[PATCH] HID: core: replace the collection tree pointers with indices

[Peter Hutterer <peter.hutterer@xxxxxxxxx>]

Introduced in c53431eb696f3c64c12c00afb81048af54b61532
"HID: core: store the collections as a basic tree".

Previously, the pointer to the parent collection was stored. If a device
exceeds 16 collections (HID_DEFAULT_NUM_COLLECTIONS), the array to store
the collections is reallocated, the pointer to the parent collection becomes
invalid.

Replace the pointers with an index-based lookup into the collections array.

Reported-by: Pandruvada, Srinivas <srinivas.pandruvada@xxxxxxxxx>
Signed-off-by: Peter Hutterer <peter.hutterer@xxxxxxxxx>
---
I have a test locally for hid-tools now but it relies on the kernel
crashing, so it's going to generate false positives. I verified the test
with printks though.

 drivers/hid/hid-core.c | 32 +++++++++++++++++++++-----------
 include/linux/hid.h    |  4 ++--
 2 files changed, 23 insertions(+), 13 deletions(-)

diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
index f41d5fe51abe..f9093dedf647 100644
--- a/drivers/hid/hid-core.c
+++ b/drivers/hid/hid-core.c
@@ -125,6 +125,7 @@ static int open_collection(struct hid_parser *parser, unsigned type)
 {
 	struct hid_collection *collection;
 	unsigned usage;
+	int collection_index;
 
 	usage = parser->local.usage[0];
 
@@ -167,13 +168,13 @@ static int open_collection(struct hid_parser *parser, unsigned type)
 	parser->collection_stack[parser->collection_stack_ptr++] =
 		parser->device->maxcollection;
 
-	collection = parser->device->collection +
-		parser->device->maxcollection++;
+	collection_index = parser->device->maxcollection++;
+	collection = parser->device->collection + collection_index;
 	collection->type = type;
 	collection->usage = usage;
 	collection->level = parser->collection_stack_ptr - 1;
-	collection->parent = parser->active_collection;
-	parser->active_collection = collection;
+	collection->parent_idx = parser->active_collection_idx;
+	parser->active_collection_idx = collection_index;
 
 	if (type == HID_COLLECTION_APPLICATION)
 		parser->device->maxapplication++;
@@ -192,8 +193,13 @@ static int close_collection(struct hid_parser *parser)
 		return -EINVAL;
 	}
 	parser->collection_stack_ptr--;
-	if (parser->active_collection)
-		parser->active_collection = parser->active_collection->parent;
+	if (parser->active_collection_idx != -1) {
+		struct hid_device *device = parser->device;
+		struct hid_collection *c;
+
+		c = &device->collection[parser->active_collection_idx];
+		parser->active_collection_idx = c->parent_idx;
+	}
 	return 0;
 }
 
@@ -819,6 +825,7 @@ static int hid_scan_report(struct hid_device *hid)
 		return -ENOMEM;
 
 	parser->device = hid;
+	parser->active_collection_idx = -1;
 	hid->group = HID_GROUP_GENERIC;
 
 	/*
@@ -1006,10 +1013,12 @@ static void hid_apply_multiplier_to_field(struct hid_device *hid,
 		usage = &field->usage[i];
 
 		collection = &hid->collection[usage->collection_index];
-		while (collection && collection != multiplier_collection)
-			collection = collection->parent;
+		while (collection->parent_idx != -1 &&
+		       collection != multiplier_collection)
+			collection = &hid->collection[collection->parent_idx];
 
-		if (collection || multiplier_collection == NULL)
+		if (collection->parent_idx != -1 ||
+		    multiplier_collection == NULL)
 			usage->resolution_multiplier = effective_multiplier;
 
 	}
@@ -1044,9 +1053,9 @@ static void hid_apply_multiplier(struct hid_device *hid,
 	 * applicable fields later.
 	 */
 	multiplier_collection = &hid->collection[multiplier->usage->collection_index];
-	while (multiplier_collection &&
+	while (multiplier_collection->parent_idx != -1 &&
 	       multiplier_collection->type != HID_COLLECTION_LOGICAL)
-		multiplier_collection = multiplier_collection->parent;
+		multiplier_collection = &hid->collection[multiplier_collection->parent_idx];
 
 	effective_multiplier = hid_calculate_multiplier(hid, multiplier);
 
@@ -1170,6 +1179,7 @@ int hid_open_report(struct hid_device *device)
 	}
 
 	parser->device = device;
+	parser->active_collection_idx = -1;
 
 	end = start + size;
 
diff --git a/include/linux/hid.h b/include/linux/hid.h
index 93db548f8761..554e82d812da 100644
--- a/include/linux/hid.h
+++ b/include/linux/hid.h
@@ -429,7 +429,7 @@ struct hid_local {
  */
 
 struct hid_collection {
-	struct hid_collection *parent;
+	int parent_idx; /* device->collection */
 	unsigned type;
 	unsigned usage;
 	unsigned level;
@@ -657,7 +657,7 @@ struct hid_parser {
 	unsigned int         *collection_stack;
 	unsigned int          collection_stack_ptr;
 	unsigned int          collection_stack_size;
-	struct hid_collection *active_collection;
+	int                   active_collection_idx; /* device->collection */
 	struct hid_device    *device;
 	unsigned int          scan_flags;
 };
-- 
2.19.2