1 files changed, 343 insertions, 0 deletions
diff --git a/src/cmd/internal/buildid/buildid.go b/src/cmd/internal/buildid/buildid.go
new file mode 100644
index 0000000..1e8855d
--- /dev/null
+++ b/src/cmd/internal/buildid/buildid.go
@@ -0,0 +1,343 @@
+// Copyright 2017 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 buildid
+
+import (
+	"bytes"
+	"debug/elf"
+	"fmt"
+	"internal/xcoff"
+	"io"
+	"io/fs"
+	"os"
+	"strconv"
+	"strings"
+)
+
+var (
+	errBuildIDMalformed = fmt.Errorf("malformed object file")
+
+	bangArch = []byte("!<arch>")
+	pkgdef   = []byte("__.PKGDEF")
+	goobject = []byte("go object ")
+	buildid  = []byte("build id ")
+)
+
+// ReadFile reads the build ID from an archive or executable file.
+func ReadFile(name string) (id string, err error) {
+	f, err := os.Open(name)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+
+	buf := make([]byte, 8)
+	if _, err := f.ReadAt(buf, 0); err != nil {
+		return "", err
+	}
+	if string(buf) != "!<arch>\n" {
+		if string(buf) == "<bigaf>\n" {
+			return readGccgoBigArchive(name, f)
+		}
+		return readBinary(name, f)
+	}
+
+	// Read just enough of the target to fetch the build ID.
+	// The archive is expected to look like:
+	//
+	//	!<arch>
+	//	__.PKGDEF       0           0     0     644     7955      `
+	//	go object darwin amd64 devel X:none
+	//	build id "b41e5c45250e25c9fd5e9f9a1de7857ea0d41224"
+	//
+	// The variable-sized strings are GOOS, GOARCH, and the experiment list (X:none).
+	// Reading the first 1024 bytes should be plenty.
+	data := make([]byte, 1024)
+	n, err := io.ReadFull(f, data)
+	if err != nil && n == 0 {
+		return "", err
+	}
+
+	tryGccgo := func() (string, error) {
+		return readGccgoArchive(name, f)
+	}
+
+	// Archive header.
+	for i := 0; ; i++ { // returns during i==3
+		j := bytes.IndexByte(data, '\n')
+		if j < 0 {
+			return tryGccgo()
+		}
+		line := data[:j]
+		data = data[j+1:]
+		switch i {
+		case 0:
+			if !bytes.Equal(line, bangArch) {
+				return tryGccgo()
+			}
+		case 1:
+			if !bytes.HasPrefix(line, pkgdef) {
+				return tryGccgo()
+			}
+		case 2:
+			if !bytes.HasPrefix(line, goobject) {
+				return tryGccgo()
+			}
+		case 3:
+			if !bytes.HasPrefix(line, buildid) {
+				// Found the object header, just doesn't have a build id line.
+				// Treat as successful, with empty build id.
+				return "", nil
+			}
+			id, err := strconv.Unquote(string(line[len(buildid):]))
+			if err != nil {
+				return tryGccgo()
+			}
+			return id, nil
+		}
+	}
+}
+
+// readGccgoArchive tries to parse the archive as a standard Unix
+// archive file, and fetch the build ID from the _buildid.o entry.
+// The _buildid.o entry is written by (*Builder).gccgoBuildIDELFFile
+// in cmd/go/internal/work/exec.go.
+func readGccgoArchive(name string, f *os.File) (string, error) {
+	bad := func() (string, error) {
+		return "", &fs.PathError{Op: "parse", Path: name, Err: errBuildIDMalformed}
+	}
+
+	off := int64(8)
+	for {
+		if _, err := f.Seek(off, io.SeekStart); err != nil {
+			return "", err
+		}
+
+		// TODO(iant): Make a debug/ar package, and use it
+		// here and in cmd/link.
+		var hdr [60]byte
+		if _, err := io.ReadFull(f, hdr[:]); err != nil {
+			if err == io.EOF {
+				// No more entries, no build ID.
+				return "", nil
+			}
+			return "", err
+		}
+		off += 60
+
+		sizeStr := strings.TrimSpace(string(hdr[48:58]))
+		size, err := strconv.ParseInt(sizeStr, 0, 64)
+		if err != nil {
+			return bad()
+		}
+
+		name := strings.TrimSpace(string(hdr[:16]))
+		if name == "_buildid.o/" {
+			sr := io.NewSectionReader(f, off, size)
+			e, err := elf.NewFile(sr)
+			if err != nil {
+				return bad()
+			}
+			s := e.Section(".go.buildid")
+			if s == nil {
+				return bad()
+			}
+			data, err := s.Data()
+			if err != nil {
+				return bad()
+			}
+			return string(data), nil
+		}
+
+		off += size
+		if off&1 != 0 {
+			off++
+		}
+	}
+}
+
+// readGccgoBigArchive tries to parse the archive as an AIX big
+// archive file, and fetch the build ID from the _buildid.o entry.
+// The _buildid.o entry is written by (*Builder).gccgoBuildIDXCOFFFile
+// in cmd/go/internal/work/exec.go.
+func readGccgoBigArchive(name string, f *os.File) (string, error) {
+	bad := func() (string, error) {
+		return "", &fs.PathError{Op: "parse", Path: name, Err: errBuildIDMalformed}
+	}
+
+	// Read fixed-length header.
+	if _, err := f.Seek(0, io.SeekStart); err != nil {
+		return "", err
+	}
+	var flhdr [128]byte
+	if _, err := io.ReadFull(f, flhdr[:]); err != nil {
+		return "", err
+	}
+	// Read first member offset.
+	offStr := strings.TrimSpace(string(flhdr[68:88]))
+	off, err := strconv.ParseInt(offStr, 10, 64)
+	if err != nil {
+		return bad()
+	}
+	for {
+		if off == 0 {
+			// No more entries, no build ID.
+			return "", nil
+		}
+		if _, err := f.Seek(off, io.SeekStart); err != nil {
+			return "", err
+		}
+		// Read member header.
+		var hdr [112]byte
+		if _, err := io.ReadFull(f, hdr[:]); err != nil {
+			return "", err
+		}
+		// Read member name length.
+		namLenStr := strings.TrimSpace(string(hdr[108:112]))
+		namLen, err := strconv.ParseInt(namLenStr, 10, 32)
+		if err != nil {
+			return bad()
+		}
+		if namLen == 10 {
+			var nam [10]byte
+			if _, err := io.ReadFull(f, nam[:]); err != nil {
+				return "", err
+			}
+			if string(nam[:]) == "_buildid.o" {
+				sizeStr := strings.TrimSpace(string(hdr[0:20]))
+				size, err := strconv.ParseInt(sizeStr, 10, 64)
+				if err != nil {
+					return bad()
+				}
+				off += int64(len(hdr)) + namLen + 2
+				if off&1 != 0 {
+					off++
+				}
+				sr := io.NewSectionReader(f, off, size)
+				x, err := xcoff.NewFile(sr)
+				if err != nil {
+					return bad()
+				}
+				data := x.CSect(".go.buildid")
+				if data == nil {
+					return bad()
+				}
+				return string(data), nil
+			}
+		}
+
+		// Read next member offset.
+		offStr = strings.TrimSpace(string(hdr[20:40]))
+		off, err = strconv.ParseInt(offStr, 10, 64)
+		if err != nil {
+			return bad()
+		}
+	}
+}
+
+var (
+	goBuildPrefix = []byte("\xff Go build ID: \"")
+	goBuildEnd    = []byte("\"\n \xff")
+
+	elfPrefix = []byte("\x7fELF")
+
+	machoPrefixes = [][]byte{
+		{0xfe, 0xed, 0xfa, 0xce},
+		{0xfe, 0xed, 0xfa, 0xcf},
+		{0xce, 0xfa, 0xed, 0xfe},
+		{0xcf, 0xfa, 0xed, 0xfe},
+	}
+)
+
+var readSize = 32 * 1024 // changed for testing
+
+// readBinary reads the build ID from a binary.
+//
+// ELF binaries store the build ID in a proper PT_NOTE section.
+//
+// Other binary formats are not so flexible. For those, the linker
+// stores the build ID as non-instruction bytes at the very beginning
+// of the text segment, which should appear near the beginning
+// of the file. This is clumsy but fairly portable. Custom locations
+// can be added for other binary types as needed, like we did for ELF.
+func readBinary(name string, f *os.File) (id string, err error) {
+	// Read the first 32 kB of the binary file.
+	// That should be enough to find the build ID.
+	// In ELF files, the build ID is in the leading headers,
+	// which are typically less than 4 kB, not to mention 32 kB.
+	// In Mach-O files, there's no limit, so we have to parse the file.
+	// On other systems, we're trying to read enough that
+	// we get the beginning of the text segment in the read.
+	// The offset where the text segment begins in a hello
+	// world compiled for each different object format today:
+	//
+	//	Plan 9: 0x20
+	//	Windows: 0x600
+	//
+	data := make([]byte, readSize)
+	_, err = io.ReadFull(f, data)
+	if err == io.ErrUnexpectedEOF {
+		err = nil
+	}
+	if err != nil {
+		return "", err
+	}
+
+	if bytes.HasPrefix(data, elfPrefix) {
+		return readELF(name, f, data)
+	}
+	for _, m := range machoPrefixes {
+		if bytes.HasPrefix(data, m) {
+			return readMacho(name, f, data)
+		}
+	}
+	return readRaw(name, data)
+}
+
+// readRaw finds the raw build ID stored in text segment data.
+func readRaw(name string, data []byte) (id string, err error) {
+	i := bytes.Index(data, goBuildPrefix)
+	if i < 0 {
+		// Missing. Treat as successful but build ID empty.
+		return "", nil
+	}
+
+	j := bytes.Index(data[i+len(goBuildPrefix):], goBuildEnd)
+	if j < 0 {
+		return "", &fs.PathError{Op: "parse", Path: name, Err: errBuildIDMalformed}
+	}
+
+	quoted := data[i+len(goBuildPrefix)-1 : i+len(goBuildPrefix)+j+1]
+	id, err = strconv.Unquote(string(quoted))
+	if err != nil {
+		return "", &fs.PathError{Op: "parse", Path: name, Err: errBuildIDMalformed}
+	}
+	return id, nil
+}
+
+// HashToString converts the hash h to a string to be recorded
+// in package archives and binaries as part of the build ID.
+// We use the first 120 bits of the hash (5 chunks of 24 bits each) and encode
+// it in base64, resulting in a 20-byte string. Because this is only used for
+// detecting the need to rebuild installed files (not for lookups
+// in the object file cache), 120 bits are sufficient to drive the
+// probability of a false "do not need to rebuild" decision to effectively zero.
+// We embed two different hashes in archives and four in binaries,
+// so cutting to 20 bytes is a significant savings when build IDs are displayed.
+// (20*4+3 = 83 bytes compared to 64*4+3 = 259 bytes for the
+// more straightforward option of printing the entire h in base64).
+func HashToString(h [32]byte) string {
+	const b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
+	const chunks = 5
+	var dst [chunks * 4]byte
+	for i := 0; i < chunks; i++ {
+		v := uint32(h[3*i])<<16 | uint32(h[3*i+1])<<8 | uint32(h[3*i+2])
+		dst[4*i+0] = b64[(v>>18)&0x3F]
+		dst[4*i+1] = b64[(v>>12)&0x3F]
+		dst[4*i+2] = b64[(v>>6)&0x3F]
+		dst[4*i+3] = b64[v&0x3F]
+	}
+	return string(dst[:])
+}