debug/dwarf: return ClassUnknown if attribute class cannot be determined

Currently, if the .debug_abbrev section of an ELF file contains
attributes that aren't known to the dwarf package and that have form
formSecOffset, the dwarf package will fail to open the DWARF data with
an error like "decoding dwarf section abbrev at offset 0x17: cannot
determine class of unknown attribute with formSecOffset". For the most
part, the class is implied by the form encoded in the abbrev section,
but formSecOffset can imply many different DWARF classes. Hence,
debug/dwarf disambiguates these using a table of known attributes.
However, it will reject the entire image if it encounters an attribute
it can't determine the class of. This is particularly unfortunate
because the caller may never even uses the offending attribute.

Fix this by introducing a ClassUnknown attribute class to use as a
fallback in these cases. This allows the dwarf package to load the
DWARF data and isolates the problem to just the affected attributes.

Fixes #12592.

Change-Id: I766227b136e9757f8b89c0b3ab8e9ddea899d94f
Reviewed-on: https://go-review.googlesource.com/14541Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: jcd . <jcd@golang.org>

src/debug/dwarf/class_string.go

@@ -4,14 +4,13 @@ package dwarf
 
 import "fmt"
 
-const _Class_name = "ClassAddressClassBlockClassConstantClassExprLocClassFlagClassLinePtrClassLocListPtrClassMacPtrClassRangeListPtrClassReferenceClassReferenceSigClassStringClassReferenceAltClassStringAlt"
+const _Class_name = "ClassUnknownClassAddressClassBlockClassConstantClassExprLocClassFlagClassLinePtrClassLocListPtrClassMacPtrClassRangeListPtrClassReferenceClassReferenceSigClassStringClassReferenceAltClassStringAlt"
 
-var _Class_index = [...]uint8{0, 12, 22, 35, 47, 56, 68, 83, 94, 111, 125, 142, 153, 170, 184}
+var _Class_index = [...]uint8{0, 12, 24, 34, 47, 59, 68, 80, 95, 106, 123, 137, 154, 165, 182, 196}
 
 func (i Class) String() string {
-	i -= 1
 	if i < 0 || i+1 >= Class(len(_Class_index)) {
-		return fmt.Sprintf("Class(%d)", i+1)
+		return fmt.Sprintf("Class(%d)", i)
 	}
 	return _Class_name[_Class_index[i]:_Class_index[i+1]]
 }

src/debug/dwarf/entry.go

@@ -193,8 +193,7 @@ func formToClass(form format, attr Attr, vers int, b *buf) Class {
 		if class, ok := attrPtrClass[attr]; ok {
 			return class
 		}
-		b.error("cannot determine class of unknown attribute with formSecOffset")
-		return 0
+		return ClassUnknown
 
 	case formExprloc:
 		return ClassExprLoc
@@ -235,6 +234,9 @@ type Entry struct {
 //    loclistptr        int64          ClassLocListPtr
 //    macptr            int64          ClassMacPtr
 //    rangelistptr      int64          ClassRangeListPtr
+//
+// For unrecognized or vendor-defined attributes, Class may be
+// ClassUnknown.
 type Field struct {
 	Attr  Attr
 	Val   interface{}
@@ -258,9 +260,12 @@ type Field struct {
 type Class int
 
 const (
+	// ClassUnknown represents values of unknown DWARF class.
+	ClassUnknown Class = iota
+
 	// ClassAddress represents values of type uint64 that are
 	// addresses on the target machine.
-	ClassAddress Class = 1 + iota
+	ClassAddress
 
 	// ClassBlock represents values of type []byte whose
 	// interpretation depends on the attribute.