ovl: make upperdir optional

Make "upperdir=" mount option optional.  If "upperdir=" is not given, then
the "workdir=" option is also optional (and ignored if given).
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/overlayfs/super.c

@@ -516,8 +516,10 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry)
 	struct ovl_fs *ufs = sb->s_fs_info;
 
 	seq_printf(m, ",lowerdir=%s", ufs->config.lowerdir);
+	if (ufs->config.upperdir) {
 		seq_printf(m, ",upperdir=%s", ufs->config.upperdir);
 		seq_printf(m, ",workdir=%s", ufs->config.workdir);
+	}
 	return 0;
 }
 
@@ -768,8 +770,8 @@ static bool ovl_workdir_ok(struct dentry *workdir, struct dentry *upperdir)
 static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 {
 	struct path lowerpath;
-	struct path upperpath;
-	struct path workpath;
+	struct path upperpath = { NULL, NULL };
+	struct path workpath = { NULL, NULL };
 	struct dentry *root_dentry;
 	struct ovl_entry *oe;
 	struct ovl_fs *ufs;
@@ -786,11 +788,17 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	if (err)
 		goto out_free_config;
 
-	/* FIXME: workdir is not needed for a R/O mount */
 	err = -EINVAL;
-	if (!ufs->config.upperdir || !ufs->config.lowerdir ||
-	    !ufs->config.workdir) {
-		pr_err("overlayfs: missing upperdir or lowerdir or workdir\n");
+	if (!ufs->config.lowerdir) {
+		pr_err("overlayfs: missing 'lowerdir'\n");
+		goto out_free_config;
+	}
+
+	sb->s_stack_depth = 0;
+	if (ufs->config.upperdir) {
+		/* FIXME: workdir is not needed for a R/O mount */
+		if (!ufs->config.workdir) {
+			pr_err("overlayfs: missing 'workdir'\n");
 			goto out_free_config;
 		}
 
@@ -811,6 +819,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 			goto out_put_workpath;
 		}
 		sb->s_stack_depth = upperpath.mnt->mnt_sb->s_stack_depth;
+	}
 
 	err = ovl_lower_dir(ufs->config.lowerdir, &lowerpath,
 			    &ufs->lower_namelen, &sb->s_stack_depth);
@@ -824,6 +833,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 		goto out_put_lowerpath;
 	}
 
+	if (ufs->config.upperdir) {
 		ufs->upper_mnt = clone_private_mount(&upperpath);
 		err = PTR_ERR(ufs->upper_mnt);
 		if (IS_ERR(ufs->upper_mnt)) {
@@ -838,6 +848,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 			       ufs->config.workdir, OVL_WORKDIR_NAME);
 			goto out_put_upper_mnt;
 		}
+	}
 
 	ufs->lower_mnt = kcalloc(1, sizeof(struct vfsmount *), GFP_KERNEL);
 	if (ufs->lower_mnt == NULL)
@@ -858,8 +869,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	ufs->lower_mnt[0] = mnt;
 	ufs->numlower = 1;
 
-	/* If the upper fs is r/o, we mark overlayfs r/o too */
-	if (ufs->upper_mnt->mnt_sb->s_flags & MS_RDONLY)
+	/* If the upper fs is r/o or nonexistent, we mark overlayfs r/o too */
+	if (!ufs->upper_mnt || (ufs->upper_mnt->mnt_sb->s_flags & MS_RDONLY))
 		sb->s_flags |= MS_RDONLY;
 
 	sb->s_d_op = &ovl_dentry_operations;