fs: create helper file_user_path() for user displayed mapped file path

Overlayfs uses backing files with "fake" overlayfs f_path and "real"
underlying f_inode, in order to use underlying inode aops for mapped
files and to display the overlayfs path in /proc/<pid>/maps.

In preparation for storing the overlayfs "fake" path instead of the
underlying "real" path in struct backing_file, define a noop helper
file_user_path() that returns f_path for now.

Use the new helper in procfs and kernel logs whenever a path of a
mapped file is displayed to users.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20231009153712.1566422-3-amir73il@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index ceafc40cc25f..057a3bbd4d27 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2513,6 +2513,20 @@ static inline const struct path *file_real_path(struct file *f)
 	return &f->f_path;
 }
 
+/*
+ * file_user_path - get the path to display for memory mapped file
+ *
+ * When mmapping a file on a stackable filesystem (e.g., overlayfs), the file
+ * stored in ->vm_file is a backing file whose f_inode is on the underlying
+ * filesystem.  When the mapped file path is displayed to user (e.g. via
+ * /proc/<pid>/maps), this helper should be used to get the path to display
+ * to the user, which is the path of the fd that user has requested to map.
+ */
+static inline const struct path *file_user_path(struct file *f)
+{
+	return &f->f_path;
+}
+
 static inline struct file *file_clone_open(struct file *file)
 {
 	return dentry_open(&file->f_path, file->f_flags, file->f_cred);