mtd: block2mtd: don't call early_lookup_bdev after the system is running

early_lookup_bdev is supposed to only be called from the early boot
code, but mdtblock_early_get_bdev is called as a general fallback when
lookup_bdev fails, which is problematic because early_lookup_bdev
bypasses all normal path based permission checking, and might cause
problems with certain container environments renaming devices.

Switch to only call early_lookup_bdev when block2mtd is built-in and the
system state in not running yet.

Note that this strictly speaking changes the kernel ABI as the PARTUUID=
and PARTLABEL= style syntax is now not available during a running
systems.  They never were intended for that, but this breaks things
we'll have to figure out a way to make them available again.  But if
avoidable in any way I'd rather avoid that.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20230531125535.676098-24-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/mtd/devices/block2mtd.c

@@ -215,13 +215,23 @@ static void block2mtd_free_device(struct block2mtd_dev *dev)
 	kfree(dev);
 }
 
-static struct block_device *mdtblock_early_get_bdev(const char *devname,
+/*
+ * This function is marked __ref because it calls the __init marked
+ * early_lookup_bdev when called from the early boot code.
+ */
+static struct block_device __ref *mdtblock_early_get_bdev(const char *devname,
 		fmode_t mode, int timeout, struct block2mtd_dev *dev)
 {
 	struct block_device *bdev = ERR_PTR(-ENODEV);
 #ifndef MODULE
 	int i;
 
+	/*
+	 * We can't use early_lookup_bdev from a running system.
+	 */
+	if (system_state >= SYSTEM_RUNNING)
+		return bdev;
+
 	/*
 	 * We might not have the root device mounted at this point.
 	 * Try to resolve the device name by other means.