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

[Nikolaus Voss]

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.

Signed-off-by: Nikolaus Voss <nikolaus.voss@xxxxxxxxxxxxxxxxxxxxx>
---
 init/do_mounts.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/init/do_mounts.c b/init/do_mounts.c
index 2c71dabe5626..239b5104cff0 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -177,6 +177,24 @@ static dev_t devt_from_partuuid(const char *uuid_str)
 	}
 	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
 
 /*
@@ -200,6 +218,8 @@ static dev_t devt_from_partuuid(const char *uuid_str)
  *	   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
@@ -221,6 +241,17 @@ 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
 
-- 
2.17.1