debug/dwarf: check for DWARFv4 AttrDataBitOffset value

AttrBitOffset is deprecated (but reserved) in DWARFv4.  This fix adds
logic to check the new AttrDataBitOffset attribute if AttrBitOffset
attribute is not present.

Fixes #46784

Change-Id: I7406dcaa4c98e95df72361fd4462c39e6be8879d
GitHub-Last-Rev: 5aa10d0491
GitHub-Pull-Request: golang/go#46790
Reviewed-on: https://go-review.googlesource.com/c/go/+/328709
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>

src/debug/dwarf/testdata/typedef.c

@@ -8,6 +8,7 @@ gcc -gdwarf-2 -m64 -c typedef.c && gcc -gdwarf-2 -m64 -o typedef.elf typedef.o
 
 OS X Mach-O:
 gcc -gdwarf-2 -m64 -c typedef.c -o typedef.macho
+gcc -gdwarf-4 -m64 -c typedef.c -o typedef.macho4
 */
 #include <complex.h>
 

src/debug/dwarf/type.go

@@ -516,7 +516,10 @@ func (d *Data) readType(name string, r typeReader, off Offset, typeCache map[Off
 		}).Basic()
 		t.Name = name
 		t.BitSize, _ = e.Val(AttrBitSize).(int64)
-		t.BitOffset, _ = e.Val(AttrBitOffset).(int64)
+		haveBitOffset := false
+		if t.BitOffset, haveBitOffset = e.Val(AttrBitOffset).(int64); !haveBitOffset {
+			t.BitOffset, _ = e.Val(AttrDataBitOffset).(int64)
+		}
 
 	case TagClassType, TagStructType, TagUnionType:
 		// Structure, union, or class type.  (DWARF v2 §5.5)
@@ -578,7 +581,9 @@ func (d *Data) readType(name string, r typeReader, off Offset, typeCache map[Off
 			haveBitOffset := false
 			f.Name, _ = kid.Val(AttrName).(string)
 			f.ByteSize, _ = kid.Val(AttrByteSize).(int64)
-			f.BitOffset, haveBitOffset = kid.Val(AttrBitOffset).(int64)
+			if f.BitOffset, haveBitOffset = kid.Val(AttrBitOffset).(int64); !haveBitOffset {
+				f.BitOffset, haveBitOffset = kid.Val(AttrDataBitOffset).(int64)
+			}
 			f.BitSize, _ = kid.Val(AttrBitSize).(int64)
 			t.Field = append(t.Field, f)
 

src/debug/dwarf/type_test.go

@@ -228,3 +228,54 @@ func TestUnsupportedTypes(t *testing.T) {
 		}
 	}
 }
+
+func TestBitOffsetsELF(t *testing.T) { testBitOffsets(t, elfData(t, "testdata/typedef.elf")) }
+
+func TestBitOffsetsMachO(t *testing.T) {
+	testBitOffsets(t, machoData(t, "testdata/typedef.macho"))
+}
+
+func TestBitOffsetsMachO4(t *testing.T) {
+	testBitOffsets(t, machoData(t, "testdata/typedef.macho4"))
+}
+
+func TestBitOffsetsELFDwarf4(t *testing.T) {
+	testBitOffsets(t, elfData(t, "testdata/typedef.elf4"))
+}
+
+func testBitOffsets(t *testing.T, d *Data) {
+	r := d.Reader()
+	for {
+		e, err := r.Next()
+		if err != nil {
+			t.Fatal("r.Next:", err)
+		}
+		if e == nil {
+			break
+		}
+
+		if e.Tag == TagStructType {
+			typ, err := d.Type(e.Offset)
+			if err != nil {
+				t.Fatal("d.Type:", err)
+			}
+
+			t1 := typ.(*StructType)
+
+			for _, field := range t1.Field {
+				// We're only testing for bitfields
+				if field.BitSize == 0 {
+					continue
+				}
+
+				// Ensure BitOffset is not zero
+				if field.BitOffset == 0 {
+					t.Errorf("bit offset of field %s in %s %s is not set", field.Name, t1.Kind, t1.StructName)
+				}
+			}
+		}
+		if e.Tag != TagCompileUnit {
+			r.SkipChildren()
+		}
+	}
+}