debug/dwarf: delay array type fixup to handle type cycles

A user encountered a debug/dwarf crash when running the dwarf2json
tool (https://github.com/volatilityfoundation/dwarf2json) on a
debug-built copy of the linux kernel. In this crash, the DWARF type
reader was trying to examine the contents of an array type while that
array type was still in the process of being constructed (due to
cycles in the type graph).

To avoid such situations, this patch extends the mechanism introduced
in https://go-review.googlesource.com/18459 (which handles typedef
types) to delay fixup of array types as well.

Change-Id: I303f6ce5db1ca4bd79da3581957dfc2bfc17cc01
Reviewed-on: https://go-review.googlesource.com/c/go/+/319329
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Yi Chou <yich@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/debug/dwarf/type.go

@@ -292,11 +292,35 @@ func (d *Data) Type(off Offset) (Type, error) {
 	return d.readType("info", d.Reader(), off, d.typeCache, nil)
 }
 
+type typeFixer struct {
+	typedefs   []*TypedefType
+	arraytypes []*Type
+}
+
+func (tf *typeFixer) recordArrayType(t *Type) {
+	if t == nil {
+		return
+	}
+	_, ok := (*t).(*ArrayType)
+	if ok {
+		tf.arraytypes = append(tf.arraytypes, t)
+	}
+}
+
+func (tf *typeFixer) apply() {
+	for _, t := range tf.typedefs {
+		t.Common().ByteSize = t.Type.Size()
+	}
+	for _, t := range tf.arraytypes {
+		zeroArray(t)
+	}
+}
+
 // readType reads a type from r at off of name. It adds types to the
 // type cache, appends new typedef types to typedefs, and computes the
 // sizes of types. Callers should pass nil for typedefs; this is used
 // for internal recursion.
-func (d *Data) readType(name string, r typeReader, off Offset, typeCache map[Offset]Type, typedefs *[]*TypedefType) (Type, error) {
+func (d *Data) readType(name string, r typeReader, off Offset, typeCache map[Offset]Type, fixups *typeFixer) (Type, error) {
 	if t, ok := typeCache[off]; ok {
 		return t, nil
 	}
@@ -311,18 +335,16 @@ func (d *Data) readType(name string, r typeReader, off Offset, typeCache map[Off
 	}
 
 	// If this is the root of the recursion, prepare to resolve
-	// typedef sizes once the recursion is done. This must be done
-	// after the type graph is constructed because it may need to
-	// resolve cycles in a different order than readType
-	// encounters them.
-	if typedefs == nil {
-		var typedefList []*TypedefType
+	// typedef sizes and perform other fixups once the recursion is
+	// done. This must be done after the type graph is constructed
+	// because it may need to resolve cycles in a different order than
+	// readType encounters them.
+	if fixups == nil {
+		var fixer typeFixer
 		defer func() {
-			for _, t := range typedefList {
-				t.Common().ByteSize = t.Type.Size()
-			}
+			fixer.apply()
 		}()
-		typedefs = &typedefList
+		fixups = &fixer
 	}
 
 	// Parse type from Entry.
@@ -376,7 +398,7 @@ func (d *Data) readType(name string, r typeReader, off Offset, typeCache map[Off
 		var t Type
 		switch toff := tval.(type) {
 		case Offset:
-			if t, err = d.readType(name, r.clone(), toff, typeCache, typedefs); err != nil {
+			if t, err = d.readType(name, r.clone(), toff, typeCache, fixups); err != nil {
 				return nil
 			}
 		case uint64:
@@ -567,7 +589,7 @@ func (d *Data) readType(name string, r typeReader, off Offset, typeCache map[Off
 			if bito == lastFieldBitOffset && t.Kind != "union" {
 				// Last field was zero width. Fix array length.
 				// (DWARF writes out 0-length arrays as if they were 1-length arrays.)
-				zeroArray(lastFieldType)
+				fixups.recordArrayType(lastFieldType)
 			}
 			lastFieldType = &f.Type
 			lastFieldBitOffset = bito
@@ -576,7 +598,7 @@ func (d *Data) readType(name string, r typeReader, off Offset, typeCache map[Off
 			b, ok := e.Val(AttrByteSize).(int64)
 			if ok && b*8 == lastFieldBitOffset {
 				// Final field must be zero width. Fix array length.
-				zeroArray(lastFieldType)
+				fixups.recordArrayType(lastFieldType)
 			}
 		}
 
@@ -719,7 +741,7 @@ func (d *Data) readType(name string, r typeReader, off Offset, typeCache map[Off
 				// Record that we need to resolve this
 				// type's size once the type graph is
 				// constructed.
-				*typedefs = append(*typedefs, t)
+				fixups.typedefs = append(fixups.typedefs, t)
 			case *PtrType:
 				b = int64(addressSize)
 			}
@@ -737,11 +759,8 @@ Error:
 }
 
 func zeroArray(t *Type) {
-	if t == nil {
-		return
-	}
-	at, ok := (*t).(*ArrayType)
-	if !ok || at.Type.Size() == 0 {
+	at := (*t).(*ArrayType)
+	if at.Type.Size() == 0 {
 		return
 	}
 	// Make a copy to avoid invalidating typeCache.