[PATCH v3 1/5] ovl: do not open non-data lower file for fsync

[Amir Goldstein <amir73il@xxxxxxxxx>]

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
ovl_upper_fdget() where the latter returns an empty struct fd in that
case to avoid the unneeded backing file open.

Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
---
 fs/overlayfs/file.c | 71 +++++++++++++++++++++++++++------------------
 1 file changed, 43 insertions(+), 28 deletions(-)

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 4504493b20be..f5d0498355d0 100644
--- a/fs/overlayfs/file.c
+++ b/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;
@@ -130,7 +117,11 @@ 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);
@@ -138,7 +129,33 @@ static int ovl_real_fdget(const struct file *file, struct fd *real)
 		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_upper_fdget(const struct file *file, struct fd *real, bool data)
+{
+	struct dentry *dentry = file_dentry(file);
+	struct path realpath;
+	enum ovl_path_type type;
+
+	if (data)
+		type = ovl_path_realdata(dentry, &realpath);
+	else
+		type = ovl_path_real(dentry, &realpath);
+
+	real->word = 0;
+	/* Not interested in lower nor in upper meta if data was requested */
+	if (!OVL_TYPE_UPPER(type) || (data && OVL_TYPE_MERGE(type)))
+		return 0;
+
+	return ovl_real_fdget_path(file, real, &realpath);
 }
 
 static int ovl_open(struct inode *inode, struct file *file)
@@ -394,16 +411,14 @@ 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);
-	if (ret)
+	/* Don't sync lower file for fear of receiving EROFS error */
+	ret = ovl_upper_fdget(file, &real, datasync);
+	if (ret || fd_empty(real))
 		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);
-		revert_creds(old_cred);
-	}
+	old_cred = ovl_override_creds(file_inode(file)->i_sb);
+	ret = vfs_fsync_range(fd_file(real), start, end, datasync);
+	revert_creds(old_cred);
 
 	fdput(real);
 
-- 
2.34.1