fuzz: fix broken qtest check at rcu_disable_atfork

The qtest_enabled check introduced in d6919e4 always returns false, as
it is called prior to configure_accelerators(). Instead of trying to
skip rcu_disable_atfork in qemu_main, simply call rcu_enable_atfork in
the fuzzer, after qemu_main returns.

Reported-by: Thomas Huth <[email protected]>
Signed-off-by: Alexander Bulekov <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>

Author: a1xndr

softmmu/vl.c

@@ -3832,17 +3832,7 @@ void qemu_init(int argc, char **argv, char **envp)
                                               machine_class);
 
     os_daemonize();
-
-    /*
-     * If QTest is enabled, keep the rcu_atfork enabled, since system processes
-     * may be forked testing purposes (e.g. fork-server based fuzzing) The fork
-     * should happen before a signle cpu instruction is executed, to prevent
-     * deadlocks. See commit 73c6e40, rcu: "completely disable pthread_atfork
-     * callbacks as soon as possible"
-     */
-    if (!qtest_enabled()) {
-        rcu_disable_atfork();
-    }
+    rcu_disable_atfork();
 
     if (pid_file && !qemu_write_pidfile(pid_file, &err)) {
         error_reportf_err(err, "cannot create PID file: ");

tests/qtest/fuzz/fuzz.c

@@ -211,5 +211,8 @@ int LLVMFuzzerInitialize(int *argc, char ***argv, char ***envp)
 
     qemu_init(result.we_wordc, result.we_wordv, NULL);
 
+    /* re-enable the rcu atfork, which was previously disabled in qemu_init */
+    rcu_enable_atfork();
+
     return 0;
 }