block: null_blk: Improve device creation with configfs

Currently, the directory name used to create a nullb device through
sysfs is not used as the device name, potentially causing headaches for
users if devices are already created through the modprobe operation
withe the nr_device module parameter not set to 0. E.g. a user can do
"mkdir /sys/kernel/config/nullb/nullb0" to create a nullb device even
though /dev/nullb0 was already created by modprobe. In this case, the
configfs nullb device will be named nullb1, causing confusion for the
user.

Simplify this by using the configfs directory name as the nullb device
name, always, unless another nullb device is already using the same
name. E.g. if modprobe created nullb0, then:

$ mkdir /sys/kernel/config/nullb/nullb0
mkdir: cannot create directory '/sys/kernel/config/nullb/nullb0': File
exists

will be reported to the user.

To implement this, the function null_find_dev_by_name() is added to
check for the existence of a nullb device with the name used for a new
configfs device directory. nullb_group_make_item() uses this new
function to check if the directory name can be used as the disk name.
Finally, null_add_dev() is modified to use the device config item name
as the disk name for a new nullb device created using configfs.
The naming of devices created though modprobe remains unchanged.

Of note is that it is possible for a user to create through configfs a
nullb device with the same name as an existing device. E.g.

$ mkdir /sys/kernel/config/nullb/null

will successfully create the nullb device named "null" but this block
device will however not appear under /dev/ since /dev/null already
exists.
Suggested-by: Joseph Bacik <josef@toxicpanda.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20220420005718.3780004-5-damien.lemoal@opensource.wdc.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk/main.c

@@ -235,6 +235,7 @@ static struct nullb_device *null_alloc_dev(void);
 static void null_free_dev(struct nullb_device *dev);
 static void null_del_dev(struct nullb *nullb);
 static int null_add_dev(struct nullb_device *dev);
+static struct nullb *null_find_dev_by_name(const char *name);
 static void null_free_device_storage(struct nullb_device *dev, bool is_cache);
 
 static inline struct nullb_device *to_nullb_device(struct config_item *item)
@@ -563,6 +564,9 @@ config_item *nullb_group_make_item(struct config_group *group, const char *name)
 {
 	struct nullb_device *dev;
 
+	if (null_find_dev_by_name(name))
+		return ERR_PTR(-EEXIST);
+
 	dev = null_alloc_dev();
 	if (!dev)
 		return ERR_PTR(-ENOMEM);
@@ -2062,7 +2066,13 @@ static int null_add_dev(struct nullb_device *dev)
 
 	null_config_discard(nullb);
 
-	sprintf(nullb->disk_name, "nullb%d", nullb->index);
+	if (config_item_name(&dev->item)) {
+		/* Use configfs dir name as the device name */
+		snprintf(nullb->disk_name, sizeof(nullb->disk_name),
+			 "%s", config_item_name(&dev->item));
+	} else {
+		sprintf(nullb->disk_name, "nullb%d", nullb->index);
+	}
 
 	rv = null_gendisk_register(nullb);
 	if (rv)
@@ -2091,6 +2101,22 @@ static int null_add_dev(struct nullb_device *dev)
 	return rv;
 }
 
+static struct nullb *null_find_dev_by_name(const char *name)
+{
+	struct nullb *nullb = NULL, *nb;
+
+	mutex_lock(&lock);
+	list_for_each_entry(nb, &nullb_list, list) {
+		if (strcmp(nb->disk_name, name) == 0) {
+			nullb = nb;
+			break;
+		}
+	}
+	mutex_unlock(&lock);
+
+	return nullb;
+}
+
 static int null_create_dev(void)
 {
 	struct nullb_device *dev;