jspawnhelper's sometimes problematic use of argv0

[Thomas Stüfe]

Thank you Daniel. Comment's fixed.

Cheers, Thomas

On Sun, Jun 18, 2023 at 2:25 AM Daniel Jarabek <jarabekit at gmail.com> wrote:

> Hi Thomas,
>
> Thank you for this quick fix!
>
> I have tested (after much difficultly getting cross compilation to work -
> a good learning experience) and this fix works for me. Using QEMU_STRACE, I
> verified that argv[0] is now the program name and argv[1] contains the fd
> data.
>
> While you are correct, that changes to jspawnhelper aren't needed (since
> it uses argv[argc-1]), you may want to update this comment here:
> https://github.com/openjdk/jdk/blob/959a61fdd483c9523764b9ba0972f59ca06db0ee/src/java.base/unix/native/jspawnhelper/jspawnhelper.c#L139
>
> -DJ
>
> On 6/17/2023 16:43, Thomas Stüfe wrote:
>
> Mind testing https://github.com/openjdk/jdk/pull/14531 ?
>
> On Sat, Jun 17, 2023 at 8:20 PM Thomas Stüfe <thomas.stuefe at gmail.com>
> wrote:
>
>> Hi Daniel,
>>
>> thank you for that thorough analysis. I opened
>> https://bugs.openjdk.org/browse/JDK-8310265 to track this issue.
>>
>> Looking at the code, this should be easy to fix, we would not even have
>> to fix jspawnhelper itself, just its spawn point.
>>
>> I did a small patch which seemed to work. But this code is notorious for
>> being tricky and I may overlooking something. Lets see.
>>
>> Cheers, Thomas
>>
>>
>>
>>
>>
>> On Sat, Jun 17, 2023 at 6:33 AM Daniel Jarabek <jarabekit at gmail.com>
>> wrote:
>>
>>> Hi,
>>>
>>> While trying to run jshell in qemu-user-static using binfmt_misc
>>> (specifically, using this docker based tool [1]), I ran into an issue.
>>> Jshell would fail to start and print `This command is not for general
>>> use and should only be run as the result of a call to
>>> ProcessBuilder.start() or Runtime.exec() in a java application` along
>>> with a stack trace including `Failed to exec spawn helper`. After
>>> searching this error, I found JDK-8296001 [2]. This issue presented a
>>> very similar scenario (using a userspace architecture emulator). I
>>> looked into this issue and I believe I have identified the root cause.
>>>
>>> My specific investigation was done around jdk17, but this issue also
>>> occurs on jdk20 and older jdk versions (though it seems like the "spawn"
>>> strategy is not used by default on linux with jdk 8 and 11).
>>>
>>> Note: I use argv0 to refer to "argv[0]" to prevent confusion with
>>> footnotes.
>>>
>>> Traditionally, argv0 contains the name of the program being executed.
>>> While this is more a convention than anything [3], it is very widely
>>> used. jspawnhelper [4] is an executable used to assist in launching
>>> processes via ProcessBuilder and Runtime.exec. Unlike the convention,
>>> jspawnhelper uses argv0 for passing data [5]. When binfmts_misc [6]
>>> executes an interpreter, it overwrites argv0, unless a flag is set. This
>>> causes jspawnhelper to fail to parse argv0 (since it's the application
>>> path, and not the data sent by the JDK), so it prints the warning about
>>> not being for general use. By setting the binfmt_misc `P`
>>> (preserve-argv0) flag, it preserves argv0 which fixes this problem.
>>>
>>> Since qemu-user correctly supports this flag, all I had to do to fix the
>>> issue in my case was run qemu-binfmt-conf.sh with `--preserve-argv0 yes`
>>> to set that flag. It's likely that in the case of the above issue, the
>>> same problem is occurring and it may have a similar resolution. This
>>> could be fixed in OpenJDK by using argv1 instead, but at least in my
>>> case of binfmts_misc, this is an external configuration issue that can
>>> be easily fixed. However, I would appreciate it if someone with
>>> permission could comment on the above issue with a link to this email
>>> for anyone who runs into this in the future.
>>>
>>> -DJ
>>>
>>> [1] https://github.com/multiarch/qemu-user-static
>>> [2] https://bugs.openjdk.org/browse/JDK-8296001
>>> [3] https://stackoverflow.com/a/2051031
>>> [4]
>>>
>>> https://github.com/openjdk/jdk/blob/master/src/java.base/unix/native/jspawnhelper/jspawnhelper.c
>>> [5]
>>>
>>> https://github.com/openjdk/jdk/blob/bcc4d36857b0907e865d0afc4447f9b0780f8101/src/java.base/unix/native/libjava/ProcessImpl_md.c#L498-L500
>>> [6] https://docs.kernel.org/admin-guide/binfmt-misc.html
>>>
>>
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