New subject: [BUG PROPOSAL]: C++ code that calls JNI_CreateJavaVM can be exited by java

25 Oct 2017

      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_open...
...
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

Re: [BUG PROPOSAL]: C++ code that calls JNI_CreateJavaVM can be exited by java

Adam Farley8

David Holmes

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