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<p>The behavior is undefined. It might crash, or not.</p>
<p>While the description says that the downcall should not upcall,
please note that the Javadoc doesn't say anywhere that ill-behaved
downcalls will produce new errors. Moreover, the javadoc says
this:</p>
<p>
<blockquote type="cite">
<p> Using this linker option when linking non trivial functions
is likely to have adverse effects, such as loss of
performance, or JVM crashes.</p>
</blockquote>
Basically, you are in undefined behavior territory. It seems to me
that you are inferring too much from the javadoc.</p>
<p>While I agree it might be desirable to detect this and log some
kind of error (see email I sent yesterday), I don't think that, as
thing stands, the javadoc is incorrect, and/or lacking
information.</p>
<p>Maurizio<br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 27/07/2023 14:40, Cheng Jin wrote:<br>
</div>
<blockquote type="cite" cite="mid:BN8PR15MB257733FE3CA75202C523ADD7F501A@BN8PR15MB2577.namprd15.prod.outlook.com">
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<p class="MsoNormal"><span style="font-size:11.0pt">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
<a href="https://urldefense.com/v3/__https://download.java.net/java/early_access/jdk21/docs/api/java.base/java/lang/foreign/Linker.Option.html*isTrivial()__;Iw!!ACWV5N9M2RV99hQ!KpMq9yrQInVdYsiAcZWCDTeOZd_GLJwrDN4n06JvbKQ194M_eabqlUaxzgDuSjbnoxQLukS4Uu8wT7q__xolFW5e$" moz-do-not-send="true">
https://download.java.net/java/early_access/jdk21/docs/api/java.base/java/lang/foreign/Linker.Option.html#isTrivial()</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">or OpenJDK
doesn’t support such behavior mixed with JNI & FFM?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:11.0pt">[1]
JniTest.java<o:p></o:p></span></b></p>
<p class="MsoNormal"><span style="font-size:11.0pt">public class
JniTest {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
private static Linker linker = Linker.nativeLinker();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> static {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
System.loadLibrary("jnitest");<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
private static final SymbolLookup nativeLibLookup =
SymbolLookup.loaderLookup();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">private
native int addJNI2Ints(int arg1, int arg2);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> public
int <b>addJNI2Ints_Upcall</b>(int arg1, int arg2) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> int
sum = arg1 + arg2;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
System.out.println("trivial: addJNI2Ints_Upcall: sum = " +
sum);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> return
sum;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
public void test_add2Ints() throws Throwable {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
FunctionDescriptor fd = FunctionDescriptor.of(JAVA_INT,
JAVA_INT, JAVA_INT);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
MemorySegment functionSymbol =
nativeLibLookup.find("add2Ints").get();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
MethodHandle mh = linker.downcallHandle(functionSymbol, fd,
<b>Linker.Option.isTrivial()</b>);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
int result = (int)mh.invokeExact(112, 123);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
System.out.println("test_add2Ints result = " + result);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> public
static void main(String[] args) throws Throwable {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
JniTest test = new JniTest();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> int
sum = test.<b>addJNI2Ints</b>(112, 123);
</span><span style="font-size:11.0pt;font-family:Wingdings">ß</span><span style="font-size:11.0pt">-- call the JNI native at first to
save the global JNIEnv & jobject<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
System.out.println("addJNI2Ints sum = " + sum);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
test.test_<b>add2Ints</b>();
</span><span style="font-size:11.0pt;font-family:Wingdings">ß</span><span style="font-size:11.0pt">-- use saved the global JNIEnv
& jobject to trigger a JNI upcall in native.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">}<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:11.0pt">[2]
JniTest.c<o:p></o:p></span></b></p>
<p class="MsoNormal"><span style="font-size:11.0pt">#include
<jni.h><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">#include
"JniTest.h"<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">JNIEnv
*globalEnv;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">jobject
globalObj;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">JNIEXPORT
int JNICALL Java_JniTest_addJNI2Ints(JNIEnv *env, jobject
thisObj, jint arg1, jint arg2) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> jint
intSum = arg1 + arg2;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
globalEnv = env;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
globalObj = thisObj;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
printf("\nJava_JniTest_addJNI2Ints: env = %p, globalObj =
%p\n", globalEnv, globalObj);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> return
intSum;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">}<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">int<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">add2Ints(int
intArg1, int intArg2)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">{<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
printf("\nadd2Ints: globalEnv = %p, globalObj = %p\n",
globalEnv, globalObj);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> jclass
clazz = (*globalEnv)->GetObjectClass(globalEnv,
globalObj);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">
jmethodID method = (*globalEnv)->GetMethodID(globalEnv,
clazz, "addJNI2Ints_Upcall", "(II)I");<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> return
(*globalEnv)->CallIntMethod(globalEnv, globalObj, method,
intArg1, intArg2);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">}<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">[3]Commands:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">./jdk21_hotspot_x86_64/bin/javac
--enable-preview --source 21 -h . JniTest.java<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">gcc -fPIC
-I"./jdk21_hotspot_x86_64/include"
-I"./jdk21_hotspot_x86_64/include/linux" -shared -g -o
libjnitest.so JniTest.c<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">./jdk21_hotspot_x86_64/bin/java
--enable-preview -Djava.library.path=./jnitests
--enable-native-access=ALL-UNNAMED
-Dforeign.restricted=permit JniTest<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">[4] Output:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Java_JniTest_addJNI2Ints:
env = 0x7f81f8028a08, globalObj = 0x7f81ff379998<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">addJNI2Ints
sum = 235<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">add2Ints:
intSum = 235<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">add2Ints:
globalEnv = 0x7f81f8028a08, globalObj = 0x7f81ff379998<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:11.0pt">trivial:
addJNI2Ints_Upcall: sum = 235
</span></b><b><span style="font-size:11.0pt;font-family:Wingdings">ß</span></b><b><span style="font-size:11.0pt"><o:p></o:p></span></b></p>
<p class="MsoNormal"><span style="font-size:11.0pt">test_add2Ints
result = 235<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Best Regards<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Cheng Jin<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<div style="border:none;border-top:solid #E1E1E1
1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span style="font-size:11.0pt" lang="EN-US">From:</span></b><span style="font-size:11.0pt" lang="EN-US"> Maurizio Cimadamore
<a class="moz-txt-link-rfc2396E" href="mailto:maurizio.cimadamore@oracle.com"><maurizio.cimadamore@oracle.com></a>
<br>
<b>Sent:</b> Wednesday, July 26, 2023 8:45 PM<br>
<b>To:</b> Cheng Jin <a class="moz-txt-link-rfc2396E" href="mailto:jincheng@ca.ibm.com"><jincheng@ca.ibm.com></a>;
<a class="moz-txt-link-abbreviated" href="mailto:panama-dev@openjdk.org">panama-dev@openjdk.org</a><br>
<b>Subject:</b> [EXTERNAL] Re: Questions about the trivial
downcall<o:p></o:p></span></p>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal" style="mso-line-height-alt:.75pt"><span style="font-size:1.0pt;color:white">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 <o:p></o:p></span></p>
</div>
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<o:p></o:p></span></p>
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<p><o:p> </o:p></p>
<div>
<p class="MsoNormal"><span style="font-size:11.0pt">On
27/07/2023 01:40, Cheng Jin wrote:<o:p></o:p></span></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<p>>> Right, JNI has not much to do with this. There's
different ways to upcall back into Java (JNI and FFM upcall
stubs).<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">If so, my
understanding is that both JNI and FFM upcall should crash
the VM in a trivial FFM downcall, correct?</span><o:p></o:p></p>
</blockquote>
<p>I believe so. Jorn will confirm.<o:p></o:p></p>
<p>Maurizio<o:p></o:p></p>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal"><span style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Best
Regards</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Cheng Jin</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> </span><o:p></o:p></p>
<div style="border:none;border-top:solid #E1E1E1
1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span style="font-size:11.0pt" lang="EN-US">From:</span></b><span style="font-size:11.0pt" lang="EN-US"> Maurizio
Cimadamore
</span><a href="mailto:maurizio.cimadamore@oracle.com" moz-do-not-send="true"><span style="font-size:11.0pt" lang="EN-US"><maurizio.cimadamore@oracle.com></span></a><span style="font-size:11.0pt" lang="EN-US">
<br>
<b>Sent:</b> Wednesday, July 26, 2023 8:29 PM<br>
<b>To:</b> Cheng Jin </span><a href="mailto:jincheng@ca.ibm.com" moz-do-not-send="true"><span style="font-size:11.0pt" lang="EN-US"><jincheng@ca.ibm.com></span></a><span style="font-size:11.0pt" lang="EN-US">;
</span><a href="mailto:panama-dev@openjdk.org" moz-do-not-send="true"><span style="font-size:11.0pt" lang="EN-US">panama-dev@openjdk.org</span></a><span style="font-size:11.0pt" lang="EN-US"><br>
<b>Subject:</b> [EXTERNAL] Re: Questions about the
trivial downcall</span><o:p></o:p></p>
</div>
<p class="MsoNormal"> <o:p></o:p></p>
<div>
<p class="MsoNormal" style="mso-line-height-alt:.75pt"><span style="font-size:1.0pt;color:white">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.
</span><o:p></o:p></p>
</div>
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<p>and does not call back into Java (e.g. using an upcall
stub). <span style="font-family:Wingdings">
ß</span>------------------------- it occurs to me that
any kind of upcall should be disallowed in such case
whether it is JNI or FFM, am I correct?
<o:p></o:p></p>
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<p>Right, JNI has not much to do with this. There's different
ways to upcall back into Java (JNI and FFM upcall stubs).<o:p></o:p></p>
<p>Your question is: what happens if we do call back when we
are in trivial mode.<o:p></o:p></p>
<p>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.<o:p></o:p></p>
<p>Maurizio<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt">Best Regards</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt">Cheng Jin</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span style="font-size:11.0pt" lang="EN-US">From:</span></b><span style="font-size:11.0pt" lang="EN-US"> Maurizio
Cimadamore
</span><a href="mailto:maurizio.cimadamore@oracle.com" moz-do-not-send="true"><span style="font-size:11.0pt" lang="EN-US"><maurizio.cimadamore@oracle.com></span></a><span style="font-size:11.0pt" lang="EN-US">
<br>
<b>Sent:</b> Wednesday, July 26, 2023 7:44 PM<br>
<b>To:</b> Cheng Jin </span><a href="mailto:jincheng@ca.ibm.com" moz-do-not-send="true"><span style="font-size:11.0pt" lang="EN-US"><jincheng@ca.ibm.com></span></a><span style="font-size:11.0pt" lang="EN-US">;
</span><a href="mailto:panama-dev@openjdk.org" moz-do-not-send="true"><span style="font-size:11.0pt" lang="EN-US">panama-dev@openjdk.org</span></a><span style="font-size:11.0pt" lang="EN-US"><br>
<b>Subject:</b> [EXTERNAL] Re: Questions about the
trivial downcall</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-line-height-alt:.75pt"><span style="font-size:1.0pt;color:white">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 </span><o:p></o:p></p>
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<p>Hi,<br>
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.<o:p></o:p></p>
<p>So, no, a trivial downcall cannot trigger upcalls (I
don't think we detect this, I believe the JVM just
crashes if you do).<o:p></o:p></p>
<p>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?).<o:p></o:p></p>
<p>Maurizio<o:p></o:p></p>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt">On 26/07/2023 22:23, Cheng
Jin wrote:</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt">Hi there,</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt">I’ve got a couple of
questions about the behavior of a trivial downcall
(with
<b>isTrivial</b> specified).</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt">Is a FFI downcall able to
trigger a JNI upcall ?</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt">If so, should a JNI upcall
should be captured in this trivial downcall?</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt">Best Regards</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt">Cheng Jin</span><o:p></o:p></p>
</blockquote>
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