Incorrect error message from erofs "backed by file" in 6.12-rc

[Gao Xiang]

Hi Christian,

On 2024/10/7 19:35, Christian Brauner wrote:
> On Sat, Oct 05, 2024 at 10:41:10PM GMT, Gao Xiang wrote:

...

>>
>> Hi Christian, if possible, could you give some other
>> idea to handle this case better? Many thanks!

Thanks for the reply!

> 
> (1) Require that the path be qualified like:
> 
>      fsconfig(<fd>, FSCONFIG_SET_STRING, "source", "file:/home/lis/src/mountcfs/cfs", 0)
> 
>      and match on it in either erofs_*_get_tree() or by adding a custom
>      function for the Opt_source/"source" parameter.

IMHO, Users could create names with the prefix `file:`,
it's somewhat strange to define a fixed prefix by the
definition of source path fc->source.

Although there could be some escape character likewise
way, but I'm not sure if it's worthwhile to work out
this in kernel.

> 
> (2) Add a erofs specific "source-file" mount option. IOW, check that
>      either "source-file" or "source" was specified but not both. You
>      could even set fc->source to "source-file" value and fail if
>      fc->source is already set. You get the idea.

I once thought to add a new mount option too, yet from
the user perpertive, I think users may not care about
the source type of an arbitary path, and the kernel also
can parse the type of the source path directly... so..


So.. I wonder if it's possible to add a new VFS interface
like get_tree_bdev_by_dev() for filesystems to specify a
device number rather than hardcoded hard-coded source path
way, e.g. I could see the potential benefits other than
the EROFS use case:

  - Filesystems can have other ways to get a bdev-based sb
    in addition to the current hard-coded source path way;

  - Some pseudo fs can use this way to generate a fs from a
    bdev.

  - Just like get_tree_nodev(), it doesn't strictly tie to
    fc->source too.

Also EROFS could lookup_bdev() (this kAPI is already
exported) itself to check if it uses get_tree_bdev_by_dev()
or get_tree_nodev()... Does it sounds good?  Many thanks!

Thanks,
Gao Xiang

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));


> 
> ?