QProcess/Linux: use the FFD_VFORK_SEMANTICS flag

... when we are not using the FFD_USE_FORK flag. We use the FFD_USE_FORK
flag when we have user code to run in setupChildProcess(). This code is
enabled for all Unix, but forkfd() honors this flag only on Linux >=
5.4.

See the commit adding the flag for more information on what the flag
does and see the comment in this commit on why it's safe to use it.

Fixes: QTBUG-17331
Change-Id: I1bee3bc466a04f19bd6efffd15f448cb23ce1e91
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>

1 files changed, 10 insertions, 3 deletions

diff --git a/src/corelib/io/qprocess_unix.cpp b/src/corelib/io/qprocess_unix.cpp
index 2186f23ab6..930007ff04 100644
--- a/src/corelib/io/qprocess_unix.cpp
+++ b/src/corelib/io/qprocess_unix.cpp
@@ -450,12 +450,19 @@ void QProcessPrivate::startProcess()
         workingDirPtr = encodedWorkingDirectory.constData();
     }
 
-    // Start the process manager, and fork off the child process.
-    // ### Qt6: revisit whether the change in behavior due to not using fork()
-    // is acceptable for derived classes.
+    // Select FFD_USE_FORK and FFD_VFORK_SEMANTICS based on whether there's
+    // user code running in the child process: if there is, we don't know what
+    // the user will want to do, so we err on the safe side and request an
+    // actual fork() (for example, the user could attempt to do some
+    // synchronization with the parent process). But if there isn't, then our
+    // code in execChild() is just a handful of dup2() and a chdir(), so it's
+    // safe with vfork semantics: suspend the parent execution until the child
+    // either execve()s or _exit()s.
     int ffdflags = FFD_CLOEXEC;
     if (typeid(*q) != typeid(QProcess))
         ffdflags |= FFD_USE_FORK;
+    else
+        ffdflags |= FFD_VFORK_SEMANTICS;
     pid_t childPid;
     forkfd = ::forkfd(ffdflags , &childPid);
     int lastForkErrno = errno;