libgo/go/cmd/go/internal/buildid/note.go

   1 // Copyright 2015 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 package buildid
   6
   7 import (
   8         "bytes"
   9         "debug/elf"
  10         "debug/macho"
  11         "encoding/binary"
  12         "fmt"
  13         "io"
  14         "os"
  15 )
  16
  17 func readAligned4(r io.Reader, sz int32) ([]byte, error) {
  18         full := (sz + 3) &^ 3
  19         data := make([]byte, full)
  20         _, err := io.ReadFull(r, data)
  21         if err != nil {
  22                 return nil, err
  23         }
  24         data = data[:sz]
  25         return data, nil
  26 }
  27
  28 func ReadELFNote(filename, name string, typ int32) ([]byte, error) {
  29         f, err := elf.Open(filename)
  30         if err != nil {
  31                 return nil, err
  32         }
  33         for _, sect := range f.Sections {
  34                 if sect.Type != elf.SHT_NOTE {
  35                         continue
  36                 }
  37                 r := sect.Open()
  38                 for {
  39                         var namesize, descsize, noteType int32
  40                         err = binary.Read(r, f.ByteOrder, &namesize)
  41                         if err != nil {
  42                                 if err == io.EOF {
  43                                         break
  44                                 }
  45                                 return nil, fmt.Errorf("read namesize failed: %v", err)
  46                         }
  47                         err = binary.Read(r, f.ByteOrder, &descsize)
  48                         if err != nil {
  49                                 return nil, fmt.Errorf("read descsize failed: %v", err)
  50                         }
  51                         err = binary.Read(r, f.ByteOrder, &noteType)
  52                         if err != nil {
  53                                 return nil, fmt.Errorf("read type failed: %v", err)
  54                         }
  55                         noteName, err := readAligned4(r, namesize)
  56                         if err != nil {
  57                                 return nil, fmt.Errorf("read name failed: %v", err)
  58                         }
  59                         desc, err := readAligned4(r, descsize)
  60                         if err != nil {
  61                                 return nil, fmt.Errorf("read desc failed: %v", err)
  62                         }
  63                         if name == string(noteName) && typ == noteType {
  64                                 return desc, nil
  65                         }
  66                 }
  67         }
  68         return nil, nil
  69 }
  70
  71 var elfGoNote = []byte("Go\x00\x00")
  72
  73 // The Go build ID is stored in a note described by an ELF PT_NOTE prog
  74 // header. The caller has already opened filename, to get f, and read
  75 // at least 4 kB out, in data.
  76 func readELFGoBuildID(filename string, f *os.File, data []byte) (buildid string, err error) {
  77         // Assume the note content is in the data, already read.
  78         // Rewrite the ELF header to set shnum to 0, so that we can pass
  79         // the data to elf.NewFile and it will decode the Prog list but not
  80         // try to read the section headers and the string table from disk.
  81         // That's a waste of I/O when all we care about is the Prog list
  82         // and the one ELF note.
  83         switch elf.Class(data[elf.EI_CLASS]) {
  84         case elf.ELFCLASS32:
  85                 data[48] = 0
  86                 data[49] = 0
  87         case elf.ELFCLASS64:
  88                 data[60] = 0
  89                 data[61] = 0
  90         }
  91
  92         const elfGoBuildIDTag = 4
  93
  94         ef, err := elf.NewFile(bytes.NewReader(data))
  95         if err != nil {
  96                 return "", &os.PathError{Path: filename, Op: "parse", Err: err}
  97         }
  98         for _, p := range ef.Progs {
  99                 if p.Type != elf.PT_NOTE || p.Filesz < 16 {
 100                         continue
 101                 }
 102
 103                 var note []byte
 104                 if p.Off+p.Filesz < uint64(len(data)) {
 105                         note = data[p.Off : p.Off+p.Filesz]
 106                 } else {
 107                         // For some linkers, such as the Solaris linker,
 108                         // the buildid may not be found in data (which
 109                         // likely contains the first 16kB of the file)
 110                         // or even the first few megabytes of the file
 111                         // due to differences in note segment placement;
 112                         // in that case, extract the note data manually.
 113                         _, err = f.Seek(int64(p.Off), io.SeekStart)
 114                         if err != nil {
 115                                 return "", err
 116                         }
 117
 118                         note = make([]byte, p.Filesz)
 119                         _, err = io.ReadFull(f, note)
 120                         if err != nil {
 121                                 return "", err
 122                         }
 123                 }
 124
 125                 filesz := p.Filesz
 126                 for filesz >= 16 {
 127                         nameSize := ef.ByteOrder.Uint32(note)
 128                         valSize := ef.ByteOrder.Uint32(note[4:])
 129                         tag := ef.ByteOrder.Uint32(note[8:])
 130                         name := note[12:16]
 131                         if nameSize == 4 && 16+valSize <= uint32(len(note)) && tag == elfGoBuildIDTag && bytes.Equal(name, elfGoNote) {
 132                                 return string(note[16 : 16+valSize]), nil
 133                         }
 134
 135                         nameSize = (nameSize + 3) &^ 3
 136                         valSize = (valSize + 3) &^ 3
 137                         notesz := uint64(12 + nameSize + valSize)
 138                         if filesz <= notesz {
 139                                 break
 140                         }
 141                         filesz -= notesz
 142                         note = note[notesz:]
 143                 }
 144         }
 145
 146         // No note. Treat as successful but build ID empty.
 147         return "", nil
 148 }
 149
 150 // The Go build ID is stored at the beginning of the Mach-O __text segment.
 151 // The caller has already opened filename, to get f, and read a few kB out, in data.
 152 // Sadly, that's not guaranteed to hold the note, because there is an arbitrary amount
 153 // of other junk placed in the file ahead of the main text.
 154 func readMachoGoBuildID(filename string, f *os.File, data []byte) (buildid string, err error) {
 155         // If the data we want has already been read, don't worry about Mach-O parsing.
 156         // This is both an optimization and a hedge against the Mach-O parsing failing
 157         // in the future due to, for example, the name of the __text section changing.
 158         if b, err := readRawGoBuildID(filename, data); b != "" && err == nil {
 159                 return b, err
 160         }
 161
 162         mf, err := macho.NewFile(f)
 163         if err != nil {
 164                 return "", &os.PathError{Path: filename, Op: "parse", Err: err}
 165         }
 166
 167         sect := mf.Section("__text")
 168         if sect == nil {
 169                 // Every binary has a __text section. Something is wrong.
 170                 return "", &os.PathError{Path: filename, Op: "parse", Err: fmt.Errorf("cannot find __text section")}
 171         }
 172
 173         // It should be in the first few bytes, but read a lot just in case,
 174         // especially given our past problems on OS X with the build ID moving.
 175         // There shouldn't be much difference between reading 4kB and 32kB:
 176         // the hard part is getting to the data, not transferring it.
 177         n := sect.Size
 178         if n > uint64(BuildIDReadSize) {
 179                 n = uint64(BuildIDReadSize)
 180         }
 181         buf := make([]byte, n)
 182         if _, err := f.ReadAt(buf, int64(sect.Offset)); err != nil {
 183                 return "", err
 184         }
 185
 186         return readRawGoBuildID(filename, buf)
 187 }