core/namespace: stop applying mount options on private cgroupfs mount

We always unshare cgroup ns for ProtectControlGroups=private/strict,
while the mount options only apply to the cgroupfs instance
in initial cgns (c.f.
https://github.com/torvalds/linux/blob/b69bb476dee99d564d65d418e9a20acca6f32c3f/kernel/cgroup/cgroup.c#L1984)
Hence let's drop the thing wholesale.

Also, as noted in the comment already, mount_private_apivfs()
internally enforces nosuid/noexec, so drop explicit flags too.

src/core/namespace.c

@@ -26,7 +26,6 @@
 #include "loopback-setup.h"
 #include "missing_syscall.h"
 #include "mkdir-label.h"
-#include "mount-setup.h"
 #include "mount-util.h"
 #include "mountpoint-util.h"
 #include "namespace-util.h"
@@ -207,14 +206,14 @@ static const MountEntry protect_control_groups_yes_table[] = {
 };
 
 /* ProtectControlGroups=private table. Note mount_private_apivfs() always use MS_NOSUID|MS_NOEXEC|MS_NODEV so
- * flags is not set here. nsdelegate has been supported since kernels >= 4.13 so it is safe to use. */
+ * flags is not set here. */
 static const MountEntry protect_control_groups_private_table[] = {
-        { "/sys/fs/cgroup",      MOUNT_PRIVATE_CGROUP2FS, false, .read_only = false, .nosuid = true, .noexec = true, .options_const = "nsdelegate"  },
+        { "/sys/fs/cgroup",      MOUNT_PRIVATE_CGROUP2FS, false, .read_only = false },
 };
 
 /* ProtectControlGroups=strict table */
 static const MountEntry protect_control_groups_strict_table[] = {
-        { "/sys/fs/cgroup",      MOUNT_PRIVATE_CGROUP2FS, false, .read_only = true,  .nosuid = true, .noexec = true, .options_const = "nsdelegate"  },
+        { "/sys/fs/cgroup",      MOUNT_PRIVATE_CGROUP2FS, false, .read_only = true },
 };
 
 /* ProtectSystem=yes table */
@@ -338,7 +337,7 @@ static bool mount_entry_read_only(const MountEntry *p) {
 static bool mount_entry_noexec(const MountEntry *p) {
         assert(p);
 
-        return p->noexec || IN_SET(p->mode, MOUNT_NOEXEC, MOUNT_INACCESSIBLE, MOUNT_PRIVATE_SYSFS, MOUNT_BIND_SYSFS, MOUNT_PROCFS);
+        return p->noexec || IN_SET(p->mode, MOUNT_NOEXEC, MOUNT_INACCESSIBLE, MOUNT_PRIVATE_SYSFS, MOUNT_BIND_SYSFS, MOUNT_PROCFS, MOUNT_PRIVATE_CGROUP2FS);
 }
 
 static bool mount_entry_exec(const MountEntry *p) {
@@ -1375,18 +1374,9 @@ static int mount_private_sysfs(const MountEntry *m, const NamespaceParameters *p
 }
 
 static int mount_private_cgroup2fs(const MountEntry *m, const NamespaceParameters *p) {
-        _cleanup_free_ char *opts = NULL;
-
         assert(m);
         assert(p);
-
-        if (cgroupfs_recursiveprot_supported()) {
-                opts = strextend_with_separator(NULL, ",", mount_entry_options(m) ?: POINTER_MAX, "memory_recursiveprot");
-                if (!opts)
-                        return -ENOMEM;
-        }
-
-        return mount_private_apivfs("cgroup2", mount_entry_path(m), "/sys/fs/cgroup", opts ?: mount_entry_options(m), p->runtime_scope);
+        return mount_private_apivfs("cgroup2", mount_entry_path(m), "/sys/fs/cgroup", /* opts = */ NULL, p->runtime_scope);
 }
 
 static int mount_procfs(const MountEntry *m, const NamespaceParameters *p) {

src/shared/mount-setup.c

@@ -52,7 +52,7 @@ typedef struct MountPoint {
         MountMode mode;
 } MountPoint;
 
-bool cgroupfs_recursiveprot_supported(void) {
+static bool cgroupfs_recursiveprot_supported(void) {
         int r;
 
         /* Added in kernel 5.7 */

src/shared/mount-setup.h

@@ -8,5 +8,3 @@ bool mount_point_ignore(const char *path);
 
 int mount_setup_early(void);
 int mount_setup(bool loaded_policy, bool leave_propagation);
-
-bool cgroupfs_recursiveprot_supported(void);