[PATCH 1/4] ovl: do not open non-data lower file for fsync

[Amir Goldstein <amir73il@gmail.com>]

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.

Fix the helper ovl_real_fdget_meta() to return an empty struct fd in
that case to avoid the unneeded backing file open.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/overlayfs/file.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 4504493b20be..3d64d00ef981 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -90,17 +90,19 @@ static int ovl_change_flags(struct file *file, unsigned int flags)
 }
 
 static int ovl_real_fdget_meta(const struct file *file, struct fd *real,
-			       bool allow_meta)
+			       bool upper_meta)
 {
 	struct dentry *dentry = file_dentry(file);
 	struct file *realfile = file->private_data;
 	struct path realpath;
 	int err;
 
-	real->word = (unsigned long)realfile;
+	real->word = 0;
 
-	if (allow_meta) {
-		ovl_path_real(dentry, &realpath);
+	if (upper_meta) {
+		ovl_path_upper(dentry, &realpath);
+		if (!realpath.dentry)
+			return 0;
 	} else {
 		/* lazy lookup and verify of lowerdata */
 		err = ovl_verify_lowerdata(dentry);
@@ -395,7 +397,7 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
 		return ret;
 
 	ret = ovl_real_fdget_meta(file, &real, !datasync);
-	if (ret)
+	if (ret || fd_empty(real))
 		return ret;
 
 	/* Don't sync lower file for fear of receiving EROFS error */
-- 
2.34.1