jspawnhelper's sometimes problematic use of argv0

[Thomas Stüfe]

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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