[PATCH 2/2] dm: Get devices using name_to_dev_t

[Dan Ehrenberg <dehrenberg@chromium.org>]

If a device is used as the root filesystem, it can't be built
off of devices which are within the root filesystem (just like
command line arguments to root=). For this reason, Linux has a
pseudo-filesystem for root= and md initialization based on the
function name_to_dev_t, which handles different ways of specifying
devices including PARTUUID and major:minor.

This patch applies name_to_dev_t to dm initialization. Rather
than assuming that all things which are not major:minor are paths
in an already-mounted filesystem, this patch first attempts
to look up the device in the filesystem, and applies name_to_dev_t
if it is not found there.

In terms of backwards compatibility, there are some cases where
behavior will be different:
- If you have a file in the current working directory named 1:2 and
  you initialze DM there, then it will try to use that file rather
  than the disk with that major/minor pair as a backing device.
- Similarly for other bdev types which name_to_dev_t knows how to
  interpret, the previous behavior was to repeatedly check for the
  existence of the file (e.g., while waiting for rootfs to come up)
  but the new behavior is to use the name_to_dev_t interpretation.
  For example, if you have a file named /dev/ubiblock0_0 which is
  a symlink to /dev/sda3, but it is not yet present when dm starts
  to initialize, then the name_to_dev_t interpretation will take
  precedence.
I believe these incompatibilities would only show up in really
strange setups with bad practices and we don't have to worry about
them.

Signed-off-by: Dan Ehrenberg <dehrenberg@chromium.org>
---
 drivers/md/dm-table.c | 20 ++++++++------------
 1 file changed, 8 insertions(+), 12 deletions(-)

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 3afae9e..3ce1e01 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -18,6 +18,7 @@
 #include <linux/mutex.h>
 #include <linux/delay.h>
 #include <linux/atomic.h>
+#include <linux/mount.h>
 
 #define DM_MSG_PREFIX "table"
 
@@ -372,23 +373,18 @@ int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
 	int r;
 	dev_t uninitialized_var(dev);
 	struct dm_dev_internal *dd;
-	unsigned int major, minor;
 	struct dm_table *t = ti->table;
-	char dummy;
+	struct block_device *bdev;
 
 	BUG_ON(!t);
 
-	if (sscanf(path, "%u:%u%c", &major, &minor, &dummy) == 2) {
-		/* Extract the major/minor numbers */
-		dev = MKDEV(major, minor);
-		if (MAJOR(dev) != major || MINOR(dev) != minor)
-			return -EOVERFLOW;
+	/* convert the path to a device */
+	bdev = lookup_bdev(path);
+	if (IS_ERR(bdev)) {
+		dev = name_to_dev_t(path);
+		if (!dev)
+			return -ENODEV;
 	} else {
-		/* convert the path to a device */
-		struct block_device *bdev = lookup_bdev(path);
-
-		if (IS_ERR(bdev))
-			return PTR_ERR(bdev);
 		dev = bdev->bd_dev;
 		bdput(bdev);
 	}
-- 
2.2.0.rc0.207.ga3a616c