ovl: do not open non-data lower file for fsync

ovl_fsync() with !datasync opens a backing file from the top most dentry
in the stack, checks if this dentry is non-upper and skips the fsync.

In case of an overlay dentry stack with lower data and lower metadata
above it, but without an upper metadata above it, the backing file is
opened from the top most lower metadata dentry and never used.

Refactor the helper ovl_real_fdget_meta() into ovl_real_fdget_path() and
open code the checks for non-upper inode in ovl_fsync(), so in that case
we can avoid the unneeded backing file open.

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

Author: amir73il

fs/overlayfs/file.c

@@ -89,32 +89,19 @@ static int ovl_change_flags(struct file *file, unsigned int flags)
 	return 0;
 }
 
-static int ovl_real_fdget_meta(const struct file *file, struct fd *real,
-			       bool allow_meta)
+static int ovl_real_fdget_path(const struct file *file, struct fd *real,
+			       struct path *realpath)
 {
-	struct dentry *dentry = file_dentry(file);
 	struct file *realfile = file->private_data;
-	struct path realpath;
-	int err;
 
 	real->word = (unsigned long)realfile;
 
-	if (allow_meta) {
-		ovl_path_real(dentry, &realpath);
-	} else {
-		/* lazy lookup and verify of lowerdata */
-		err = ovl_verify_lowerdata(dentry);
-		if (err)
-			return err;
-
-		ovl_path_realdata(dentry, &realpath);
-	}
-	if (!realpath.dentry)
+	if (WARN_ON_ONCE(!realpath->dentry))
 		return -EIO;
 
 	/* Has it been copied up since we'd opened it? */
-	if (unlikely(file_inode(realfile) != d_inode(realpath.dentry))) {
-		struct file *f = ovl_open_realfile(file, &realpath);
+	if (unlikely(file_inode(realfile) != d_inode(realpath->dentry))) {
+		struct file *f = ovl_open_realfile(file, realpath);
 		if (IS_ERR(f))
 			return PTR_ERR(f);
 		real->word = (unsigned long)f | FDPUT_FPUT;
@@ -130,15 +117,26 @@ static int ovl_real_fdget_meta(const struct file *file, struct fd *real,
 
 static int ovl_real_fdget(const struct file *file, struct fd *real)
 {
-	if (d_is_dir(file_dentry(file))) {
+	struct dentry *dentry = file_dentry(file);
+	struct path realpath;
+	int err;
+
+	if (d_is_dir(dentry)) {
 		struct file *f = ovl_dir_real_file(file, false);
 		if (IS_ERR(f))
 			return PTR_ERR(f);
 		real->word = (unsigned long)f;
 		return 0;
 	}
 
-	return ovl_real_fdget_meta(file, real, false);
+	/* lazy lookup and verify of lowerdata */
+	err = ovl_verify_lowerdata(dentry);
+	if (err)
+		return err;
+
+	ovl_path_realdata(dentry, &realpath);
+
+	return ovl_real_fdget_path(file, real, &realpath);
 }
 
 static int ovl_open(struct inode *inode, struct file *file)
@@ -396,6 +394,9 @@ static ssize_t ovl_splice_write(struct pipe_inode_info *pipe, struct file *out,
 
 static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
 {
+	struct dentry *dentry = file_dentry(file);
+	enum ovl_path_type type;
+	struct path upperpath;
 	struct fd real;
 	const struct cred *old_cred;
 	int ret;
@@ -404,16 +405,19 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
 	if (ret <= 0)
 		return ret;
 
-	ret = ovl_real_fdget_meta(file, &real, !datasync);
+	/* Don't sync lower file for fear of receiving EROFS error */
+	type = ovl_path_type(dentry);
+	if (!OVL_TYPE_UPPER(type) || (datasync && OVL_TYPE_MERGE(type)))
+		return 0;
+
+	ovl_path_upper(dentry, &upperpath);
+	ret = ovl_real_fdget_path(file, &real, &upperpath);
 	if (ret)
 		return ret;
 
-	/* Don't sync lower file for fear of receiving EROFS error */
-	if (file_inode(fd_file(real)) == ovl_inode_upper(file_inode(file))) {
-		old_cred = ovl_override_creds(file_inode(file)->i_sb);
-		ret = vfs_fsync_range(fd_file(real), start, end, datasync);
-		ovl_revert_creds(old_cred);
-	}
+	old_cred = ovl_override_creds(file_inode(file)->i_sb);
+	ret = vfs_fsync_range(fd_file(real), start, end, datasync);
+	ovl_revert_creds(old_cred);
 
 	fdput(real);