bcache: don't attach backing with duplicate UUID

This can happen e.g. during disk cloning.

This is an incomplete fix: it does not catch duplicate UUIDs earlier
when things are still unattached. It does not unregister the device.
Further changes to cope better with this are planned but conflict with
Coly's ongoing improvements to handling device errors. In the meantime,
one can manually stop the device after this has happened.

Attempts to attach a duplicate device result in:

[ 136.372404] loop: module loaded
[ 136.424461] bcache: register_bdev() registered backing device loop0
[ 136.424464] bcache: bch_cached_dev_attach() Tried to attach loop0 but duplicate UUID already attached

My test procedure is:

dd if=/dev/sdb1 of=imgfile bs=1024 count=262144
losetup -f imgfile

Signed-off-by: Michael Lyle <[e-mail]>
Reviewed-by: Tang Junhui <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/md/bcache/super.c

···
 	uint32_t rtime = cpu_to_le32(get_seconds());
 	struct uuid_entry *u;
 	char buf[BDEVNAME_SIZE];
+	struct cached_dev *exist_dc, *t;
 
 	bdevname(dc->bdev, buf);
 
···
 		pr_err("Couldn't attach %s: block size less than set's block size",
 		       buf);
 		return -EINVAL;
+	}
+
+	/* Check whether already attached */
+	list_for_each_entry_safe(exist_dc, t, &c->cached_devs, list) {
+		if (!memcmp(dc->sb.uuid, exist_dc->sb.uuid, 16)) {
+			pr_err("Tried to attach %s but duplicate UUID already attached",
+				buf);
+
+			return -EINVAL;
+		}
 	}
 
 	u = uuid_find(c, dc->sb.uuid);