// Copyright 2009 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.
// DWARF debug information entry parser.
// An entry is a sequence of data items of a given format.
// The first word in the entry is an index into what DWARF
// calls the ``abbreviation table.'' An abbreviation is really
// just a type descriptor: it's an array of attribute tag/value format pairs.
package dwarf
import (
"errors"
"strconv"
)
// a single entry's description: a sequence of attributes
type abbrev struct {
tag Tag
children bool
field []afield
}
type afield struct {
attr Attr
fmt format
class Class
}
// a map from entry format ids to their descriptions
type abbrevTable map[uint32]abbrev
// ParseAbbrev returns the abbreviation table that starts at byte off
// in the .debug_abbrev section.
func (d *Data) parseAbbrev(off uint64, vers int) (abbrevTable, error) {
if m, ok := d.abbrevCache[off]; ok {
return m, nil
}
data := d.abbrev
if off > uint64(len(data)) {
data = nil
} else {
data = data[off:]
}
b := makeBuf(d, unknownFormat{}, "abbrev", 0, data)
// Error handling is simplified by the buf getters
// returning an endless stream of 0s after an error.
m := make(abbrevTable)
for {
// Table ends with id == 0.
id := uint32(b.uint())
if id == 0 {
break
}
// Walk over attributes, counting.
n := 0
b1 := b // Read from copy of b.
b1.uint()
b1.uint8()
for {
tag := b1.uint()
fmt := b1.uint()
if tag == 0 && fmt == 0 {
break
}
n++
}
if b1.err != nil {
return nil, b1.err
}
// Walk over attributes again, this time writing them down.
var a abbrev
a.tag = Tag(b.uint())
a.children = b.uint8() != 0
a.field = make([]afield, n)
for i := range a.field {
a.field[i].attr = Attr(b.uint())
a.field[i].fmt = format(b.uint())
a.field[i].class = formToClass(a.field[i].fmt, a.field[i].attr, vers, &b)
}
b.uint()
b.uint()
m[id] = a
}
if b.err != nil {
return nil, b.err
}
d.abbrevCache[off] = m
return m, nil
}
// attrIsExprloc indicates attributes that allow exprloc values that
// are encoded as block values in DWARF 2 and 3. See DWARF 4, Figure
// 20.
var attrIsExprloc = map[Attr]bool{
AttrLocation: true,
AttrByteSize: true,
AttrBitOffset: true,
AttrBitSize: true,
AttrStringLength: true,
AttrLowerBound: true,
AttrReturnAddr: true,
AttrStrideSize: true,
AttrUpperBound: true,
AttrCount: true,
AttrDataMemberLoc: true,
AttrFrameBase: true,
AttrSegment: true,
AttrStaticLink: true,
AttrUseLocation: true,
AttrVtableElemLoc: true,
AttrAllocated: true,
AttrAssociated: true,
AttrDataLocation: true,
AttrStride: true,
}
// attrPtrClass indicates the *ptr class of attributes that have
// encoding formSecOffset in DWARF 4 or formData* in DWARF 2 and 3.
var attrPtrClass = map[Attr]Class{
AttrLocation: ClassLocListPtr,
AttrStmtList: ClassLinePtr,
AttrStringLength: ClassLocListPtr,
AttrReturnAddr: ClassLocListPtr,
AttrStartScope: ClassRangeListPtr,
AttrDataMemberLoc: ClassLocListPtr,
AttrFrameBase: ClassLocListPtr,
AttrMacroInfo: ClassMacPtr,
AttrSegment: ClassLocListPtr,
AttrStaticLink: ClassLocListPtr,
AttrUseLocation: ClassLocListPtr,
AttrVtableElemLoc: ClassLocListPtr,
AttrRanges: ClassRangeListPtr,
}
// formToClass returns the DWARF 4 Class for the given form. If the
// DWARF version is less then 4, it will disambiguate some forms
// depending on the attribute.
func formToClass(form format, attr Attr, vers int, b *buf) Class {
switch form {
default:
b.error("cannot determine class of unknown attribute form")
return 0
case formAddr:
return ClassAddress
case formDwarfBlock1, formDwarfBlock2, formDwarfBlock4, formDwarfBlock:
// In DWARF 2 and 3, ClassExprLoc was encoded as a
// block. DWARF 4 distinguishes ClassBlock and
// ClassExprLoc, but there are no attributes that can
// be both, so we also promote ClassBlock values in
// DWARF 4 that should be ClassExprLoc in case
// producers get this wrong.
if attrIsExprloc[attr] {
return ClassExprLoc
}
return ClassBlock
case formData1, formData2, formData4, formData8, formSdata, formUdata:
// In DWARF 2 and 3, ClassPtr was encoded as a
// constant. Unlike ClassExprLoc/ClassBlock, some
// DWARF 4 attributes need to distinguish Class*Ptr
// from ClassConstant, so we only do this promotion
// for versions 2 and 3.
if class, ok := attrPtrClass[attr]; vers < 4 && ok {
return class
}
return ClassConstant
case formFlag, formFlagPresent:
return ClassFlag
case formRefAddr, formRef1, formRef2, formRef4, formRef8, formRefUdata:
return ClassReference
case formRefSig8:
return ClassReferenceSig
case formString, formStrp:
return ClassString
case formSecOffset:
// DWARF 4 defines four *ptr classes, but doesn't
// distinguish them in the encoding. Disambiguate
// these classes using the attribute.
if class, ok := attrPtrClass[attr]; ok {
return class
}
return ClassUnknown
case formExprloc:
return ClassExprLoc
case formGnuRefAlt:
return ClassReferenceAlt
case formGnuStrpAlt:
return ClassStringAlt
}
}
// An entry is a sequence of attribute/value pairs.
type Entry struct {
Offset Offset // offset of Entry in DWARF info
Tag Tag // tag (kind of Entry)
Children bool // whether Entry is followed by children
Field []Field
}
// A Field is a single attribute/value pair in an Entry.
//
// A value can be one of several "attribute classes" defined by DWARF.
// The Go types corresponding to each class are:
//
// DWARF class Go type Class
// ----------- ------- -----
// address uint64 ClassAddress
// block []byte ClassBlock
// constant int64 ClassConstant
// flag bool ClassFlag
// reference
// to info dwarf.Offset ClassReference
// to type unit uint64 ClassReferenceSig
// string string ClassString
// exprloc []byte ClassExprLoc
// lineptr int64 ClassLinePtr
// loclistptr int64 ClassLocListPtr
// macptr int64 ClassMacPtr
// rangelistptr int64 ClassRangeListPtr
//
// For unrecognized or vendor-defined attributes, Class may be
// ClassUnknown.
type Field struct {
Attr Attr
Val interface{}
Class Class
}
// A Class is the DWARF 4 class of an attribute value.
//
// In general, a given attribute's value may take on one of several
// possible classes defined by DWARF, each of which leads to a
// slightly different interpretation of the attribute.
//
// DWARF version 4 distinguishes attribute value classes more finely
// than previous versions of DWARF. The reader will disambiguate
// coarser classes from earlier versions of DWARF into the appropriate
// DWARF 4 class. For example, DWARF 2 uses "constant" for constants
// as well as all types of section offsets, but the reader will
// canonicalize attributes in DWARF 2 files that refer to section
// offsets to one of the Class*Ptr classes, even though these classes
// were only defined in DWARF 3.
type Class int
const (
// ClassUnknown represents values of unknown DWARF class.
ClassUnknown Class = iota
// ClassAddress represents values of type uint64 that are
// addresses on the target machine.
ClassAddress
// ClassBlock represents values of type []byte whose
// interpretation depends on the attribute.
ClassBlock
// ClassConstant represents values of type int64 that are
// constants. The interpretation of this constant depends on
// the attribute.
ClassConstant
// ClassExprLoc represents values of type []byte that contain
// an encoded DWARF expression or location description.
ClassExprLoc
// ClassFlag represents values of type bool.
ClassFlag
// ClassLinePtr represents values that are an int64 offset
// into the "line" section.
ClassLinePtr
// ClassLocListPtr represents values that are an int64 offset
// into the "loclist" section.
ClassLocListPtr
// ClassMacPtr represents values that are an int64 offset into
// the "mac" section.
ClassMacPtr
// ClassMacPtr represents values that are an int64 offset into
// the "rangelist" section.
ClassRangeListPtr
// ClassReference represents values that are an Offset offset
// of an Entry in the info section (for use with Reader.Seek).
// The DWARF specification combines ClassReference and
// ClassReferenceSig into class "reference".
ClassReference
// ClassReferenceSig represents values that are a uint64 type
// signature referencing a type Entry.
ClassReferenceSig
// ClassString represents values that are strings. If the
// compilation unit specifies the AttrUseUTF8 flag (strongly
// recommended), the string value will be encoded in UTF-8.
// Otherwise, the encoding is unspecified.
ClassString
// ClassReferenceAlt represents values of type int64 that are
// an offset into the DWARF "info" section of an alternate
// object file.
ClassReferenceAlt
// ClassStringAlt represents values of type int64 that are an
// offset into the DWARF string section of an alternate object
// file.
ClassStringAlt
)
//go:generate stringer -type=Class
func (i Class) GoString() string {
return "dwarf." + i.String()
}
// Val returns the value associated with attribute Attr in Entry,
// or nil if there is no such attribute.
//
// A common idiom is to merge the check for nil return with
// the check that the value has the expected dynamic type, as in:
// v, ok := e.Val(AttrSibling).(int64)
//
func (e *Entry) Val(a Attr) interface{} {
if f := e.AttrField(a); f != nil {
return f.Val
}
return nil
}
// AttrField returns the Field associated with attribute Attr in
// Entry, or nil if there is no such attribute.
func (e *Entry) AttrField(a Attr) *Field {
for i, f := range e.Field {
if f.Attr == a {
return &e.Field[i]
}
}
return nil
}
// An Offset represents the location of an Entry within the DWARF info.
// (See Reader.Seek.)
type Offset uint32
// Entry reads a single entry from buf, decoding
// according to the given abbreviation table.
func (b *buf) entry(atab abbrevTable, ubase Offset) *Entry {
off := b.off
id := uint32(b.uint())
if id == 0 {
return &Entry{}
}
a, ok := atab[id]
if !ok {
b.error("unknown abbreviation table index")
return nil
}
e := &Entry{
Offset: off,
Tag: a.tag,
Children: a.children,
Field: make([]Field, len(a.field)),
}
for i := range e.Field {
e.Field[i].Attr = a.field[i].attr
e.Field[i].Class = a.field[i].class
fmt := a.field[i].fmt
if fmt == formIndirect {
fmt = format(b.uint())
}
var val interface{}
switch fmt {
default:
b.error("unknown entry attr format 0x" + strconv.FormatInt(int64(fmt), 16))
// address
case formAddr:
val = b.addr()
// block
case formDwarfBlock1:
val = b.bytes(int(b.uint8()))
case formDwarfBlock2:
val = b.bytes(int(b.uint16()))
case formDwarfBlock4:
val = b.bytes(int(b.uint32()))
case formDwarfBlock:
val = b.bytes(int(b.uint()))
// constant
case formData1:
val = int64(b.uint8())
case formData2:
val = int64(b.uint16())
case formData4:
val = int64(b.uint32())
case formData8:
val = int64(b.uint64())
case formSdata:
val = int64(b.int())
case formUdata:
val = int64(b.uint())
// flag
case formFlag:
val = b.uint8() == 1
// New in DWARF 4.
case formFlagPresent:
// The attribute is implicitly indicated as present, and no value is
// encoded in the debugging information entry itself.
val = true
// reference to other entry
case formRefAddr:
vers := b.format.version()
if vers == 0 {
b.error("unknown version for DW_FORM_ref_addr")
} else if vers == 2 {
val = Offset(b.addr())
} else {
is64, known := b.format.dwarf64()
if !known {
b.error("unknown size for DW_FORM_ref_addr")
} else if is64 {
val = Offset(b.uint64())
} else {
val = Offset(b.uint32())
}
}
case formRef1:
val = Offset(b.uint8()) + ubase
case formRef2:
val = Offset(b.uint16()) + ubase
case formRef4:
val = Offset(b.uint32()) + ubase
case formRef8:
val = Offset(b.uint64()) + ubase
case formRefUdata:
val = Offset(b.uint()) + ubase
// string
case formString:
val = b.string()
case formStrp:
off := b.uint32() // offset into .debug_str
if b.err != nil {
return nil
}
b1 := makeBuf(b.dwarf, unknownFormat{}, "str", 0, b.dwarf.str)
b1.skip(int(off))
val = b1.string()
if b1.err != nil {
b.err = b1.err
return nil
}
// lineptr, loclistptr, macptr, rangelistptr
// New in DWARF 4, but clang can generate them with -gdwarf-2.
// Section reference, replacing use of formData4 and formData8.
case formSecOffset, formGnuRefAlt, formGnuStrpAlt:
is64, known := b.format.dwarf64()
if !known {
b.error("unknown size for form 0x" + strconv.FormatInt(int64(fmt), 16))
} else if is64 {
val = int64(b.uint64())
} else {
val = int64(b.uint32())
}
// exprloc
// New in DWARF 4.
case formExprloc:
val = b.bytes(int(b.uint()))
// reference
// New in DWARF 4.
case formRefSig8:
// 64-bit type signature.
val = b.uint64()
}
e.Field[i].Val = val
}
if b.err != nil {
return nil
}
return e
}
// A Reader allows reading Entry structures from a DWARF ``info'' section.
// The Entry structures are arranged in a tree. The Reader's Next function
// return successive entries from a pre-order traversal of the tree.
// If an entry has children, its Children field will be true, and the children
// follow, terminated by an Entry with Tag 0.
type Reader struct {
b buf
d *Data
err error
unit int
lastChildren bool // .Children of last entry returned by Next
lastSibling Offset // .Val(AttrSibling) of last entry returned by Next
}
// Reader returns a new Reader for Data.
// The reader is positioned at byte offset 0 in the DWARF ``info'' section.
func (d *Data) Reader() *Reader {
r := &Reader{d: d}
r.Seek(0)
return r
}
// AddressSize returns the size in bytes of addresses in the current compilation
// unit.
func (r *Reader) AddressSize() int {
return r.d.unit[r.unit].asize
}
// Seek positions the Reader at offset off in the encoded entry stream.
// Offset 0 can be used to denote the first entry.
func (r *Reader) Seek(off Offset) {
d := r.d
r.err = nil
r.lastChildren = false
if off == 0 {
if len(d.unit) == 0 {
return
}
u := &d.unit[0]
r.unit = 0
r.b = makeBuf(r.d, u, "info", u.off, u.data)
return
}
i := d.offsetToUnit(off)
if i == -1 {
r.err = errors.New("offset out of range")
return
}
u := &d.unit[i]
r.unit = i
r.b = makeBuf(r.d, u, "info", off, u.data[off-u.off:])
}
// maybeNextUnit advances to the next unit if this one is finished.
func (r *Reader) maybeNextUnit() {
for len(r.b.data) == 0 && r.unit+1 < len(r.d.unit) {
r.unit++
u := &r.d.unit[r.unit]
r.b = makeBuf(r.d, u, "info", u.off, u.data)
}
}
// Next reads the next entry from the encoded entry stream.
// It returns nil, nil when it reaches the end of the section.
// It returns an error if the current offset is invalid or the data at the
// offset cannot be decoded as a valid Entry.
func (r *Reader) Next() (*Entry, error) {
if r.err != nil {
return nil, r.err
}
r.maybeNextUnit()
if len(r.b.data) == 0 {
return nil, nil
}
u := &r.d.unit[r.unit]
e := r.b.entry(u.atable, u.base)
if r.b.err != nil {
r.err = r.b.err
return nil, r.err
}
if e != nil {
r.lastChildren = e.Children
if r.lastChildren {
r.lastSibling, _ = e.Val(AttrSibling).(Offset)
}
} else {
r.lastChildren = false
}
return e, nil
}
// SkipChildren skips over the child entries associated with
// the last Entry returned by Next. If that Entry did not have
// children or Next has not been called, SkipChildren is a no-op.
func (r *Reader) SkipChildren() {
if r.err != nil || !r.lastChildren {
return
}
// If the last entry had a sibling attribute,
// that attribute gives the offset of the next
// sibling, so we can avoid decoding the
// child subtrees.
if r.lastSibling >= r.b.off {
r.Seek(r.lastSibling)
return
}
for {
e, err := r.Next()
if err != nil || e == nil || e.Tag == 0 {
break
}
if e.Children {
r.SkipChildren()
}
}
}
// clone returns a copy of the reader. This is used by the typeReader
// interface.
func (r *Reader) clone() typeReader {
return r.d.Reader()
}
// offset returns the current buffer offset. This is used by the
// typeReader interface.
func (r *Reader) offset() Offset {
return r.b.off
}
// SeekPC returns the Entry for the compilation unit that includes pc,
// and positions the reader to read the children of that unit. If pc
// is not covered by any unit, SeekPC returns ErrUnknownPC and the
// position of the reader is undefined.
//
// Because compilation units can describe multiple regions of the
// executable, in the worst case SeekPC must search through all the
// ranges in all the compilation units. Each call to SeekPC starts the
// search at the compilation unit of the last call, so in general
// looking up a series of PCs will be faster if they are sorted. If
// the caller wishes to do repeated fast PC lookups, it should build
// an appropriate index using the Ranges method.
func (r *Reader) SeekPC(pc uint64) (*Entry, error) {
unit := r.unit
for i := 0; i < len(r.d.unit); i++ {
if unit >= len(r.d.unit) {
unit = 0
}
r.err = nil
r.lastChildren = false
r.unit = unit
u := &r.d.unit[unit]
r.b = makeBuf(r.d, u, "info", u.off, u.data)
e, err := r.Next()
if err != nil {
return nil, err
}
ranges, err := r.d.Ranges(e)
if err != nil {
return nil, err
}
for _, pcs := range ranges {
if pcs[0] <= pc && pc < pcs[1] {
return e, nil
}
}
unit++
}
return nil, ErrUnknownPC
}
// Ranges returns the PC ranges covered by e, a slice of [low,high) pairs.
// Only some entry types, such as TagCompileUnit or TagSubprogram, have PC
// ranges; for others, this will return nil with no error.
func (d *Data) Ranges(e *Entry) ([][2]uint64, error) {
var ret [][2]uint64
low, lowOK := e.Val(AttrLowpc).(uint64)
var high uint64
var highOK bool
highField := e.AttrField(AttrHighpc)
if highField != nil {
switch highField.Class {
case ClassAddress:
high, highOK = highField.Val.(uint64)
case ClassConstant:
off, ok := highField.Val.(int64)
if ok {
high = low + uint64(off)
highOK = true
}
}
}
if lowOK && highOK {
ret = append(ret, [2]uint64{low, high})
}
ranges, rangesOK := e.Val(AttrRanges).(int64)
if rangesOK && d.ranges != nil {
// The initial base address is the lowpc attribute
// of the enclosing compilation unit.
// Although DWARF specifies the lowpc attribute,
// comments in gdb/dwarf2read.c say that some versions
// of GCC use the entrypc attribute, so we check that too.
var cu *Entry
if e.Tag == TagCompileUnit {
cu = e
} else {
i := d.offsetToUnit(e.Offset)
if i == -1 {
return nil, errors.New("no unit for entry")
}
u := &d.unit[i]
b := makeBuf(d, u, "info", u.off, u.data)
cu = b.entry(u.atable, u.base)
if b.err != nil {
return nil, b.err
}
}
var base uint64
if cuEntry, cuEntryOK := cu.Val(AttrEntrypc).(uint64); cuEntryOK {
base = cuEntry
} else if cuLow, cuLowOK := cu.Val(AttrLowpc).(uint64); cuLowOK {
base = cuLow
}
u := &d.unit[d.offsetToUnit(e.Offset)]
buf := makeBuf(d, u, "ranges", Offset(ranges), d.ranges[ranges:])
for len(buf.data) > 0 {
low = buf.addr()
high = buf.addr()
if low == 0 && high == 0 {
break
}
if low == ^uint64(0)>>uint((8-u.addrsize())*8) {
base = high
} else {
ret = append(ret, [2]uint64{base + low, base + high})
}
}
}
return ret, nil
}