Questions about the trivial downcall

Thu Jul 27 13:40:19 UTC 2023

But I tried a simple example to indirectly trigger a JNI upcall within a trivial FFM downcall by saving a global JNIEnv within a JNI downcall at first which works good as follows. Does it mean that a FFM trivial downcall literally doesn’t stop a JNI upcall which seems inconsistent with the description of https://download.java.net/java/early_access/jdk21/docs/api/java.base/java/lang/foreign/Linker.Option.html#isTrivial()
or OpenJDK doesn’t support such behavior mixed with JNI & FFM?

[1] JniTest.java
public class JniTest {
        private static Linker linker = Linker.nativeLinker();
   static {
      System.loadLibrary("jnitest");
   }
        private static final SymbolLookup nativeLibLookup = SymbolLookup.loaderLookup();

private native int addJNI2Ints(int arg1, int arg2);

    public int addJNI2Ints_Upcall(int arg1, int arg2) {
      int sum = arg1 + arg2;
      System.out.println("trivial: addJNI2Ints_Upcall: sum = " + sum);
      return sum;
    }

        public void test_add2Ints() throws Throwable {
                FunctionDescriptor fd = FunctionDescriptor.of(JAVA_INT, JAVA_INT, JAVA_INT);
                MemorySegment functionSymbol = nativeLibLookup.find("add2Ints").get();
                MethodHandle mh = linker.downcallHandle(functionSymbol, fd, Linker.Option.isTrivial());
                int result = (int)mh.invokeExact(112, 123);
                System.out.println("test_add2Ints result = " + result);
        }

   public static void main(String[] args) throws Throwable {
      JniTest test = new JniTest();
      int sum = test.addJNI2Ints(112, 123); <---- call the JNI native at first to save the global JNIEnv & jobject
      System.out.println("addJNI2Ints sum = " + sum);
      test.test_add2Ints(); <---- use saved the global JNIEnv & jobject to trigger a JNI upcall in native.
   }
}

[2] JniTest.c
#include <jni.h>
#include "JniTest.h"

JNIEnv *globalEnv;
jobject globalObj;

JNIEXPORT int JNICALL Java_JniTest_addJNI2Ints(JNIEnv *env, jobject thisObj, jint arg1, jint arg2) {
    jint intSum = arg1 + arg2;
    globalEnv = env;
    globalObj = thisObj;
    printf("\nJava_JniTest_addJNI2Ints: env = %p, globalObj = %p\n", globalEnv, globalObj);
    return intSum;
}

int
add2Ints(int intArg1, int intArg2)
{
        printf("\nadd2Ints: globalEnv = %p, globalObj = %p\n", globalEnv, globalObj);
    jclass clazz = (*globalEnv)->GetObjectClass(globalEnv, globalObj);
    jmethodID method = (*globalEnv)->GetMethodID(globalEnv, clazz, "addJNI2Ints_Upcall", "(II)I");
    return (*globalEnv)->CallIntMethod(globalEnv, globalObj, method, intArg1, intArg2);
}

[3]Commands:
./jdk21_hotspot_x86_64/bin/javac  --enable-preview --source 21  -h . JniTest.java
gcc -fPIC -I"./jdk21_hotspot_x86_64/include" -I"./jdk21_hotspot_x86_64/include/linux" -shared -g  -o libjnitest.so  JniTest.c
./jdk21_hotspot_x86_64/bin/java --enable-preview  -Djava.library.path=./jnitests  --enable-native-access=ALL-UNNAMED -Dforeign.restricted=permit   JniTest

[4] Output:
Java_JniTest_addJNI2Ints: env = 0x7f81f8028a08, globalObj = 0x7f81ff379998
addJNI2Ints sum = 235
add2Ints: intSum = 235
add2Ints: globalEnv = 0x7f81f8028a08, globalObj = 0x7f81ff379998

trivial: addJNI2Ints_Upcall: sum = 235 <--
test_add2Ints result = 235

Best Regards
Cheng Jin

From: Maurizio Cimadamore <maurizio.cimadamore at oracle.com>
Sent: Wednesday, July 26, 2023 8:45 PM
To: Cheng Jin <jincheng at ca.ibm.com>; panama-dev at openjdk.org
Subject: [EXTERNAL] Re: Questions about the trivial downcall

On 27/07/2023 01: 40, Cheng Jin wrote: >> Right, JNI has not much to do with this. There's different ways to upcall back into Java (JNI and FFM upcall stubs). If so, my understanding is that both JNI and FFM upcall should crash the VM
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On 27/07/2023 01:40, Cheng Jin wrote:

>> Right, JNI has not much to do with this. There's different ways to upcall back into Java (JNI and FFM upcall stubs).
If so, my understanding is that both JNI and FFM upcall should crash the VM in a trivial FFM downcall, correct?

I believe so. Jorn will confirm.

Maurizio

Best Regards
Cheng Jin

From: Maurizio Cimadamore <maurizio.cimadamore at oracle.com><mailto:maurizio.cimadamore at oracle.com>
Sent: Wednesday, July 26, 2023 8:29 PM
To: Cheng Jin <jincheng at ca.ibm.com><mailto:jincheng at ca.ibm.com>; panama-dev at openjdk.org<mailto:panama-dev at openjdk.org>
Subject: [EXTERNAL] Re: Questions about the trivial downcall

and does not call back into Java (e. g. using an upcall stub). ß------------------------- it occurs to me that any kind of upcall should be disallowed in such case whether it is JNI or FFM, am I correct? Right, JNI has not much to do with this. 
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and does not call back into Java (e.g. using an upcall stub). <--------------------------- it occurs to me that any kind of upcall should be disallowed in such case whether it is JNI or FFM, am I correct?

Right, JNI has not much to do with this. There's different ways to upcall back into Java (JNI and FFM upcall stubs).

Your question is: what happens if we do call back when we are in trivial mode.

I believe the answer is "we crash" (but not in a "nice way"), and I'm not too sure we can do much to prevent that. I believe Jorn knows more on that topic.

Maurizio

Best Regards
Cheng Jin

From: Maurizio Cimadamore <maurizio.cimadamore at oracle.com><mailto:maurizio.cimadamore at oracle.com>
Sent: Wednesday, July 26, 2023 7:44 PM
To: Cheng Jin <jincheng at ca.ibm.com><mailto:jincheng at ca.ibm.com>; panama-dev at openjdk.org<mailto:panama-dev at openjdk.org>
Subject: [EXTERNAL] Re: Questions about the trivial downcall

Hi, a trivial downcall is supposed to target a native function that (a) terminates quickly (so as not to block the GC for too long) and (b) does not upcall into Java. So, no, a trivial downcall cannot trigger upcalls (I don't think we detect
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Hi,
a trivial downcall is supposed to target a native function that (a) terminates quickly (so as not to block the GC for too long) and (b) does not upcall into Java.

So, no, a trivial downcall cannot trigger upcalls (I don't think we detect this, I believe the JVM just crashes if you do).

Also I notice that you speak of "JNI" upcall. Is that what you really mean - e.g. a trivial FFM downcall making an upcall using JNI? In general, I think it's a bit hard for downcalls to do anything that has to do with JNI because the invoked native function is not passed a JNIEnv - FFM really just bridges the Java code with the native library function you want to call, nothing more. (I suppose that function could get its hands on the VM using the JNI attach API, but I feel that's a different question from what you were asking?).

Maurizio
On 26/07/2023 22:23, Cheng Jin wrote:
Hi there,

I’ve got a couple of questions about the behavior of a trivial downcall (with isTrivial specified).

Is a FFI downcall able to trigger a JNI upcall ?
If so, should a JNI upcall should be captured in this trivial downcall?

Best Regards
Cheng Jin
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