[BitcodeReader] Check if we can create a null constant for type.

[fhahn]

Summary

We cannot create null constants for certain types, e.g. VoidTy,
FunctionTy or LabelTy. getNullValue asserts if we pass in an
unsupported type. We should also check for opaque types, but I'm not
sure how.

This fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=14795.

[fhahn]

ping

[fhahn]

ping

[vsk]

LGTM.

llvm/trunk/lib/Bitcode/Reader/BitcodeReader.cpp

case bitc::CST_CODE_SETTYPE:   // SETTYPE: [typeid]
       if (Record[0] >= TypeList.size() || !TypeList[Record[0]])
         return error("Invalid record");
       if (TypeList[Record[0]] == VoidType)
         return error("Invalid constant type");
       CurFullTy = TypeList[Record[0]];
       CurTy = flattenPointerTypes(CurFullTy);
       continue;  // Skip the ValueList manipulation.
     case bitc::CST_CODE_NULL:      // NULL
+      if (CurTy->isVoidTy() || CurTy->isFunctionTy() || CurTy->isLabelTy())
+        return error("Invalid type for a constant null value");
       V = Constant::getNullValue(CurTy);
       break;
     case bitc::CST_CODE_INTEGER:   // INTEGER: [intval]
       if (!CurTy->isIntegerTy() || Record.empty())
         return error("Invalid record");
       V = ConstantInt::get(CurTy, decodeSignRotatedValue(Record[0]));
       break;
     case bitc::CST_CODE_WIDE_INTEGER: {// WIDE_INTEGER: [n x intval]

llvm/trunk/test/Bitcode/invalid-type-for-null-constant.ll

+; Bitcode with a CST_CODE_NULL with void type.
+
+; RUN: not llvm-dis %s.bc -o - 2>&1 | FileCheck %s
+
+; CHECK: error: Invalid type for a constant null value
+