selftests/bpf: Add a new cgroup helper get_cgroup_hierarchy_id()

A new cgroup helper function, get_cgroup1_hierarchy_id(), has been
introduced to obtain the ID of a cgroup1 hierarchy based on the provided
cgroup name. This cgroup name can be obtained from the /proc/self/cgroup
file.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20231111090034.4248-6-laoar.shao@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/cgroup_helpers.c

@@ -637,3 +637,55 @@ unsigned long long get_classid_cgroup_id(void)
  format_classid_path(cgroup_workdir);
  return get_cgroup_id_from_path(cgroup_workdir);
 }
+
+/**
+ * get_cgroup1_hierarchy_id - Retrieves the ID of a cgroup1 hierarchy from the cgroup1 subsys name.
+ * @subsys_name: The cgroup1 subsys name, which can be retrieved from /proc/self/cgroup. It can be
+ * a named cgroup like "name=systemd", a controller name like "net_cls", or multi-contollers like
+ * "net_cls,net_prio".
+ */
+int get_cgroup1_hierarchy_id(const char *subsys_name)
+{
+ char *c, *c2, *c3, *c4;
+ bool found = false;
+ char line[1024];
+ FILE *file;
+ int i, id;
+
+ if (!subsys_name)
+ return -1;
+
+ file = fopen("/proc/self/cgroup", "r");
+ if (!file) {
+ log_err("fopen /proc/self/cgroup");
+ return -1;
+ }
+
+ while (fgets(line, 1024, file)) {
+ i = 0;
+ for (c = strtok_r(line, ":", &c2); c && i < 2; c = strtok_r(NULL, ":", &c2)) {
+ if (i == 0) {
+ id = strtol(c, NULL, 10);
+ } else if (i == 1) {
+ if (!strcmp(c, subsys_name)) {
+ found = true;
+ break;
+ }
+
+ /* Multiple subsystems may share one single mount point */
+ for (c3 = strtok_r(c, ",", &c4); c3;
+ c3 = strtok_r(NULL, ",", &c4)) {
+ if (!strcmp(c, subsys_name)) {
+ found = true;
+ break;
+ }
+ }
+ }
+ i++;
+ }
+ if (found)
+ break;
+ }
+ fclose(file);
+ return found ? id : -1;
+}

tools/testing/selftests/bpf/cgroup_helpers.h

@@ -20,6 +20,7 @@ int get_root_cgroup(void);
 int create_and_get_cgroup(const char *relative_path);
 void remove_cgroup(const char *relative_path);
 unsigned long long get_cgroup_id(const char *relative_path);
+int get_cgroup1_hierarchy_id(const char *subsys_name);
 
 int join_cgroup(const char *relative_path);
 int join_root_cgroup(void);