ntpd: improve postponed hostname resolution

Run the namelookup from the main loop so a misspelled first ntp server
name does not block everything forever.

This fixes the following situation which would block forever:
  $ sudo ./busybox ntpd -dn -p foobar  -p pool.ntp.org
  ntpd: bad address 'foobar'
  ntpd: bad address 'foobar'
  ntpd: bad address 'foobar'
  ...

New behavior:
  ntpd: bad address 'foobar'
  ntpd: sending query to 137.190.2.4
  ntpd: reply from 137.190.2.4: offset:-1.009775 delay:0.175550 status:0x24 strat:1 refid:0x00535047 rootdelay:0.000000 reach:0x01
  ntpd: sending query to 137.190.2.4
  ntpd: reply from 137.190.2.4: offset:-1.009605 delay:0.175461 status:0x24 strat:1 refid:0x00535047 rootdelay:0.000000 reach:0x03
  ntpd: sending query to 137.190.2.4
  ntpd: reply from 137.190.2.4: offset:-1.005327 delay:0.167027 status:0x24 strat:1 refid:0x00535047 rootdelay:0.000000 reach:0x07
  ntpd: sending query to 137.190.2.4
  ntpd: bad address 'foobar'
  ntpd: reply from 137.190.2.4: offset:-1.046349 delay:0.248705 status:0x24 strat:1 refid:0x00535047 rootdelay:0.000000 reach:0x0f

This patch is based on Kaarle Ritvanens work.
http://lists.busybox.net/pipermail/busybox/2016-May/084197.html

function                                             old     new   delta
ntpd_main                                           1061    1079     +18
ntp_init                                             556     560      +4
resolve_peer_hostname                                 81      75      -6
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/1 up/down: 22/-6)              Total: 16 bytes

Signed-off-by: Kaarle Ritvanen <kaarle.ritvanen@datakunkku.fi>
Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 34 insertions, 39 deletions

diff --git a/networking/ntpd.c b/networking/ntpd.c
index b7fa5dce9..bfd5705fc 100644
--- a/networking/ntpd.c
+++ b/networking/ntpd.c
@@ -155,6 +155,7 @@
 #define RETRY_INTERVAL    32    /* on send/recv error, retry in N secs (need to be power of 2) */
 #define NOREPLY_INTERVAL 512    /* sent, but got no reply: cap next query by this many seconds */
 #define RESPONSE_INTERVAL 16    /* wait for reply up to N secs */
+#define HOSTNAME_INTERVAL  5    /* hostname lookup failed. Wait N secs for next try */
 
 /* Step threshold (sec). std ntpd uses 0.128.
  */
@@ -790,28 +791,20 @@ reset_peer_stats(peer_t *p, double offset)
         VERB6 bb_error_msg("%s->lastpkt_recv_time=%f", p->p_dotted, p->lastpkt_recv_time);
 }
 
-static void
-resolve_peer_hostname(peer_t *p, int loop_on_fail)
+static len_and_sockaddr*
+resolve_peer_hostname(peer_t *p)
 {
-        len_and_sockaddr *lsa;
-
- again:
-        lsa = host2sockaddr(p->p_hostname, 123);
-        if (!lsa) {
-                /* error message already emitted by host2sockaddr() */
-                if (!loop_on_fail)
-                        return;
-//FIXME: do this to avoid infinite looping on typo in a hostname?
-//well... in which case, what is a good value for loop_on_fail?
-                //if (--loop_on_fail == 0)
-                //      xfunc_die();
-                sleep(5);
-                goto again;
+        len_and_sockaddr *lsa = host2sockaddr(p->p_hostname, 123);
+        if (lsa) {
+                free(p->p_lsa);
+                free(p->p_dotted);
+                p->p_lsa = lsa;
+                p->p_dotted = xmalloc_sockaddr2dotted_noport(&lsa->u.sa);
+        } else {
+                /* error message is emitted by host2sockaddr() */
+                set_next(p, HOSTNAME_INTERVAL);
         }
-        free(p->p_lsa);
-        free(p->p_dotted);
-        p->p_lsa = lsa;
-        p->p_dotted = xmalloc_sockaddr2dotted_noport(&lsa->u.sa);
+        return lsa;
 }
 
 static void
@@ -822,28 +815,28 @@ add_peers(const char *s)
 
         p = xzalloc(sizeof(*p) + strlen(s));
         strcpy(p->p_hostname, s);
-        resolve_peer_hostname(p, /*loop_on_fail=*/ 1);
+        p->p_fd = -1;
+        p->p_xmt_msg.m_status = MODE_CLIENT | (NTP_VERSION << 3);
+        p->next_action_time = G.cur_time; /* = set_next(p, 0); */
+        reset_peer_stats(p, STEP_THRESHOLD);
 
         /* Names like N.<country2chars>.pool.ntp.org are randomly resolved
          * to a pool of machines. Sometimes different N's resolve to the same IP.
          * It is not useful to have two peers with same IP. We skip duplicates.
          */
-        for (item = G.ntp_peers; item != NULL; item = item->link) {
-                peer_t *pp = (peer_t *) item->data;
-                if (strcmp(p->p_dotted, pp->p_dotted) == 0) {
-                        bb_error_msg("duplicate peer %s (%s)", s, p->p_dotted);
-                        free(p->p_lsa);
-                        free(p->p_dotted);
-                        free(p);
-                        return;
+        if (resolve_peer_hostname(p)) {
+                for (item = G.ntp_peers; item != NULL; item = item->link) {
+                        peer_t *pp = (peer_t *) item->data;
+                        if (pp->p_dotted && strcmp(p->p_dotted, pp->p_dotted) == 0) {
+                                bb_error_msg("duplicate peer %s (%s)", s, p->p_dotted);
+                                free(p->p_lsa);
+                                free(p->p_dotted);
+                                free(p);
+                                return;
+                        }
                 }
         }
 
-        p->p_fd = -1;
-        p->p_xmt_msg.m_status = MODE_CLIENT | (NTP_VERSION << 3);
-        p->next_action_time = G.cur_time; /* = set_next(p, 0); */
-        reset_peer_stats(p, STEP_THRESHOLD);
-
         llist_add_to(&G.ntp_peers, p);
         G.peer_cnt++;
 }
@@ -871,6 +864,11 @@ do_sendto(int fd,
 static void
 send_query_to_peer(peer_t *p)
 {
+        if (!p->p_lsa) {
+                if (!resolve_peer_hostname(p))
+                        return;
+        }
+
         /* Why do we need to bind()?
          * See what happens when we don't bind:
          *
@@ -2238,7 +2236,7 @@ static NOINLINE void ntp_init(char **argv)
                         IF_FEATURE_NTPD_SERVER("I:") /* compat */
                         "d" /* compat */
                         "46aAbgL", /* compat, ignored */
-                        &peers,&G.script_name,
+                        &peers, &G.script_name,
 #if ENABLE_FEATURE_NTPD_SERVER
                         &G.if_name,
 #endif
@@ -2263,9 +2261,6 @@ static NOINLINE void ntp_init(char **argv)
         if (opts & OPT_N)
                 setpriority(PRIO_PROCESS, 0, -15);
 
-        /* add_peers() calls can retry DNS resolution (possibly forever).
-         * Daemonize before them, or else boot can stall forever.
-         */
         if (!(opts & OPT_n)) {
                 bb_daemonize_or_rexec(DAEMON_DEVNULL_STDIO, argv);
                 logmode = LOGMODE_NONE;
@@ -2400,7 +2395,7 @@ int ntpd_main(int argc UNUSED_PARAM, char **argv)
 
                                         /* What if don't see it because it changed its IP? */
                                         if (p->reachable_bits == 0)
-                                                resolve_peer_hostname(p, /*loop_on_fail=*/ 0);
+                                                resolve_peer_hostname(p);
 
                                         set_next(p, timeout);
                                 }