wifi: cfg80211: Annotate struct cfg80211_mbssid_elems with __counted_by

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 checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct cfg80211_mbssid_elems.
Additionally, since the element count member must be set before accessing
the annotated flexible array member, move its initialization earlier.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Johannes Berg <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: Paolo Abeni <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Gustavo A. R. Silva <[email protected]>
Reviewed-by: Jeff Johnson <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Johannes Berg <[email protected]>

Author: kees

include/net/cfg80211.h

@@ -1187,7 +1187,7 @@ struct cfg80211_mbssid_elems {
 	struct {
 		const u8 *data;
 		size_t len;
-	} elem[];
+	} elem[] __counted_by(cnt);
 };
 
 /**

net/wireless/nl80211.c

@@ -5438,13 +5438,13 @@ nl80211_parse_mbssid_elems(struct wiphy *wiphy, struct nlattr *attrs)
 	elems = kzalloc(struct_size(elems, elem, num_elems), GFP_KERNEL);
 	if (!elems)
 		return ERR_PTR(-ENOMEM);
+	elems->cnt = num_elems;
 
 	nla_for_each_nested(nl_elems, attrs, rem_elems) {
 		elems->elem[i].data = nla_data(nl_elems);
 		elems->elem[i].len = nla_len(nl_elems);
 		i++;
 	}
-	elems->cnt = num_elems;
 	return elems;
 }