cfg80211: Use const more consistently in for_each_element macros

Enforce the first argument to be a correct type of a pointer to struct
element and avoid unnecessary typecasts from const to non-const pointers
(the change in validate_ie_attr() is needed to make this part work). In
addition, avoid signed/unsigned comparison within for_each_element() and
mark struct element packed just in case.
Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/linux/ieee80211.h

@@ -3284,16 +3284,16 @@ struct element {
 	u8 id;
 	u8 datalen;
 	u8 data[];
-};
+} __packed;
 
 /* element iteration helpers */
-#define for_each_element(element, _data, _datalen)			\
-	for (element = (void *)(_data);					\
-	     (u8 *)(_data) + (_datalen) - (u8 *)element >=		\
-		sizeof(*element) &&					\
-	     (u8 *)(_data) + (_datalen) - (u8 *)element >=		\
-		sizeof(*element) + element->datalen;			\
-	     element = (void *)(element->data + element->datalen))
+#define for_each_element(_elem, _data, _datalen)			\
+	for (_elem = (const struct element *)(_data);			\
+	     (const u8 *)(_data) + (_datalen) - (const u8 *)_elem >=	\
+		(int)sizeof(*_elem) &&					\
+	     (const u8 *)(_data) + (_datalen) - (const u8 *)_elem >=	\
+		(int)sizeof(*_elem) + _elem->datalen;			\
+	     _elem = (const struct element *)(_elem->data + _elem->datalen))
 
 #define for_each_element_id(element, _id, data, datalen)		\
 	for_each_element(element, data, datalen)			\
@@ -3330,7 +3330,7 @@ struct element {
 static inline bool for_each_element_completed(const struct element *element,
 					      const void *data, size_t datalen)
 {
-	return (u8 *)element == (u8 *)data + datalen;
+	return (const u8 *)element == (const u8 *)data + datalen;
 }
 
 #endif /* LINUX_IEEE80211_H */

net/wireless/nl80211.c

@@ -205,7 +205,7 @@ static int validate_ie_attr(const struct nlattr *attr,
 {
 	const u8 *data = nla_data(attr);
 	unsigned int len = nla_len(attr);
-	struct element *elem;
+	const struct element *elem;
 
 	for_each_element(elem, data, len) {
 		/* nothing */