[PATCH 3/4] cgroup: Use __counted_by for cgroup::ancestors

["Michal Koutný" <mkoutny@suse.com>]

cgroup::ancestors includes self, i.e. root cgroups have one ancestor but
their level is 0. Change the value that we store inside struct cgroup
and use an inlined helper where we need to know the level. This way we
preserve the concept of 0-based levels and we can utilize __counted_by
constraint to guard ancestors access. (We could've used level value as a
counter for _low_ancestors but that would have no benefit since we never
access data through this flexible array alias.)

Cc: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Signed-off-by: Michal Koutný <mkoutny@suse.com>
---
 include/linux/cgroup-defs.h | 19 ++++++++-----------
 include/linux/cgroup.h      |  2 +-
 kernel/cgroup/cgroup.c      |  3 ++-
 3 files changed, 11 insertions(+), 13 deletions(-)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 9247e437da5ce..8ce1ae9bea909 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -475,14 +475,6 @@ struct cgroup {
 
 	unsigned long flags;		/* "unsigned long" so bitops work */
 
-	/*
-	 * The depth this cgroup is at.  The root is at depth zero and each
-	 * step down the hierarchy increments the level.  This along with
-	 * ancestors[] can determine whether a given cgroup is a
-	 * descendant of another without traversing the hierarchy.
-	 */
-	int level;
-
 	/* Maximum allowed descent tree depth */
 	int max_depth;
 
@@ -625,13 +617,18 @@ struct cgroup {
 	struct bpf_local_storage __rcu  *bpf_cgrp_storage;
 #endif
 
-	/* All ancestors including self */
 	union {
 		struct {
-			void *_sentinel[0]; /* XXX to avoid 'flexible array member in a struct with no named members' */
-			struct cgroup *ancestors[];
+			int nr_ancestors;	/* do not use directly but via cgroup_level() */
+			/*
+			 * All ancestors including self.
+			 * ancestors[] can determine whether a given cgroup is a
+			 * descendant of another without traversing the hierarchy.
+			 */
+			struct cgroup *ancestors[] __counted_by(nr_ancestors);
 		};
 		struct {
+			int _nr_ancestors;	/* auxiliary padding, see nr_ancestors above */
 			struct cgroup *_root_ancestor;
 			struct cgroup *_low_ancestors[];
 		};
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 0290878ebad26..45f720b9ecedd 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -534,7 +534,7 @@ static inline struct cgroup *cgroup_parent(struct cgroup *cgrp)
  */
 static inline int cgroup_level(struct cgroup *cgrp)
 {
-	return cgrp->level;
+	return cgrp->nr_ancestors - 1;
 }
 
 /**
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index e011f1dd6d87f..5110d3e13d125 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -2197,6 +2197,7 @@ int cgroup_setup_root(struct cgroup_root *root, u16 ss_mask)
 	}
 	root_cgrp->kn = kernfs_root_to_node(root->kf_root);
 	WARN_ON_ONCE(cgroup_ino(root_cgrp) != 1);
+	root_cgrp->nr_ancestors = 1; /* stored in _root_ancestor */
 	root_cgrp->ancestors[0] = root_cgrp;
 
 	ret = css_populate_dir(&root_cgrp->self);
@@ -5869,7 +5870,7 @@ static struct cgroup *cgroup_create(struct cgroup *parent, const char *name,
 
 	cgrp->self.parent = &parent->self;
 	cgrp->root = root;
-	cgrp->level = level;
+	cgrp->nr_ancestors = parent->nr_ancestors + 1;
 
 	/*
 	 * Now that init_cgroup_housekeeping() has been called and cgrp->self
-- 
2.52.0