telnetd: at Alexander Kriegisch <Alexander@kriegisch.name> insistence

add an option to close sessions as soon as child exits.
Maybe it should be a CONFIG option. OTOH, maybe it should be always on,
as it mimics, say, getty's behaviour.

function                                             old     new   delta
handle_sigchld                                         -      49     +49
telnetd_main                                        1312    1355     +43
.rodata                                           123429  123466     +37
packed_usage                                       22770   22806     +36
make_new_session                                     525     532      +7
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 4/0 up/down: 172/0)             Total: 172 bytes
   text    data     bss     dec     hex filename
 676285    2538   12104  690927   a8aef busybox_old
 676421    2538   12104  691063   a8b77 busybox_unstripped

2 files changed, 66 insertions, 35 deletions

diff --git a/include/usage.h b/include/usage.h
index 210e1f246..1e05532d5 100644
--- a/include/usage.h
+++ b/include/usage.h
@@ -3517,6 +3517,8 @@ USE_FEATURE_RUN_PARTS_FANCY("\n	-l	Prints names of all matching files even when
        "\n\nOptions:" \
        "\n      -l LOGIN        Exec LOGIN on connect" \
        "\n      -f issue_file   Display issue_file instead of /etc/issue" \
+       "\n      -K              Close connection as soon as login exits" \
+       "\n                      (normally wait until all programs close slave pty)" \
         USE_FEATURE_TELNETD_STANDALONE( \
        "\n      -p PORT         Port to listen to" \
        "\n      -b ADDR         Address to bind to" \
diff --git a/networking/telnetd.c b/networking/telnetd.c
index 85c2ebc44..96620d949 100644
--- a/networking/telnetd.c
+++ b/networking/telnetd.c
@@ -37,7 +37,7 @@
 struct tsession {
         struct tsession *next;
         int sockfd_read, sockfd_write, ptyfd;
-        /*int shell_pid;*/
+        int shell_pid;
 
         /* two circular buffers */
         /*char *buf1, *buf2;*/
@@ -265,7 +265,7 @@ make_new_session(
         }
         if (pid > 0) {
                 /* Parent */
-                /*ts->shell_pid = pid;*/
+                ts->shell_pid = pid;
                 return ts;
         }
 
@@ -305,7 +305,8 @@ make_new_session(
         login_argv[0] = loginpath;
         login_argv[1] = NULL;
         execvp(loginpath, (char **)login_argv);
-        /* Safer with vfork, and we shouldn't send this to the client anyway */
+        /* Safer with vfork, and we shouldn't send message
+         * to remote clients anyway */
         _exit(1); /*bb_perror_msg_and_die("execv %s", loginpath);*/
 }
 
@@ -357,6 +358,24 @@ void free_session(struct tsession *ts);
 
 #endif
 
+static void handle_sigchld(int sig)
+{
+        pid_t pid;
+        struct tsession *ts;
+
+        pid = waitpid(-1, &sig, WNOHANG);
+        if (pid > 0) {
+                ts = sessions;
+                while (ts) {
+                        if (ts->shell_pid == pid) {
+                                ts->shell_pid = -1;
+                                return;
+                        }
+                        ts = ts->next;
+                }
+        }
+}
+
 
 int telnetd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int telnetd_main(int argc, char **argv)
@@ -379,29 +398,35 @@ int telnetd_main(int argc, char **argv)
         };
 #endif
         enum {
-                OPT_INETD      = (1 << 2) * ENABLE_FEATURE_TELNETD_STANDALONE, /* -i */
-                OPT_PORT       = (1 << 3) * ENABLE_FEATURE_TELNETD_STANDALONE, /* -p */
-                OPT_FOREGROUND = (1 << 5) * ENABLE_FEATURE_TELNETD_STANDALONE, /* -F */
+                OPT_WATCHCHILD = (1 << 2), /* -K */
+                OPT_INETD      = (1 << 3) * ENABLE_FEATURE_TELNETD_STANDALONE, /* -i */
+                OPT_PORT       = (1 << 4) * ENABLE_FEATURE_TELNETD_STANDALONE, /* -p */
+                OPT_FOREGROUND = (1 << 6) * ENABLE_FEATURE_TELNETD_STANDALONE, /* -F */
         };
 
-        /* If !STANDALONE, we accept (and ignore) -i, thus people
+        /* Even if !STANDALONE, we accept (and ignore) -i, thus people
          * don't need to guess whether it's ok to pass -i to us */
-        opt = getopt32(argv, "f:l:i" USE_FEATURE_TELNETD_STANDALONE("p:b:F"),
+        opt = getopt32(argv, "f:l:Ki" USE_FEATURE_TELNETD_STANDALONE("p:b:F"),
                         &issuefile, &loginpath
                         USE_FEATURE_TELNETD_STANDALONE(, &opt_portnbr, &opt_bindaddr));
+        if (!IS_INETD /*&& !re_execed*/) {
+                /* inform that we start in standalone mode?
+                 * May be useful when people forget to give -i */
+                /*bb_error_msg("listening for connections");*/
+                if (!(opt & OPT_FOREGROUND)) {
+                        /* DAEMON_CHDIR_ROOT was giving inconsistent
+                         * behavior with/wthout -F, -i */
+                        bb_daemonize_or_rexec(0 /*DAEMON_CHDIR_ROOT*/, argv);
+                }
+        }
         /* Redirect log to syslog early, if needed */
         if (IS_INETD || !(opt & OPT_FOREGROUND)) {
                 openlog(applet_name, 0, LOG_USER);
                 logmode = LOGMODE_SYSLOG;
         }
-        //if (opt & 1) // -f
-        //if (opt & 2) // -l
         USE_FEATURE_TELNETD_STANDALONE(
-                if (opt & OPT_PORT) // -p
+                if (opt & OPT_PORT)
                         portnbr = xatou16(opt_portnbr);
-                //if (opt & 8) // -b
-                //if (opt & 0x10) // -F
-                //if (opt & 0x20) // -i
         );
 
         /* Used to check access(loginpath, X_OK) here. Pointless.
@@ -413,12 +438,8 @@ int telnetd_main(int argc, char **argv)
                 if (!sessions) /* pty opening or vfork problem, exit */
                         return 1; /* make_new_session prints error message */
         } else {
-//vda: inform that we start in standalone mode?
                 master_fd = create_and_bind_stream_or_die(opt_bindaddr, portnbr);
                 xlisten(master_fd, 1);
-                if (!(opt & OPT_FOREGROUND))
-//vda: NOMMU?
-                        bb_daemonize(DAEMON_CHDIR_ROOT);
         }
 #else
         sessions = make_new_session();
@@ -429,6 +450,9 @@ int telnetd_main(int argc, char **argv)
         /* We don't want to die if just one session is broken */
         signal(SIGPIPE, SIG_IGN);
 
+        if (opt & OPT_WATCHCHILD)
+                signal(SIGCHLD, handle_sigchld);
+
 /*
    This is how the buffers are used. The arrows indicate the movement
    of data.
@@ -450,14 +474,6 @@ int telnetd_main(int argc, char **argv)
  again:
         FD_ZERO(&rdfdset);
         FD_ZERO(&wrfdset);
-        if (!IS_INETD) {
-                FD_SET(master_fd, &rdfdset);
-                /* This is needed because free_session() does not
-                 * take into account master_fd when it finds new
-                 * maxfd among remaining fd's: */
-                if (master_fd > maxfd)
-                        maxfd = master_fd;
-        }
 
         /* Select on the master socket, all telnet sockets and their
          * ptys if there is room in their session buffers.
@@ -465,15 +481,28 @@ int telnetd_main(int argc, char **argv)
          * before each select. Can be a problem with 500+ connections. */
         ts = sessions;
         while (ts) {
-                if (ts->size1 > 0)       /* can write to pty */
-                        FD_SET(ts->ptyfd, &wrfdset);
-                if (ts->size1 < BUFSIZE) /* can read from socket */
-                        FD_SET(ts->sockfd_read, &rdfdset);
-                if (ts->size2 > 0)       /* can write to socket */
-                        FD_SET(ts->sockfd_write, &wrfdset);
-                if (ts->size2 < BUFSIZE) /* can read from pty */
-                        FD_SET(ts->ptyfd, &rdfdset);
-                ts = ts->next;
+                struct tsession *next = ts->next; /* in case we free ts. */
+                if (ts->shell_pid == -1) {
+                        free_session(ts);
+                } else {
+                        if (ts->size1 > 0)       /* can write to pty */
+                                FD_SET(ts->ptyfd, &wrfdset);
+                        if (ts->size1 < BUFSIZE) /* can read from socket */
+                                FD_SET(ts->sockfd_read, &rdfdset);
+                        if (ts->size2 > 0)       /* can write to socket */
+                                FD_SET(ts->sockfd_write, &wrfdset);
+                        if (ts->size2 < BUFSIZE) /* can read from pty */
+                                FD_SET(ts->ptyfd, &rdfdset);
+                }
+                ts = next;
+        }
+        if (!IS_INETD) {
+                FD_SET(master_fd, &rdfdset);
+                /* This is needed because free_session() does not
+                 * take into account master_fd when it finds new
+                 * maxfd among remaining fd's */
+                if (master_fd > maxfd)
+                        maxfd = master_fd;
         }
 
         count = select(maxfd + 1, &rdfdset, &wrfdset, NULL, NULL);