cgroups: fix prune_init_scope()

src/lxc/cgroups/cgroup.c

@@ -106,15 +106,3 @@ void cgroup_exit(struct cgroup_ops *ops)
 
 	return;
 }
-
-#define INIT_SCOPE "/init.scope"
-char *prune_init_scope(char *cg)
-{
-	if (is_empty_string(cg))
-		return NULL;
-
-	if (strnequal(cg, INIT_SCOPE, STRLITERALLEN(INIT_SCOPE)))
-		return cg + STRLITERALLEN(INIT_SCOPE);
-
-	return cg;
-}

src/lxc/cgroups/cgroup.h

@@ -207,8 +207,6 @@ __hidden extern struct cgroup_ops *cgroup_init(struct lxc_conf *conf);
 __hidden extern void cgroup_exit(struct cgroup_ops *ops);
 define_cleanup_function(struct cgroup_ops *, cgroup_exit);
 
-__hidden extern char *prune_init_scope(char *cg);
-
 __hidden extern int cgroup_attach(const struct lxc_conf *conf, const char *name,
 				  const char *lxcpath, pid_t pid);
 __hidden extern int cgroup_get(const char *name, const char *lxcpath,

src/lxc/cgroups/cgroup_utils.c

@@ -159,3 +159,32 @@ int cgroup_tree_prune(int dfd, const char *path)
 
 	return 0;
 }
+
+#define INIT_SCOPE "/init.scope"
+char *prune_init_scope(char *path)
+{
+	char *slash = path;
+	size_t len;
+
+	/*
+	 * This function can only be called on information parsed from
+	 * /proc/<pid>/cgroup. The file displays the current cgroup of the
+	 * process as absolute paths. So if we are passed a non-absolute path
+	 * things are way wrong.
+	 */
+	if (!abspath(path))
+		return ret_set_errno(NULL, EINVAL);
+
+	len = strlen(path);
+	if (len < STRLITERALLEN(INIT_SCOPE))
+		return path;
+
+	slash += (len - STRLITERALLEN(INIT_SCOPE));
+	if (strequal(slash, INIT_SCOPE)) {
+		if (slash == path)
+			slash++;
+		*slash = '\0';
+	}
+
+	return path;
+}

src/lxc/cgroups/cgroup_utils.h

@@ -43,4 +43,11 @@ static inline bool cgns_supported(void)
 
 __hidden extern int cgroup_tree_prune(int dfd, const char *path);
 
+/*
+ * This function can only be called on information parsed from
+ * /proc/<pid>/cgroup or on absolute paths and it will verify the latter and
+ * return NULL if a relative path is passed.
+ */
+__hidden extern char *prune_init_scope(char *path);
+
 #endif /* __LXC_CGROUP_UTILS_H */