debug/dwarf: factor parsing of unit lengths

Many headers in DWARF sections have a "unit length" that can be either
4 bytes or 12 bytes and indicates both the length of the unit and
whether the unit is in 32-bit or 64-bit format.

Currently, we implement unit length parsing in four different places.
Add a "unitLength" method to buf that parses a unit length and use it
in these four places.

Change-Id: I7950b91caaa92aa5e19aa63debc8ae46178ecc4d
Reviewed-on: https://go-review.googlesource.com/7281Reviewed-by: Nigel Tao <nigeltao@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/debug/dwarf/buf.go

@@ -163,6 +163,17 @@ func (b *buf) addr() uint64 {
 	return 0
 }
 
+func (b *buf) unitLength() (length Offset, dwarf64 bool) {
+	length = Offset(b.uint32())
+	if length == 0xffffffff {
+		dwarf64 = true
+		length = Offset(b.uint64())
+	} else if length >= 0xfffffff0 {
+		b.error("unit length has reserved value")
+	}
+	return
+}
+
 func (b *buf) error(s string) {
 	if b.err == nil {
 		b.data = nil

src/debug/dwarf/line.go

@@ -177,14 +177,7 @@ func (r *LineReader) readHeader() error {
 
 	// Read basic header fields [DWARF2 6.2.4].
 	hdrOffset := buf.off
-	dwarf64 := false
-	unitLength := Offset(buf.uint32())
-	if unitLength == 0xffffffff {
-		dwarf64 = true
-		unitLength = Offset(buf.uint64())
-	} else if unitLength >= 0xfffffff0 {
-		return DecodeError{"line", hdrOffset, fmt.Sprintf("total length field has reserved value %#x", unitLength)}
-	}
+	unitLength, dwarf64 := buf.unitLength()
 	r.endOffset = buf.off + unitLength
 	if r.endOffset > buf.off+Offset(len(buf.data)) {
 		return DecodeError{"line", hdrOffset, fmt.Sprintf("line table end %d exceeds section size %d", r.endOffset, buf.off+Offset(len(buf.data)))}

src/debug/dwarf/typeunit.go

@@ -27,17 +27,11 @@ func (d *Data) parseTypes(name string, types []byte) error {
 	b := makeBuf(d, unknownFormat{}, name, 0, types)
 	for len(b.data) > 0 {
 		base := b.off
-		dwarf64 := false
-		n := b.uint32()
-		if n == 0xffffffff {
-			n64 := b.uint64()
-			if n64 != uint64(uint32(n64)) {
+		n, dwarf64 := b.unitLength()
+		if n != Offset(uint32(n)) {
 			b.error("type unit length overflow")
 			return b.err
 		}
-			n = uint32(n64)
-			dwarf64 = true
-		}
 		hdroff := b.off
 		vers := b.uint16()
 		if vers != 4 {
@@ -79,7 +73,7 @@ func (d *Data) parseTypes(name string, types []byte) error {
 			unit: unit{
 				base:   base,
 				off:    boff,
-				data:   b.bytes(int(Offset(n) - (b.off - hdroff))),
+				data:   b.bytes(int(n - (b.off - hdroff))),
 				atable: atable,
 				asize:  int(asize),
 				vers:   int(vers),

src/debug/dwarf/unit.go

@@ -41,15 +41,11 @@ func (d *Data) parseUnits() ([]unit, error) {
 	nunit := 0
 	b := makeBuf(d, unknownFormat{}, "info", 0, d.info)
 	for len(b.data) > 0 {
-		len := b.uint32()
-		if len == 0xffffffff {
-			len64 := b.uint64()
-			if len64 != uint64(uint32(len64)) {
+		len, _ := b.unitLength()
+		if len != Offset(uint32(len)) {
 			b.error("unit length overflow")
 			break
 		}
-			len = uint32(len64)
-		}
 		b.skip(int(len))
 		nunit++
 	}
@@ -63,11 +59,8 @@ func (d *Data) parseUnits() ([]unit, error) {
 	for i := range units {
 		u := &units[i]
 		u.base = b.off
-		n := b.uint32()
-		if n == 0xffffffff {
-			u.is64 = true
-			n = uint32(b.uint64())
-		}
+		var n Offset
+		n, u.is64 = b.unitLength()
 		vers := b.uint16()
 		if vers != 2 && vers != 3 && vers != 4 {
 			b.error("unsupported DWARF version " + strconv.Itoa(int(vers)))