fs: prepare for stackable filesystems backing file helpers

In preparation for factoring out some backing file io helpers from
overlayfs, move backing_file_open() into a new file fs/backing-file.c
and header.

Add a MAINTAINERS entry for stackable filesystems and add a Kconfig
FS_STACK which stackable filesystems need to select.

For now, the backing_file struct, the backing_file alloc/free functions
and the backing_file_real_path() accessor remain internal to file_table.c.
We may change that in the future.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/include/linux/backing-file.h b/include/linux/backing-file.h
new file mode 100644
index 000000000000..55c9e804f780
--- /dev/null
+++ b/include/linux/backing-file.h
@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Common helpers for stackable filesystems and backing files.
+ *
+ * Copyright (C) 2023 CTERA Networks.
+ */
+
+#ifndef _LINUX_BACKING_FILE_H
+#define _LINUX_BACKING_FILE_H
+
+#include <linux/file.h>
+
+struct file *backing_file_open(const struct path *user_path, int flags,
+			       const struct path *real_path,
+			       const struct cred *cred);
+
+#endif /* _LINUX_BACKING_FILE_H */