wifi: cfg80211: Annotate struct cfg80211_pmsr_request 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_pmsr_request.
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

@@ -3948,7 +3948,7 @@ struct cfg80211_pmsr_request {
 
 	struct list_head list;
 
-	struct cfg80211_pmsr_request_peer peers[];
+	struct cfg80211_pmsr_request_peer peers[] __counted_by(n_peers);
 };
 
 /**

net/wireless/pmsr.c

@@ -291,6 +291,7 @@ int nl80211_pmsr_start(struct sk_buff *skb, struct genl_info *info)
 	req = kzalloc(struct_size(req, peers, count), GFP_KERNEL);
 	if (!req)
 		return -ENOMEM;
+	req->n_peers = count;
 
 	if (info->attrs[NL80211_ATTR_TIMEOUT])
 		req->timeout = nla_get_u32(info->attrs[NL80211_ATTR_TIMEOUT]);
@@ -321,8 +322,6 @@ int nl80211_pmsr_start(struct sk_buff *skb, struct genl_info *info)
 			goto out_err;
 		idx++;
 	}
-
-	req->n_peers = count;
 	req->cookie = cfg80211_assign_cookie(rdev);
 	req->nl_portid = info->snd_portid;