debug/dwarf: more graceful handling of unsupported types

Enhance the type decoder to do a better job handling unknown type
tags. DWARF has a number of type DIEs that this package doesn't handle
(things like "pointer to member" types in C++); avoid crashing for
such types, but instead return a placeholder "UnsupportedType" object
(this idea suggested by Austin). This provides a compromise between
implementing the entire kitchen sink and simply returning an error
outright on any unknown type DIE.

Fixes #29601.

Change-Id: I2eeffa094c86ef3a2c358ee42e8e629d74cec2ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/158797Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/debug/dwarf/testdata/cppunsuptypes.cc

+// Copyright 2019 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.
+
+// cppunsuptypes.elf built with g++ 7.3
+//    g++ -g -c -o cppunsuptypes.elf cppunsuptypes.cc
+
+int i = 3;
+double d = 3;
+
+// anonymous reference type
+int &culprit = i;
+
+// named reference type
+typedef double &dref;
+dref dr = d;
+
+// incorporated into another type
+typedef struct {
+  dref q;
+  int &r;
+} hasrefs;
+
+hasrefs hr = { d, i };
+
+// This code is intended to trigger a DWARF "pointer to member" type DIE
+struct CS { int dm; };
+
+int foo()
+{
+  int CS::* pdm = &CS::dm;
+  CS cs = {42};
+  return cs.*pdm;
+}

src/debug/dwarf/type.go

@@ -261,6 +261,20 @@ func (t *TypedefType) String() string { return t.Name }
 
 func (t *TypedefType) Size() int64 { return t.Type.Size() }
 
+// An UnsupportedType is a placeholder returned in situations where we
+// encounter a type that isn't supported.
+type UnsupportedType struct {
+	CommonType
+	Tag Tag
+}
+
+func (t *UnsupportedType) String() string {
+	if t.Name != "" {
+		return t.Name
+	}
+	return t.Name + "(unsupported type " + t.Tag.String() + ")"
+}
+
 // typeReader is used to read from either the info section or the
 // types section.
 type typeReader interface {
@@ -680,6 +694,16 @@ func (d *Data) readType(name string, r typeReader, off Offset, typeCache map[Off
 		typ = t
 		typeCache[off] = t
 		t.Name, _ = e.Val(AttrName).(string)
+
+	default:
+		// This is some other type DIE that we're currently not
+		// equipped to handle. Return an abstract "unsupported type"
+		// object in such cases.
+		t := new(UnsupportedType)
+		typ = t
+		typeCache[off] = t
+		t.Tag = e.Tag
+		t.Name, _ = e.Val(AttrName).(string)
 	}
 
 	if err != nil {

src/debug/dwarf/type_test.go

@@ -9,6 +9,8 @@ import (
 	"debug/elf"
 	"debug/macho"
 	"debug/pe"
+	"fmt"
+	"strconv"
 	"testing"
 )
 
@@ -168,3 +170,61 @@ func TestTypedefCycle(t *testing.T) {
 		}
 	}
 }
+
+var unsupportedTypeTests = []string{
+	// varname:typename:string:size
+	"culprit::(unsupported type ReferenceType):8",
+	"pdm::(unsupported type PtrToMemberType):-1",
+}
+
+func TestUnsupportedTypes(t *testing.T) {
+	// Issue 29601:
+	// When reading DWARF from C++ load modules, we can encounter
+	// oddball type DIEs. These will be returned as "UnsupportedType"
+	// objects; check to make sure this works properly.
+	d := elfData(t, "testdata/cppunsuptypes.elf")
+	r := d.Reader()
+	seen := make(map[string]bool)
+	for {
+		e, err := r.Next()
+		if err != nil {
+			t.Fatal("r.Next:", err)
+		}
+		if e == nil {
+			break
+		}
+		if e.Tag == TagVariable {
+			vname, _ := e.Val(AttrName).(string)
+			tAttr := e.Val(AttrType)
+			typOff, ok := tAttr.(Offset)
+			if !ok {
+				t.Errorf("variable at offset %v has no type", e.Offset)
+				continue
+			}
+			typ, err := d.Type(typOff)
+			if err != nil {
+				t.Errorf("err in type decode: %v\n", err)
+				continue
+			}
+			unsup, isok := typ.(*UnsupportedType)
+			if !isok {
+				continue
+			}
+			tag := vname + ":" + unsup.Name + ":" + unsup.String() +
+				":" + strconv.FormatInt(unsup.Size(), 10)
+			seen[tag] = true
+		}
+	}
+	dumpseen := false
+	for _, v := range unsupportedTypeTests {
+		if !seen[v] {
+			t.Errorf("missing %s", v)
+			dumpseen = true
+		}
+	}
+	if dumpseen {
+		for k, _ := range seen {
+			fmt.Printf("seen: %s\n", k)
+		}
+	}
+}