// findBodyOffset does the minimum work to verify the file has a header
// and returns the file body offset.
func (f *File) findBodyOffset() (int64, error) {
	var buf [fileHeaderLen]byte
	if _, err := f.zipr.ReadAt(buf[:], f.headerOffset); err != nil {
		return 0, err
	}
	b := readBuf(buf[:])
	if sig := b.uint32(); sig != fileHeaderSignature {
		return 0, ErrFormat
	}
	b = b[22:] // skip over most of the header

	var z *Reader
	var err error
	var raw []byte
	if zt.Source != nil {
		rat, size := zt.Source()
		z, err = NewReader(rat, size)
		raw = make([]byte, size)
		if _, err := rat.ReadAt(raw, 0); err != nil {
			t.Errorf("ReadAt error=%v", err)
			return
		}
	} else {
		path := filepath.Join("testdata", zt.Name)
		if zt.Obscured {
			tf, err := obscuretestdata.DecodeToTempFile(path)
			if err != nil {

pkg debug/plan9obj, method (*File) Symbols() ([]Sym, error)
pkg debug/plan9obj, method (*Section) Data() ([]uint8, error)
pkg debug/plan9obj, method (*Section) Open() io.ReadSeeker
pkg debug/plan9obj, method (Section) ReadAt([]uint8, int64) (int, error)
pkg debug/plan9obj, type File struct
pkg debug/plan9obj, type File struct, Sections []*Section
pkg debug/plan9obj, type File struct, embedded FileHeader
pkg debug/plan9obj, type FileHeader struct

pkg debug/macho, method (LoadCmd) GoString() string
pkg debug/macho, method (LoadCmd) String() string
pkg debug/macho, method (Section) ReadAt([]uint8, int64) (int, error)
pkg debug/macho, method (Segment) Raw() []uint8
pkg debug/macho, method (Segment) ReadAt([]uint8, int64) (int, error)
pkg debug/macho, method (Symtab) Raw() []uint8
pkg debug/macho, type Cpu uint32
pkg debug/macho, type Dylib struct

		return 0, io.EOF
	}
	r.prevRune = -1
	n = copy(b, r.s[r.i:])
	r.i += int64(n)
	return
}

// ReadAt implements the [io.ReaderAt] interface.
func (r *Reader) ReadAt(b []byte, off int64) (n int, err error) {
	// cannot modify state - see io.ReaderAt
	if off < 0 {
		return 0, errors.New("bytes.Reader.ReadAt: negative offset")
	}
	if off >= int64(len(r.s)) {
		return 0, io.EOF
	}
	n = copy(b, r.s[off:])

	}{
		{0, 10, "0123456789", nil},
		{1, 10, "123456789", io.EOF},
		{1, 9, "123456789", nil},
		{11, 10, "", io.EOF},
		{0, 0, "", nil},
		{-1, 0, "", "bytes.Reader.ReadAt: negative offset"},
	}
	for i, tt := range tests {
		b := make([]byte, tt.n)
		rn, err := r.ReadAt(b, tt.off)
		got := string(b[:rn])
		if got != tt.want {
			t.Errorf("%d. got %q; want %q", i, got, tt.want)
		}

	for i := 0; i < len(all); i++ {
		for j := 0; j < len(all)-i; j++ {
			buf := make([]byte, j)
			n, err := b.ReadAt(buf, int64(i))
			if err != nil || n != len(buf) {
				t.Errorf("ReadAt(%d, %d) = %d, %v; want %d, nil", i, j, n, err, len(buf))
			}
			if !bytes.Equal(buf, all[i:i+j]) {
				t.Errorf("ReadAt(%d, %d) = %q; want %q", i, j, buf, all[i:i+j])
			}
		}
	}
}