selftests/bpf: Run cgroup1_hierarchy test in own mount namespace

The cgroup1_hierarchy test uses setup_classid_environment to setup
cgroupv1 environment. The problem is that the environment is set in
/sys/fs/cgroup and therefore, if not run under an own mount namespace,
effectively deletes all system cgroups:

    $ ls /sys/fs/cgroup | wc -l
    27
    $ sudo ./test_progs -t cgroup1_hierarchy
    #41/1    cgroup1_hierarchy/test_cgroup1_hierarchy:OK
    #41/2    cgroup1_hierarchy/test_root_cgid:OK
    #41/3    cgroup1_hierarchy/test_invalid_level:OK
    #41/4    cgroup1_hierarchy/test_invalid_cgid:OK
    #41/5    cgroup1_hierarchy/test_invalid_hid:OK
    #41/6    cgroup1_hierarchy/test_invalid_cgrp_name:OK
    #41/7    cgroup1_hierarchy/test_invalid_cgrp_name2:OK
    #41/8    cgroup1_hierarchy/test_sleepable_prog:OK
    #41      cgroup1_hierarchy:OK
    Summary: 1/8 PASSED, 0 SKIPPED, 0 FAILED
    $ ls /sys/fs/cgroup | wc -l
    1

To avoid this, run setup_cgroup_environment first which will create an
own mount namespace. This only affects the cgroupv1_hierarchy test as
all other cgroup1 test progs already run setup_cgroup_environment prior
to running setup_classid_environment.

Also add a comment to the header of setup_classid_environment to warn
against this invalid usage in future.

Fixes: 36076923 ("selftests/bpf: Add selftests for cgroup1 hierarchy")
Signed-off-by: Viktor Malik <vmalik@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240429112311.402497-1-vmalik@redhat.com

tools/testing/selftests/bpf/cgroup_helpers.c

@@ -508,6 +508,9 @@ int cgroup_setup_and_join(const char *path) {
 /**
  * setup_classid_environment() - Setup the cgroupv1 net_cls environment
  *
+ * This function should only be called in a custom mount namespace, e.g.
+ * created by running setup_cgroup_environment.
+ *
  * After calling this function, cleanup_classid_environment should be called
  * once testing is complete.
  *

tools/testing/selftests/bpf/prog_tests/cgroup1_hierarchy.c

@@ -87,9 +87,12 @@ void test_cgroup1_hierarchy(void)
 		goto destroy;
 
 	/* Setup cgroup1 hierarchy */
+	err = setup_cgroup_environment();
+	if (!ASSERT_OK(err, "setup_cgroup_environment"))
+		goto destroy;
 	err = setup_classid_environment();
 	if (!ASSERT_OK(err, "setup_classid_environment"))
-		goto destroy;
+		goto cleanup_cgroup;
 
 	err = join_classid();
 	if (!ASSERT_OK(err, "join_cgroup1"))
@@ -153,6 +156,8 @@ void test_cgroup1_hierarchy(void)
 
 cleanup:
 	cleanup_classid_environment();
+cleanup_cgroup:
+	cleanup_cgroup_environment();
 destroy:
 	test_cgroup1_hierarchy__destroy(skel);
 }