diff --git a/src/corelib/io/qprocess.cpp b/src/corelib/io/qprocess.cpp
index 49ba04068a7..16447033ca1 100644
--- a/src/corelib/io/qprocess.cpp
+++ b/src/corelib/io/qprocess.cpp
@@ -1574,7 +1574,7 @@ std::function<void(void)> QProcess::childProcessModifier() const
     Sets the \a modifier function for the child process, for Unix systems
     (including \macos; for Windows, see setCreateProcessArgumentsModifier()).
     The function contained by the \a modifier argument will be invoked in the
-    child process after \c{fork()} is completed and QProcess has set up the
+    child process after \c{fork()} or \c{vfork()} is completed and QProcess has set up the
     standard file descriptors for the child process, but before \c{execve()},
     inside start(). The modifier is useful to change certain properties of the
     child process, such as setting up additional file descriptors or closing
@@ -1595,6 +1595,15 @@ std::function<void(void)> QProcess::childProcessModifier() const
     "async-signal-safe" is advised). Most of the Qt API is unsafe inside this
     callback, including qDebug(), and may lead to deadlocks.
 
+    \note On some systems (notably, Linux), QProcess will use \c{vfork()}
+    semantics to start the child process, so this function must obey even
+    stricter constraints. First, because it is still sharing memory with the
+    parent process, it must not write to any non-local variable and must obey
+    proper ordering semantics when reading from them, to avoid data races.
+    Second, even more library functions may misbehave; therefore, this function
+    should only make use of low-level system calls, such as \c{read()},
+    \c{write()}, \c{setsid()}, \c{nice()}, and similar.
+
     \sa childProcessModifier()
 */
 void QProcess::setChildProcessModifier(const std::function<void(void)> &modifier)
diff --git a/src/corelib/io/qprocess_p.h b/src/corelib/io/qprocess_p.h
index 1331aef1693..929c06480b5 100644
--- a/src/corelib/io/qprocess_p.h
+++ b/src/corelib/io/qprocess_p.h
@@ -279,9 +279,6 @@ public:
     bool openChannels();
     bool openChannelsForDetached();
     bool openChannel(Channel &channel);
-#if defined(Q_OS_UNIX)
-    void commitChannels();
-#endif
     void closeChannel(Channel *channel);
     void closeWriteChannel();
     void closeChannels();
@@ -308,7 +305,8 @@ public:
     void start(QIODevice::OpenMode mode);
     void startProcess();
 #if defined(Q_OS_UNIX)
-    void execChild(const char *workingDirectory, char **argv, char **envp);
+    void commitChannels() const;
+    void execChild(const char *workingDirectory, char **argv, char **envp) const;
 #endif
     bool processStarted(QString *errorMessage = nullptr);
     void processFinished();
diff --git a/src/corelib/io/qprocess_unix.cpp b/src/corelib/io/qprocess_unix.cpp
index 449fd14c5c2..7607901021a 100644
--- a/src/corelib/io/qprocess_unix.cpp
+++ b/src/corelib/io/qprocess_unix.cpp
@@ -358,7 +358,7 @@ bool QProcessPrivate::openChannel(Channel &channel)
     }
 }
 
-void QProcessPrivate::commitChannels()
+void QProcessPrivate::commitChannels() const
 {
     // copy the stdin socket if asked to (without closing on exec)
     if (stdinChannel.pipe[0] != INVALID_Q_PIPE)
@@ -516,7 +516,12 @@ void QProcessPrivate::startProcess()
         ::fcntl(stderrChannel.pipe[0], F_SETFL, ::fcntl(stderrChannel.pipe[0], F_GETFL) | O_NONBLOCK);
 }
 
-void QProcessPrivate::execChild(const char *workingDir, char **argv, char **envp)
+// IMPORTANT:
+//
+// This function is called in a vfork() context on some OSes (notably, Linux
+// with forkfd), so it MUST NOT modify any non-local variable because it's
+// still sharing memory with the parent process.
+void QProcessPrivate::execChild(const char *workingDir, char **argv, char **envp) const
 {
     ::signal(SIGPIPE, SIG_DFL);         // reset the signal that we ignored
 
@@ -556,7 +561,6 @@ void QProcessPrivate::execChild(const char *workingDir, char **argv, char **envp
 report_errno:
     error.code = errno;
     qt_safe_write(childStartedPipe[1], &error, sizeof(error));
-    childStartedPipe[1] = -1;
 }
 
 bool QProcessPrivate::processStarted(QString *errorMessage)