cgroup: export list of delegatable control files using sysfs

Delegatable cgroup v2 control files may require special handling
(e.g. chowning), and the exact list of such files varies between
kernel versions (and likely to be extended in the future).

To guarantee correctness of this list and simplify the life
of userspace (systemd, first of all), let's export the list
via /sys/kernel/cgroup/delegate pseudo-file.

Format is siple: each control file name is printed on a new line.
Example:
  $ cat /sys/kernel/cgroup/delegate
  cgroup.procs
  cgroup.subtree_control

Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: kernel-team@fb.com
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index d6ed725f36d99d21808e41b92d99de8ad4ea2262..eed92ed624e56a02f84b58ce4b651faab97b26ed 100644 (file)
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5832,3 +5832,64 @@ int cgroup_bpf_update(struct cgroup *cgrp, struct bpf_prog *prog,
        return ret;
 }
 #endif /* CONFIG_CGROUP_BPF */
+
+#ifdef CONFIG_SYSFS
+static ssize_t show_delegatable_files(struct cftype *files, char *buf,
+                                     ssize_t size, const char *prefix)
+{
+       struct cftype *cft;
+       ssize_t ret = 0;
+
+       for (cft = files; cft && cft->name[0] != '\0'; cft++) {
+               if (!(cft->flags & CFTYPE_NS_DELEGATABLE))
+                       continue;
+
+               if (prefix)
+                       ret += snprintf(buf + ret, size - ret, "%s.", prefix);
+
+               ret += snprintf(buf + ret, size - ret, "%s\n", cft->name);
+
+               if (unlikely(ret >= size)) {
+                       WARN_ON(1);
+                       break;
+               }
+       }
+
+       return ret;
+}
+
+static ssize_t delegate_show(struct kobject *kobj, struct kobj_attribute *attr,
+                             char *buf)
+{
+       struct cgroup_subsys *ss;
+       int ssid;
+       ssize_t ret = 0;
+
+       ret = show_delegatable_files(cgroup_base_files, buf, PAGE_SIZE - ret,
+                                    NULL);
+
+       for_each_subsys(ss, ssid)
+               ret += show_delegatable_files(ss->dfl_cftypes, buf + ret,
+                                             PAGE_SIZE - ret,
+                                             cgroup_subsys_name[ssid]);
+
+       return ret;
+}
+static struct kobj_attribute cgroup_delegate_attr = __ATTR_RO(delegate);
+
+static struct attribute *cgroup_sysfs_attrs[] = {
+       &cgroup_delegate_attr.attr,
+       NULL,
+};
+
+static const struct attribute_group cgroup_sysfs_attr_group = {
+       .attrs = cgroup_sysfs_attrs,
+       .name = "cgroup",
+};
+
+static int __init cgroup_sysfs_init(void)
+{
+       return sysfs_create_group(kernel_kobj, &cgroup_sysfs_attr_group);
+}
+subsys_initcall(cgroup_sysfs_init);
+#endif /* CONFIG_SYSFS */