tools: Fix use of fcntl(F_SETFD) during socket activation

Blindly setting FD_CLOEXEC without a read-modify-write will
inadvertently clear any other intentionally-set bits, such as a
proposed new bit for designating a fd that must behave in 32-bit mode.
However, we cannot use our wrapper qemu_set_cloexec(), because that
wrapper intentionally abort()s on failure, whereas the probe here
intentionally tolerates failure to deal with incorrect socket
activation gracefully.  Instead, fix the code to do the proper
read-modify-write.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200420175309.75894-3-eblake@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

util/systemd.c

@@ -23,6 +23,7 @@ unsigned int check_socket_activation(void)
     unsigned long nr_fds;
     unsigned int i;
     int fd;
+    int f;
     int err;
 
     s = getenv("LISTEN_PID");
@@ -54,7 +55,8 @@ unsigned int check_socket_activation(void)
     /* So the file descriptors don't leak into child processes. */
     for (i = 0; i < nr_fds; ++i) {
         fd = FIRST_SOCKET_ACTIVATION_FD + i;
-        if (fcntl(fd, F_SETFD, FD_CLOEXEC) == -1) {
+        f = fcntl(fd, F_GETFD);
+        if (f == -1 || fcntl(fd, F_SETFD, f | FD_CLOEXEC) == -1) {
             /* If we cannot set FD_CLOEXEC then it probably means the file
              * descriptor is invalid, so socket activation has gone wrong
              * and we should exit.