jspawnhelper's sometimes problematic use of argv0

[Daniel Jarabek]

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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