Questions about the trivial downcall

[Maurizio Cimadamore]

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>
> *Sent:* Wednesday, July 26, 2023 8:29 PM
> *To:* Cheng Jin <jincheng at ca.ibm.com>; 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. 
>
> ZjQcmQRYFpfptBannerStart
>
> *This Message Is From an External Sender *
>
> This message came from outside your organization.
>
> *  Report Suspicious * 
> <https://us-phishalarm-ewt.proofpoint.com/EWT/v1/PjiDSg!2c-gyJS_6lP6sKYb2IHEjb2TaeBqM7xJQ9RStijLaImfla_LrFEK8T2PB5_umxDwUuL8X7BGAHosOeXRLCglJfzdYnJaiHF6gQ$>  ‌ 
>
>
> ZjQcmQRYFpfptBannerEnd
>
>     and does not call back into Java (e.g. using an upcall stub).
>     ß------------------------- it occurs to me that any kind of upcall
>     should be disallowed in such case whether it is JNI or FFM, am I
>     correct?
>
> Right, JNI has not much to do with this. There's different ways to 
> upcall back into Java (JNI and FFM upcall stubs).
>
> Your question is: what happens if we do call back when we are in 
> trivial mode.
>
> I believe the answer is "we crash" (but not in a "nice way"), and I'm 
> not too sure we can do much to prevent that. I believe Jorn knows more 
> on that topic.
>
> Maurizio
>
>     Best Regards
>
>     Cheng Jin
>
>     *From:*Maurizio Cimadamore <maurizio.cimadamore at oracle.com>
>     <mailto:maurizio.cimadamore at oracle.com>
>     *Sent:* Wednesday, July 26, 2023 7:44 PM
>     *To:* Cheng Jin <jincheng at ca.ibm.com>
>     <mailto:jincheng at ca.ibm.com>; panama-dev at openjdk.org
>     *Subject:* [EXTERNAL] Re: Questions about the trivial downcall
>
>     Hi, a trivial downcall is supposed to target a native function
>     that (a) terminates quickly (so as not to block the GC for too
>     long) and (b) does not upcall into Java. So, no, a trivial
>     downcall cannot trigger upcalls (I don't think we detect
>
>     ZjQcmQRYFpfptBannerStart
>
>     *This Message Is From an External Sender *
>
>     This message came from outside your organization.
>
>     *  Report Suspicious *
>     <https://us-phishalarm-ewt.proofpoint.com/EWT/v1/PjiDSg!2c-r455VanMQ2qYReaGl7Z4QRx46ATMxAjK9nbXsj13QlLeimn0PtcIAb-U80cJ-A34Q8dYfoQ6JN5nMYOpBz-Hn3cuAGTZLJQ$>  ‌
>
>
>     ZjQcmQRYFpfptBannerEnd
>
>     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
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/panama-dev/attachments/20230727/8fcb6717/attachment-0001.htm>