debug/dwarf: add support for line tables

This implements a LineReader for line tables that parallels the
existing Reader for debug entries.

This code is partly based on the debug subrepo's fork of dwarf, but it
is a more complete (and, I believe, correct) implementation of the
spec and exposes a more general API.  While the debug subrepo's
implementation exposed only a PC-to-line function, this version
exposes the line table rows to the caller.  This way the caller can
make its own trade-offs when implementing PC-to-line (or line-to-PC),
such as whether or not to build an index for fast lookup.

Change-Id: Ie157bc817f55e940b6f2e1ae010c5a4e1f29c5c8
Reviewed-on: https://go-review.googlesource.com/6734
Reviewed-by: Nigel Tao <nigeltao@golang.org>

src/debug/dwarf/const.go

@@ -452,3 +452,31 @@ const (
 	encUnsignedChar   = 0x08
 	encImaginaryFloat = 0x09
 )
+
+// Statement program standard opcode encodings.
+const (
+	lnsCopy           = 1
+	lnsAdvancePC      = 2
+	lnsAdvanceLine    = 3
+	lnsSetFile        = 4
+	lnsSetColumn      = 5
+	lnsNegateStmt     = 6
+	lnsSetBasicBlock  = 7
+	lnsConstAddPC     = 8
+	lnsFixedAdvancePC = 9
+
+	// DWARF 3
+	lnsSetPrologueEnd   = 10
+	lnsSetEpilogueBegin = 11
+	lnsSetISA           = 12
+)
+
+// Statement program extended opcode encodings.
+const (
+	lneEndSequence = 1
+	lneSetAddress  = 2
+	lneDefineFile  = 3
+
+	// DWARF 4
+	lneSetDiscriminator = 4
+)

src/debug/dwarf/line.go

@@ -0,0 +1,597 @@
+// Copyright 2015 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 dwarf
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"path"
+)
+
+// A LineReader reads a sequence of LineEntry structures from a DWARF
+// "line" section for a single compilation unit. LineEntries occur in
+// order of increasing PC and each LineEntry gives metadata for the
+// instructions from that LineEntry's PC to just before the next
+// LineEntry's PC. The last entry will have its EndSequence field set.
+type LineReader struct {
+	buf buf
+
+	// Original .debug_line section data. Used by Seek.
+	section []byte
+
+	// Header information
+	version              uint16
+	minInstructionLength int
+	maxOpsPerInstruction int
+	defaultIsStmt        bool
+	lineBase             int
+	lineRange            int
+	opcodeBase           int
+	opcodeLengths        []int
+	directories          []string
+	fileEntries          []*LineFile
+
+	programOffset Offset // section offset of line number program
+	endOffset     Offset // section offset of byte following program
+
+	initialFileEntries int // initial length of fileEntries
+
+	// Current line number program state machine registers
+	state     LineEntry // public state
+	fileIndex int       // private state
+}
+
+// A LineEntry is a row in a DWARF line table.
+type LineEntry struct {
+	// Address is the program-counter value of a machine
+	// instruction generated by the compiler. This LineEntry
+	// applies to each instruction from Address to just before the
+	// Address of the next LineEntry.
+	Address uint64
+
+	// OpIndex is the index of an operation within a VLIW
+	// instruction. The index of the first operation is 0. For
+	// non-VLIW architectures, it will always be 0. Address and
+	// OpIndex together form an operation pointer that can
+	// reference any individual operation within the instruction
+	// stream.
+	OpIndex int
+
+	// File is the source file corresponding to these
+	// instructions.
+	File *LineFile
+
+	// Line is the source code line number corresponding to these
+	// instructions. Lines are numbered beginning at 1. It may be
+	// 0 if these instructions cannot be attributed to any source
+	// line.
+	Line int
+
+	// Column is the column number within the source line of these
+	// instructions. Columns are numbered beginning at 1. It may
+	// be 0 to indicate the "left edge" of the line.
+	Column int
+
+	// IsStmt indicates that Address is a recommended breakpoint
+	// location, such as the beginning of a line, statement, or a
+	// distinct subpart of a statement.
+	IsStmt bool
+
+	// BasicBlock indicates that Address is the beginning of a
+	// basic block.
+	BasicBlock bool
+
+	// PrologueEnd indicates that Address is one (of possibly
+	// many) PCs where execution should be suspended for a
+	// breakpoint on entry to the containing function.
+	//
+	// Added in DWARF 3.
+	PrologueEnd bool
+
+	// EpilogueBegin indicates that Address is one (of possibly
+	// many) PCs where execution should be suspended for a
+	// breakpoint on exit from this function.
+	//
+	// Added in DWARF 3.
+	EpilogueBegin bool
+
+	// ISA is the instruction set architecture for these
+	// instructions. Possible ISA values should be defined by the
+	// applicable ABI specification.
+	//
+	// Added in DWARF 3.
+	ISA int
+
+	// Discriminator is an arbitrary integer indicating the block
+	// to which these instructions belong. It serves to
+	// distinguish among multiple blocks that may all have with
+	// the same source file, line, and column. Where only one
+	// block exists for a given source position, it should be 0.
+	//
+	// Added in DWARF 3.
+	Discriminator int
+
+	// EndSequence indicates that Address is the first byte after
+	// the end of a sequence of target machine instructions. If it
+	// is set, only this and the Address field are meaningful. A
+	// line number table may contain information for multiple
+	// potentially disjoint instruction sequences. The last entry
+	// in a line table should always have EndSequence set.
+	EndSequence bool
+}
+
+// A LineFile is a source file referenced by a DWARF line table entry.
+type LineFile struct {
+	Name   string
+	Mtime  uint64 // Implementation defined modification time, or 0 if unknown
+	Length int    // File length, or 0 if unknown
+}
+
+// LineReader returns a new reader for the line table of compilation
+// unit cu, which must be an Entry with tag TagCompileUnit.
+//
+// If this compilation unit has no line table, it returns nil, nil.
+func (d *Data) LineReader(cu *Entry) (*LineReader, error) {
+	if d.line == nil {
+		// No line tables available.
+		return nil, nil
+	}
+
+	// Get line table information from cu.
+	off, ok := cu.Val(AttrStmtList).(int64)
+	if !ok {
+		// cu has no line table.
+		return nil, nil
+	}
+	if off > int64(len(d.line)) {
+		return nil, errors.New("AttrStmtList value out of range")
+	}
+	// AttrCompDir is optional if all file names are absolute. Use
+	// the empty string if it's not present.
+	compDir, _ := cu.Val(AttrCompDir).(string)
+
+	// Create the LineReader.
+	u := &d.unit[d.offsetToUnit(cu.Offset)]
+	buf := makeBuf(d, u, "line", Offset(off), d.line[off:])
+	// The compilation directory is implicitly directories[0].
+	r := LineReader{buf: buf, section: d.line, directories: []string{compDir}}
+
+	// Read the header.
+	if err := r.readHeader(); err != nil {
+		return nil, err
+	}
+
+	// Initialize line reader state.
+	r.Reset()
+
+	return &r, nil
+}
+
+// readHeader reads the line number program header from r.buf and sets
+// all of the header fields in r.
+func (r *LineReader) readHeader() error {
+	buf := &r.buf
+
+	// Read basic header fields [DWARF2 6.2.4].
+	hdrOffset := buf.off
+	dwarf64 := false
+	unitLength := Offset(buf.uint32())
+	if unitLength == 0xffffffff {
+		dwarf64 = true
+		unitLength = Offset(buf.uint64())
+	} else if unitLength >= 0xfffffff0 {
+		return DecodeError{"line", hdrOffset, fmt.Sprintf("total length field has reserved value %#x", unitLength)}
+	}
+	r.endOffset = buf.off + unitLength
+	if r.endOffset > buf.off+Offset(len(buf.data)) {
+		return DecodeError{"line", hdrOffset, fmt.Sprintf("line table end %d exceeds section size %d", r.endOffset, buf.off+Offset(len(buf.data)))}
+	}
+	r.version = buf.uint16()
+	if buf.err == nil && (r.version < 2 || r.version > 4) {
+		// DWARF goes to all this effort to make new opcodes
+		// backward-compatible, and then adds fields right in
+		// the middle of the header in new versions, so we're
+		// picky about only supporting known line table
+		// versions.
+		return DecodeError{"line", hdrOffset, fmt.Sprintf("unknown line table version %d", r.version)}
+	}
+	var headerLength Offset
+	if dwarf64 {
+		headerLength = Offset(buf.uint64())
+	} else {
+		headerLength = Offset(buf.uint32())
+	}
+	r.programOffset = buf.off + headerLength
+	r.minInstructionLength = int(buf.uint8())
+	if r.version >= 4 {
+		// [DWARF4 6.2.4]
+		r.maxOpsPerInstruction = int(buf.uint8())
+	} else {
+		r.maxOpsPerInstruction = 1
+	}
+	r.defaultIsStmt = buf.uint8() != 0
+	r.lineBase = int(int8(buf.uint8()))
+	r.lineRange = int(buf.uint8())
+
+	// Validate header.
+	if buf.err != nil {
+		return buf.err
+	}
+	if r.maxOpsPerInstruction == 0 {
+		return DecodeError{"line", hdrOffset, "invalid maximum operations per instruction: 0"}
+	}
+	if r.lineRange == 0 {
+		return DecodeError{"line", hdrOffset, "invalid line range: 0"}
+	}
+
+	// Read standard opcode length table. This table starts with opcode 1.
+	r.opcodeBase = int(buf.uint8())
+	r.opcodeLengths = make([]int, r.opcodeBase)
+	for i := 1; i < r.opcodeBase; i++ {
+		r.opcodeLengths[i] = int(buf.uint8())
+	}
+
+	// Validate opcode lengths.
+	if buf.err != nil {
+		return buf.err
+	}
+	for i, length := range r.opcodeLengths {
+		if known, ok := knownOpcodeLengths[i]; ok && known != length {
+			return DecodeError{"line", hdrOffset, fmt.Sprintf("opcode %d expected to have length %d, but has length %d", i, known, length)}
+		}
+	}
+
+	// Read include directories table. The caller already set
+	// directories[0] to the compilation directory.
+	for {
+		directory := buf.string()
+		if buf.err != nil {
+			return buf.err
+		}
+		if len(directory) == 0 {
+			break
+		}
+		if !path.IsAbs(directory) {
+			// Relative paths are implicitly relative to
+			// the compilation directory.
+			directory = path.Join(r.directories[0], directory)
+		}
+		r.directories = append(r.directories, directory)
+	}
+
+	// Read file name list. File numbering starts with 1, so leave
+	// the first entry nil.
+	r.fileEntries = make([]*LineFile, 1)
+	for {
+		if done, err := r.readFileEntry(); err != nil {
+			return err
+		} else if done {
+			break
+		}
+	}
+	r.initialFileEntries = len(r.fileEntries)
+
+	return buf.err
+}
+
+// readFileEntry reads a file entry from either the header or a
+// DW_LNE_define_file extended opcode and adds it to r.fileEntries. A
+// true return value indicates that there are no more entries to read.
+func (r *LineReader) readFileEntry() (bool, error) {
+	name := r.buf.string()
+	if r.buf.err != nil {
+		return false, r.buf.err
+	}
+	if len(name) == 0 {
+		return true, nil
+	}
+	off := r.buf.off
+	dirIndex := int(r.buf.uint())
+	if !path.IsAbs(name) {
+		if dirIndex >= len(r.directories) {
+			return false, DecodeError{"line", off, "directory index too large"}
+		}
+		name = path.Join(r.directories[dirIndex], name)
+	}
+	mtime := r.buf.uint()
+	length := int(r.buf.uint())
+
+	r.fileEntries = append(r.fileEntries, &LineFile{name, mtime, length})
+	return false, nil
+}
+
+// updateFile updates r.state.File after r.fileIndex has
+// changed or r.fileEntries has changed.
+func (r *LineReader) updateFile() {
+	if r.fileIndex < len(r.fileEntries) {
+		r.state.File = r.fileEntries[r.fileIndex]
+	} else {
+		r.state.File = nil
+	}
+}
+
+// Next sets *entry to the next row in this line table and moves to
+// the next row. If there are no more entries and the line table is
+// properly terminated, it returns io.EOF.
+//
+// Rows are always in order of increasing entry.Address, but
+// entry.Line may go forward or backward.
+func (r *LineReader) Next(entry *LineEntry) error {
+	if r.buf.err != nil {
+		return r.buf.err
+	}
+
+	// Execute opcodes until we reach an opcode that emits a line
+	// table entry.
+	for {
+		if len(r.buf.data) == 0 {
+			return io.EOF
+		}
+		emit := r.step(entry)
+		if r.buf.err != nil {
+			return r.buf.err
+		}
+		if emit {
+			return nil
+		}
+	}
+}
+
+// knownOpcodeLengths gives the opcode lengths (in varint arguments)
+// of known standard opcodes.
+var knownOpcodeLengths = map[int]int{
+	lnsCopy:             0,
+	lnsAdvancePC:        1,
+	lnsAdvanceLine:      1,
+	lnsSetFile:          1,
+	lnsNegateStmt:       0,
+	lnsSetBasicBlock:    0,
+	lnsConstAddPC:       0,
+	lnsSetPrologueEnd:   0,
+	lnsSetEpilogueBegin: 0,
+	lnsSetISA:           1,
+	// lnsFixedAdvancePC takes a uint8 rather than a varint; it's
+	// unclear what length the header is supposed to claim, so
+	// ignore it.
+}
+
+// step processes the next opcode and updates r.state. If the opcode
+// emits a row in the line table, this updates *entry and returns
+// true.
+func (r *LineReader) step(entry *LineEntry) bool {
+	opcode := int(r.buf.uint8())
+
+	if opcode >= r.opcodeBase {
+		// Special opcode [DWARF2 6.2.5.1, DWARF4 6.2.5.1]
+		adjustedOpcode := opcode - r.opcodeBase
+		r.advancePC(adjustedOpcode / r.lineRange)
+		lineDelta := r.lineBase + int(adjustedOpcode)%r.lineRange
+		r.state.Line += lineDelta
+		goto emit
+	}
+
+	switch opcode {
+	case 0:
+		// Extended opcode [DWARF2 6.2.5.3]
+		length := Offset(r.buf.uint())
+		startOff := r.buf.off
+		opcode := r.buf.uint8()
+
+		switch opcode {
+		case lneEndSequence:
+			r.state.EndSequence = true
+			*entry = r.state
+			r.resetState()
+
+		case lneSetAddress:
+			r.state.Address = r.buf.addr()
+
+		case lneDefineFile:
+			if done, err := r.readFileEntry(); err != nil {
+				r.buf.err = err
+				return false
+			} else if done {
+				r.buf.err = DecodeError{"line", startOff, "malformed DW_LNE_define_file operation"}
+				return false
+			}
+			r.updateFile()
+
+		case lneSetDiscriminator:
+			// [DWARF4 6.2.5.3]
+			r.state.Discriminator = int(r.buf.uint())
+		}
+
+		r.buf.skip(int(startOff + length - r.buf.off))
+
+		if opcode == lneEndSequence {
+			return true
+		}
+
+	// Standard opcodes [DWARF2 6.2.5.2]
+	case lnsCopy:
+		goto emit
+
+	case lnsAdvancePC:
+		r.advancePC(int(r.buf.uint()))
+
+	case lnsAdvanceLine:
+		r.state.Line += int(r.buf.int())
+
+	case lnsSetFile:
+		r.fileIndex = int(r.buf.uint())
+		r.updateFile()
+
+	case lnsSetColumn:
+		r.state.Column = int(r.buf.uint())
+
+	case lnsNegateStmt:
+		r.state.IsStmt = !r.state.IsStmt
+
+	case lnsSetBasicBlock:
+		r.state.BasicBlock = true
+
+	case lnsConstAddPC:
+		r.advancePC((255 - r.opcodeBase) / r.lineRange)
+
+	case lnsFixedAdvancePC:
+		r.state.Address += uint64(r.buf.uint16())
+
+	// DWARF3 standard opcodes [DWARF3 6.2.5.2]
+	case lnsSetPrologueEnd:
+		r.state.PrologueEnd = true
+
+	case lnsSetEpilogueBegin:
+		r.state.EpilogueBegin = true
+
+	case lnsSetISA:
+		r.state.ISA = int(r.buf.uint())
+
+	default:
+		// Unhandled standard opcode. Skip the number of
+		// arguments that the prologue says this opcode has.
+		for i := 0; i < r.opcodeLengths[opcode]; i++ {
+			r.buf.uint()
+		}
+	}
+	return false
+
+emit:
+	*entry = r.state
+	r.state.BasicBlock = false
+	r.state.PrologueEnd = false
+	r.state.EpilogueBegin = false
+	r.state.Discriminator = 0
+	return true
+}
+
+// advancePC advances "operation pointer" (the combination of Address
+// and OpIndex) in r.state by opAdvance steps.
+func (r *LineReader) advancePC(opAdvance int) {
+	opIndex := r.state.OpIndex + opAdvance
+	r.state.Address += uint64(r.minInstructionLength * (opIndex / r.maxOpsPerInstruction))
+	r.state.OpIndex = opIndex % r.maxOpsPerInstruction
+}
+
+// A LineReaderPos represents a position in a line table.
+type LineReaderPos struct {
+	// off is the current offset in the DWARF line section.
+	off Offset
+	// numFileEntries is the length of fileEntries.
+	numFileEntries int
+	// state and fileIndex are the statement machine state at
+	// offset off.
+	state     LineEntry
+	fileIndex int
+}
+
+// Tell returns the current position in the line table.
+func (r *LineReader) Tell() LineReaderPos {
+	return LineReaderPos{r.buf.off, len(r.fileEntries), r.state, r.fileIndex}
+}
+
+// Seek restores the line table reader to a position returned by Tell.
+//
+// The argument pos must have been returned by a call to Tell on this
+// line table.
+func (r *LineReader) Seek(pos LineReaderPos) {
+	r.buf.off = pos.off
+	r.buf.data = r.section[r.buf.off:r.endOffset]
+	r.fileEntries = r.fileEntries[:pos.numFileEntries]
+	r.state = pos.state
+	r.fileIndex = pos.fileIndex
+}
+
+// Reset repositions the line table reader at the beginning of the
+// line table.
+func (r *LineReader) Reset() {
+	// Reset buffer to the line number program offset.
+	r.buf.off = r.programOffset
+	r.buf.data = r.section[r.buf.off:r.endOffset]
+
+	// Reset file entries list.
+	r.fileEntries = r.fileEntries[:r.initialFileEntries]
+
+	// Reset line number program state.
+	r.resetState()
+}
+
+// resetState resets r.state to its default values
+func (r *LineReader) resetState() {
+	// Reset the state machine registers to the defaults given in
+	// [DWARF4 6.2.2].
+	r.state = LineEntry{
+		Address:       0,
+		OpIndex:       0,
+		File:          nil,
+		Line:          1,
+		Column:        0,
+		IsStmt:        r.defaultIsStmt,
+		BasicBlock:    false,
+		PrologueEnd:   false,
+		EpilogueBegin: false,
+		ISA:           0,
+		Discriminator: 0,
+	}
+	r.fileIndex = 1
+	r.updateFile()
+}
+
+// ErrUnknownPC is the error returned by LineReader.ScanPC when the
+// seek PC is not covered by any entry in the line table.
+var ErrUnknownPC = errors.New("ErrUnknownPC")
+
+// SeekPC sets *entry to the LineEntry that includes pc and positions
+// the reader on the next entry in the line table. If necessary, this
+// will seek backwards to find pc.
+//
+// If pc is not covered by any entry in this line table, SeekPC
+// returns ErrUnknownPC. In this case, *entry and the final seek
+// position are unspecified.
+//
+// Note that DWARF line tables only permit sequential, forward scans.
+// Hence, in the worst case, this takes time linear in the size of the
+// line table. If the caller wishes to do repeated fast PC lookups, it
+// should build an appropriate index of the line table.
+func (r *LineReader) SeekPC(pc uint64, entry *LineEntry) error {
+	if err := r.Next(entry); err != nil {
+		return err
+	}
+	if entry.Address > pc {
+		// We're too far. Start at the beginning of the table.
+		r.Reset()
+		if err := r.Next(entry); err != nil {
+			return err
+		}
+		if entry.Address > pc {
+			// The whole table starts after pc.
+			r.Reset()
+			return ErrUnknownPC
+		}
+	}
+
+	// Scan until we pass pc, then back up one.
+	for {
+		var next LineEntry
+		pos := r.Tell()
+		if err := r.Next(&next); err != nil {
+			if err == io.EOF {
+				return ErrUnknownPC
+			}
+			return err
+		}
+		if next.Address > pc {
+			if entry.EndSequence {
+				// pc is in a hole in the table.
+				return ErrUnknownPC
+			}
+			// entry is the desired entry. Back up the
+			// cursor to "next" and return success.
+			r.Seek(pos)
+			return nil
+		}
+		*entry = next
+	}
+}