RFR: 8210549: Runtime.exec: in closeDescriptors(), use FD_CLOEXEC instead of close()

[Roger Riggs]

On Mon, 19 May 2025 12:23:07 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> Hi, please consider the following patch.
> 
> This patch replaces the existing close-file-descriptors-logic we follow before exec'ing a target binary: instead of explicitly closing the file descriptors, we mark them as CLOEXEC. That simplifies the logic: it gets rid of the awkward tiptoeing around the fact that we need to keep alive a few file descriptors: the fail pipe fd needs to be kept open right up to the exec(), and we cause opening internal file descriptors during our iteration of open file handles from /proc.
> 
> This patch also makes future developments easier: I am working on improving logging during child process spawning (https://bugs.openjdk.org/browse/JDK-8357100), and there we have a similar problem where we need to keep a logfile fd open right up to the point exec() happens).
> 
> Note: Using fcntl() with FD_CLOEXEC should work on all our POSIX platforms, since we rely on it already, see unconditional use of that flag here: https://github.com/openjdk/jdk/blob/3acfa9e4e7be2f37ac55f97348aad4f74ba802a0/src/java.base/unix/native/libjava/childproc.c#L408-L409
> 
> This patch also fixes two subtle bugs:
> - we didn't check the return value of the close() inside closeAllFileDescriptors
> - the final fcntl for the fail pipe was subtly wrong (should have or'd the FD_CLOEXEC flag with the existing state before setting it)
> 
> ----
> 
> Testing:
> 
> We already have the PipelineLeak test, but I also added a new test that checks that we don't accidentally leak file descriptors even if those had been opened outside the JVM and without FD_CLOEXEC.
> 
> - in the parent JVM, the test opens a file in native code without FD_CLOEXEC
> - test then spawns a child program that checks that no file descriptors beyond the expected stdin/out/err are open
> 
> I verified that the test correctly detects a broken implementation that leaks file descriptors. 
> 
> I verified that with this patch, we close all file descriptors. I also verified the fallback path (where we brute-force-iterate all descriptors up to _SC_OPEN_MAX).
> 
> I ran manually all tests from test/jdk/java/base/Process*, and verified that these tests run as part of the GHAs, which are green.

Leaving it to 26 is good.  One review is ok.  
If we were pushing for JDK 25, a second review would be warranted.

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PR Comment: https://git.openjdk.org/jdk/pull/25301#issuecomment-2901193939