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>

1 files changed, 1 insertions, 5 deletions

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index d95cc88e9dc2..10d814bcd487 100644
--- a/include/linux/cgroup-defs.h
+++ b/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