Filed: https://bugs.openjdk.java.net/browse/JDK-8190187 "JVM options that cause the VM to "do X then exit" are incompatible with the JNI Invocation API" David On 26/10/2017 1:23 AM, Adam Farley8 wrote:
Thanks David :)
Please let me know what the bug number is when you have time.
Everyone else: Any volunteers to champion this?
I'm on IRC 9-5 from monday-friday if anyone wants to discuss it.
Best Regards
Adam Farley
P.S. Here's the summary including David's addition, minus the chevrons:
1) Exit(0) (during VM startup) is a big bug because it imitates successful completion of external cpp code accessing JNI methods. 2) One solution for (1) is to specify a new return code for JNI. 3) The supplied code (diff) generates, facilitates, and handles that return code for the exit(0) scenario: -agentlib:jdwp=help 4) The exit(0) problem (in general) is worth fixing, however we may choose not to support the use of this new code in the jdwp example case. 5) The supplied test confirms that the supplied code works (run via unzip, and then bash TestStart.sh <path to jdk home dir that contains bin dir>). 6) To implement this new return code, plus the code that handles it, we would need to follow the CSR process to modify the JNI spec. 7) To implement the jdwp scenario fix, if we choose to support it at all, we would also need to use the CSR process the JVM TI spec. 8) To address all of the worst instances of exit(0), we would need to search for exit(0) and raise a bug for each significant one (or group). 9) To solve (8) in one bug would be a lot of work, arguably too much work for a single bug. 10) If the new return code is identified as the appropriate solution to this problem, we will need to agree on the right name and return code value.
Also, here's a shortlist of the main questions I recall being raised here, plus answers people have given.
A) What are the potential solutions for the exit(0) problem? i) New JNI Return Code. ii) Remove the info-only options, at least via the JNI, and return a standard error code if they are used. iii) Remove the info-only options, at least via the JNI, and filter them out if they are used. iv) Retain existing behaviour, and document a need for the user to filter out help options before starting the VM via the JNI.
B) What are the criteria for the "Best" solution? i) It must prevent "exit(0)" calls. ii) It must be proven to work. iii) It should require minimal (none, ideally) behaviour change from the java.exe user. iv) It should allow the external cpp code accessing the JNI to complete normally, without being prematurely terminated.
C) Which solutions meet the (B) criteria? i) Both the "new return code" and the "remove info-only options" solutions meet the (B) criteria.
D) Is it right to have any "Do X and then exit." arguments at all, and (if no) what would be the alternatives? i) Given the VM is a loadable shared library the answer should be No. We should not have any "Do X and then exit" arguments. The alternatives would be a mix of: - added Dcmds to "Do X" - added launcher smarts to recognize options that need a Dcmd and "then exit".
Note from David: I find the notion of "help" options problematic for a shared library. But I don't have a good answer for how else to document things.
That all said I think this ship has sailed and we're unlikely to want to invest the time and effort in trying to clean this up this way.
From: David Holmes <david.holmes@oracle.com> To: Adam Farley8 <adam.farley@uk.ibm.com> Cc: Alan Bateman <Alan.Bateman@oracle.com>, core-libs-dev@openjdk.java.net, hotspot-runtime-dev@openjdk.java.net, thomas.stuefe@gmail.com Date: 25/10/2017 13:53 Subject: Re: [BUG PROPOSAL]: C++ code that calls JNI_CreateJavaVM can be exited by java ------------------------------------------------------------------------
Hi Adam,
On 18/10/2017 9:17 PM, Adam Farley8 wrote:
Hi All,
Updated summary, based on the responses:
I think this is a good summary. Thanks.
I can file a bug for this, but it will take some work to see how to fit this into the existing specifications and file CSR requests for those changes. This won't make 18.3 (or whatever it gets called). You will need a "champion" to help flesh this out in full and work with you on those CSR requests. I can't volunteer to do that at this time.
1) Exit(0) (during VM startup) is a big bug because it imitates successful completion of external cpp code accessing JNI methods. 2) One solution for (1) is to specify a new return code for JNI. 3) The supplied code (diff) generates, facilitates, and handles that return code for the exit(0) scenario: -agentlib:jdwp=help 4) The exit(0) problem (in general) is worth fixing, however we may choose not to support the use of this new code in the jdwp example case. 5) The supplied test confirms that the supplied code works (run via unzip, and then bash TestStart.sh <path to jdk home dir that contains bin dir>). 6) To implement this new return code, plus the code that handles it, we would need to follow the CSR process to modify the JNI spec. 7) To implement the jdwp scenario fix, if we choose to support it at all, we would also need to use the CSR process the JVM TI spec. 8) To address all of the worst instances of exit(0), we would need to search for exit(0) and raise a bug for each significant one (or group). 9) To solve (8) in one bug would be a lot of work, arguably too much work for a single bug. 10) If the new return code is identified as the appropriate solution to this problem, we will need to agree on the right name and return code value.
Also, here's a shortlist of the main questions I recall being raised here, plus answers people have given.
A) What are the potential solutions for the exit(0) problem? i) New JNI Return Code. ii) Remove the info-only options, at least via the JNI, and return a standard error code if they are used. iii) Remove the info-only options, at least via the JNI, and filter them out if they are used. iv) Retain existing behaviour, and document a need for the user to filter out help options before starting the VM via the JNI.
B) What are the criteria for the "Best" solution? i) It must prevent "exit(0)" calls. ii) It must be proven to work. iii) It should require minimal (none, ideally) behaviour change from the java.exe user. iv) It should allow the external cpp code accessing the JNI to complete normally, without being prematurely terminated.
C) Which solutions meet the (B) criteria? i) Both the "new return code" and the "remove info-only options" solutions meet the (B) criteria.
D) Is it right to have any "Do X and then exit." arguments at all, and (if no) what would be the alternatives? i) ?
Given the VM is a loadable shared library the answer should be No. We should not have any "Do X and then exit" arguments. The alternatives would be a mix of: - added Dcmds to "Do X" - added launcher smarts to recognize options that need a Dcmd and "then exit".
I find the notion of "help" options problematic for a shared library. But I don't have a good answer for how else to document things.
That all said I think this ship has sailed and we're unlikely to want to invest the time and effort in trying to clean this up this way.
Thanks, David
Best Regards
Adam Farley
P.S. As per Alan and David's emails, the exit(#) references have been removed entirely, as discussing them alongside the original exit(0) problem risks scope creep.
This bug, if raised, should only cover the exit(0) cases. I believe we have consensus here.
From: David Holmes <david.holmes@oracle.com> To: Adam Farley8 <adam.farley@uk.ibm.com>, Alan Bateman <Alan.Bateman@oracle.com>, core-libs-dev@openjdk.java.net, hotspot-runtime-dev@openjdk.java.net, thomas.stuefe@gmail.com Date: 13/10/2017 14:31 Subject: Re: [BUG PROPOSAL]: C++ code that calls JNI_CreateJavaVM can be exited by java ------------------------------------------------------------------------
Hi Adam,
On 13/10/2017 10:16 PM, Adam Farley8 wrote: > Hi All, > > Here's a summary of the email below (which is intended, partly, as a > summary of the emails before it). > > Let me know if you agree/disagree with any of these points. > > 1) Exit(#) during vm startup is a bug because it should return a code > regardless of the state of the VM.
Yes it's a bug but not one that is likely to be addressed in any foreseeable timeframe. There are simply too many "exit on error" paths. If we were to start using C++ exceptions within the VM that might provide a way to quickly get back to the CreateJavaVM routine where we could return an error code - but that is itself a major project that has barely even been discussed AFAIK. (Compiler folk have talked about it because compiler paths are fairly self-contained - though that was before Graal and AOT.)
> 2) Exit(0) is an *especially* big bug because it imitates successful > completion of external cpp code accessing JNI methods. > 3) One solution is to specify a new return code for JNI.
A solution for 2) yes.
> 4) The supplied code (diff) generates, facilitates, and handles that > return code for the exit(0) scenario: -agentlib:jdwp=help > 5) The supplied test confirms that the supplied code works (run via > unzip, and then bash TestStart.sh <path to jdk home dir that contains > bin dir>). > 6) To implement this new return code, plus the code that handles it, we > would need to follow the CSR process. > 7) To implement the fix for the scenario used as an example of the new > return code's use, we would need to modify the JVM TI spec.
Yes you have demonstrated a potential solution for the agent case. The question is, is it the right solution? Is it a worthwhile solution? (As I've said I'd prefer not to have any "do something then exit" VM arguments.) And can we make it fit into the existing specs without contorting things too much.
I still think it easier/preferable for whatever loads the VM to filter out the VM args that trigger this behaviour. I mean if you pass -Xshare:dump you don't have any right to expect a functioning VM after JNI_CreateJavaVM returns - at least I don't think so. Just don't do it. Yes it is an imperfection in the invocation API, but life isn't perfect.
> 8) To address all of the worst instances of exit(#), we would need to > search for exit(#) and raise a bug for each significant one (or group). > 9) To solve (8) in one bug would be a lot of work, arguably too much > work for a single bug.
This is simply impractical. You may be able to pick off a few low-hanging cases, but that won't really make any practical difference.
> 10) If the new return code is chosen as the appropriate solution to this > problem, we may need to choose a better name for the return code. > > Is this a fair assessment of the current state of the debate?
It's a fair summary of your position and proposal.
Cheers, David
> I'm on IRC every weekday from 9am-5pm (4pm fridays) BST (GMT+1) if > anyone wants to discuss this in real-time on the openjdk channel. > > Best Regards > > Adam Farley > > > > -- Previous Email -- > > Hi David, Alan, > > You are right in that the changes to HotSpot would be nontrivial. > I see a number of places in (e.g.) arguments.cpp that seem to > exit in the same manner as Xlog (such as -Xinternalversion). > > I would advise ploughing through the CSR process to alter the > JNI spec, and simultaneously identify some key paths that can > be raised as bugs. That way, when people have time to address > these issues, the mechanism to handle a silent exit is already > in place. > > The JDWP fix can be raised separately as one of these bugs, if > it would make things simpler. > > As for the name, JNI_SILENT_EXIT is a placeholder, and can be > readily changed. Do you have any suggestions? > > Lastly, in an ideal world, the VM initialisation should never exit(#). It > should return a return code that tells the caller something, pass or > fail, messy or tidy. That way, if someone is using the JNI as part of > something bigger (like a database or a web server), one of these > scenarios is just a bug, rather than a world-ender like exit(#). > > And now for the individual messages. :) > > David: Having help data returned by the launcher seems like a > good way to avoid exit(0) calls, but I'm not sure how we'd prevent > a JNI-caller using those options. Ultimately, to be sure, we'd have > to remove the logic for those options, centralise the data to better > enable launcher access, and add some logic in there so it can find > any other help data (e.g. from the jdwp agent library). I feel this would > be a bigger task than adding the new return code and changing the > vm, plus it wouldn't provide for any non-help scenarios where the > vm wants to shut down without error during initialisation. > > Alan: I should mention that the silent exit solution is already in > use in the OpenJ9 VM. Not all of the exit paths have been > resolved, but many have. > > The code is open and can be found here: <https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_eclipse_openj9&d=DwIC-g&c=jf_iaSHvJObTbx-siA1ZOg&r=P5m8KWUXJf-CeVJc0hDGD9AQ2LkcXDC0PMV9ntVw5Ho&m=c_McLiDSKtlzF_gNlWcAvBTNbDqyGHyW325GY3_3QkU&s=0QVowxRejNRAXI0Vv5whKQFWGTO36XVICmPYCG8EqIU&e=> > https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_eclipse_open... > > And though the silent exit code is disabled for the time being, it > can be re-enabled by entering this class: > > runtime/vm/jvminit.c > > and altering line 2343 ( ctrl-f for exit(1) if it's not there). > > I won't paste the full code here in case people are concerned > about contamination, but I would assert that this code (and the > associated vm files) prove that the concept is possible. > > Note that that code should not be enabled until after we've > integrated the code that can handle a silent exit. > > Best Regards > > Adam Farley > > P.S. Thank you both for your efforts on this. :) > > > > From: David Holmes <david.holmes@oracle.com> > To: Alan Bateman <Alan.Bateman@oracle.com>, Adam Farley8 > <adam.farley@uk.ibm.com>, core-libs-dev@openjdk.java.net, > hotspot-runtime-dev@openjdk.java.net, thomas.stuefe@gmail.com > Date: 15/09/2017 12:03 > Subject: Re: [BUG PROPOSAL]: C++ code that calls JNI_CreateJavaVM can be > exited by java > ------------------------------------------------------------------------ > > > > > > On 15/09/2017 8:17 PM, Alan Bateman wrote: > > On 15/09/2017 02:47, David Holmes wrote: > >> Hi Adam, > >> > >> I am still very much torn over this one. I think the idea of > >> print-and-exit flags for a potentially hosted library like the JVM is > >> just wrong - we should never have done that, but we did. Fixing that > >> by moving the flags to the launcher is far from trivial**. Endorsing > >> and encouraging these sorts of flag by adding JNI support seems to be > >> sending the wrong message. > >> > >> ** I can envisage a "help xxx" Dcmd that can read back the info from > >> the VM. The launcher can send the Dcmd, print the output and exit. The > >> launcher would not need to know what the xxx values mean, but would > >> have to intercept the existing ones. > >> > >> Another option is just to be aware of these flags (are there more than > >> jdwp and Xlog?) and deal with them specially in your custom launcher - > >> either filter them out and ignore them, or else launch the VM in its > >> own process to respond to them. > >> > >> Any changes to the JNI specification need to go through the CSR process. > > Yes, it would require an update to the JNI spec, also a change to the > > JVM TI spec where Agent_OnLoad returning a non-0 value is specified to > > terminates the VM. The name and value needs discussion too, esp. as the > > JNI spec uses negative values for failure. > > > > In any case, I'm also torn over this one as it's a corner case that is > > only interesting for custom launchers that load agents with options that > > print usage messages. It wouldn't be hard to have the Agent_OnLoad > > specify a printf hook that the agent could use for output although there > > are complications with agents such as JDWP that also announce their > > transport end point. Beyond that there is still the issue of the custom > > launcher that would need to know to destroy the VM without reporting an > > error. > > > > So what happened to the more meaty part to this which is fixing the > > various cases in HotSpot that terminate the process during > > initialization? I would expect some progress could be made on those > > cases while trying to decide whether to rev the JNI and JVM TI specs to > > cover the help case. > > Trying to eliminate the vm_exit_during_initialization paths in hotspot > is a huge undertaking IMHO. > > David > > > > > -Alan > > Unless stated otherwise above: > IBM United Kingdom Limited - Registered in England and Wales with number > 741598. > Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU