There exists a function os::fopen in HS code . This function sets "close-on-exec" flags (when possible) to the opened filedescriptor.
It might have some benefits to use os::fopen more in HS code, currently there are still quite a few calls directly to fopen .

see also the thread
https://mail.openjdk.java.net/pipermail/hotspot-dev/2020-January/040641.html
on hotspot-dev about the topic.

Comment from Thomas Stuefe :
Just a note, it is still possible for file descriptors to escape into child processes since you cannot guarantee that all forks happen via Runtime.exec():
- native third party code may fork.
- hotspot may fork to run tools for error reporting

Also note that the code which closes fds in Runtime.exec() may in theory fail to close all fds.