RFR: JDK-8245432: Lookup::defineHiddenClass should throw UnsupportedClassVersionError if the given bytes are of an unsupported major or minor version

[Mandy Chung]

On 5/28/20 6:55 AM, Alan Bateman wrote:
> On 28/05/2020 05:13, Mandy Chung wrote:
>> Updated webrev:
>> http://cr.openjdk.java.net/~mchung/jdk15/webrevs/8245432/webrev.01/
>>
>> I modify this patch to check the class file version and throws CFE if 
>> unsupported before creating ClassReader.  This also fixes JDK-8245061 
>> that it reads the value of `this_class` as a constant (as ASM will 
>> throw an exception if it's not a constant) and also ensures that 
>> `this_class` is a CONSTANT_Class_info by checking the descriptor 
>> string from Type.
> I don't want to complicate this further but the --enable-preview 
> handling in the VM doesn't seem to expose an internal property that 
> allows code in the libraries know if preview feature are enabled or 
> not. I was assuming that isSupportedClassFileVersion would need to 
> check that.
>

The runtime will ensure if --enable-preview is set if a class file with 
minor is loaded.   I will prefer to keep VM::isSupportedClassFileVersion 
to validate the given major/minor version.  At runtime, it will fail 
when such class file is loaded if preview is not enabled.

I'll add a comment to describe that.

> Will readUnsignedShort throw AIOBE if the offset is beyond the length? 

Good catch.  I add the check to throw CFE.
+            private static int readUnsignedShort(byte[] bytes, int 
offset) {
+                if (offset >= bytes.length) {
+                    throw new ClassFormatError("Invalid ClassFile 
structure");
+                }
+                return ((bytes[offset] & 0xFF) << 8) | (bytes[offset + 
1] & 0xFF);
+            }

> Also are you sure about "& 0xCAFEBABE", I assumed it would just check 
> the magic number is equal to that.

It should check ==.  Fixed.

thanks
Mandy