[PATCH v4 6/9] vfs: introduce file_modified() helper

[Amir Goldstein <amir73il@gmail.com>]

The combination of file_remove_privs() and file_update_mtime() is
quite common in filesystem ->write_iter() methods.

Modelled after the helper file_accessed(), introduce file_modified()
and use it from generic_remap_file_range_prep().

Note that the order of calling file_remove_privs() before
file_update_mtime() in the helper was matched to the more common order by
filesystems and not the current order in generic_remap_file_range_prep().

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/inode.c         | 20 ++++++++++++++++++++
 fs/read_write.c    | 21 +++------------------
 include/linux/fs.h |  2 ++
 3 files changed, 25 insertions(+), 18 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index df6542ec3b88..4348cfb14562 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1899,6 +1899,26 @@ int file_update_time(struct file *file)
 }
 EXPORT_SYMBOL(file_update_time);
 
+/* Caller must hold the file's inode lock */
+int file_modified(struct file *file)
+{
+	int err;
+
+	/*
+	 * Clear the security bits if the process is not being run by root.
+	 * This keeps people from modifying setuid and setgid binaries.
+	 */
+	err = file_remove_privs(file);
+	if (err)
+		return err;
+
+	if (unlikely(file->f_mode & FMODE_NOCMTIME))
+		return 0;
+
+	return file_update_time(file);
+}
+EXPORT_SYMBOL(file_modified);
+
 int inode_needs_sync(struct inode *inode)
 {
 	if (IS_SYNC(inode))
diff --git a/fs/read_write.c b/fs/read_write.c
index b0fb1176b628..cec7e7b1f693 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -1980,25 +1980,10 @@ int generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
 		return ret;
 
 	/* If can't alter the file contents, we're done. */
-	if (!(remap_flags & REMAP_FILE_DEDUP)) {
-		/* Update the timestamps, since we can alter file contents. */
-		if (!(file_out->f_mode & FMODE_NOCMTIME)) {
-			ret = file_update_time(file_out);
-			if (ret)
-				return ret;
-		}
+	if (!(remap_flags & REMAP_FILE_DEDUP))
+		ret = file_modified(file_out);
 
-		/*
-		 * Clear the security bits if the process is not being run by
-		 * root.  This keeps people from modifying setuid and setgid
-		 * binaries.
-		 */
-		ret = file_remove_privs(file_out);
-		if (ret)
-			return ret;
-	}
-
-	return 0;
+	return ret;
 }
 EXPORT_SYMBOL(generic_remap_file_range_prep);
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e4d382c4342a..79ffa2958bd8 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2177,6 +2177,8 @@ static inline void file_accessed(struct file *file)
 		touch_atime(&file->f_path);
 }
 
+extern int file_modified(struct file *file);
+
 int sync_inode(struct inode *inode, struct writeback_control *wbc);
 int sync_inode_metadata(struct inode *inode, int wait);
 
-- 
2.17.1