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 <jarabe...@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 >
