mdmon: avoid writes in the startup path for mdmon on root arrays

When killing a previous monitor be careful not to cause writes to the
filesystem until the reads necessary to get the monitor operational have
completed.

The code is already prepared for errors creating the pid and socket
files, so simply defer creation of these files until after the first
call to manage().

Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

managemon.c

@@ -680,6 +680,12 @@ void do_manager(struct supertype *container)
 			read_sock(container);
 
 			if (container->sock < 0 || socket_hup_requested) {
+				/* If this fails, we hope it already exists
+				 * pid file lives in /var/run/mdadm/mdXX.pid
+				 */
+				mkdir("/var", 0600);
+				mkdir("/var/run", 0600);
+				mkdir("/var/run/mdadm", 0600);
 				close(container->sock);
 				container->sock = make_control_sock(container->devname);
 				make_pidfile(container->devname, 0);

mdmon.c

@@ -113,6 +113,14 @@ static struct superswitch *find_metadata_methods(char *vers)
 	return NULL;
 }
 
+static int test_pidfile(char *devname)
+{
+	char path[100];
+	struct stat st;
+
+	sprintf(path, "/var/run/mdadm/%s.pid", devname);
+	return stat(path, &st);
+}
 
 int make_pidfile(char *devname, int o_excl)
 {
@@ -149,27 +157,30 @@ int is_container_member(struct mdstat_ent *mdstat, char *container)
 	return 1;
 }
 
-void remove_pidfile(char *devname);
+pid_t devname2mdmon(char *devname)
-static void try_kill_monitor(char *devname)
+{
+	char buf[100];
+	pid_t pid = -1;
+	int fd;
+
+	sprintf(buf, "/var/run/mdadm/%s.pid", devname);
+	fd = open(buf, O_RDONLY|O_NOATIME);
+	if (fd < 0)
+		return -1;
+
+	if (read(fd, buf, sizeof(buf)) > 0)
+		sscanf(buf, "%d\n", &pid);
+	close(fd);
+
+	return pid;
+}
+
+static void try_kill_monitor(pid_t pid, char *devname)
 {
 	char buf[100];
 	int fd;
-	pid_t pid;
 	struct mdstat_ent *mdstat;
 
-	sprintf(buf, "/var/run/mdadm/%s.pid", devname);
-	fd = open(buf, O_RDONLY);
-	if (fd < 0)
-		return;
-
-	if (read(fd, buf, sizeof(buf)) < 0) {
-		close(fd);
-		return;
-	}
-
-	close(fd);
-	pid = strtoul(buf, NULL, 10);
-
 	/* first rule of survival... don't off yourself */
 	if (pid == getpid())
 		return;
@@ -197,7 +208,6 @@ static void try_kill_monitor(char *devname)
 			WaitClean(buf, 0);
 		}
 	free_mdstat(mdstat);
-	remove_pidfile(devname);
 }
 
 void remove_pidfile(char *devname)
@@ -355,6 +365,7 @@ int mdmon(char *devname, int devnum, int scan, char *switchroot)
 	int pfd[2];
 	int status;
 	int ignore;
+	pid_t victim = -1;
 
 	dprintf("starting mdmon for %s in %s\n",
 		devname, switchroot ? : "/");
@@ -400,6 +411,7 @@ int mdmon(char *devname, int devnum, int scan, char *switchroot)
 	container->devname = devname;
 	container->arrays = NULL;
 	container->subarray[0] = 0;
+	container->sock = -1;
 
 	if (!container->devname) {
 		fprintf(stderr, "mdmon: failed to allocate container name string\n");
@@ -464,12 +476,9 @@ int mdmon(char *devname, int devnum, int scan, char *switchroot)
 
 	if (switchroot) {
 		/* we assume we assume that /sys /proc /dev are available in
-		 * the new root (see nash:setuproot)
+		 * the new root
-		 *
-		 * kill any monitors in the current namespace and change
-		 * to the new one
 		 */
-		try_kill_monitor(container->devname);
+		victim = devname2mdmon(container->devname);
 		if (chroot(switchroot) != 0) {
 			fprintf(stderr, "mdmon: failed to chroot to '%s': %s\n",
 				switchroot, strerror(errno));
@@ -477,40 +486,15 @@ int mdmon(char *devname, int devnum, int scan, char *switchroot)
 		}
 	}
 
-	/* If this fails, we hope it already exists 
-	 * pid file lives in /var/run/mdadm/mdXX.pid
-	 */
-	mkdir("/var", 0600);
-	mkdir("/var/run", 0600);
-	mkdir("/var/run/mdadm", 0600);
 	ignore = chdir("/");
-	if (make_pidfile(container->devname, O_EXCL) < 0) {
+	if (victim < 0 && test_pidfile(container->devname) == 0) {
 		if (ping_monitor(container->devname) == 0) {
 			fprintf(stderr, "mdmon: %s already managed\n",
 				container->devname);
 			exit(3);
-		} else {
+		} else if (victim < 0)
-			int err;
+			victim = devname2mdmon(container->devname);
-
-			/* cleanup the old monitor, this one is taking over */
-			try_kill_monitor(container->devname);
-			err = make_pidfile(container->devname, 0);
-			if (err < 0) {
-				fprintf(stderr, "mdmon: %s Cannot create pidfile\n",
-					container->devname);
-				if (err == -EROFS) {
-					/* FIXME implement a mechanism to
-					 * prevent duplicate monitor instances
-					 */
-					fprintf(stderr,
-						"mdmon: continuing on read-only file system\n");
-				} else
-					exit(3);
-			}
-		}
 	}
-	container->sock = make_control_sock(container->devname);
-
 	if (container->ss->load_super(container, mdfd, devname)) {
 		fprintf(stderr, "mdmon: Cannot load metadata for %s\n",
 			devname);
@@ -544,6 +528,8 @@ int mdmon(char *devname, int devnum, int scan, char *switchroot)
 		exit(2);
 	}
 
+	if (victim > -1)
+		try_kill_monitor(victim, container->devname);
 	do_manager(container);
 
 	exit(0);