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 <[e-mail]>

MAINTAINERS

···
 F:	fs/iomap/
 F:	include/linux/iomap.h
 
+FILESYSTEMS [STACKABLE]
+M:	Miklos Szeredi <[e-mail]>
+M:	Amir Goldstein <[e-mail]>
+L:	[e-mail]
+L:	[e-mail]
+S:	Maintained
+F:	fs/backing-file.c
+F:	include/linux/backing-file.h
+
 FINTEK F75375S HARDWARE MONITOR AND FAN CONTROLLER DRIVER
 M:	Riku Voipio <[e-mail]>
 L:	[e-mail]

fs/Kconfig

···
 config FS_IOMAP
 	bool
 
+# Stackable filesystems
+config FS_STACK
+	bool
+
 config BUFFER_HEAD
 	bool
 

fs/Makefile

···
 obj-$(CONFIG_BINFMT_ELF_FDPIC)	+= binfmt_elf_fdpic.o
 obj-$(CONFIG_BINFMT_FLAT)	+= binfmt_flat.o
 
+obj-$(CONFIG_FS_STACK)		+= backing-file.o
 obj-$(CONFIG_FS_MBCACHE)	+= mbcache.o
 obj-$(CONFIG_FS_POSIX_ACL)	+= posix_acl.o
 obj-$(CONFIG_NFS_COMMON)	+= nfs_common/

fs/backing-file.c

···
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Common helpers for stackable filesystems and backing files.
+ *
+ * Copyright (C) 2023 CTERA Networks.
+ */
+
+#include <linux/fs.h>
+#include <linux/backing-file.h>
+
+#include "internal.h"
+
+/**
+ * backing_file_open - open a backing file for kernel internal use
+ * @user_path:	path that the user reuqested to open
+ * @flags:	open flags
+ * @real_path:	path of the backing file
+ * @cred:	credentials for open
+ *
+ * Open a backing file for a stackable filesystem (e.g., overlayfs).
+ * @user_path may be on the stackable filesystem and @real_path on the
+ * underlying filesystem.  In this case, we want to be able to return the
+ * @user_path of the stackable filesystem. This is done by embedding the
+ * returned file into a container structure that also stores the stacked
+ * file's path, which can be retrieved using backing_file_user_path().
+ */
+struct file *backing_file_open(const struct path *user_path, int flags,
+			       const struct path *real_path,
+			       const struct cred *cred)
+{
+	struct file *f;
+	int error;
+
+	f = alloc_empty_backing_file(flags, cred);
+	if (IS_ERR(f))
+		return f;
+
+	path_get(user_path);
+	*backing_file_user_path(f) = *user_path;
+	error = vfs_open(real_path, f);
+	if (error) {
+		fput(f);
+		f = ERR_PTR(error);
+	}
+
+	return f;
+}
+EXPORT_SYMBOL_GPL(backing_file_open);

fs/open.c

···
 }
 EXPORT_SYMBOL_GPL(kernel_file_open);
 
-/**
- * backing_file_open - open a backing file for kernel internal use
- * @user_path:	path that the user reuqested to open
- * @flags:	open flags
- * @real_path:	path of the backing file
- * @cred:	credentials for open
- *
- * Open a backing file for a stackable filesystem (e.g., overlayfs).
- * @user_path may be on the stackable filesystem and @real_path on the
- * underlying filesystem.  In this case, we want to be able to return the
- * @user_path of the stackable filesystem. This is done by embedding the
- * returned file into a container structure that also stores the stacked
- * file's path, which can be retrieved using backing_file_user_path().
- */
-struct file *backing_file_open(const struct path *user_path, int flags,
-			       const struct path *real_path,
-			       const struct cred *cred)
-{
-	struct file *f;
-	int error;
-
-	f = alloc_empty_backing_file(flags, cred);
-	if (IS_ERR(f))
-		return f;
-
-	path_get(user_path);
-	*backing_file_user_path(f) = *user_path;
-	f->f_path = *real_path;
-	error = do_dentry_open(f, d_inode(real_path->dentry), NULL);
-	if (error) {
-		fput(f);
-		f = ERR_PTR(error);
-	}
-
-	return f;
-}
-EXPORT_SYMBOL_GPL(backing_file_open);
-
 #define WILL_CREATE(flags)	(flags & (O_CREAT | __O_TMPFILE))
 #define O_PATH_FLAGS		(O_DIRECTORY | O_NOFOLLOW | O_PATH | O_CLOEXEC)
 

fs/overlayfs/Kconfig

···
 # SPDX-License-Identifier: GPL-2.0-only
 config OVERLAY_FS
 	tristate "Overlay filesystem support"
+	select FS_STACK
 	select EXPORTFS
 	help
 	  An overlay filesystem combines two filesystems - an 'upper' filesystem

fs/overlayfs/file.c

···
 #include <linux/security.h>
 #include <linux/mm.h>
 #include <linux/fs.h>
+#include <linux/backing-file.h>
 #include "overlayfs.h"
 
 #include "../internal.h"	/* for sb_init_dio_done_wq */

include/linux/backing-file.h

···
+/* 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 */

include/linux/fs.h

···
 			 const struct cred *creds);
 struct file *dentry_create(const struct path *path, int flags, umode_t mode,
 			   const struct cred *cred);
-struct file *backing_file_open(const struct path *user_path, int flags,
-			       const struct path *real_path,
-			       const struct cred *cred);
 struct path *backing_file_user_path(struct file *f);
 
 /*