init/do_mounts.c: add root=PARTLABEL=<name> support

Support referencing the root partition label from GPT as argument
to the root= option on the kernel command line in analogy to
referencing the partition uuid as root=PARTUUID=<uuid>.

Specifying the partition label instead of the uuid is often much
easier, e.g. in embedded environments when there is an
A/B rootfs partition scheme for interruptible firmware updates
(i.e. rootfsA/ rootfsB).

The partition label can be queried with the blkid command.

Link: http://lkml.kernel.org/r/20180822060904.828E510665E@pc-niv.weinmann.com
Signed-off-by: Nikolaus Voss <nikolaus.voss@loewensteinmedical.de>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Sasha Levin <Alexander.Levin@microsoft.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

init/do_mounts.c

@@ -167,6 +167,24 @@ done:
 	}
 	return res;
 }
+
+/**
+ * match_dev_by_label - callback for finding a partition using its label
+ * @dev:	device passed in by the caller
+ * @data:	opaque pointer to the label to match
+ *
+ * Returns 1 if the device matches, and 0 otherwise.
+ */
+static int match_dev_by_label(struct device *dev, const void *data)
+{
+	const char *label = data;
+	struct hd_struct *part = dev_to_part(dev);
+
+	if (part->info && !strcmp(label, part->info->volname))
+		return 1;
+
+	return 0;
+}
 #endif
 
 /*
@@ -190,6 +208,8 @@ done:
  *	   a partition with a known unique id.
  *	8) <major>:<minor> major and minor number of the device separated by
  *	   a colon.
+ *	9) PARTLABEL=<name> with name being the GPT partition label.
+ *	   MSDOS partitions do not support labels!
  *
  *	If name doesn't have fall into the categories above, we return (0,0).
  *	block_class is used to check if something is a disk name. If the disk
@@ -211,6 +231,17 @@ dev_t name_to_dev_t(const char *name)
 		if (!res)
 			goto fail;
 		goto done;
+	} else if (strncmp(name, "PARTLABEL=", 10) == 0) {
+		struct device *dev;
+
+		dev = class_find_device(&block_class, NULL, name + 10,
+					&match_dev_by_label);
+		if (!dev)
+			goto fail;
+
+		res = dev->devt;
+		put_device(dev);
+		goto done;
 	}
 #endif