selftests/cgroup: add tests for cloning into cgroups

Expand the cgroup test-suite to include tests for CLONE_INTO_CGROUP.
This adds the following tests:
- CLONE_INTO_CGROUP manages to clone a process directly into a correctly
  delegated cgroup
- CLONE_INTO_CGROUP fails to clone a process into a cgroup that has been
  removed after we've opened an fd to it
- CLONE_INTO_CGROUP fails to clone a process into an invalid domain
  cgroup
- CLONE_INTO_CGROUP adheres to the no internal process constraint
- CLONE_INTO_CGROUP works with the freezer feature

Cc: Tejun Heo <tj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: cgroups@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Acked-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

tools/testing/selftests/clone3/clone3_selftests.h

@@ -5,12 +5,24 @@
 
 #define _GNU_SOURCE
 #include <sched.h>
+#include <linux/sched.h>
+#include <linux/types.h>
 #include <stdint.h>
 #include <syscall.h>
-#include <linux/types.h>
+#include <sys/wait.h>
+
+#include "../kselftest.h"
 
 #define ptr_to_u64(ptr) ((__u64)((uintptr_t)(ptr)))
 
+#ifndef CLONE_INTO_CGROUP
+#define CLONE_INTO_CGROUP 0x200000000ULL /* Clone into a specific cgroup given the right permissions. */
+#endif
+
+#ifndef CLONE_ARGS_SIZE_VER0
+#define CLONE_ARGS_SIZE_VER0 64
+#endif
+
 #ifndef __NR_clone3
 #define __NR_clone3 -1
 struct clone_args {
@@ -22,10 +34,13 @@ struct clone_args {
 	__aligned_u64 stack;
 	__aligned_u64 stack_size;
 	__aligned_u64 tls;
+#define CLONE_ARGS_SIZE_VER1 80
 	__aligned_u64 set_tid;
 	__aligned_u64 set_tid_size;
+#define CLONE_ARGS_SIZE_VER2 88
+	__aligned_u64 cgroup;
 };
-#endif
+#endif /* __NR_clone3 */
 
 static pid_t sys_clone3(struct clone_args *args, size_t size)
 {