RFC: Fix cgroup_find_parent function

There are more issues with cgroup_find_parent function:

1.
The cgroup_find_parent expects, that it's argument is group in
cg_mount_table[0] controller, which is not always the case.

IMHO the function should accept complete struct cgroup and find real parent.

2.
when checking the st_dev of the group and it's parent to prevent
"underflow" to real filesystem, actually the parent and it's parent are
checked instead of group and it's parent.

In addition, I enhanced the function to return real error code when something
goes wrong.

Open question is, if the function should return just char* as parent's
name or whole struct cgroup*. The second case is more natural - we should
work with groups and not group names, but in some cases just the name is
needed and whole cgroup creation would be unnecessary overhead (I have
prepared cgcdelete tool, which needs just parent's name).

Signed-off-by: Jan Safranek <jsafrane@redhat.com>

1 files changed, 57 insertions, 31 deletions

diff --git a/src/api.c b/src/api.c
index 1012e56..4e1b622 100644
--- a/src/api.c
+++ b/src/api.c
@@ -1356,60 +1356,78 @@ err:
 /**
  * Find the parent of the specified directory. It returns the parent (the
  * parent is usually name/.. unless name is a mount point.
+ *
+ * @param cgroup The cgroup
+ * @param parent Output, name of parent's group (if the group has parent) or
+ *	NULL, if the provided cgroup is the root group and has no parent.
+ *	Caller is responsible to free the returned string!
+ * @return 0 on success, >0 on error.
  */
-char *cgroup_find_parent(char *name)
+int cgroup_find_parent(struct cgroup *cgroup, char **parent)
 {
-	char child[FILENAME_MAX];
-	char *parent = NULL;
+	char child_path[FILENAME_MAX];
+	char *parent_path = NULL;
 	struct stat stat_child, stat_parent;
-	char *type = NULL;
-	char *dir = NULL;
+	char *controller = NULL;
+	char *dir = NULL, *parent_dir = NULL;
+	int ret = 0;
+
+	*parent = NULL;
 
 	pthread_rwlock_rdlock(&cg_mount_table_lock);
-	type = cg_mount_table[0].name;
-	if (!cg_build_path_locked(name, child, type)) {
+	controller = cgroup->controller[0]->name;
+	if (!cg_build_path_locked(cgroup->name, child_path, controller)) {
 		pthread_rwlock_unlock(&cg_mount_table_lock);
-		return NULL;
+		return ECGFAIL;
 	}
 	pthread_rwlock_unlock(&cg_mount_table_lock);
 
-	cgroup_dbg("path is %s\n", child);
-	dir = dirname(child);
-	cgroup_dbg("directory name is %s\n", dir);
+	cgroup_dbg("path is %s\n", child_path);
 
-	if (asprintf(&parent, "%s/..", dir) < 0)
-		return NULL;
+	if (asprintf(&parent_path, "%s/..", child_path) < 0)
+		return ECGFAIL;
 
-	cgroup_dbg("parent's name is %s\n", parent);
+	cgroup_dbg("parent's name is %s\n", parent_path);
 
-	if (stat(dir, &stat_child) < 0)
+	if (stat(child_path, &stat_child) < 0) {
+		last_errno = errno;
+		ret = ECGOTHER;
 		goto free_parent;
+	}
 
-	if (stat(parent, &stat_parent) < 0)
+	if (stat(parent_path, &stat_parent) < 0) {
+		last_errno = errno;
+		ret = ECGOTHER;
 		goto free_parent;
+	}
 
 	/*
 	 * Is the specified "name" a mount point?
 	 */
 	if (stat_parent.st_dev != stat_child.st_dev) {
-		cgroup_dbg("parent is a mount point\n");
-		strcpy(parent, ".");
+		*parent = NULL;
+		ret = 0;
+		cgroup_dbg("Parent is on different device\n");
 	} else {
-		dir = strdup(name);
-		if (!dir)
+		dir = strdup(cgroup->name);
+		cgroup_dbg("group name is %s\n", dir);
+		if (!dir) {
+			ret = ECGFAIL;
 			goto free_parent;
-		dir = dirname(dir);
-		if (strcmp(dir, ".") == 0)
-			strcpy(parent, "..");
-		else
-			strcpy(parent, dir);
-	}
+		}
 
-	return parent;
+		parent_dir = dirname(dir);
+		cgroup_dbg("parent's group name is %s\n", parent_dir);
+		*parent = strdup(parent_dir);
+		free(dir);
+
+		if (*parent == NULL)
+			ret = ECGFAIL;
+	}
 
 free_parent:
-	free(parent);
-	return NULL;
+	free(parent_path);
+	return ret;
 }
 
 /**
@@ -1427,10 +1445,18 @@ int cgroup_create_cgroup_from_parent(struct cgroup *cgroup,
 	if (!cgroup_initialized)
 		return ECGROUPNOTINITIALIZED;
 
-	parent = cgroup_find_parent(cgroup->name);
-	if (!parent)
+	ret = cgroup_find_parent(cgroup, &parent);
+	if (ret)
 		return ret;
 
+	if (parent == NULL) {
+		/*
+		 * The group to create is root group!
+		 * TODO: find better error code?
+		 */
+		return ECGFAIL;
+	}
+
 	cgroup_dbg("parent is %s\n", parent);
 	parent_cgroup = cgroup_new_cgroup(parent);
 	if (!parent_cgroup)