vfs: Provide a mount_pseudo-replacement for the new mount API

Provide a function, init_pseudo(), that provides a common
infrastructure for converting pseudo-filesystems that can never be
mountable.

[AV: once all users of mount_pseudo_xattr() get converted, it will be folded
into pseudo_fs_get_tree()]
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-fsdevel@vger.kernel.org

fs/libfs.c

@@ -16,6 +16,8 @@
 #include <linux/exportfs.h>
 #include <linux/writeback.h>
 #include <linux/buffer_head.h> /* sync_mapping_buffers */
+#include <linux/fs_context.h>
+#include <linux/pseudo_fs.h>
 
 #include <linux/uaccess.h>
 
@@ -235,6 +237,50 @@ static const struct super_operations simple_super_operations = {
 	.statfs		= simple_statfs,
 };
 
+static int pseudo_fs_get_tree(struct fs_context *fc)
+{
+	struct pseudo_fs_context *ctx = fc->fs_private;
+	struct dentry *root;
+
+	root = mount_pseudo_xattr(fc->fs_type,
+				  ctx->ops, ctx->xattr,
+			          ctx->dops, ctx->magic);
+	if (IS_ERR(root))
+		return PTR_ERR(root);
+
+	fc->root = root;
+	return 0;
+}
+
+static void pseudo_fs_free(struct fs_context *fc)
+{
+	kfree(fc->fs_private);
+}
+
+static const struct fs_context_operations pseudo_fs_context_ops = {
+	.free		= pseudo_fs_free,
+	.get_tree	= pseudo_fs_get_tree,
+};
+
+/*
+ * Common helper for pseudo-filesystems (sockfs, pipefs, bdev - stuff that
+ * will never be mountable)
+ */
+struct pseudo_fs_context *init_pseudo(struct fs_context *fc,
+					unsigned long magic)
+{
+	struct pseudo_fs_context *ctx;
+
+	ctx = kzalloc(sizeof(struct pseudo_fs_context), GFP_KERNEL);
+	if (likely(ctx)) {
+		ctx->magic = magic;
+		fc->fs_private = ctx;
+		fc->ops = &pseudo_fs_context_ops;
+	}
+	return ctx;
+}
+EXPORT_SYMBOL(init_pseudo);
+
 /*
  * Common helper for pseudo-filesystems (sockfs, pipefs, bdev - stuff that
  * will never be mountable)

include/linux/pseudo_fs.h

+#ifndef __LINUX_PSEUDO_FS__
+#define __LINUX_PSEUDO_FS__
+
+#include <linux/fs_context.h>
+
+struct pseudo_fs_context {
+	const struct super_operations *ops;
+	const struct xattr_handler **xattr;
+	const struct dentry_operations *dops;
+	unsigned long magic;
+};
+
+struct pseudo_fs_context *init_pseudo(struct fs_context *fc,
+				      unsigned long magic);
+
+#endif