RFR: 8377907: (process) Race in ProcessBuilder can cause JVM hangs

[Thomas Stuefe]

On Thu, 26 Feb 2026 15:16:33 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> When starting child processes from Java, we bootstrap the child process after fork and before exec. As part of that process, up to five pipes are handed to the child: three for stdin/out/err, respectively, and two internal communication pipes (fail and childenv).
> 
> If, concurrently with our invocation of `ProcessBuilder.start()`, third-party native code forks a child of its own, the natively forked child carries copies of these pipes. It then may keep these pipes open. This results in various forms of communication errors, most likely hangs - either in `ProcessBuilder.start()`, or in customer code. 
> 
> In the customer case that started this investigation, `ProcessBuilder.start()` hung intermittently when using a third-party Eclipse library that happened to perform forks natively.
> 
> The JVM has no full control over what happens in its process, since we allow native code to run. Therefore, native forks can happen, and we have to work around them. 
> 
> The fix makes sure that the pipes we use in ProcessBuilder are always tagged with CLOEXEC. Since forks are typically followed by execs, this will close any file descriptors that were accidentally inherited.
> 
> ### FORK/VFORK mode
> 
> Here, it is sufficient to open all our pipes with O_CLOEXEC.
> 
> The caveat here is that only Linux offers an API to do that cleanly: `pipe2(2)` ([1]). On MacOS and AIX, we don't have `pipe2(2)`, so we need to emulate that behavior using `pipe(2)` and `fcntl(2)` in quick succession. That is still racy, since we did not completely close the time window within which pipe file descriptors are not O_CLOEXEC. But this is the best we can do.
> 
> ### POSIX_SPAWN mode
> 
> Creating the pipes with CLOEXEC alone is not sufficient. With `posix_spawn(3)`, we exec twice: first to load the jspawnhelper (inside `posix_spawn(3)`), a second time to load the target binary. Pipes created with O_CLOEXEC would not survive the first exec.
> 
> Therefore, instead of manually `dup2(2)`'ing our file descriptors after the first exec in jspawnhelper itself, we set up dup2 file actions to let posix_spawn do the dup'ing. According to POSIX, these dup2 file actions will be processed before the kernel closes the inherited CLOEXEC file descriptors.
> 
> Unfortunately, macOS is again not POSIX-compliant, since the macOS kernel can close CLOEXEC file descriptors before posix_spawn processes them in its dup2 file actions. As a workaround for that bug, we create temporary copies of the pipe file descriptors that are untagged with CLOEXEC and use ...

ping @prrace and @simonis - could you give this a look? Thanks!

test/jdk/java/lang/ProcessBuilder/JspawnhelperProtocol.java line 61:

> 59:             // Route any stdout from the child process - be it jspawnhelper error messages or the output of "/bin/pwd" -
> 60:             // through to the parent process.
> 61:             p = new ProcessBuilder(CMD).inheritIO().start();

Reviewer Note:

JspawnhelperProtocol had a subtle issue: it implicitly relied on standard fork() redirection rules (everything redirected to the parent process) at the very beginning of jspawnhelper main(). This is because we scan in the grandfather process for the "posix_spawn:xx" string produced in the child process, but in the parent process, we don't set up Java inheritIO, instead just calling Runtime.exec(). 

I think this works more by accident, since before we dup2 the stdout fd in jspawnhelper, fork default redirection apply.

Since we now set up filedescriptors before launching jspawnhelper, this does not work anymore. We need to replace Runtime.exec() with ProcessBuilder.inheritIO in the parent process, and move the output scanning code from parent to grandparent.

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PR Comment: https://git.openjdk.org/jdk/pull/29939#issuecomment-3971005281
PR Review Comment: https://git.openjdk.org/jdk/pull/29939#discussion_r2859717998