RFR: 8025922 JNI access to Strings need to check if the value field is non-null

[Daniel D. Daugherty]

On 10/4/13 6:24 AM, David Holmes wrote:
> On 4/10/2013 10:02 PM, Staffan Larsen wrote:
>>
>> On 4 okt 2013, at 13:54, David Holmes <David.Holmes at oracle.com> wrote:
>>
>>> Hi Staffan,
>>>
>>> On 4/10/2013 9:23 PM, Staffan Larsen wrote:
>>>> If the String constructor has not completed, then we can't access 
>>>> the String value. It is still possible to do this from JNI, but it 
>>>> should not crash the VM.
>>>>
>>>> This was found while fixing a debugger issue (JDK-8021897) where 
>>>> stepping into the String constructor crashes the JVM. The fix for 
>>>> JDK-8021897 requires changes on the JDK side. Those changes will 
>>>> also include a JDI test for that issue, but won't be pushed until 
>>>> this change reaches jdk8-tl.
>>>
>>> Other than the debugger case is there any other case where these JNI 
>>> calls can occur before the constructor has completed? At the moment 
>>> I'm not sure if this is really a JNI problem or a debugger problem - 
>>> surely there are many objects that the debugger may access partially 
>>> initialized which might then lead to errors/crashes?
>>
>>  From JNI you can do AllocObject() and then GetStringLength() and it 
>> will cause the same crash. I'd love to include a test for this, but 
>> alas, our test harness does not support native code.
>
> Hmmmm. So that exposes a hole in the JNI specification. You can create 
> unconstructed objects and then invoke JNI functions on them and there 
> is no clear spec as to what should happen. What you have coded seems 
> quite reasonable but for the get*Chars method contradicts the 
> specification which states that it "Returns NULL if and only if an 
> invocation of this function has thrown an exception." And the only 
> exception listed is OOME.

Maybe I'm reading it wrong, but for GetStringChars(), I see:

http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/functions.html#wp17158

 > GetStringChars
 >
 > const jchar * GetStringChars(JNIEnv *env, jstring string,
 > jboolean *isCopy);

<snip>

 > RETURNS:
 >
 > Returns a pointer to a Unicode string, or NULL if the operation fails.


And for GetStringUTFChars(), I see:

http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/functions.html#wp17265

 > GetStringUTFChars
 >
 > const char * GetStringUTFChars(JNIEnv *env, jstring string,
 > jboolean *isCopy);

<snip>

 > RETURNS:
 >
 > Returns a pointer to a modified UTF-8 string, or NULL if the 
operation fails.


I'm not finding this phrase:

     "Returns NULL if and only if an invocation of this function has 
thrown an exception."

associated with either of these functions.

Dan


>
> Impossible to meet that spec given the existence of AllocObject.
>
> Reviewed. ;-)
>
> Thanks,
> David
>
>
>
>> /Staffan
>>