After fork(), a signalfd-created file descriptor still returns signal info

for the parent.  Close and recreate a new signalfd after forking into the
background.

ndhc/arp.c

@@ -136,7 +136,7 @@ void arp_success(struct client_state_t *cs)
     if (client_config.quit_after_lease)
         exit(EXIT_SUCCESS);
     if (!client_config.foreground)
-        background();
+        background(cs);
 }
 
 typedef uint32_t aliased_uint32_t __attribute__((__may_alias__));

ndhc/ndhc.c

@@ -181,20 +181,6 @@ static void perform_release(void)
     cs.timeout = -1;
 }
 
-static void setup_signals()
-{
-    sigset_t mask;
-    sigemptyset(&mask);
-    sigaddset(&mask, SIGUSR1);
-    sigaddset(&mask, SIGUSR2);
-    sigaddset(&mask, SIGTERM);
-    if (sigprocmask(SIG_BLOCK, &mask, NULL) < 0)
-        suicide("sigprocmask failed");
-    cs.signalFd = signalfd(-1, &mask, SFD_NONBLOCK);
-    if (cs.signalFd < 0)
-        suicide("signalfd failed");
-}
-
 static void signal_dispatch()
 {
     int t, off = 0;
@@ -234,7 +220,7 @@ static void do_work(void)
     cs.epollFd = epoll_create1(0);
     if (cs.epollFd == -1)
         suicide("epoll_create1 failed");
-    epoll_add(&cs, cs.signalFd);
+    setup_signals(&cs);
     change_listen_mode(&cs, LM_RAW);
     handle_timeout(&cs);
 
@@ -387,8 +373,6 @@ int main(int argc, char **argv)
         memcpy(client_config.clientid + 3, client_config.arp, 6);
     }
 
-    setup_signals();
-
     if (chdir(chroot_dir)) {
         printf("Failed to chdir(%s)!\n", chroot_dir);
         exit(EXIT_FAILURE);

ndhc/sys.c

@@ -3,7 +3,9 @@
 #include <stdio.h>
 #include <string.h>
 #include <errno.h>
+#include <signal.h>
 #include <sys/epoll.h>
+#include <sys/signalfd.h>
 #include "config.h"
 #include "log.h"
 #include "pidfile.h"
@@ -11,14 +13,38 @@
 
 char pidfile[MAX_PATH_LENGTH] = PID_FILE_DEFAULT;
 
-void background(void)
+void setup_signals(struct client_state_t *cs)
+{
+    sigset_t mask;
+    sigemptyset(&mask);
+    sigaddset(&mask, SIGUSR1);
+    sigaddset(&mask, SIGUSR2);
+    sigaddset(&mask, SIGTERM);
+    if (sigprocmask(SIG_BLOCK, &mask, NULL) < 0)
+        suicide("sigprocmask failed");
+    if (cs->signalFd >= 0) {
+        epoll_del(cs, cs->signalFd);
+        close(cs->signalFd);
+    }
+    cs->signalFd = signalfd(-1, &mask, SFD_NONBLOCK);
+    if (cs->signalFd < 0)
+        suicide("signalfd failed");
+    epoll_add(cs, cs->signalFd);
+}
+
+// @cs can be NULL
+void background(struct client_state_t *cs)
 {
     static char called;
-    if (!called && daemon(0, 0) == -1) {
-        perror("fork");
-        exit(EXIT_SUCCESS);
+    if (!called) {
+        called = 1;  /* Do not fork again. */
+        if (daemon(0, 0) == -1) {
+            perror("fork");
+            exit(EXIT_SUCCESS);
+        }
+        if (cs)
+            setup_signals(cs);
     }
-    called = 1;  /* Do not fork again. */
     if (file_exists(pidfile, "w") == -1) {
         log_line("cannot open pidfile for write!");
     } else
@@ -46,3 +72,4 @@ void epoll_del(struct client_state_t *cs, int fd)
     if (r == -1)
         suicide("epoll_del failed %s", strerror(errno));
 }
+

ndhc/sys.h

@@ -3,6 +3,7 @@
 
 #include <sys/time.h>
 #include "ndhc-defines.h"
+#include "config.h"
 
 static inline unsigned long long curms()
 {
@@ -13,7 +14,8 @@ static inline unsigned long long curms()
 
 extern char pidfile[MAX_PATH_LENGTH];
 
-void background(void);
+void setup_signals(struct client_state_t *cs);
+void background(struct client_state_t *cs);
 void epoll_add(struct client_state_t *cs, int fd);
 void epoll_del(struct client_state_t *cs, int fd);
 

ndhc/timeout.c

@@ -22,7 +22,7 @@ static void init_selecting_timeout(struct client_state_t *cs)
     } else {
         if (client_config.background_if_no_lease) {
             log_line("No lease, going to background.");
-            background();
+            background(cs);
         } else if (client_config.abort_if_no_lease) {
             log_line("No lease, failing.");
             exit(EXIT_FAILURE);