cgroup: replace cftype->mode with CFTYPE_WORLD_WRITABLE

cftype->mode allows controllers to give arbitrary permissions to
interface knobs.  Except for "cgroup.event_control", the existing uses
are spurious.

* Some explicitly specify S_IRUGO | S_IWUSR even though that's the
  default.

* "cpuset.memory_pressure" specifies S_IRUGO while also setting a
  write callback which returns -EACCES.  All it needs to do is simply
  not setting a write callback.

"cgroup.event_control" uses cftype->mode to make the file
world-writable.  It's a misdesigned interface and we don't want
controllers to be tweaking interface file permissions in general.
This patch removes cftype->mode and all its spurious uses and
implements CFTYPE_WORLD_WRITABLE for "cgroup.event_control" which is
marked as compatibility-only.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>

include/linux/cgroup-defs.h

@@ -76,6 +76,7 @@ enum {
 	CFTYPE_ONLY_ON_ROOT	= (1 << 0),	/* only create on root cgrp */
 	CFTYPE_NOT_ON_ROOT	= (1 << 1),	/* don't create on root cgrp */
 	CFTYPE_NO_PREFIX	= (1 << 3),	/* (DON'T USE FOR NEW FILES) no subsys prefix */
+	CFTYPE_WORLD_WRITABLE	= (1 << 4),	/* (DON'T USE FOR NEW FILES) S_IWUGO */
 
 	/* internal flags, do not use outside cgroup core proper */
 	__CFTYPE_ONLY_ON_DFL	= (1 << 16),	/* only on default hierarchy */
@@ -324,11 +325,6 @@ struct cftype {
 	 */
 	char name[MAX_CFTYPE_NAME];
 	unsigned long private;
-	/*
-	 * If not 0, file mode is set to this value, otherwise it will
-	 * be figured out automatically
-	 */
-	umode_t mode;
 
 	/*
 	 * The maximum length of string, excluding trailing nul, that can

kernel/cgroup.c

@@ -1139,23 +1139,21 @@ static char *cgroup_file_name(struct cgroup *cgrp, const struct cftype *cft,
  * cgroup_file_mode - deduce file mode of a control file
  * @cft: the control file in question
  *
- * returns cft->mode if ->mode is not 0
- * returns S_IRUGO|S_IWUSR if it has both a read and a write handler
- * returns S_IRUGO if it has only a read handler
- * returns S_IWUSR if it has only a write hander
+ * S_IRUGO for read, S_IWUSR for write.
  */
 static umode_t cgroup_file_mode(const struct cftype *cft)
 {
 	umode_t mode = 0;
 
-	if (cft->mode)
-		return cft->mode;
-
 	if (cft->read_u64 || cft->read_s64 || cft->seq_show)
 		mode |= S_IRUGO;
 
-	if (cft->write_u64 || cft->write_s64 || cft->write)
-		mode |= S_IWUSR;
+	if (cft->write_u64 || cft->write_s64 || cft->write) {
+		if (cft->flags & CFTYPE_WORLD_WRITABLE)
+			mode |= S_IWUGO;
+		else
+			mode |= S_IWUSR;
+	}
 
 	return mode;
 }
@@ -4371,7 +4369,6 @@ static struct cftype cgroup_dfl_base_files[] = {
 		.seq_show = cgroup_pidlist_show,
 		.private = CGROUP_FILE_PROCS,
 		.write = cgroup_procs_write,
-		.mode = S_IRUGO | S_IWUSR,
 	},
 	{
 		.name = "cgroup.controllers",
@@ -4406,7 +4403,6 @@ static struct cftype cgroup_legacy_base_files[] = {
 		.seq_show = cgroup_pidlist_show,
 		.private = CGROUP_FILE_PROCS,
 		.write = cgroup_procs_write,
-		.mode = S_IRUGO | S_IWUSR,
 	},
 	{
 		.name = "cgroup.clone_children",
@@ -4426,7 +4422,6 @@ static struct cftype cgroup_legacy_base_files[] = {
 		.seq_show = cgroup_pidlist_show,
 		.private = CGROUP_FILE_TASKS,
 		.write = cgroup_tasks_write,
-		.mode = S_IRUGO | S_IWUSR,
 	},
 	{
 		.name = "notify_on_release",

kernel/cpuset.c

@@ -1597,9 +1597,6 @@ static int cpuset_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
 	case FILE_MEMORY_PRESSURE_ENABLED:
 		cpuset_memory_pressure_enabled = !!val;
 		break;
-	case FILE_MEMORY_PRESSURE:
-		retval = -EACCES;
-		break;
 	case FILE_SPREAD_PAGE:
 		retval = update_flag(CS_SPREAD_PAGE, cs, val);
 		break;
@@ -1866,9 +1863,6 @@ static struct cftype files[] = {
 	{
 		.name = "memory_pressure",
 		.read_u64 = cpuset_read_u64,
-		.write_u64 = cpuset_write_u64,
-		.private = FILE_MEMORY_PRESSURE,
-		.mode = S_IRUGO,
 	},
 
 	{

mm/memcontrol.c

@@ -4060,8 +4060,7 @@ static struct cftype mem_cgroup_legacy_files[] = {
 	{
 		.name = "cgroup.event_control",		/* XXX: for compat */
 		.write = memcg_write_event_control,
-		.flags = CFTYPE_NO_PREFIX,
-		.mode = S_IWUGO,
+		.flags = CFTYPE_NO_PREFIX | CFTYPE_WORLD_WRITABLE,
 	},
 	{
 		.name = "swappiness",