, removes, and returns a varint from *pp.
   252  func (t *LineTable) readvarint(pp *[]byte) uint32 {
   253  	var v, shift uint32
   254  	p := *pp
   255  	for shift = 0; ; shift += 7 {
   256  		b := p[0]
   257  		p = p[1:]
   258  		v |= (uint32(b) & 0x7F) << shift
   259  		if b&0x80 == 0 {
   260  			break
   261  		}
   262  	}
   263  	*pp = p
   264  	return v
   265  }
   266  
   267  // string returns a Go string found at off.
   268  func (t *LineTable) string(off uint32) string {
   269  	for i := off; ; i++ {
   270  		if t.Data[i] == 0 {
   271  			return string(t.Data[off:i])
   272  		}
   273  	}
   274  }
   275  
   276  // step advances to the next pc, value pair in the encoded table.
   277  func (t *LineTable) step(p *[]byte, pc *uint64, val *int32, first bool) bool {
   278  	uvdelta := t.readvarint(p)
   279  	if uvdelta == 0 && !first {
   280  		return false
   281  	}
   282  	if uvdelta&1 != 0 {
   283  		uvdelta = ^(uvdelta >> 1)
   284  	} else {
   285  		uvdelta >>= 1
   286  	}
   287  	vdelta := int32(uvdelta)
   288  	pcdelta := t.readvarint(p) * t.quantum
   289  	*pc += uint64(pcdelta)
   290  	*val += vdelta
   291  	return true
   292  }
   293  
   294  // pcvalue reports the value associated with the target pc.
   295  // off is the offset to the beginning of the pc-value table,
   296  // and entry is the start PC for the corresponding function.
   297  func (t *LineTable) pcvalue(off uint32, entry, targetpc uint64) int32 {
   298  	p := t.Data[off:]
   299  
   300  	val := int32(-1)
   301  	pc := entry
   302  	for t.step(&p, &pc, &val, pc == entry) {
   303  		if targetpc < pc {
   304  			return val
   305  		}
   306  	}
   307  	return -1
   308  }
   309  
   310  // findFileLine scans one function in the binary looking for a
   311  // program counter in the given file on the given line.
   312  // It does so by running the pc-value tables mapping program counter
   313  // to file number. Since most functions come from a single file, these
   314  // are usually short and quick to scan. If a file match is found, then the
   315  // code goes to the expense of looking for a simultaneous line number match.
   316  func (t *LineTable) findFileLine(entry uint64, filetab, linetab uint32, filenum, line int32) uint64 {
   317  	if filetab == 0 || linetab == 0 {
   318  		return 0
   319  	}
   320  
   321  	fp := t.Data[filetab:]
   322  	fl := t.Data[linetab:]
   323  	fileVal := int32(-1)
   324  	filePC := entry
   325  	lineVal := int32(-1)
   326  	linePC := entry
   327  	fileStartPC := filePC
   328  	for t.step(&fp, &filePC, &fileVal, filePC == entry) {
   329  		if fileVal == filenum && fileStartPC < filePC {
   330  			// fileVal is in effect starting at fileStartPC up to
   331  			// but not including filePC, and it's the file we want.
   332  			// Run the PC table looking for a matching line number
   333  			// or until we reach filePC.
   334  			lineStartPC := linePC
   335  			for linePC < filePC && t.step(&fl, &linePC, &lineVal, linePC == entry) {
   336  				// lineVal is in effect until linePC, and lineStartPC < filePC.
   337  				if lineVal == line {
   338  					if fileStartPC <= lineStartPC {
   339  						return lineStartPC
   340  					}
   341  					if fileStartPC < linePC {
   342  						return fileStartPC
   343  					}
   344  				}
   345  				lineStartPC = linePC
   346  			}
   347  		}
   348  		fileStartPC = filePC
   349  	}
   350  	return 0
   351  }
   352  
   353  // go12PCToLine maps program counter to line number for the Go 1.2 pcln table.
   354  func (t *LineTable) go12PCToLine(pc uint64) (line int) {
   355  	defer func() {
   356  		if recover() != nil {
   357  			line = -1
   358  		}
   359  	}()
   360  
   361  	f := t.findFunc(pc)
   362  	if f == nil {
   363  		return -1
   364  	}
   365  	entry := t.uintptr(f)
   366  	linetab := t.binary.Uint32(f[t.ptrsize+5*4:])
   367  	return int(t.pcvalue(linetab, entry, pc))
   368  }
   369  
   370  // go12PCToFile maps program counter to file name for the Go 1.2 pcln table.
   371  func (t *LineTable) go12PCToFile(pc uint64) (file string) {
   372  	defer func() {
   373  		if recover() != nil {
   374  			file = ""
   375  		}
   376  	}()
   377  
   378  	f := t.findFunc(pc)
   379  	if f == nil {
   380  		return ""
   381  	}
   382  	entry := t.uintptr(f)
   383  	filetab := t.binary.Uint32(f[t.ptrsize+4*4:])
   384  	fno := t.pcvalue(filetab, entry, pc)
   385  	if fno <= 0 {
   386  		return ""
   387  	}
   388  	return t.string(t.binary.Uint32(t.filetab[4*fno:]))
   389  }
   390  
   391  // go12LineToPC maps a (file, line) pair to a program counter for the Go 1.2 pcln table.
   392  func (t *LineTable) go12LineToPC(file string, line int) (pc uint64) {
   393  	defer func() {
   394  		if recover() != nil {
   395  			pc = 0
   396  		}
   397  	}()
   398  
   399  	t.initFileMap()
   400  	filenum := t.fileMap[file]
   401  	if filenum == 0 {
   402  		return 0
   403  	}
   404  
   405  	// Scan all functions.
   406  	// If this turns out to be a bottleneck, we could build a map[int32][]int32
   407  	// mapping file number to a list of functions with code from that file.
   408  	for i := uint32(0); i < t.nfunctab; i++ {
   409  		f := t.Data[t.uintptr(t.functab[2*t.ptrsize*i+t.ptrsize:]):]
   410  		entry := t.uintptr(f)
   411  		filetab := t.binary.Uint32(f[t.ptrsize+4*4:])
   412  		linetab := t.binary.Uint32(f[t.ptrsize+5*4:])
   413  		pc := t.findFileLine(entry, filetab, linetab, int32(filenum), int32(line))
   414  		if pc != 0 {
   415  			return pc
   416  		}
   417  	}
   418  	return 0
   419  }
   420  
   421  // initFileMap initializes the map from file name to file number.
   422  func (t *LineTable) initFileMap() {
   423  	t.mu.Lock()
   424  	defer t.mu.Unlock()
   425  
   426  	if t.fileMap != nil {
   427  		return
   428  	}
   429  	m := make(map[string]uint32)
   430  
   431  	for i := uint32(1); i < t.nfiletab; i++ {
   432  		s := t.string(t.binary.Uint32(t.filetab[4*i:]))
   433  		m[s] = i
   434  	}
   435  	t.fileMap = m
   436  }
   437  
   438  // go12MapFiles adds to m a key for every file in the Go 1.2 LineTable.
   439  // Every key maps to obj. That's not a very interesting map, but it provides
   440  // a way for callers to obtain the list of files in the program.
   441  func (t *LineTable) go12MapFiles(m map[string]*Obj, obj *Obj) {
   442  	defer func() {
   443  		recover()
   444  	}()
   445  
   446  	t.initFileMap()
   447  	for file := range t.fileMap {
   448  		m[file] = obj
   449  	}
   450  }
   451  

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