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>

3 files changed, 25 insertions, 2 deletions

diff --git a/libbb/loop.c b/libbb/loop.c
index abf05dc4f..204fcc982 100644
--- a/libbb/loop.c
+++ b/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) {
diff --git a/scripts/defconfig b/scripts/defconfig
index d13f5b1b4..0a748febc 100644
--- a/scripts/defconfig
+++ b/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
 
 #
diff --git a/util-linux/Config.in b/util-linux/Config.in
index acd74a2c6..a59cc1ddf 100644
--- a/util-linux/Config.in
+++ b/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