mount: add an optional capability to create new /dev/loopN as needed

Signed-off-by: Lauri Kasanen <curaga@operamail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/loop.c

@@ -104,14 +104,24 @@ int FAST_FUNC set_loop(char **device, const char *file, unsigned long long offse
 
 	/* Find a loop device.  */
 	try = *device ? *device : dev;
-	for (i = 0; rc; i++) {
+	for (i = 0; rc && i < 256; i++) {
 		sprintf(dev, LOOP_FORMAT, i);
 
-		/* Ran out of block devices, return failure.  */
+		IF_FEATURE_MOUNT_LOOP_CREATE(errno = 0;)
 		if (stat(try, &statbuf) != 0 || !S_ISBLK(statbuf.st_mode)) {
+			if (ENABLE_FEATURE_MOUNT_LOOP_CREATE
+			 && errno == ENOENT
+			 && try == dev
+			) {
+				/* Node doesn't exist, try to create it.  */
+				if (mknod(dev, S_IFBLK|0644, makedev(7, i)) == 0)
+					goto try_to_open;
+			}
+			/* Ran out of block devices, return failure.  */
 			rc = -ENOENT;
 			break;
 		}
+ try_to_open:
 		/* Open the sucker and check its loopiness.  */
 		dfd = open(try, mode);
 		if (dfd < 0 && errno == EROFS) {

scripts/defconfig

@@ -574,6 +574,7 @@ CONFIG_FEATURE_UMOUNT_ALL=y
 # Common options for mount/umount
 #
 CONFIG_FEATURE_MOUNT_LOOP=y
+# CONFIG_FEATURE_MOUNT_LOOP_CREATE is not set
 # CONFIG_FEATURE_MTAB_SUPPORT is not set
 
 #

util-linux/Config.in

@@ -901,6 +901,18 @@ config FEATURE_MOUNT_LOOP
 	  specify an offset or cryptographic options to the loopback device.
 	  (If you don't want umount to free the loop device, use "umount -D".)
 
+config FEATURE_MOUNT_LOOP_CREATE
+	bool "Create new loopback devices if needed"
+	default n
+	depends on FEATURE_MOUNT_LOOP
+	help
+	  Linux kernels >= 2.6.24 support unlimited loopback devices. They are
+	  allocated for use when trying to use a loop device. The loop device
+	  must however exist.
+
+	  This feature lets mount to try to create next /dev/loopN device
+	  if it does not find a free one.
+
 config FEATURE_MTAB_SUPPORT
 	bool "Support for the old /etc/mtab file"
 	default n