net/sched: wrap open coded Qdics class filter counter

The 'filter_cnt' counter is used to control a Qdisc class lifetime.
Each filter referecing this class by its id will eventually
increment/decrement this counter in their respective
'add/update/delete' routines.
As these operations are always serialized under rtnl lock, we don't
need an atomic type like 'refcount_t'.

It also means that we lose the overflow/underflow checks already
present in refcount_t, which are valuable to hunt down bugs
where the unsigned counter wraps around as it aids automated tools
like syzkaller to scream in such situations.

Wrap the open coded increment/decrement into helper functions and
add overflow checks to the operations.

Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 26 insertions, 0 deletions

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 15be2d96b06d..f232512505f8 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -599,6 +599,7 @@ get_default_qdisc_ops(const struct net_device *dev, int ntx)
 
 struct Qdisc_class_common {
 	u32			classid;
+	unsigned int		filter_cnt;
 	struct hlist_node	hnode;
 };
 
@@ -633,6 +634,31 @@ qdisc_class_find(const struct Qdisc_class_hash *hash, u32 id)
 	return NULL;
 }
 
+static inline bool qdisc_class_in_use(const struct Qdisc_class_common *cl)
+{
+	return cl->filter_cnt > 0;
+}
+
+static inline void qdisc_class_get(struct Qdisc_class_common *cl)
+{
+	unsigned int res;
+
+	if (check_add_overflow(cl->filter_cnt, 1, &res))
+		WARN(1, "Qdisc class overflow");
+
+	cl->filter_cnt = res;
+}
+
+static inline void qdisc_class_put(struct Qdisc_class_common *cl)
+{
+	unsigned int res;
+
+	if (check_sub_overflow(cl->filter_cnt, 1, &res))
+		WARN(1, "Qdisc class underflow");
+
+	cl->filter_cnt = res;
+}
+
 static inline int tc_classid_to_hwtc(struct net_device *dev, u32 classid)
 {
 	u32 hwtc = TC_H_MIN(classid) - TC_H_MIN_PRIORITY;