cmd/link: Mach-O (OS X) file formatter

See CL 48870044 for basic structure.

R=iant
CC=golang-codereviews
https://golang.org/cl/48910043

src/cmd/link/macho.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.
+
+// Mach-O (Darwin) object file writing.
+
+package main
+
+import (
+	"debug/macho"
+	"encoding/binary"
+	"io"
+	"strings"
+)
+
+// machoFormat is the implementation of formatter.
+type machoFormat struct{}
+
+// machoHeader and friends are data structures
+// corresponding to the Mach-O file header
+// to be written to disk.
+
+const (
+	macho64Bit     = 1 << 24
+	machoSubCPU386 = 3
+)
+
+// machoArch describes a Mach-O target architecture.
+type machoArch struct {
+	CPU    uint32
+	SubCPU uint32
+}
+
+// machoHeader is the Mach-O file header.
+type machoHeader struct {
+	machoArch
+	FileType uint32
+	Loads    []*machoLoad
+	Segments []*machoSegment
+	p        *Prog // for reporting errors
+}
+
+// machoLoad is a Mach-O load command.
+type machoLoad struct {
+	Type uint32
+	Data []uint32
+}
+
+// machoSegment is a Mach-O segment.
+type machoSegment struct {
+	Name       string
+	VirtAddr   Addr
+	VirtSize   Addr
+	FileOffset Addr
+	FileSize   Addr
+	Prot1      uint32
+	Prot2      uint32
+	Flags      uint32
+	Sections   []*machoSection
+}
+
+// machoSection is a Mach-O section, inside a segment.
+type machoSection struct {
+	Name    string
+	Segment string
+	Addr    Addr
+	Size    Addr
+	Offset  uint32
+	Align   uint32
+	Reloc   uint32
+	Nreloc  uint32
+	Flags   uint32
+	Res1    uint32
+	Res2    uint32
+}
+
+// layout positions the segments and sections in p
+// to make room for the Mach-O file header.
+// That is, it edits their VirtAddr fields to adjust for the presence
+// of the Mach-O header at the beginning of the address space.
+func (machoFormat) headerSize(p *Prog) (virt, file Addr) {
+	var h machoHeader
+	h.init(p)
+	size := Addr(h.size())
+	size = round(size, 4096)
+	p.HeaderSize = size
+	return size, size
+}
+
+// write writes p to w as a Mach-O executable.
+// layout(p) must have already been called,
+// and the number, sizes, and addresses of the segments
+// and sections must not have been modified since the call.
+func (machoFormat) write(w io.Writer, p *Prog) {
+	var h machoHeader
+	h.init(p)
+	off := Addr(0)
+	enc := h.encode()
+	w.Write(enc)
+	off += Addr(len(enc))
+	for _, seg := range p.Segments {
+		if seg.FileOffset < off {
+			h.p.errorf("mach-o error: invalid file offset")
+		}
+		w.Write(make([]byte, int(seg.FileOffset-off)))
+		if seg.FileSize != Addr(len(seg.Data)) {
+			h.p.errorf("mach-o error: invalid file size")
+		}
+		w.Write(seg.Data)
+		off = seg.FileOffset + Addr(len(seg.Data))
+	}
+}
+
+// Conversion of Prog to macho data structures.
+
+// machoArches maps from GOARCH to machoArch.
+var machoArches = map[string]machoArch{
+	"amd64": {
+		CPU:    uint32(macho.CpuAmd64),
+		SubCPU: uint32(machoSubCPU386),
+	},
+}
+
+// init initializes the header h to describe p.
+func (h *machoHeader) init(p *Prog) {
+	h.p = p
+	h.Segments = nil
+	h.Loads = nil
+	var ok bool
+	h.machoArch, ok = machoArches[p.GOARCH]
+	if !ok {
+		p.errorf("mach-o: unknown target GOARCH %q", p.GOARCH)
+		return
+	}
+	h.FileType = uint32(macho.TypeExec)
+
+	mseg := h.addSegment(p, "__PAGEZERO", nil)
+	mseg.VirtSize = p.UnmappedSize
+
+	for _, seg := range p.Segments {
+		h.addSegment(p, "__"+strings.ToUpper(seg.Name), seg)
+	}
+
+	var data []uint32
+	switch h.CPU {
+	default:
+		p.errorf("mach-o: unknown cpu %#x for GOARCH %q", h.CPU, p.GOARCH)
+	case uint32(macho.CpuAmd64):
+		data = make([]uint32, 2+42)
+		data[0] = 4                  // thread type
+		data[1] = 42                 // word count
+		data[2+32] = uint32(p.Entry) // RIP register, in two parts
+		data[2+32+1] = uint32(p.Entry >> 32)
+	}
+
+	h.Loads = append(h.Loads, &machoLoad{
+		Type: uint32(macho.LoadCmdUnixThread),
+		Data: data,
+	})
+}
+
+// addSegment adds to h a Mach-O segment like seg with the given name.
+func (h *machoHeader) addSegment(p *Prog, name string, seg *Segment) *machoSegment {
+	mseg := &machoSegment{
+		Name: name,
+	}
+	h.Segments = append(h.Segments, mseg)
+	if seg == nil {
+		return mseg
+	}
+
+	mseg.VirtAddr = seg.VirtAddr
+	mseg.VirtSize = seg.VirtSize
+	mseg.FileOffset = round(seg.FileOffset, 4096)
+	mseg.FileSize = seg.FileSize
+
+	if name == "__TEXT" {
+		// Initially RWX, then just RX
+		mseg.Prot1 = 7
+		mseg.Prot2 = 5
+
+		// Text segment maps Mach-O header, needed by dynamic linker.
+		mseg.VirtAddr -= p.HeaderSize
+		mseg.VirtSize += p.HeaderSize
+		mseg.FileOffset -= p.HeaderSize
+		mseg.FileSize += p.HeaderSize
+	} else {
+		// RW
+		mseg.Prot1 = 3
+		mseg.Prot2 = 3
+	}
+
+	for _, sect := range seg.Sections {
+		h.addSection(mseg, seg, sect)
+	}
+	return mseg
+}
+
+// addSection adds to mseg a Mach-O section like sect, inside seg, with the given name.
+func (h *machoHeader) addSection(mseg *machoSegment, seg *Segment, sect *Section) {
+	msect := &machoSection{
+		Name:    "__" + sect.Name,
+		Segment: mseg.Name,
+		// Reloc: sect.RelocOffset,
+		// NumReloc: sect.RelocLen / 8,
+		Addr: sect.VirtAddr,
+		Size: sect.Size,
+	}
+	mseg.Sections = append(mseg.Sections, msect)
+
+	for 1<<msect.Align < sect.Align {
+		msect.Align++
+	}
+
+	if off := sect.VirtAddr - seg.VirtAddr; off < seg.FileSize {
+		// Data in file.
+		if sect.Size > seg.FileSize-off {
+			h.p.errorf("mach-o error: section crosses file boundary")
+		}
+		msect.Offset = uint32(seg.FileOffset + off)
+	} else {
+		// Zero filled.
+		msect.Flags |= 1
+	}
+
+	if sect.Name == "text" {
+		msect.Flags |= 0x400 // contains executable instructions
+	}
+}
+
+// A machoWriter helps write Mach-O headers.
+// It is basically a buffer with some helper routines for writing integers.
+type machoWriter struct {
+	dst   []byte
+	tmp   [8]byte
+	order binary.ByteOrder
+	is64  bool
+	p     *Prog
+}
+
+// if64 returns x if w is writing a 64-bit object file; otherwise it returns y.
+func (w *machoWriter) if64(x, y interface{}) interface{} {
+	if w.is64 {
+		return x
+	}
+	return y
+}
+
+// encode encodes each of the given arguments into the writer.
+// It encodes uint32, []uint32, uint64, and []uint64 by writing each value
+// in turn in the correct byte order for the output file.
+// It encodes an Addr as a uint64 if writing a 64-bit output file, or else as a uint32.
+// It encodes []byte and string by writing the raw bytes (no length prefix).
+// It skips nil values in the args list.
+func (w *machoWriter) encode(args ...interface{}) {
+	for _, arg := range args {
+		switch arg := arg.(type) {
+		default:
+			w.p.errorf("mach-o error: cannot encode %T", arg)
+		case nil:
+			// skip
+		case []byte:
+			w.dst = append(w.dst, arg...)
+		case string:
+			w.dst = append(w.dst, arg...)
+		case uint32:
+			w.order.PutUint32(w.tmp[:], arg)
+			w.dst = append(w.dst, w.tmp[:4]...)
+		case []uint32:
+			for _, x := range arg {
+				w.order.PutUint32(w.tmp[:], x)
+				w.dst = append(w.dst, w.tmp[:4]...)
+			}
+		case uint64:
+			w.order.PutUint64(w.tmp[:], arg)
+			w.dst = append(w.dst, w.tmp[:8]...)
+		case Addr:
+			if w.is64 {
+				w.order.PutUint64(w.tmp[:], uint64(arg))
+				w.dst = append(w.dst, w.tmp[:8]...)
+			} else {
+				if Addr(uint32(arg)) != arg {
+					w.p.errorf("mach-o error: truncating address %#x to uint32", arg)
+				}
+				w.order.PutUint32(w.tmp[:], uint32(arg))
+				w.dst = append(w.dst, w.tmp[:4]...)
+			}
+		}
+	}
+}
+
+// segmentSize returns the size of the encoding of seg in bytes.
+func (w *machoWriter) segmentSize(seg *machoSegment) int {
+	if w.is64 {
+		return 18*4 + 20*4*len(seg.Sections)
+	}
+	return 14*4 + 22*4*len(seg.Sections)
+}
+
+// zeroPad returns the string s truncated or padded with NULs to n bytes.
+func zeroPad(s string, n int) string {
+	if len(s) >= n {
+		return s[:n]
+	}
+	return s + strings.Repeat("\x00", n-len(s))
+}
+
+// size returns the encoded size of the header.
+func (h *machoHeader) size() int {
+	// Could write separate code, but encoding is cheap; encode and throw it away.
+	return len(h.encode())
+}
+
+// encode returns the Mach-O encoding of the header.
+func (h *machoHeader) encode() []byte {
+	w := &machoWriter{p: h.p}
+	w.is64 = h.CPU&macho64Bit != 0
+	switch h.SubCPU {
+	default:
+		h.p.errorf("mach-o error: unknown CPU")
+	case machoSubCPU386:
+		w.order = binary.LittleEndian
+	}
+
+	loadSize := 0
+	for _, seg := range h.Segments {
+		loadSize += w.segmentSize(seg)
+	}
+	for _, l := range h.Loads {
+		loadSize += 4 * (2 + len(l.Data))
+	}
+
+	w.encode(
+		w.if64(macho.Magic64, macho.Magic32),
+		uint32(h.CPU),
+		uint32(h.SubCPU),
+		uint32(h.FileType),
+		uint32(len(h.Loads)+len(h.Segments)),
+		uint32(loadSize),
+		uint32(1),
+		w.if64(uint32(0), nil),
+	)
+
+	for _, seg := range h.Segments {
+		w.encode(
+			w.if64(uint32(macho.LoadCmdSegment64), uint32(macho.LoadCmdSegment)),
+			uint32(w.segmentSize(seg)),
+			zeroPad(seg.Name, 16),
+			seg.VirtAddr,
+			seg.VirtSize,
+			seg.FileOffset,
+			seg.FileSize,
+			seg.Prot1,
+			seg.Prot2,
+			uint32(len(seg.Sections)),
+			seg.Flags,
+		)
+		for _, sect := range seg.Sections {
+			w.encode(
+				zeroPad(sect.Name, 16),
+				zeroPad(seg.Name, 16),
+				sect.Addr,
+				sect.Size,
+				sect.Offset,
+				sect.Align,
+				sect.Reloc,
+				sect.Nreloc,
+				sect.Flags,
+				sect.Res1,
+				sect.Res2,
+				w.if64(uint32(0), nil),
+			)
+		}
+	}
+
+	for _, load := range h.Loads {
+		w.encode(
+			load.Type,
+			uint32(4*(2+len(load.Data))),
+			load.Data,
+		)
+	}
+
+	return w.dst
+}

src/cmd/link/macho_test.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 main
+
+import (
+	"bytes"
+	"debug/macho"
+	"encoding/binary"
+	"fmt"
+	"io/ioutil"
+	"strings"
+	"testing"
+)
+
+// Test macho writing by checking that each generated prog can be written
+// and then read back using debug/macho to get the same prog.
+// Also check against golden testdata file.
+var machoWriteTests = []struct {
+	name   string
+	golden bool
+	prog   *Prog
+}{
+	// amd64 exit 9
+	{
+		name:   "exit9",
+		golden: true,
+		prog: &Prog{
+			GOARCH:       "amd64",
+			UnmappedSize: 0x1000,
+			Entry:        0x1000,
+			Segments: []*Segment{
+				{
+					Name:       "text",
+					VirtAddr:   0x1000,
+					VirtSize:   13,
+					FileOffset: 0,
+					FileSize:   13,
+					Data: []byte{
+						0xb8, 0x01, 0x00, 0x00, 0x02, // MOVL $0x2000001, AX
+						0xbf, 0x09, 0x00, 0x00, 0x00, // MOVL $9, DI
+						0x0f, 0x05, // SYSCALL
+						0xf4, // HLT
+					},
+					Sections: []*Section{
+						{
+							Name:     "text",
+							VirtAddr: 0x1000,
+							Size:     13,
+							Align:    64,
+						},
+					},
+				},
+			},
+		},
+	},
+
+	// amd64 write hello world & exit 9
+	{
+		name:   "hello",
+		golden: true,
+		prog: &Prog{
+			GOARCH:       "amd64",
+			UnmappedSize: 0x1000,
+			Entry:        0x1000,
+			Segments: []*Segment{
+				{
+					Name:       "text",
+					VirtAddr:   0x1000,
+					VirtSize:   35,
+					FileOffset: 0,
+					FileSize:   35,
+					Data: []byte{
+						0xb8, 0x04, 0x00, 0x00, 0x02, // MOVL $0x2000001, AX
+						0xbf, 0x01, 0x00, 0x00, 0x00, // MOVL $1, DI
+						0xbe, 0x00, 0x30, 0x00, 0x00, // MOVL $0x3000, SI
+						0xba, 0x0c, 0x00, 0x00, 0x00, // MOVL $12, DX
+						0x0f, 0x05, // SYSCALL
+						0xb8, 0x01, 0x00, 0x00, 0x02, // MOVL $0x2000001, AX
+						0xbf, 0x09, 0x00, 0x00, 0x00, // MOVL $9, DI
+						0x0f, 0x05, // SYSCALL
+						0xf4, // HLT
+					},
+					Sections: []*Section{
+						{
+							Name:     "text",
+							VirtAddr: 0x1000,
+							Size:     35,
+							Align:    64,
+						},
+					},
+				},
+				{
+					Name:       "data",
+					VirtAddr:   0x2000,
+					VirtSize:   12,
+					FileOffset: 0x1000,
+					FileSize:   12,
+					Data:       []byte("hello world\n"),
+					Sections: []*Section{
+						{
+							Name:     "data",
+							VirtAddr: 0x2000,
+							Size:     12,
+							Align:    64,
+						},
+					},
+				},
+			},
+		},
+	},
+
+	// amd64 write hello world from rodata & exit 0
+	{
+		name:   "helloro",
+		golden: true,
+		prog: &Prog{
+			GOARCH:       "amd64",
+			UnmappedSize: 0x1000,
+			Entry:        0x1000,
+			Segments: []*Segment{
+				{
+					Name:       "text",
+					VirtAddr:   0x1000,
+					VirtSize:   0x100c,
+					FileOffset: 0,
+					FileSize:   0x100c,
+					Data: concat(
+						[]byte{
+							0xb8, 0x04, 0x00, 0x00, 0x02, // MOVL $0x2000001, AX
+							0xbf, 0x01, 0x00, 0x00, 0x00, // MOVL $1, DI
+							0xbe, 0x00, 0x30, 0x00, 0x00, // MOVL $0x3000, SI
+							0xba, 0x0c, 0x00, 0x00, 0x00, // MOVL $12, DX
+							0x0f, 0x05, // SYSCALL
+							0xb8, 0x01, 0x00, 0x00, 0x02, // MOVL $0x2000001, AX
+							0xbf, 0x00, 0x00, 0x00, 0x00, // MOVL $0, DI
+							0x0f, 0x05, // SYSCALL
+							0xf4, // HLT
+						},
+						make([]byte, 0x1000-35),
+						[]byte("hello world\n"),
+					),
+					Sections: []*Section{
+						{
+							Name:     "text",
+							VirtAddr: 0x1000,
+							Size:     35,
+							Align:    64,
+						},
+						{
+							Name:     "rodata",
+							VirtAddr: 0x2000,
+							Size:     12,
+							Align:    64,
+						},
+					},
+				},
+			},
+		},
+	},
+}
+
+func concat(xs ...[]byte) []byte {
+	var out []byte
+	for _, x := range xs {
+		out = append(out, x...)
+	}
+	return out
+}
+
+func TestMachoWrite(t *testing.T) {
+	for _, tt := range machoWriteTests {
+		name := tt.prog.GOARCH + "." + tt.name
+		prog := cloneProg(tt.prog)
+		var f machoFormat
+		vsize, fsize := f.headerSize(prog)
+		shiftProg(prog, vsize, fsize)
+		var buf bytes.Buffer
+		f.write(&buf, prog)
+		if false { // enable to debug
+			ioutil.WriteFile("a.out", buf.Bytes(), 0777)
+		}
+		read, err := machoRead(machoArches[tt.prog.GOARCH], buf.Bytes())
+		if err != nil {
+			t.Errorf("%s: reading mach-o output:\n\t%v", name, err)
+			continue
+		}
+		diffs := diffProg(read, prog)
+		if diffs != nil {
+			t.Errorf("%s: mismatched prog:\n\t%s", name, strings.Join(diffs, "\n\t"))
+			continue
+		}
+		if !tt.golden {
+			continue
+		}
+		checkGolden(t, buf.Bytes(), "testdata/macho."+name)
+	}
+}
+
+// machoRead reads the mach-o file in data and returns a corresponding prog.
+func machoRead(arch machoArch, data []byte) (*Prog, error) {
+	f, err := macho.NewFile(bytes.NewReader(data))
+	if err != nil {
+		return nil, err
+	}
+
+	var errors []string
+	errorf := func(format string, args ...interface{}) {
+		errors = append(errors, fmt.Sprintf(format, args...))
+	}
+
+	magic := uint32(0xFEEDFACE)
+	if arch.CPU&macho64Bit != 0 {
+		magic |= 1
+	}
+	if f.Magic != magic {
+		errorf("header: Magic = %#x, want %#x", f.Magic, magic)
+	}
+	if f.Cpu != macho.CpuAmd64 {
+		errorf("header: CPU = %#x, want %#x", f.Cpu, macho.CpuAmd64)
+	}
+	if f.SubCpu != 3 {
+		errorf("header: SubCPU = %#x, want %#x", f.SubCpu, 3)
+	}
+	if f.Type != 2 {
+		errorf("header: FileType = %d, want %d", f.Type, 2)
+	}
+	if f.Flags != 1 {
+		errorf("header: Flags = %d, want %d", f.Flags, 1)
+	}
+
+	msects := f.Sections
+	var limit uint64
+	prog := new(Prog)
+	for _, load := range f.Loads {
+		switch load := load.(type) {
+		default:
+			errorf("unexpected macho load %T %x", load, load.Raw())
+
+		case macho.LoadBytes:
+			if len(load) < 8 || len(load)%4 != 0 {
+				errorf("unexpected load length %d", len(load))
+				continue
+			}
+			cmd := f.ByteOrder.Uint32(load)
+			switch macho.LoadCmd(cmd) {
+			default:
+				errorf("unexpected macho load cmd %s", macho.LoadCmd(cmd))
+			case macho.LoadCmdUnixThread:
+				data := make([]uint32, len(load[8:])/4)
+				binary.Read(bytes.NewReader(load[8:]), f.ByteOrder, data)
+				if len(data) != 44 {
+					errorf("macho thread len(data) = %d, want 42", len(data))
+					continue
+				}
+				if data[0] != 4 {
+					errorf("macho thread type = %d, want 4", data[0])
+				}
+				if data[1] != uint32(len(data))-2 {
+					errorf("macho thread desc len = %d, want %d", data[1], uint32(len(data))-2)
+					continue
+				}
+				for i, val := range data[2:] {
+					switch i {
+					default:
+						if val != 0 {
+							errorf("macho thread data[%d] = %#x, want 0", i, val)
+						}
+					case 32:
+						prog.Entry = Addr(val)
+					case 33:
+						prog.Entry |= Addr(val) << 32
+					}
+				}
+			}
+
+		case *macho.Segment:
+			if load.Addr < limit {
+				errorf("segments out of order: %q at %#x after %#x", load.Name, load.Addr, limit)
+			}
+			limit = load.Addr + load.Memsz
+			if load.Name == "__PAGEZERO" || load.Addr == 0 && load.Filesz == 0 {
+				if load.Name != "__PAGEZERO" {
+					errorf("segment with Addr=0, Filesz=0 is named %q, want %q", load.Name, "__PAGEZERO")
+				} else if load.Addr != 0 || load.Filesz != 0 {
+					errorf("segment %q has Addr=%#x, Filesz=%d, want Addr=%#x, Filesz=%d", load.Name, load.Addr, load.Filesz, 0, 0)
+				}
+				prog.UnmappedSize = Addr(load.Memsz)
+				continue
+			}
+
+			if !strings.HasPrefix(load.Name, "__") {
+				errorf("segment name %q does not begin with %q", load.Name, "__")
+			}
+			if strings.ToUpper(load.Name) != load.Name {
+				errorf("segment name %q is not all upper case", load.Name)
+			}
+
+			seg := &Segment{
+				Name:       strings.ToLower(strings.TrimPrefix(load.Name, "__")),
+				VirtAddr:   Addr(load.Addr),
+				VirtSize:   Addr(load.Memsz),
+				FileOffset: Addr(load.Offset),
+				FileSize:   Addr(load.Filesz),
+			}
+			prog.Segments = append(prog.Segments, seg)
+
+			data, err := load.Data()
+			if err != nil {
+				errorf("loading data from %q: %v", load.Name, err)
+			}
+			seg.Data = data
+
+			var maxprot, prot uint32
+			if load.Name == "__TEXT" {
+				maxprot, prot = 7, 5
+			} else {
+				maxprot, prot = 3, 3
+			}
+			if load.Maxprot != maxprot || load.Prot != prot {
+				errorf("segment %q protection is %d, %d, want %d, %d",
+					load.Maxprot, load.Prot, maxprot, prot)
+			}
+
+			for len(msects) > 0 && msects[0].Addr < load.Addr+load.Memsz {
+				msect := msects[0]
+				msects = msects[1:]
+
+				if msect.Offset > 0 && prog.HeaderSize == 0 {
+					prog.HeaderSize = Addr(msect.Offset)
+					if seg.FileOffset != 0 {
+						errorf("initial segment %q does not map header", load.Name)
+					}
+					seg.VirtAddr += prog.HeaderSize
+					seg.VirtSize -= prog.HeaderSize
+					seg.FileOffset += prog.HeaderSize
+					seg.FileSize -= prog.HeaderSize
+					seg.Data = seg.Data[prog.HeaderSize:]
+				}
+
+				if msect.Addr < load.Addr {
+					errorf("section %q at address %#x is missing segment", msect.Name, msect.Addr)
+					continue
+				}
+
+				if !strings.HasPrefix(msect.Name, "__") {
+					errorf("section name %q does not begin with %q", msect.Name, "__")
+				}
+				if strings.ToLower(msect.Name) != msect.Name {
+					errorf("section name %q is not all lower case", msect.Name)
+				}
+				if msect.Seg != load.Name {
+					errorf("section %q is lists segment name %q, want %q",
+						msect.Name, msect.Seg, load.Name)
+				}
+				if uint64(msect.Offset) != uint64(load.Offset)+msect.Addr-load.Addr {
+					errorf("section %q file offset is %#x, want %#x",
+						msect.Name, msect.Offset, load.Offset+msect.Addr-load.Addr)
+				}
+				if msect.Reloff != 0 || msect.Nreloc != 0 {
+					errorf("section %q has reloff %d,%d, want %d,%d",
+						msect.Name, msect.Reloff, msect.Nreloc, 0, 0)
+				}
+				flags := uint32(0)
+				if msect.Name == "__text" {
+					flags = 0x400
+				}
+				if msect.Offset == 0 {
+					flags = 1
+				}
+				if msect.Flags != flags {
+					errorf("section %q flags = %#x, want %#x", msect.Flags, flags)
+				}
+				sect := &Section{
+					Name:     strings.ToLower(strings.TrimPrefix(msect.Name, "__")),
+					VirtAddr: Addr(msect.Addr),
+					Size:     Addr(msect.Size),
+					Align:    1 << msect.Align,
+				}
+				seg.Sections = append(seg.Sections, sect)
+			}
+		}
+	}
+
+	for _, msect := range msects {
+		errorf("section %q has no segment", msect.Name)
+	}
+
+	limit = 0
+	for _, msect := range f.Sections {
+		if msect.Addr < limit {
+			errorf("sections out of order: %q at %#x after %#x", msect.Name, msect.Addr, limit)
+		}
+		limit = msect.Addr + msect.Size
+	}
+
+	err = nil
+	if errors != nil {
+		err = fmt.Errorf("%s", strings.Join(errors, "\n\t"))
+	}
+	return prog, err
+}