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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 elf implements access to ELF object files.
package elf
import (
"bytes"
"debug/dwarf"
"encoding/binary"
"fmt"
"io"
"os"
)
// TODO: error reporting detail
/*
* Internal ELF representation
*/
// A FileHeader represents an ELF file header.
type FileHeader struct {
Class Class
Data Data
Version Version
OSABI OSABI
ABIVersion uint8
ByteOrder binary.ByteOrder
Type Type
Machine Machine
}
// A File represents an open ELF file.
type File struct {
FileHeader
Sections []*Section
Progs []*Prog
closer io.Closer
}
// A SectionHeader represents a single ELF section header.
type SectionHeader struct {
Name string
Type SectionType
Flags SectionFlag
Addr uint64
Offset uint64
Size uint64
Link uint32
Info uint32
Addralign uint64
Entsize uint64
}
// A Section represents a single section in an ELF file.
type Section struct {
SectionHeader
// Embed ReaderAt for ReadAt method.
// Do not embed SectionReader directly
// to avoid having Read and Seek.
// If a client wants Read and Seek it must use
// Open() to avoid fighting over the seek offset
// with other clients.
io.ReaderAt
sr *io.SectionReader
}
// Data reads and returns the contents of the ELF section.
func (s *Section) Data() ([]byte, os.Error) {
dat := make([]byte, s.sr.Size())
n, err := s.sr.ReadAt(dat, 0)
return dat[0:n], err
}
// Open returns a new ReadSeeker reading the ELF section.
func (s *Section) Open() io.ReadSeeker { return io.NewSectionReader(s.sr, 0, 1<<63-1) }
// A ProgHeader represents a single ELF program header.
type ProgHeader struct {
Type ProgType
Flags ProgFlag
Vaddr uint64
Paddr uint64
Filesz uint64
Memsz uint64
Align uint64
}
// A Prog represents a single ELF program header in an ELF binary.
type Prog struct {
ProgHeader
// Embed ReaderAt for ReadAt method.
// Do not embed SectionReader directly
// to avoid having Read and Seek.
// If a client wants Read and Seek it must use
// Open() to avoid fighting over the seek offset
// with other clients.
io.ReaderAt
sr *io.SectionReader
}
// Open returns a new ReadSeeker reading the ELF program body.
func (p *Prog) Open() io.ReadSeeker { return io.NewSectionReader(p.sr, 0, 1<<63-1) }
// A Symbol represents an entry in an ELF symbol table section.
type Symbol struct {
Name uint32
Info, Other byte
Section uint32
Value, Size uint64
}
/*
* ELF reader
*/
type FormatError struct {
off int64
msg string
val interface{}
}
func (e *FormatError) String() string {
msg := e.msg
if e.val != nil {
msg += fmt.Sprintf(" '%v' ", e.val)
}
msg += fmt.Sprintf("in record at byte %#x", e.off)
return msg
}
// Open opens the named file using os.Open and prepares it for use as an ELF binary.
func Open(name string) (*File, os.Error) {
f, err := os.Open(name, os.O_RDONLY, 0)
if err != nil {
return nil, err
}
ff, err := NewFile(f)
if err != nil {
f.Close()
return nil, err
}
ff.closer = f
return ff, nil
}
// Close closes the File.
// If the File was created using NewFile directly instead of Open,
// Close has no effect.
func (f *File) Close() os.Error {
var err os.Error
if f.closer != nil {
err = f.closer.Close()
f.closer = nil
}
return err
}
// NewFile creates a new File for acecssing an ELF binary in an underlying reader.
// The ELF binary is expected to start at position 0 in the ReaderAt.
func NewFile(r io.ReaderAt) (*File, os.Error) {
sr := io.NewSectionReader(r, 0, 1<<63-1)
// Read and decode ELF identifier
var ident [16]uint8
if _, err := r.ReadAt(&ident, 0); err != nil {
return nil, err
}
if ident[0] != '\x7f' || ident[1] != 'E' || ident[2] != 'L' || ident[3] != 'F' {
return nil, &FormatError{0, "bad magic number", ident[0:4]}
}
f := new(File)
f.Class = Class(ident[EI_CLASS])
switch f.Class {
case ELFCLASS32:
case ELFCLASS64:
// ok
default:
return nil, &FormatError{0, "unknown ELF class", f.Class}
}
f.Data = Data(ident[EI_DATA])
switch f.Data {
case ELFDATA2LSB:
f.ByteOrder = binary.LittleEndian
case ELFDATA2MSB:
f.ByteOrder = binary.BigEndian
default:
return nil, &FormatError{0, "unknown ELF data encoding", f.Data}
}
f.Version = Version(ident[EI_VERSION])
if f.Version != EV_CURRENT {
return nil, &FormatError{0, "unknown ELF version", f.Version}
}
f.OSABI = OSABI(ident[EI_OSABI])
f.ABIVersion = ident[EI_ABIVERSION]
// Read ELF file header
var shoff int64
var shentsize, shnum, shstrndx int
shstrndx = -1
switch f.Class {
case ELFCLASS32:
hdr := new(Header32)
sr.Seek(0, 0)
if err := binary.Read(sr, f.ByteOrder, hdr); err != nil {
return nil, err
}
f.Type = Type(hdr.Type)
f.Machine = Machine(hdr.Machine)
if v := Version(hdr.Version); v != f.Version {
return nil, &FormatError{0, "mismatched ELF version", v}
}
shoff = int64(hdr.Shoff)
shentsize = int(hdr.Shentsize)
shnum = int(hdr.Shnum)
shstrndx = int(hdr.Shstrndx)
case ELFCLASS64:
hdr := new(Header64)
sr.Seek(0, 0)
if err := binary.Read(sr, f.ByteOrder, hdr); err != nil {
return nil, err
}
f.Type = Type(hdr.Type)
f.Machine = Machine(hdr.Machine)
if v := Version(hdr.Version); v != f.Version {
return nil, &FormatError{0, "mismatched ELF version", v}
}
shoff = int64(hdr.Shoff)
shentsize = int(hdr.Shentsize)
shnum = int(hdr.Shnum)
shstrndx = int(hdr.Shstrndx)
}
if shstrndx < 0 || shstrndx >= shnum {
return nil, &FormatError{0, "invalid ELF shstrndx", shstrndx}
}
// Read program headers
// TODO
// Read section headers
f.Sections = make([]*Section, shnum)
names := make([]uint32, shnum)
for i := 0; i < shnum; i++ {
off := shoff + int64(i)*int64(shentsize)
sr.Seek(off, 0)
s := new(Section)
switch f.Class {
case ELFCLASS32:
sh := new(Section32)
if err := binary.Read(sr, f.ByteOrder, sh); err != nil {
return nil, err
}
names[i] = sh.Name
s.SectionHeader = SectionHeader{
Type: SectionType(sh.Type),
Flags: SectionFlag(sh.Flags),
Addr: uint64(sh.Addr),
Offset: uint64(sh.Off),
Size: uint64(sh.Size),
Link: uint32(sh.Link),
Info: uint32(sh.Info),
Addralign: uint64(sh.Addralign),
Entsize: uint64(sh.Entsize),
}
case ELFCLASS64:
sh := new(Section64)
if err := binary.Read(sr, f.ByteOrder, sh); err != nil {
return nil, err
}
names[i] = sh.Name
s.SectionHeader = SectionHeader{
Type: SectionType(sh.Type),
Flags: SectionFlag(sh.Flags),
Offset: uint64(sh.Off),
Size: uint64(sh.Size),
Addr: uint64(sh.Addr),
Link: uint32(sh.Link),
Info: uint32(sh.Info),
Addralign: uint64(sh.Addralign),
Entsize: uint64(sh.Entsize),
}
}
s.sr = io.NewSectionReader(r, int64(s.Offset), int64(s.Size))
s.ReaderAt = s.sr
f.Sections[i] = s
}
// Load section header string table.
s := f.Sections[shstrndx]
shstrtab := make([]byte, s.Size)
if _, err := r.ReadAt(shstrtab, int64(s.Offset)); err != nil {
return nil, err
}
for i, s := range f.Sections {
var ok bool
s.Name, ok = getString(shstrtab, int(names[i]))
if !ok {
return nil, &FormatError{shoff + int64(i*shentsize), "bad section name index", names[i]}
}
}
return f, nil
}
func (f *File) getSymbols() ([]Symbol, os.Error) {
switch f.Class {
case ELFCLASS64:
return f.getSymbols64()
}
return nil, os.ErrorString("not implemented")
}
// GetSymbols returns a slice of Symbols from parsing the symbol table.
func (f *File) getSymbols64() ([]Symbol, os.Error) {
var symtabSection *Section
for _, section := range f.Sections {
if section.Type == SHT_SYMTAB {
symtabSection = section
break
}
}
if symtabSection == nil {
return nil, os.ErrorString("no symbol section")
}
data, err := symtabSection.Data()
if err != nil {
return nil, os.ErrorString("cannot load symbol section")
}
symtab := bytes.NewBuffer(data)
if symtab.Len()%Sym64Size != 0 {
return nil, os.ErrorString("length of symbol section is not a multiple of Sym64Size")
}
// The first entry is all zeros.
var skip [Sym64Size]byte
symtab.Read(skip[0:])
symbols := make([]Symbol, symtab.Len()/Sym64Size)
i := 0
var sym Sym64
for symtab.Len() > 0 {
binary.Read(symtab, f.ByteOrder, &sym)
symbols[i].Name = sym.Name
symbols[i].Info = sym.Info
symbols[i].Other = sym.Other
symbols[i].Section = uint32(sym.Shndx)
symbols[i].Value = sym.Value
symbols[i].Size = sym.Size
i++
}
return symbols, nil
}
// getString extracts a string from an ELF string table.
func getString(section []byte, start int) (string, bool) {
if start < 0 || start >= len(section) {
return "", false
}
for end := start; end < len(section); end++ {
if section[end] == 0 {
return string(section[start:end]), true
}
}
return "", false
}
// Section returns a section with the given name, or nil if no such
// section exists.
func (f *File) Section(name string) *Section {
for _, s := range f.Sections {
if s.Name == name {
return s
}
}
return nil
}
// applyRelocations applies relocations to dst. rels is a relocations section
// in RELA format.
func (f *File) applyRelocations(dst []byte, rels []byte) os.Error {
if f.Class == ELFCLASS64 && f.Machine == EM_X86_64 {
return f.applyRelocationsAMD64(dst, rels)
}
return os.ErrorString("not implemented")
}
func (f *File) applyRelocationsAMD64(dst []byte, rels []byte) os.Error {
if len(rels)%Sym64Size != 0 {
return os.ErrorString("length of relocation section is not a multiple of Sym64Size")
}
symbols, err := f.getSymbols()
if err != nil {
return err
}
b := bytes.NewBuffer(rels)
var rela Rela64
for b.Len() > 0 {
binary.Read(b, f.ByteOrder, &rela)
symNo := rela.Info >> 32
t := R_X86_64(rela.Info & 0xffff)
if symNo >= uint64(len(symbols)) {
continue
}
sym := &symbols[symNo]
if SymType(sym.Info&0xf) != STT_SECTION {
// We don't handle non-section relocations for now.
continue
}
switch t {
case R_X86_64_64:
if rela.Off+8 >= uint64(len(dst)) || rela.Addend < 0 {
continue
}
f.ByteOrder.PutUint64(dst[rela.Off:rela.Off+8], uint64(rela.Addend))
case R_X86_64_32:
if rela.Off+4 >= uint64(len(dst)) || rela.Addend < 0 {
continue
}
f.ByteOrder.PutUint32(dst[rela.Off:rela.Off+4], uint32(rela.Addend))
}
}
return nil
}
func (f *File) DWARF() (*dwarf.Data, os.Error) {
// There are many other DWARF sections, but these
// are the required ones, and the debug/dwarf package
// does not use the others, so don't bother loading them.
var names = [...]string{"abbrev", "info", "str"}
var dat [len(names)][]byte
for i, name := range names {
name = ".debug_" + name
s := f.Section(name)
if s == nil {
continue
}
b, err := s.Data()
if err != nil && uint64(len(b)) < s.Size {
return nil, err
}
dat[i] = b
}
// If there's a relocation table for .debug_info, we have to process it
// now otherwise the data in .debug_info is invalid for x86-64 objects.
rela := f.Section(".rela.debug_info")
if rela != nil && rela.Type == SHT_RELA && f.Machine == EM_X86_64 {
data, err := rela.Data()
if err != nil {
return nil, err
}
err = f.applyRelocations(dat[1], data)
if err != nil {
return nil, err
}
}
abbrev, info, str := dat[0], dat[1], dat[2]
return dwarf.New(abbrev, nil, nil, info, nil, nil, nil, str)
}