cfg80211: add and use strongly typed element iteration macros

BugLink: https://bugs.launchpad.net/bugs/1848780

commit 0f3b07f0 upstream.

Rather than always iterating elements from frames with pure
u8 pointers, add a type "struct element" that encapsulates
the id/datalen/data format of them.

Then, add the element iteration macros
 * for_each_element
 * for_each_element_id
 * for_each_element_extid

which take, as their first 'argument', such a structure and
iterate through a given u8 array interpreting it as elements.

While at it and since we'll need it, also add
 * for_each_subelement
 * for_each_subelement_id
 * for_each_subelement_extid

which instead of taking data/length just take an outer element
and use its data/datalen.

Also add for_each_element_completed() to determine if any of
the loops above completed, i.e. it was able to parse all of
the elements successfully and no data remained.

Use for_each_element_id() in cfg80211_find_ie_match() as the
first user of this.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

include/linux/ieee80211.h

@@ -2550,4 +2550,57 @@ static inline bool ieee80211_action_contains_tpc(struct sk_buff *skb)
 	return true;
 }
 
+struct element {
+	u8 id;
+	u8 datalen;
+	u8 data[];
+};
+
+/* 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_id(element, _id, data, datalen)		\
+	for_each_element(element, data, datalen)			\
+		if (element->id == (_id))
+
+#define for_each_element_extid(element, extid, data, datalen)		\
+	for_each_element(element, data, datalen)			\
+		if (element->id == WLAN_EID_EXTENSION &&		\
+		    element->datalen > 0 &&				\
+		    element->data[0] == (extid))
+
+#define for_each_subelement(sub, element)				\
+	for_each_element(sub, (element)->data, (element)->datalen)
+
+#define for_each_subelement_id(sub, id, element)			\
+	for_each_element_id(sub, id, (element)->data, (element)->datalen)
+
+#define for_each_subelement_extid(sub, extid, element)			\
+	for_each_element_extid(sub, extid, (element)->data, (element)->datalen)
+
+/**
+ * for_each_element_completed - determine if element parsing consumed all data
+ * @element: element pointer after for_each_element() or friends
+ * @data: same data pointer as passed to for_each_element() or friends
+ * @datalen: same data length as passed to for_each_element() or friends
+ *
+ * This function returns %true if all the data was parsed or considered
+ * while walking the elements. Only use this if your for_each_element()
+ * loop cannot be broken out of, otherwise it always returns %false.
+ *
+ * If some data was malformed, this returns %false since the last parsed
+ * element will not fill the whole remaining data.
+ */
+static inline bool for_each_element_completed(const struct element *element,
+					      const void *data, size_t datalen)
+{
+	return (u8 *)element == (u8 *)data + datalen;
+}
+
 #endif /* LINUX_IEEE80211_H */