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// 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.
// Package gosym implements access to the Go symbol
// and line number tables embedded in Go binaries generated
// by the gc compilers.
package gosym
// The table format is a variant of the format used in Plan 9's a.out
// format, documented at https://9p.io/magic/man2html/6/a.out.
// The best reference for the differences between the Plan 9 format
// and the Go format is the runtime source, specifically ../../runtime/symtab.c.
import (
"bytes"
"encoding/binary"
"fmt"
"strconv"
"strings"
)
/*
* Symbols
*/
// A Sym represents a single symbol table entry.
type Sym struct {
Value uint64
Type byte
Name string
GoType uint64
// If this symbol is a function symbol, the corresponding Func
Func *Func
}
// Static reports whether this symbol is static (not visible outside its file).
func (s *Sym) Static() bool { return s.Type >= 'a' }
// PackageName returns the package part of the symbol name,
// or the empty string if there is none.
func (s *Sym) PackageName() string {
pathend := strings.LastIndex(s.Name, "/")
if pathend < 0 {
pathend = 0
}
if i := strings.Index(s.Name[pathend:], "."); i != -1 {
return s.Name[:pathend+i]
}
return ""
}
// ReceiverName returns the receiver type name of this symbol,
// or the empty string if there is none.
func (s *Sym) ReceiverName() string {
pathend := strings.LastIndex(s.Name, "/")
if pathend < 0 {
pathend = 0
}
l := strings.Index(s.Name[pathend:], ".")
r := strings.LastIndex(s.Name[pathend:], ".")
if l == -1 || r == -1 || l == r {
return ""
}
return s.Name[pathend+l+1 : pathend+r]
}
// BaseName returns the symbol name without the package or receiver name.
func (s *Sym) BaseName() string {
if i := strings.LastIndex(s.Name, "."); i != -1 {
return s.Name[i+1:]
}
return s.Name
}
// A Func collects information about a single function.
type Func struct {
Entry uint64
*Sym
End uint64
Params []*Sym // nil for Go 1.3 and later binaries
Locals []*Sym // nil for Go 1.3 and later binaries
FrameSize int
LineTable *LineTable
Obj *Obj
}
// An Obj represents a collection of functions in a symbol table.
//
// The exact method of division of a binary into separate Objs is an internal detail
// of the symbol table format.
//
// In early versions of Go each source file became a different Obj.
//
// In Go 1 and Go 1.1, each package produced one Obj for all Go sources
// and one Obj per C source file.
//
// In Go 1.2, there is a single Obj for the entire program.
type Obj struct {
// Funcs is a list of functions in the Obj.
Funcs []Func
// In Go 1.1 and earlier, Paths is a list of symbols corresponding
// to the source file names that produced the Obj.
// In Go 1.2, Paths is nil.
// Use the keys of Table.Files to obtain a list of source files.
Paths []Sym // meta
}
/*
* Symbol tables
*/
// Table represents a Go symbol table. It stores all of the
// symbols decoded from the program and provides methods to translate
// between symbols, names, and addresses.
type Table struct {
Syms []Sym // nil for Go 1.3 and later binaries
Funcs []Func
Files map[string]*Obj // nil for Go 1.2 and later binaries
Objs []Obj // nil for Go 1.2 and later binaries
go12line *LineTable // Go 1.2 line number table
}
type sym struct {
value uint64
gotype uint64
typ byte
name []byte
}
var (
littleEndianSymtab = []byte{0xFD, 0xFF, 0xFF, 0xFF, 0x00, 0x00, 0x00}
bigEndianSymtab = []byte{0xFF, 0xFF, 0xFF, 0xFD, 0x00, 0x00, 0x00}
oldLittleEndianSymtab = []byte{0xFE, 0xFF, 0xFF, 0xFF, 0x00, 0x00}
)
func walksymtab(data []byte, fn func(sym) error) error {
if len(data) == 0 { // missing symtab is okay
return nil
}
var order binary.ByteOrder = binary.BigEndian
newTable := false
switch {
case bytes.HasPrefix(data, oldLittleEndianSymtab):
// Same as Go 1.0, but little endian.
// Format was used during interim development between Go 1.0 and Go 1.1.
// Should not be widespread, but easy to support.
data = data[6:]
order = binary.LittleEndian
case bytes.HasPrefix(data, bigEndianSymtab):
newTable = true
case bytes.HasPrefix(data, littleEndianSymtab):
newTable = true
order = binary.LittleEndian
}
var ptrsz int
if newTable {
if len(data) < 8 {
return &DecodingError{len(data), "unexpected EOF", nil}
}
ptrsz = int(data[7])
if ptrsz != 4 && ptrsz != 8 {
return &DecodingError{7, "invalid pointer size", ptrsz}
}
data = data[8:]
}
var s sym
p := data
for len(p) >= 4 {
var typ byte
if newTable {
// Symbol type, value, Go type.
typ = p[0] & 0x3F
wideValue := p[0]&0x40 != 0
goType := p[0]&0x80 != 0
if typ < 26 {
typ += 'A'
} else {
typ += 'a' - 26
}
s.typ = typ
p = p[1:]
if wideValue {
if len(p) < ptrsz {
return &DecodingError{len(data), "unexpected EOF", nil}
}
// fixed-width value
if ptrsz == 8 {
s.value = order.Uint64(p[0:8])
p = p[8:]
} else {
s.value = uint64(order.Uint32(p[0:4]))
p = p[4:]
}
} else {
// varint value
s.value = 0
shift := uint(0)
for len(p) > 0 && p[0]&0x80 != 0 {
s.value |= uint64(p[0]&0x7F) << shift
shift += 7
p = p[1:]
}
if len(p) == 0 {
return &DecodingError{len(data), "unexpected EOF", nil}
}
s.value |= uint64(p[0]) << shift
p = p[1:]
}
if goType {
if len(p) < ptrsz {
return &DecodingError{len(data), "unexpected EOF", nil}
}
// fixed-width go type
if ptrsz == 8 {
s.gotype = order.Uint64(p[0:8])
p = p[8:]
} else {
s.gotype = uint64(order.Uint32(p[0:4]))
p = p[4:]
}
}
} else {
// Value, symbol type.
s.value = uint64(order.Uint32(p[0:4]))
if len(p) < 5 {
return &DecodingError{len(data), "unexpected EOF", nil}
}
typ = p[4]
if typ&0x80 == 0 {
return &DecodingError{len(data) - len(p) + 4, "bad symbol type", typ}
}
typ &^= 0x80
s.typ = typ
p = p[5:]
}
// Name.
var i int
var nnul int
for i = 0; i < len(p); i++ {
if p[i] == 0 {
nnul = 1
break
}
}
switch typ {
case 'z', 'Z':
p = p[i+nnul:]
for i = 0; i+2 <= len(p); i += 2 {
if p[i] == 0 && p[i+1] == 0 {
nnul = 2
break
}
}
}
if len(p) < i+nnul {
return &DecodingError{len(data), "unexpected EOF", nil}
}
s.name = p[0:i]
i += nnul
p = p[i:]
if !newTable {
if len(p) < 4 {
return &DecodingError{len(data), "unexpected EOF", nil}
}
// Go type.
s.gotype = uint64(order.Uint32(p[:4]))
p = p[4:]
}
fn(s)
}
return nil
}
// NewTable decodes the Go symbol table (the ".gosymtab" section in ELF),
// returning an in-memory representation.
// Starting with Go 1.3, the Go symbol table no longer includes symbol data.
func NewTable(symtab []byte, pcln *LineTable) (*Table, error) {
var n int
err := walksymtab(symtab, func(s sym) error {
n++
return nil
})
if err != nil {
return nil, err
}
var t Table
if pcln.isGo12() {
t.go12line = pcln
}
fname := make(map[uint16]string)
t.Syms = make([]Sym, 0, n)
nf := 0
nz := 0
lasttyp := uint8(0)
err = walksymtab(symtab, func(s sym) error {
n := len(t.Syms)
t.Syms = t.Syms[0 : n+1]
ts := &t.Syms[n]
ts.Type = s.typ
ts.Value = s.value
ts.GoType = s.gotype
switch s.typ {
default:
// rewrite name to use . instead of · (c2 b7)
w := 0
b := s.name
for i := 0; i < len(b); i++ {
if b[i] == 0xc2 && i+1 < len(b) && b[i+1] == 0xb7 {
i++
b[i] = '.'
}
b[w] = b[i]
w++
}
ts.Name = string(s.name[0:w])
case 'z', 'Z':
if lasttyp != 'z' && lasttyp != 'Z' {
nz++
}
for i := 0; i < len(s.name); i += 2 {
eltIdx := binary.BigEndian.Uint16(s.name[i : i+2])
elt, ok := fname[eltIdx]
if !ok {
return &DecodingError{-1, "bad filename code", eltIdx}
}
if n := len(ts.Name); n > 0 && ts.Name[n-1] != '/' {
ts.Name += "/"
}
ts.Name += elt
}
}
switch s.typ {
case 'T', 't', 'L', 'l':
nf++
case 'f':
fname[uint16(s.value)] = ts.Name
}
lasttyp = s.typ
return nil
})
if err != nil {
return nil, err
}
t.Funcs = make([]Func, 0, nf)
t.Files = make(map[string]*Obj)
var obj *Obj
if t.go12line != nil {
// Put all functions into one Obj.
t.Objs = make([]Obj, 1)
obj = &t.Objs[0]
t.go12line.go12MapFiles(t.Files, obj)
} else {
t.Objs = make([]Obj, 0, nz)
}
// Count text symbols and attach frame sizes, parameters, and
// locals to them. Also, find object file boundaries.
lastf := 0
for i := 0; i < len(t.Syms); i++ {
sym := &t.Syms[i]
switch sym.Type {
case 'Z', 'z': // path symbol
if t.go12line != nil {
// Go 1.2 binaries have the file information elsewhere. Ignore.
break
}
// Finish the current object
if obj != nil {
obj.Funcs = t.Funcs[lastf:]
}
lastf = len(t.Funcs)
// Start new object
n := len(t.Objs)
t.Objs = t.Objs[0 : n+1]
obj = &t.Objs[n]
// Count & copy path symbols
var end int
for end = i + 1; end < len(t.Syms); end++ {
if c := t.Syms[end].Type; c != 'Z' && c != 'z' {
break
}
}
obj.Paths = t.Syms[i:end]
i = end - 1 // loop will i++
// Record file names
depth := 0
for j := range obj.Paths {
s := &obj.Paths[j]
if s.Name == "" {
depth--
} else {
if depth == 0 {
t.Files[s.Name] = obj
}
depth++
}
}
case 'T', 't', 'L', 'l': // text symbol
if n := len(t.Funcs); n > 0 {
t.Funcs[n-1].End = sym.Value
}
if sym.Name == "runtime.etext" || sym.Name == "etext" {
continue
}
// Count parameter and local (auto) syms
var np, na int
var end int
countloop:
for end = i + 1; end < len(t.Syms); end++ {
switch t.Syms[end].Type {
case 'T', 't', 'L', 'l', 'Z', 'z':
break countloop
case 'p':
np++
case 'a':
na++
}
}
// Fill in the function symbol
n := len(t.Funcs)
t.Funcs = t.Funcs[0 : n+1]
fn := &t.Funcs[n]
sym.Func = fn
fn.Params = make([]*Sym, 0, np)
fn.Locals = make([]*Sym, 0, na)
fn.Sym = sym
fn.Entry = sym.Value
fn.Obj = obj
if t.go12line != nil {
// All functions share the same line table.
// It knows how to narrow down to a specific
// function quickly.
fn.LineTable = t.go12line
} else if pcln != nil {
fn.LineTable = pcln.slice(fn.Entry)
pcln = fn.LineTable
}
for j := i; j < end; j++ {
s := &t.Syms[j]
switch s.Type {
case 'm':
fn.FrameSize = int(s.Value)
case 'p':
n := len(fn.Params)
fn.Params = fn.Params[0 : n+1]
fn.Params[n] = s
case 'a':
n := len(fn.Locals)
fn.Locals = fn.Locals[0 : n+1]
fn.Locals[n] = s
}
}
i = end - 1 // loop will i++
}
}
if t.go12line != nil && nf == 0 {
t.Funcs = t.go12line.go12Funcs()
}
if obj != nil {
obj.Funcs = t.Funcs[lastf:]
}
return &t, nil
}
// PCToFunc returns the function containing the program counter pc,
// or nil if there is no such function.
func (t *Table) PCToFunc(pc uint64) *Func {
funcs := t.Funcs
for len(funcs) > 0 {
m := len(funcs) / 2
fn := &funcs[m]
switch {
case pc < fn.Entry:
funcs = funcs[0:m]
case fn.Entry <= pc && pc < fn.End:
return fn
default:
funcs = funcs[m+1:]
}
}
return nil
}
// PCToLine looks up line number information for a program counter.
// If there is no information, it returns fn == nil.
func (t *Table) PCToLine(pc uint64) (file string, line int, fn *Func) {
if fn = t.PCToFunc(pc); fn == nil {
return
}
if t.go12line != nil {
file = t.go12line.go12PCToFile(pc)
line = t.go12line.go12PCToLine(pc)
} else {
file, line = fn.Obj.lineFromAline(fn.LineTable.PCToLine(pc))
}
return
}
// LineToPC looks up the first program counter on the given line in
// the named file. It returns UnknownPathError or UnknownLineError if
// there is an error looking up this line.
func (t *Table) LineToPC(file string, line int) (pc uint64, fn *Func, err error) {
obj, ok := t.Files[file]
if !ok {
return 0, nil, UnknownFileError(file)
}
if t.go12line != nil {
pc := t.go12line.go12LineToPC(file, line)
if pc == 0 {
return 0, nil, &UnknownLineError{file, line}
}
return pc, t.PCToFunc(pc), nil
}
abs, err := obj.alineFromLine(file, line)
if err != nil {
return
}
for i := range obj.Funcs {
f := &obj.Funcs[i]
pc := f.LineTable.LineToPC(abs, f.End)
if pc != 0 {
return pc, f, nil
}
}
return 0, nil, &UnknownLineError{file, line}
}
// LookupSym returns the text, data, or bss symbol with the given name,
// or nil if no such symbol is found.
func (t *Table) LookupSym(name string) *Sym {
// TODO(austin) Maybe make a map
for i := range t.Syms {
s := &t.Syms[i]
switch s.Type {
case 'T', 't', 'L', 'l', 'D', 'd', 'B', 'b':
if s.Name == name {
return s
}
}
}
return nil
}
// LookupFunc returns the text, data, or bss symbol with the given name,
// or nil if no such symbol is found.
func (t *Table) LookupFunc(name string) *Func {
for i := range t.Funcs {
f := &t.Funcs[i]
if f.Sym.Name == name {
return f
}
}
return nil
}
// SymByAddr returns the text, data, or bss symbol starting at the given address.
func (t *Table) SymByAddr(addr uint64) *Sym {
for i := range t.Syms {
s := &t.Syms[i]
switch s.Type {
case 'T', 't', 'L', 'l', 'D', 'd', 'B', 'b':
if s.Value == addr {
return s
}
}
}
return nil
}
/*
* Object files
*/
// This is legacy code for Go 1.1 and earlier, which used the
// Plan 9 format for pc-line tables. This code was never quite
// correct. It's probably very close, and it's usually correct, but
// we never quite found all the corner cases.
//
// Go 1.2 and later use a simpler format, documented at golang.org/s/go12symtab.
func (o *Obj) lineFromAline(aline int) (string, int) {
type stackEnt struct {
path string
start int
offset int
prev *stackEnt
}
noPath := &stackEnt{"", 0, 0, nil}
tos := noPath
pathloop:
for _, s := range o.Paths {
val := int(s.Value)
switch {
case val > aline:
break pathloop
case val == 1:
// Start a new stack
tos = &stackEnt{s.Name, val, 0, noPath}
case s.Name == "":
// Pop
if tos == noPath {
return "<malformed symbol table>", 0
}
tos.prev.offset += val - tos.start
tos = tos.prev
default:
// Push
tos = &stackEnt{s.Name, val, 0, tos}
}
}
if tos == noPath {
return "", 0
}
return tos.path, aline - tos.start - tos.offset + 1
}
func (o *Obj) alineFromLine(path string, line int) (int, error) {
if line < 1 {
return 0, &UnknownLineError{path, line}
}
for i, s := range o.Paths {
// Find this path
if s.Name != path {
continue
}
// Find this line at this stack level
depth := 0
var incstart int
line += int(s.Value)
pathloop:
for _, s := range o.Paths[i:] {
val := int(s.Value)
switch {
case depth == 1 && val >= line:
return line - 1, nil
case s.Name == "":
depth--
if depth == 0 {
break pathloop
} else if depth == 1 {
line += val - incstart
}
default:
if depth == 1 {
incstart = val
}
depth++
}
}
return 0, &UnknownLineError{path, line}
}
return 0, UnknownFileError(path)
}
/*
* Errors
*/
// UnknownFileError represents a failure to find the specific file in
// the symbol table.
type UnknownFileError string
func (e UnknownFileError) Error() string { return "unknown file: " + string(e) }
// UnknownLineError represents a failure to map a line to a program
// counter, either because the line is beyond the bounds of the file
// or because there is no code on the given line.
type UnknownLineError struct {
File string
Line int
}
func (e *UnknownLineError) Error() string {
return "no code at " + e.File + ":" + strconv.Itoa(e.Line)
}
// DecodingError represents an error during the decoding of
// the symbol table.
type DecodingError struct {
off int
msg string
val interface{}
}
func (e *DecodingError) Error() string {
msg := e.msg
if e.val != nil {
msg += fmt.Sprintf(" '%v'", e.val)
}
msg += fmt.Sprintf(" at byte %#x", e.off)
return msg
}