wifi: mac80211: Add __counted_by for struct ieee802_11_elems and use struct_size()

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

While there, use struct_size() helper, instead of the open-coded
version, to calculate the size for the allocation of the whole
flexible structure including, of course, the flexible-array member.

This code was found with the help of Coccinelle, and audited and
fixed manually.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/ZSQ/jcmTAf/PKHg/@workSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/ieee80211_i.h

@@ -1746,7 +1746,7 @@ struct ieee802_11_elems {
 	 */
 	size_t scratch_len;
 	u8 *scratch_pos;
-	u8 scratch[];
+	u8 scratch[] __counted_by(scratch_len);
 };
 
 static inline struct ieee80211_local *hw_to_local(

net/mac80211/util.c

@@ -1625,7 +1625,7 @@ ieee802_11_parse_elems_full(struct ieee80211_elems_parse_params *params)
 	int nontransmitted_profile_len = 0;
 	size_t scratch_len = 3 * params->len;
 
-	elems = kzalloc(sizeof(*elems) + scratch_len, GFP_ATOMIC);
+	elems = kzalloc(struct_size(elems, scratch, scratch_len), GFP_ATOMIC);
 	if (!elems)
 		return NULL;
 	elems->ie_start = params->start;