[PATCH 1/2] fs/super.c: introduce get_tree_bdev_by_dev()

[Gao Xiang]

As Allison reported [1], currently get_tree_bdev() will store
"Can't lookup blockdev" error message.  Although it makes sense for
pure bdev-based fses, this message may mislead users who try to use
EROFS file-backed mounts since get_tree_nodev() is used as a fallback
then.

Add get_tree_bdev_by_dev() to specify a device number explicitly
instead of the hardcoded fc->source as mentioned in [2], there are
other benefits like:
  - Filesystems can have other ways to get a bdev-based sb
    in addition to the current hard-coded source path;

  - Pseudo-filesystems can utilize this method to generate a
    filesystem from given device numbers too.

  - Like get_tree_nodev(), it doesn't strictly tie to fc->source
    either.

[1] https://lore.kernel.org/r/CAOYeF9VQ8jKVmpy5Zy9DNhO6xmWSKMB-DO8yvBB0XvBE7=3Ugg@xxxxxxxxxxxxxx
[2] https://lore.kernel.org/r/b9565874-7018-46ef-b123-b524a1dffb21@xxxxxxxxxxxxxxxxx
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>
---
 fs/super.c                 | 41 ++++++++++++++++++++++++++------------
 include/linux/fs_context.h |  3 +++
 2 files changed, 31 insertions(+), 13 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 1db230432960..8cc8350b9ba6 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1596,26 +1596,17 @@ int setup_bdev_super(struct super_block *sb, int sb_flags,
 EXPORT_SYMBOL_GPL(setup_bdev_super);
 
 /**
- * get_tree_bdev - Get a superblock based on a single block device
+ * get_tree_bdev_by_dev - Get a bdev-based superblock with a given device number
  * @fc: The filesystem context holding the parameters
  * @fill_super: Helper to initialise a new superblock
+ * @dev: The device number indicating the target block device
  */
-int get_tree_bdev(struct fs_context *fc,
+int get_tree_bdev_by_dev(struct fs_context *fc,
 		int (*fill_super)(struct super_block *,
-				  struct fs_context *))
+				  struct fs_context *), dev_t dev)
 {
 	struct super_block *s;
 	int error = 0;
-	dev_t dev;
-
-	if (!fc->source)
-		return invalf(fc, "No source specified");
-
-	error = lookup_bdev(fc->source, &dev);
-	if (error) {
-		errorf(fc, "%s: Can't lookup blockdev", fc->source);
-		return error;
-	}
 
 	fc->sb_flags |= SB_NOSEC;
 	s = sget_dev(fc, dev);
@@ -1644,6 +1635,30 @@ int get_tree_bdev(struct fs_context *fc,
 	fc->root = dget(s->s_root);
 	return 0;
 }
+EXPORT_SYMBOL_GPL(get_tree_bdev_by_dev);
+
+/**
+ * get_tree_bdev - Get a superblock based on a single block device
+ * @fc: The filesystem context holding the parameters
+ * @fill_super: Helper to initialise a new superblock
+ */
+int get_tree_bdev(struct fs_context *fc,
+		int (*fill_super)(struct super_block *,
+				  struct fs_context *))
+{
+	int error;
+	dev_t dev;
+
+	if (!fc->source)
+		return invalf(fc, "No source specified");
+
+	error = lookup_bdev(fc->source, &dev);
+	if (error) {
+		errorf(fc, "%s: Can't lookup blockdev", fc->source);
+		return error;
+	}
+	return get_tree_bdev_by_dev(fc, fill_super, dev);
+}
 EXPORT_SYMBOL(get_tree_bdev);
 
 static int test_bdev_super(struct super_block *s, void *data)
diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h
index c13e99cbbf81..54f23589ad5b 100644
--- a/include/linux/fs_context.h
+++ b/include/linux/fs_context.h
@@ -160,6 +160,9 @@ extern int get_tree_keyed(struct fs_context *fc,
 
 int setup_bdev_super(struct super_block *sb, int sb_flags,
 		struct fs_context *fc);
+int get_tree_bdev_by_dev(struct fs_context *fc,
+			 int (*fill_super)(struct super_block *sb,
+					   struct fs_context *fc), dev_t dev);
 extern int get_tree_bdev(struct fs_context *fc,
 			       int (*fill_super)(struct super_block *sb,
 						 struct fs_context *fc));
-- 
2.43.5