block: check disk exist before trying to add partition

If disk have been deleted, we should return fail for ioctl
BLKPG_DEL_PARTITION. Otherwise, the directory /sys/class/block
may remain invalid symlinks file. The race as following:

blkdev_open
				del_gendisk
				    disk->flags &= ~GENHD_FL_UP;
				    blk_drop_partitions
blkpg_ioctl
    bdev_add_partition
    add_partition
        device_add
	    device_add_class_symlinks

ioctl may add_partition after del_gendisk() have tried to delete
partitions. Then, symlinks file will be created.
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Link: https://lore.kernel.org/r/20210610023241.3646241-1-yuyufen@huawei.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/partitions/core.c

@@ -453,17 +453,26 @@ int bdev_add_partition(struct block_device *bdev, int partno,
 		sector_t start, sector_t length)
 {
 	struct block_device *part;
+	struct gendisk *disk = bdev->bd_disk;
+	int ret;
 
-	mutex_lock(&bdev->bd_disk->open_mutex);
-	if (partition_overlaps(bdev->bd_disk, start, length, -1)) {
-		mutex_unlock(&bdev->bd_disk->open_mutex);
-		return -EBUSY;
+	mutex_lock(&disk->open_mutex);
+	if (!(disk->flags & GENHD_FL_UP)) {
+		ret = -ENXIO;
+		goto out;
 	}
 
-	part = add_partition(bdev->bd_disk, partno, start, length,
+	if (partition_overlaps(disk, start, length, -1)) {
+		ret = -EBUSY;
+		goto out;
+	}
+
+	part = add_partition(disk, partno, start, length,
 			ADDPART_FLAG_NONE, NULL);
-	mutex_unlock(&bdev->bd_disk->open_mutex);
-	return PTR_ERR_OR_ZERO(part);
+	ret = PTR_ERR_OR_ZERO(part);
+out:
+	mutex_unlock(&disk->open_mutex);
+	return ret;
 }
 
 int bdev_del_partition(struct block_device *bdev, int partno)