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kernel/SOURCES/1332-fs-prepare-for-stackable-filesystems-backing-file-helpers.patch

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From ecbb1b105f3616d4342dba0f840dcf9265dfbea0 Mon Sep 17 00:00:00 2001
From: Ondrej Mosnacek <omosnace@redhat.com>
Date: Wed, 3 Jun 2026 10:42:36 +0200
Subject: [PATCH] fs: prepare for stackable filesystems backing file helpers
JIRA: https://issues.redhat.com/browse/RHEL-179443
CVE: CVE-2026-46054
commit f91a704f7161c2cf0fcd41fa9fbec4355b813fff
Author: Amir Goldstein <amir73il@gmail.com>
Date: Mon Oct 2 17:19:46 2023 +0300
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>
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index f5dcab467670..3a29f2d3a2b1 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -7475,6 +7475,15 @@ F: fs/mnt_idmapping.c
F: include/linux/mnt_idmapping.*
F: tools/testing/selftests/mount_setattr/
+FILESYSTEMS [STACKABLE]
+M: Miklos Szeredi <miklos@szeredi.hu>
+M: Amir Goldstein <amir73il@gmail.com>
+L: linux-fsdevel@vger.kernel.org
+L: linux-unionfs@vger.kernel.org
+S: Maintained
+F: fs/backing-file.c
+F: include/linux/backing-file.h
+
FINTEK F75375S HARDWARE MONITOR AND FAN CONTROLLER DRIVER
M: Riku Voipio <riku.voipio@iki.fi>
L: linux-hwmon@vger.kernel.org
diff --git a/fs/Kconfig b/fs/Kconfig
index 5378e55f87d3..a9c6fa9cff1f 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -18,6 +18,10 @@ config VALIDATE_FS_PARSER
config FS_IOMAP
bool
+# Stackable filesystems
+config FS_STACK
+ bool
+
config BUFFER_HEAD
bool
diff --git a/fs/Makefile b/fs/Makefile
index 0da17ff145c6..716c9fe04dec 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -41,6 +41,7 @@ obj-$(CONFIG_COMPAT_BINFMT_ELF) += compat_binfmt_elf.o
obj-$(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/
diff --git a/fs/backing-file.c b/fs/backing-file.c
new file mode 100644
index 000000000000..04b33036f709
--- /dev/null
+++ b/fs/backing-file.c
@@ -0,0 +1,48 @@
+// 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);
diff --git a/fs/open.c b/fs/open.c
index 45547548a0e5..4260d61560d4 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1141,44 +1141,6 @@ struct file *kernel_file_open(const struct path *path, int flags,
}
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)
diff --git a/fs/overlayfs/Kconfig b/fs/overlayfs/Kconfig
index 6708e54b0e30..148d9567b5c3 100644
--- a/fs/overlayfs/Kconfig
+++ b/fs/overlayfs/Kconfig
@@ -1,6 +1,7 @@
# 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
diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index cd0770bb3020..634a96a65bfd 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -13,6 +13,7 @@
#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 */
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 */
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 1927fcdad989..5f3ca25c77e5 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2586,9 +2586,6 @@ struct file *dentry_open(const struct path *path, int flags,
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);
/*
--
2.50.1 (Apple Git-155)
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