bb_daemonize_or_rexec(): add flag to double-fork; use it in start-stop-daemon

Add a DAEMON_DOUBLE_FORK flag to make bb_daemonize double-fork so it isn't a
session leader, and hence doesn't get a controlling tty on Linux if a tty is
ever opened, similar to how libdaemon's daemon_fork or the big
start-stop-daemon does it - And use it in start-stop-daemon.

For details, see http://www.win.tue.nl/~aeb/linux/lk/lk-10.html#ss10.3

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

3 files changed, 11 insertions, 2 deletions

diff --git a/debianutils/start_stop_daemon.c b/debianutils/start_stop_daemon.c
index 02609c04f..495ed0a09 100644
--- a/debianutils/start_stop_daemon.c
+++ b/debianutils/start_stop_daemon.c
@@ -474,7 +474,7 @@ int start_stop_daemon_main(int argc UNUSED_PARAM, char **argv)
         *--argv = startas;
         if (opt & OPT_BACKGROUND) {
 #if BB_MMU
-                bb_daemonize(DAEMON_DEVNULL_STDIO + DAEMON_CLOSE_EXTRA_FDS);
+                bb_daemonize(DAEMON_DEVNULL_STDIO + DAEMON_CLOSE_EXTRA_FDS + DAEMON_DOUBLE_FORK);
                 /* DAEMON_DEVNULL_STDIO is superfluous -
                  * it's always done by bb_daemonize() */
 #else
diff --git a/include/libbb.h b/include/libbb.h
index 53224fa35..09e8d28e7 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -966,6 +966,7 @@ enum {
         DAEMON_DEVNULL_STDIO = 2,
         DAEMON_CLOSE_EXTRA_FDS = 4,
         DAEMON_ONLY_SANITIZE = 8, /* internal use */
+        DAEMON_DOUBLE_FORK = 16, /* double fork to avoid controlling tty */
 };
 #if BB_MMU
   enum { re_execed = 0 };
diff --git a/libbb/vfork_daemon_rexec.c b/libbb/vfork_daemon_rexec.c
index a75eafbd3..ed1f86f0c 100644
--- a/libbb/vfork_daemon_rexec.c
+++ b/libbb/vfork_daemon_rexec.c
@@ -253,11 +253,19 @@ void FAST_FUNC bb_daemonize_or_rexec(int flags, char **argv)
         if (!(flags & DAEMON_ONLY_SANITIZE)) {
                 if (fork_or_rexec(argv))
                         exit(EXIT_SUCCESS); /* parent */
-                /* if daemonizing, make sure we detach from stdio & ctty */
+                /* if daemonizing, detach from stdio & ctty */
                 setsid();
                 dup2(fd, 0);
                 dup2(fd, 1);
                 dup2(fd, 2);
+                if (flags & DAEMON_DOUBLE_FORK) {
+                        /* On Linux, session leader can acquire ctty
+                         * unknowingly, by opening a tty.
+                         * Prevent this: stop being a session leader.
+                         */
+                        if (fork_or_rexec(argv))
+                                exit(EXIT_SUCCESS); /* parent */
+                }
         }
         while (fd > 2) {
                 close(fd--);