clone3: add build-time CLONE_ARGS_SIZE_VER* validity checks

CLONE_ARGS_SIZE_VER* macros are defined explicitly and not via
the offsets of the relevant struct clone_args fields, which makes
it rather error-prone, so it probably makes sense to add some
compile-time checks for them (including the one that breaks
on struct clone_args extension as a reminder to add a relevant
size macro and a similar check).  Function copy_clone_args_from_user
seems to be a good place for such checks.

Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20200412202658.GA31499@asgard.redhat.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

kernel/fork.c

@@ -2605,6 +2605,14 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
 	struct clone_args args;
 	pid_t *kset_tid = kargs->set_tid;
 
+	BUILD_BUG_ON(offsetofend(struct clone_args, tls) !=
+		     CLONE_ARGS_SIZE_VER0);
+	BUILD_BUG_ON(offsetofend(struct clone_args, set_tid_size) !=
+		     CLONE_ARGS_SIZE_VER1);
+	BUILD_BUG_ON(offsetofend(struct clone_args, cgroup) !=
+		     CLONE_ARGS_SIZE_VER2);
+	BUILD_BUG_ON(sizeof(struct clone_args) != CLONE_ARGS_SIZE_VER2);
+
 	if (unlikely(usize > PAGE_SIZE))
 		return -E2BIG;
 	if (unlikely(usize < CLONE_ARGS_SIZE_VER0))