wifi: cfg80211: Annotate struct cfg80211_acl_data 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_acl_data.
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 <johannes@sipsolutions.net>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Link: https://lore.kernel.org/r/20230817211531.4193219-1-keescook@chromium.orgSigned-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/cfg80211.h

@@ -1282,7 +1282,7 @@ struct cfg80211_acl_data {
 	int n_acl_entries;
 
 	/* Keep it last */
-	struct mac_address mac_addrs[];
+	struct mac_address mac_addrs[] __counted_by(n_acl_entries);
 };
 
 /**

net/wireless/nl80211.c

@@ -4889,13 +4889,12 @@ static struct cfg80211_acl_data *parse_acl_data(struct wiphy *wiphy,
 	acl = kzalloc(struct_size(acl, mac_addrs, n_entries), GFP_KERNEL);
 	if (!acl)
 		return ERR_PTR(-ENOMEM);
+	acl->n_acl_entries = n_entries;
 
 	nla_for_each_nested(attr, info->attrs[NL80211_ATTR_MAC_ADDRS], tmp) {
 		memcpy(acl->mac_addrs[i].addr, nla_data(attr), ETH_ALEN);
 		i++;
 	}
-
-	acl->n_acl_entries = n_entries;
 	acl->acl_policy = acl_policy;
 
 	return acl;