debug/macho: Add support for opening fat/universal binaries.

New testdata was created from existing using:
$ lipo gcc-386-darwin-exec gcc-amd64-darwin-exec -create -output fat-gcc-386-amd64-darwin-exec

Fixes #7250.

LGTM=dave
R=golang-codereviews, dave, josharian, bradfitz
CC=golang-codereviews
https://golang.org/cl/60190043

src/pkg/debug/macho/fat.go

+// Copyright 2014 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 macho
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+	"os"
+)
+
+// A FatFile is a Mach-O universal binary that contains at least one architecture.
+type FatFile struct {
+	Magic  uint32
+	Arches []FatArch
+	closer io.Closer
+}
+
+// A FatArchHeader represents a fat header for a specific image architecture.
+type FatArchHeader struct {
+	Cpu    Cpu
+	SubCpu uint32
+	Offset uint32
+	Size   uint32
+	Align  uint32
+}
+
+const fatArchHeaderSize = 5 * 4
+
+// A FatArch is a Mach-O File inside a FatFile.
+type FatArch struct {
+	FatArchHeader
+	*File
+}
+
+// ErrNotFat is returned from NewFatFile or OpenFat when the file is not a
+// universal binary but may be a thin binary, based on its magic number.
+var ErrNotFat = &FormatError{0, "not a fat Mach-O file", nil}
+
+// NewFatFile creates a new FatFile for accessing all the Mach-O images in a
+// universal binary. The Mach-O binary is expected to start at position 0 in
+// the ReaderAt.
+func NewFatFile(r io.ReaderAt) (*FatFile, error) {
+	var ff FatFile
+	sr := io.NewSectionReader(r, 0, 1<<63-1)
+
+	// Read the fat_header struct, which is always in big endian.
+	// Start with the magic number.
+	err := binary.Read(sr, binary.BigEndian, &ff.Magic)
+	if err != nil {
+		return nil, &FormatError{0, "error reading magic number", nil}
+	} else if ff.Magic != MagicFat {
+		// See if this is a Mach-O file via its magic number. The magic
+		// must be converted to little endian first though.
+		var buf [4]byte
+		binary.BigEndian.PutUint32(buf[:], ff.Magic)
+		leMagic := binary.LittleEndian.Uint32(buf[:])
+		if leMagic == Magic32 || leMagic == Magic64 {
+			return nil, ErrNotFat
+		} else {
+			return nil, &FormatError{0, "invalid magic number", nil}
+		}
+	}
+	offset := int64(4)
+
+	// Read the number of FatArchHeaders that come after the fat_header.
+	var narch uint32
+	err = binary.Read(sr, binary.BigEndian, &narch)
+	if err != nil {
+		return nil, &FormatError{offset, "invalid fat_header", nil}
+	}
+	offset += 4
+
+	if narch < 1 {
+		return nil, &FormatError{offset, "file contains no images", nil}
+	}
+
+	// Combine the Cpu and SubCpu (both uint32) into a uint64 to make sure
+	// there are not duplicate architectures.
+	seenArches := make(map[uint64]bool, narch)
+	// Make sure that all images are for the same MH_ type.
+	var machoType Type
+
+	// Following the fat_header comes narch fat_arch structs that index
+	// Mach-O images further in the file.
+	ff.Arches = make([]FatArch, narch)
+	for i := uint32(0); i < narch; i++ {
+		fa := &ff.Arches[i]
+		err = binary.Read(sr, binary.BigEndian, &fa.FatArchHeader)
+		if err != nil {
+			return nil, &FormatError{offset, "invalid fat_arch header", nil}
+		}
+		offset += fatArchHeaderSize
+
+		fr := io.NewSectionReader(r, int64(fa.Offset), int64(fa.Size))
+		fa.File, err = NewFile(fr)
+		if err != nil {
+			return nil, err
+		}
+
+		// Make sure the architecture for this image is not duplicate.
+		seenArch := (uint64(fa.Cpu) << 32) | uint64(fa.SubCpu)
+		if o, k := seenArches[seenArch]; o || k {
+			return nil, &FormatError{offset, fmt.Sprintf("duplicate architecture cpu=%v, subcpu=%#x", fa.Cpu, fa.SubCpu), nil}
+		}
+		seenArches[seenArch] = true
+
+		// Make sure the Mach-O type matches that of the first image.
+		if i == 0 {
+			machoType = fa.Type
+		} else {
+			if fa.Type != machoType {
+				return nil, &FormatError{offset, fmt.Sprintf("Mach-O type for architecture #%d (type=%#x) does not match first (type=%#x)", i, fa.Type, machoType), nil}
+			}
+		}
+	}
+
+	return &ff, nil
+}
+
+// OpenFat opens the named file using os.Open and prepares it for use as a Mach-O
+// universal binary.
+func OpenFat(name string) (ff *FatFile, err error) {
+	f, err := os.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	ff, err = NewFatFile(f)
+	if err != nil {
+		f.Close()
+		return nil, err
+	}
+	ff.closer = f
+	return
+}
+
+func (ff *FatFile) Close() error {
+	var err error
+	if ff.closer != nil {
+		err = ff.closer.Close()
+		ff.closer = nil
+	}
+	return err
+}

src/pkg/debug/macho/file_test.go

@@ -165,3 +165,46 @@ func TestOpenFailure(t *testing.T) {
 		t.Errorf("open %s: succeeded unexpectedly", filename)
 	}
 }
+
+func TestOpenFat(t *testing.T) {
+	ff, err := OpenFat("testdata/fat-gcc-386-amd64-darwin-exec")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if ff.Magic != MagicFat {
+		t.Errorf("OpenFat: got magic number %#x, want %#x", ff.Magic, MagicFat)
+	}
+	if len(ff.Arches) != 2 {
+		t.Errorf("OpenFat: got %d architectures, want 2", len(ff.Arches))
+	}
+
+	for i := range ff.Arches {
+		arch := &ff.Arches[i]
+		ftArch := &fileTests[i]
+
+		if arch.Cpu != ftArch.hdr.Cpu || arch.SubCpu != ftArch.hdr.SubCpu {
+			t.Error("OpenFat: architecture #%d got cpu=%#x subtype=%#x, expected cpu=%#x, subtype=%#x", i, arch.Cpu, arch.SubCpu, ftArch.hdr.Cpu, ftArch.hdr.SubCpu)
+		}
+
+		if !reflect.DeepEqual(arch.FileHeader, ftArch.hdr) {
+			t.Errorf("OpenFat header:\n\tgot %#v\n\twant %#v\n", arch.FileHeader, ftArch.hdr)
+		}
+	}
+}
+
+func TestOpenFatFailure(t *testing.T) {
+	filename := "file.go" // not a Mach-O file
+	if _, err := OpenFat(filename); err == nil {
+		t.Errorf("OpenFat %s: succeeded unexpectedly", filename)
+	}
+
+	filename = "testdata/gcc-386-darwin-exec" // not a fat Mach-O
+	ff, err := OpenFat(filename)
+	if err != ErrNotFat {
+		t.Errorf("OpenFat %s: got %v, want ErrNotFat", err)
+	}
+	if ff != nil {
+		t.Errorf("OpenFat %s: got %v, want nil", ff)
+	}
+}

src/pkg/debug/macho/macho.go

@@ -28,14 +28,17 @@ const (
 const (
 	Magic32  uint32 = 0xfeedface
 	Magic64  uint32 = 0xfeedfacf
+	MagicFat uint32 = 0xcafebabe
 )
 
-// A Type is a Mach-O file type, either an object or an executable.
+// A Type is the Mach-O file type, e.g. an object file, executable, or dynamic library.
 type Type uint32
 
 const (
 	TypeObj    Type = 1
 	TypeExec   Type = 2
+	TypeDylib  Type = 6
+	TypeBundle Type = 8
 )
 
 // A Cpu is a Mach-O cpu type.