On Mon, 9 Mar 2026 06:57:02 GMT, Thomas Stuefe <[email protected]> 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 descr... > > Thomas Stuefe has updated the pull request incrementally with one additional > commit since the last revision: > > attempt a different fix for the MacOS kernel-closes-CLOEXEC-fds-too-early > problem I like this approach, and would give it a bit more bake-time before approving. src/java.base/unix/native/libjava/childproc.c line 435: > 433: (closeSafely(p->childenv[0]) == -1) || > 434: (closeSafely(p->childenv[1]) == -1) || > 435: (closeSafely(p->fail[0]) == -1)) The errno sent by WhyJohnnyCantExec will be meaningless if any of these are true. It would useful debugging info to insert some kind of identifying code in the upper bits of errno for each of the `goto WhyCan'tJohnnyExec`. ------------- PR Review: https://git.openjdk.org/jdk/pull/29939#pullrequestreview-3924465096 PR Review Comment: https://git.openjdk.org/jdk/pull/29939#discussion_r2913603521
