ovl: switch to mounter creds in readdir

In preparation for more permission checking, override credentials for
directory operations on the underlying filesystems.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/readdir.c

@@ -743,8 +743,10 @@ static int ovl_iterate(struct file *file, struct dir_context *ctx)
 	struct ovl_dir_file *od = file->private_data;
 	struct dentry *dentry = file->f_path.dentry;
 	struct ovl_cache_entry *p;
+	const struct cred *old_cred;
 	int err;
 
+	old_cred = ovl_override_creds(dentry->d_sb);
 	if (!ctx->pos)
 		ovl_dir_reset(file);
 
@@ -758,17 +760,20 @@ static int ovl_iterate(struct file *file, struct dir_context *ctx)
 		    (ovl_same_fs(dentry->d_sb) &&
 		     (ovl_is_impure_dir(file) ||
 		      OVL_TYPE_MERGE(ovl_path_type(dentry->d_parent))))) {
-			return ovl_iterate_real(file, ctx);
+			err = ovl_iterate_real(file, ctx);
+		} else {
+			err = iterate_dir(od->realfile, ctx);
 		}
-		return iterate_dir(od->realfile, ctx);
+		goto out;
 	}
 
 	if (!od->cache) {
 		struct ovl_dir_cache *cache;
 
 		cache = ovl_cache_get(dentry);
+		err = PTR_ERR(cache);
 		if (IS_ERR(cache))
-			return PTR_ERR(cache);
+			goto out;
 
 		od->cache = cache;
 		ovl_seek_cursor(od, ctx->pos);
@@ -780,7 +785,7 @@ static int ovl_iterate(struct file *file, struct dir_context *ctx)
 			if (!p->ino) {
 				err = ovl_cache_update_ino(&file->f_path, p);
 				if (err)
-					return err;
+					goto out;
 			}
 			if (!dir_emit(ctx, p->name, p->len, p->ino, p->type))
 				break;
@@ -788,7 +793,10 @@ static int ovl_iterate(struct file *file, struct dir_context *ctx)
 		od->cursor = p->l_node.next;
 		ctx->pos++;
 	}
-	return 0;
+	err = 0;
+out:
+	revert_creds(old_cred);
+	return err;
 }
 
 static loff_t ovl_dir_llseek(struct file *file, loff_t offset, int origin)
@@ -834,7 +842,14 @@ static loff_t ovl_dir_llseek(struct file *file, loff_t offset, int origin)
 static struct file *ovl_dir_open_realfile(struct file *file,
 					  struct path *realpath)
 {
-	return ovl_path_open(realpath, O_RDONLY | (file->f_flags & O_LARGEFILE));
+	struct file *res;
+	const struct cred *old_cred;
+
+	old_cred = ovl_override_creds(file_inode(file)->i_sb);
+	res = ovl_path_open(realpath, O_RDONLY | (file->f_flags & O_LARGEFILE));
+	revert_creds(old_cred);
+
+	return res;
 }
 
 static int ovl_dir_fsync(struct file *file, loff_t start, loff_t end,