buildid.go

Documentation: cmd/internal/buildid

     1  // Copyright 2017 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  	"fmt"
    11  	"internal/xcoff"
    12  	"io"
    13  	"io/fs"
    14  	"os"
    15  	"strconv"
    16  	"strings"
    17  )
    18  
    19  var (
    20  	errBuildIDMalformed = fmt.Errorf("malformed object file")
    21  
    22  	bangArch = []byte("!<arch>")
    23  	pkgdef   = []byte("__.PKGDEF")
    24  	goobject = []byte("go object ")
    25  	buildid  = []byte("build id ")
    26  )
    27  
    28  // ReadFile reads the build ID from an archive or executable file.
    29  func ReadFile(name string) (id string, err error) {
    30  	f, err := os.Open(name)
    31  	if err != nil {
    32  		return "", err
    33  	}
    34  	defer f.Close()
    35  
    36  	buf := make([]byte, 8)
    37  	if _, err := f.ReadAt(buf, 0); err != nil {
    38  		return "", err
    39  	}
    40  	if string(buf) != "!<arch>\n" {
    41  		if string(buf) == "<bigaf>\n" {
    42  			return readGccgoBigArchive(name, f)
    43  		}
    44  		return readBinary(name, f)
    45  	}
    46  
    47  	// Read just enough of the target to fetch the build ID.
    48  	// The archive is expected to look like:
    49  	//
    50  	//	!<arch>
    51  	//	__.PKGDEF       0           0     0     644     7955      `
    52  	//	go object darwin amd64 devel X:none
    53  	//	build id "b41e5c45250e25c9fd5e9f9a1de7857ea0d41224"
    54  	//
    55  	// The variable-sized strings are GOOS, GOARCH, and the experiment list (X:none).
    56  	// Reading the first 1024 bytes should be plenty.
    57  	data := make([]byte, 1024)
    58  	n, err := io.ReadFull(f, data)
    59  	if err != nil && n == 0 {
    60  		return "", err
    61  	}
    62  
    63  	tryGccgo := func() (string, error) {
    64  		return readGccgoArchive(name, f)
    65  	}
    66  
    67  	// Archive header.
    68  	for i := 0; ; i++ { // returns during i==3
    69  		j := bytes.IndexByte(data, '\n')
    70  		if j < 0 {
    71  			return tryGccgo()
    72  		}
    73  		line := data[:j]
    74  		data = data[j+1:]
    75  		switch i {
    76  		case 0:
    77  			if !bytes.Equal(line, bangArch) {
    78  				return tryGccgo()
    79  			}
    80  		case 1:
    81  			if !bytes.HasPrefix(line, pkgdef) {
    82  				return tryGccgo()
    83  			}
    84  		case 2:
    85  			if !bytes.HasPrefix(line, goobject) {
    86  				return tryGccgo()
    87  			}
    88  		case 3:
    89  			if !bytes.HasPrefix(line, buildid) {
    90  				// Found the object header, just doesn't have a build id line.
    91  				// Treat as successful, with empty build id.
    92  				return "", nil
    93  			}
    94  			id, err := strconv.Unquote(string(line[len(buildid):]))
    95  			if err != nil {
    96  				return tryGccgo()
    97  			}
    98  			return id, nil
    99  		}
   100  	}
   101  }
   102  
   103  // readGccgoArchive tries to parse the archive as a standard Unix
   104  // archive file, and fetch the build ID from the _buildid.o entry.
   105  // The _buildid.o entry is written by (*Builder).gccgoBuildIDELFFile
   106  // in cmd/go/internal/work/exec.go.
   107  func readGccgoArchive(name string, f *os.File) (string, error) {
   108  	bad := func() (string, error) {
   109  		return "", &fs.PathError{Op: "parse", Path: name, Err: errBuildIDMalformed}
   110  	}
   111  
   112  	off := int64(8)
   113  	for {
   114  		if _, err := f.Seek(off, io.SeekStart); err != nil {
   115  			return "", err
   116  		}
   117  
   118  		// TODO(iant): Make a debug/ar package, and use it
   119  		// here and in cmd/link.
   120  		var hdr [60]byte
   121  		if _, err := io.ReadFull(f, hdr[:]); err != nil {
   122  			if err == io.EOF {
   123  				// No more entries, no build ID.
   124  				return "", nil
   125  			}
   126  			return "", err
   127  		}
   128  		off += 60
   129  
   130  		sizeStr := strings.TrimSpace(string(hdr[48:58]))
   131  		size, err := strconv.ParseInt(sizeStr, 0, 64)
   132  		if err != nil {
   133  			return bad()
   134  		}
   135  
   136  		name := strings.TrimSpace(string(hdr[:16]))
   137  		if name == "_buildid.o/" {
   138  			sr := io.NewSectionReader(f, off, size)
   139  			e, err := elf.NewFile(sr)
   140  			if err != nil {
   141  				return bad()
   142  			}
   143  			s := e.Section(".go.buildid")
   144  			if s == nil {
   145  				return bad()
   146  			}
   147  			data, err := s.Data()
   148  			if err != nil {
   149  				return bad()
   150  			}
   151  			return string(data), nil
   152  		}
   153  
   154  		off += size
   155  		if off&1 != 0 {
   156  			off++
   157  		}
   158  	}
   159  }
   160  
   161  // readGccgoBigArchive tries to parse the archive as an AIX big
   162  // archive file, and fetch the build ID from the _buildid.o entry.
   163  // The _buildid.o entry is written by (*Builder).gccgoBuildIDXCOFFFile
   164  // in cmd/go/internal/work/exec.go.
   165  func readGccgoBigArchive(name string, f *os.File) (string, error) {
   166  	bad := func() (string, error) {
   167  		return "", &fs.PathError{Op: "parse", Path: name, Err: errBuildIDMalformed}
   168  	}
   169  
   170  	// Read fixed-length header.
   171  	if _, err := f.Seek(0, io.SeekStart); err != nil {
   172  		return "", err
   173  	}
   174  	var flhdr [128]byte
   175  	if _, err := io.ReadFull(f, flhdr[:]); err != nil {
   176  		return "", err
   177  	}
   178  	// Read first member offset.
   179  	offStr := strings.TrimSpace(string(flhdr[68:88]))
   180  	off, err := strconv.ParseInt(offStr, 10, 64)
   181  	if err != nil {
   182  		return bad()
   183  	}
   184  	for {
   185  		if off == 0 {
   186  			// No more entries, no build ID.
   187  			return "", nil
   188  		}
   189  		if _, err := f.Seek(off, io.SeekStart); err != nil {
   190  			return "", err
   191  		}
   192  		// Read member header.
   193  		var hdr [112]byte
   194  		if _, err := io.ReadFull(f, hdr[:]); err != nil {
   195  			return "", err
   196  		}
   197  		// Read member name length.
   198  		namLenStr := strings.TrimSpace(string(hdr[108:112]))
   199  		namLen, err := strconv.ParseInt(namLenStr, 10, 32)
   200  		if err != nil {
   201  			return bad()
   202  		}
   203  		if namLen == 10 {
   204  			var nam [10]byte
   205  			if _, err := io.ReadFull(f, nam[:]); err != nil {
   206  				return "", err
   207  			}
   208  			if string(nam[:]) == "_buildid.o" {
   209  				sizeStr := strings.TrimSpace(string(hdr[0:20]))
   210  				size, err := strconv.ParseInt(sizeStr, 10, 64)
   211  				if err != nil {
   212  					return bad()
   213  				}
   214  				off += int64(len(hdr)) + namLen + 2
   215  				if off&1 != 0 {
   216  					off++
   217  				}
   218  				sr := io.NewSectionReader(f, off, size)
   219  				x, err := xcoff.NewFile(sr)
   220  				if err != nil {
   221  					return bad()
   222  				}
   223  				data := x.CSect(".go.buildid")
   224  				if data == nil {
   225  					return bad()
   226  				}
   227  				return string(data), nil
   228  			}
   229  		}
   230  
   231  		// Read next member offset.
   232  		offStr = strings.TrimSpace(string(hdr[20:40]))
   233  		off, err = strconv.ParseInt(offStr, 10, 64)
   234  		if err != nil {
   235  			return bad()
   236  		}
   237  	}
   238  }
   239  
   240  var (
   241  	goBuildPrefix = []byte("\xff Go build ID: \"")
   242  	goBuildEnd    = []byte("\"\n \xff")
   243  
   244  	elfPrefix = []byte("\x7fELF")
   245  
   246  	machoPrefixes = [][]byte{
   247  		{0xfe, 0xed, 0xfa, 0xce},
   248  		{0xfe, 0xed, 0xfa, 0xcf},
   249  		{0xce, 0xfa, 0xed, 0xfe},
   250  		{0xcf, 0xfa, 0xed, 0xfe},
   251  	}
   252  )
   253  
   254  var readSize = 32 * 1024 // changed for testing
   255  
   256  // readBinary reads the build ID from a binary.
   257  //
   258  // ELF binaries store the build ID in a proper PT_NOTE section.
   259  //
   260  // Other binary formats are not so flexible. For those, the linker
   261  // stores the build ID as non-instruction bytes at the very beginning
   262  // of the text segment, which should appear near the beginning
   263  // of the file. This is clumsy but fairly portable. Custom locations
   264  // can be added for other binary types as needed, like we did for ELF.
   265  func readBinary(name string, f *os.File) (id string, err error) {
   266  	// Read the first 32 kB of the binary file.
   267  	// That should be enough to find the build ID.
   268  	// In ELF files, the build ID is in the leading headers,
   269  	// which are typically less than 4 kB, not to mention 32 kB.
   270  	// In Mach-O files, there's no limit, so we have to parse the file.
   271  	// On other systems, we're trying to read enough that
   272  	// we get the beginning of the text segment in the read.
   273  	// The offset where the text segment begins in a hello
   274  	// world compiled for each different object format today:
   275  	//
   276  	//	Plan 9: 0x20
   277  	//	Windows: 0x600
   278  	//
   279  	data := make([]byte, readSize)
   280  	_, err = io.ReadFull(f, data)
   281  	if err == io.ErrUnexpectedEOF {
   282  		err = nil
   283  	}
   284  	if err != nil {
   285  		return "", err
   286  	}
   287  
   288  	if bytes.HasPrefix(data, elfPrefix) {
   289  		return readELF(name, f, data)
   290  	}
   291  	for _, m := range machoPrefixes {
   292  		if bytes.HasPrefix(data, m) {
   293  			return readMacho(name, f, data)
   294  		}
   295  	}
   296  	return readRaw(name, data)
   297  }
   298  
   299  // readRaw finds the raw build ID stored in text segment data.
   300  func readRaw(name string, data []byte) (id string, err error) {
   301  	i := bytes.Index(data, goBuildPrefix)
   302  	if i < 0 {
   303  		// Missing. Treat as successful but build ID empty.
   304  		return "", nil
   305  	}
   306  
   307  	j := bytes.Index(data[i+len(goBuildPrefix):], goBuildEnd)
   308  	if j < 0 {
   309  		return "", &fs.PathError{Op: "parse", Path: name, Err: errBuildIDMalformed}
   310  	}
   311  
   312  	quoted := data[i+len(goBuildPrefix)-1 : i+len(goBuildPrefix)+j+1]
   313  	id, err = strconv.Unquote(string(quoted))
   314  	if err != nil {
   315  		return "", &fs.PathError{Op: "parse", Path: name, Err: errBuildIDMalformed}
   316  	}
   317  	return id, nil
   318  }
   319  
   320  // HashToString converts the hash h to a string to be recorded
   321  // in package archives and binaries as part of the build ID.
   322  // We use the first 120 bits of the hash (5 chunks of 24 bits each) and encode
   323  // it in base64, resulting in a 20-byte string. Because this is only used for
   324  // detecting the need to rebuild installed files (not for lookups
   325  // in the object file cache), 120 bits are sufficient to drive the
   326  // probability of a false "do not need to rebuild" decision to effectively zero.
   327  // We embed two different hashes in archives and four in binaries,
   328  // so cutting to 20 bytes is a significant savings when build IDs are displayed.
   329  // (20*4+3 = 83 bytes compared to 64*4+3 = 259 bytes for the
   330  // more straightforward option of printing the entire h in base64).
   331  func HashToString(h [32]byte) string {
   332  	const b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
   333  	const chunks = 5
   334  	var dst [chunks * 4]byte
   335  	for i := 0; i < chunks; i++ {
   336  		v := uint32(h[3*i])<<16 | uint32(h[3*i+1])<<8 | uint32(h[3*i+2])
   337  		dst[4*i+0] = b64[(v>>18)&0x3F]
   338  		dst[4*i+1] = b64[(v>>12)&0x3F]
   339  		dst[4*i+2] = b64[(v>>6)&0x3F]
   340  		dst[4*i+3] = b64[v&0x3F]
   341  	}
   342  	return string(dst[:])
   343  }
   344
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